SolidWorks Flow Simulation 2012

5/19/2018 SolidWorks Flow Simulation 2012

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3/90Solving Engineering Problems with Flow Simulation 2012 i

Solving Engineering Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1 Simulating Engineering Tasks with Flow Simulation . . . . . . . . . . . . . . . 1-4Selecting Geometrical and Physical Features of the Task. . . . . . . . . . . . . . . . . . . . 1-4Creating the Model and the Flow Simulation Project. . . . . . . . . . . . . . . . . . . . . . . 1-5

2 Solving Engineering Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6

Strategy of Solving the Engineering Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6Settings for Resolving the Geometrical Features of the Model and for Obtaining the Required

Solution Accuracy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7Monitoring the Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9Viewing and Analyzing the Obtained Solution. . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

Estimating the Reliability and Adequacy of the Obtained Solution . . . . . . . . . . . . . 1-103 Frequent Errors and Improper Actions . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11

Advanced Knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

1 Mesh - Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Types of Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1Mesh Construction Stages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3Basic Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4

Control Planes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5Resolving Small Features by Using the Control Planes. . . . . . . . . . . . . . . . . . . . . 2-5Contracting the Basic Mesh. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5Resolving Small Solid Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7Curvature Refinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7Tolerance Refinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9Narrow Channel Refinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10Local Mesh Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12

Contents


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Recommendations for Creating the Computational Mesh . . . . . . . . . . . . . . . . . . . 2-13

2 Mesh-associated Tools. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-14

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-14Visualizing the Basic Mesh Before Constructing the Initial Mesh. . . . . . . . . . . . . 2-14Enhanced Capabilities of the Results Loading . . . . . . . . . . . . . . . . . . . . . . . . . . 2-14

Viewing the Initial Computational Mesh Saved in the .cpt Files . . . . . . . . . . . . . . 2-16Viewing the Computational Mesh Cells with the Mesh Option . . . . . . . . . . . . . . . 2-16Visualizing the Real Computational Geometry . . . . . . . . . . . . . . . . . . . . . . . . . 2-17Switching off the Interpolation and Extrapolation of Calculation Results . . . . . . . . 2-19Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20

3 Meshing - Additional Insight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20

Initial Mesh Generation Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-21Basic Mesh Generation and Resolving the Interface . . . . . . . . . . . . . . . . . . . . 2-21Narrow Channel Refinement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-23Thin walls resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-25Square Difference Refinement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-26Mesh Diagnostic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-28

Refinements at Interfaces Between Substances . . . . . . . . . . . . . . . . . . . . . . . . . 2-28Small Solid Features Refinement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-28Curvature Refinement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-29SSFRL or CRL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-30Tolerance Refinement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-31

Local Mesh Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-31The "Optimize thin walls resolution" option . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-32Postamble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-33

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-334 Calculation Control Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-33

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-33Finishing the Calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-34Refinement of the Computational Mesh During Calculation . . . . . . . . . . . . . . . . . 2-36

5 Flow Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-38

What is Flow Freezing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-38How It Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-38

Flow Freezing in a Permanent Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-39

Flow Freezing in a Periodic Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-40Advanced Features Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

1 Cavitation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Physical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2Engineering Cavitation Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2Isothermal Cavitation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3


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Examples of use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

2 Steam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Physical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Example of use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

3 Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

Physical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12Example of use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

4 Real Gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

Physical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15

Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16Example of use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19

5 Rotation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20

Physical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20Local Rotating Regions - Additional Information. . . . . . . . . . . . . . . . . . . . . . . 3-21

Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22Global Rotating Reference Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22Local Rotating Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23

Examples of Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26


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1

Solving Engineering Problems

Introduction

Engineering problem is them problem connected with designing certain object or system.

There are three general approaches to solveing engineering problems:

an experimental approach: a hardware rig or prototype, i.e., the full-scale object

and/or its model, is manufactured and the experiments needed for designing the

object are conducted with this hardware;

a computational approach: the computations needed for designing the object are

performed and their results are directly used for designing the object, without

conducting any experiments;

a computational-experimental approachcombines computations and

experiments (with the manufactured full-scale object and/or its model) needed for

designing the object, their sequence and contents depending on the solved problem,

e.g. iterative procedures may be run.

Each of the first two approaches has advantages and disadvantages.

The purely experimental approach, being properly conducted, does not require additional

validations of the obtained results, but it is very expensive, even if it is realized on the

object models, since testing facilities and hardware are required anyway. Moreover, if the

object models are tested, the obtained results must be scaled to the full-scale object, so

some computations are required anyway.


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The purely computational approach, being properly performed, is substantially less

expensive than the experimental one, both in terms of finances and time, but it requires

assurance in adequacy of the obtained computational results. Naturally, such assurance

must be based on numerous verifications and validations of the used computational codes,

both from mathematical and physical viewpoints, i.e., both on the mathematical accuracy

of the obtained results (the results adequacy to the used mathematical model) and on the

adequacy of the used mathematical model to the governing physical processes, that is

validated by comparing the computations with the available experimental data.

The third approach, if it combines experiments and computations reasonably, joins the

advantages of both of the first two above-mentioned approaches and avoids their

disadvantages. Complex engineering problems are solved mainly in this way. A

computational code validated on available experimental data allows of quickly selecting

the optimal object design and/or its optimal operating mode. Then necessary experiments

are conducted to verify the selection.

When selecting from the world market a computational code that is most suitable for

solving your problems, it is necessary to take into account the following suggestions. Anycomputational code is based, firstly, on a mathematical model of the governing physical

processes (expressed in the form of a set of differential and/or integral equations derived

from physical laws, and include, if necessary, semi-empirical and empirical constants

and/or relationships) and, secondly, a method of solving these equations. Since the

equations of the mathematical model cannot be solved analytically, they are solved in a

discrete form on a computational mesh, so the solution of the mathematical problem is

obtained with a certain degree of accuracy. Naturally, the accuracy of the solution of the

mathematical problem depends on both the method of discretising the differential and/or

integral equations and on the method of solving the obtained discrete equations. Once

these methods have been selected, the accuracy of solution of the mathematical problem

depends on how well the computational mesh resolves the problem regions of non-linear

behavior. To provide good accuracy, the mesh has to be rather fine in these regions.

Moreover, a usual way of estimating the accuracy of solution of the mathematical problem

consists of obtaining solutions on several different meshes, from coarser to finer. So, if

beginning from some mesh in this set, the difference in the interesting physical parameters

between the solutions obtained on the finer and coarser meshes becomes negligible from

the viewpoint of the engineering problem, i.e., the solution flattens, then the accuracy of

solution of the mathematical problem required for solving this engineering problem is

considered to be attained, since the so-called solution mesh convergence is attained.

Naturally, the solution of the mathematical problem can differ from the experimental

values (i.e., from the solution of the physical problem, if it is known), and this differencedepends, firstly, from the conformity of the mathematical model and the simulated

physical processes, and, secondly, on the error, which these experimental values have been

measured with and which are known and tend to decrease upon increasing the number of

tests peroformed to measure them.


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Solving Engineering Problems with Flow Simulation 2012 1-3

Correspondingly, the computational codes presented on the market differ from each other

not only in their cost, but also in accuracy of mathematical simulation of the physical

problems, as well as in the procedure of specifying the initial data, in the amount of users

time needed for this specification, in the procedure of solving a problem and the computer

memory and CPU time needed for obtaining a solution of the required accuracy, and at last

in the procedures of processing and visualization of the obtained results and the users

time needed for that.

Naturally, a highly accurate solution requires a fine computational mesh, and consequently

rather substantial computer memory and CPU time, as well as, in some cases, increased

user time and efforts for specifying the initial data for the calculation. As a result, if the

time needed to solve an engineering problem with a computational code exceeds some

threshold time, then either the engineering problem becomes irrelevant, e.g. because your

competitors have out-distanced you by this time, or alternative approaches, which may be

not so accurate, but are surely faster, are used instead in order to solve this problem at

given time span.

Before getting acquainted with the recommended procedure of obtaining a reliable andrather accurate solution of an engineering problem with Flow Simulation, it is expedient to

consider Flow Simulation features governing the below-described strategy of solving

engineering problems with Flow Simulation.

Since Flow Simulation is based on solving time-dependent Navier-Stokes equations,

steady-state problems are solved through a steady-state approach. To obtain the

steady-state solution quicker, a method of local time stepping is employed over the

computational domain considered. A multigrid method is used for accelerating the

solution convergence and suppressing parasitic oscillations. The computational domain is

designed as a parallelepiped enveloping the model with planes orthogonal to the axes of

the SolidWorks models Cartesian Global coordinate system. The computational mesh isbuilt by dividing the computational domain into parallelepiped cells with its sides

orthogonal to the Global coordinate system axes. (The cells lying outside the fluid-filled

regions and outside solids with heat conduction inside do not participate in the subsequent

calculations). Procedures of the computational mesh refinement (splitting) are used to

resolve the model features better, such as high-curvature surfaces in contact with fluid,

thin walls surrounded by fluid, narrow flow passages (gaps), and the specified insulators

boundaries. During the subsequent calculations during the solving of the problem the

computational mesh can be refined additionally (if that is allowed by the user-defined

settings) to better resolve the high-gradient flow and solid regions revealed in these

calculations (Solution-Adaptive Meshing).


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Since steady-state problems are solved in Flow Simulation through the steady-state

approach, it is necessary to determine the termination moment for the calculation properly.

If the calculation is finished too early, i.e., when the steady state solution has not been

attained yet, then the obtained solution can depend on the specified initial conditions and

so be not very reliable. On the contrary, if the calculation is finished too late, then some

time has been wasted uselessly. To optimize the termination moment for the calculation

and to determine physical parameters of interest (e.g. a force acting on a model surface, or

a model hydraulic resistance) with a sufficient degree of accuracy, you can specify them as

the calculation goals.

The way to simulate an engineering problem with SolidWorks+Flow Simulation correctly

and adequately from the physical viewpoint, i.e. to state the corresponding model

problem, and to solve this model problem properly and reliably with Flow Simulation, is

described in the chapters Simulating Engineering Tasks with Flow Simulationand

Solving Engineering Tasks.

1 Simulating Engineering Tasks with Flow Simulation

It is necessary to remember that a fast but inaccurate beginning will cost you more efforts

and time spent not only for specifying the initial data, but, even worse, for the subsequent

calculations, until they finally become reliable. Therefore, we strongly recommend that

you carefully read this section.

Selecting Geometrical and Physical Features of the Task

Before you start to create a SolidWorks model and a Flow Simulation project, it is

necessary to select the engineering problems geometrical and physical features that mostsubstantially influence this problems solution - first of all, those, which are important for

estimating the possibility of solving this problem with Flow Simulation. For example,

if the problem contains movable parts, then it is necessary to estimate the

importance of taking into account their motions when solving the problem, and, if

these motions are important, to estimate the possibility of solving this problem with

a quasi-stationary approach, since model parts motions during a calculation are not

considered in Flow Simulation (however, you may specify a translational and/or

rotational motion of the specific wall or a rotating reference frame),

if the problem includes several fluids, or fluid and solid, then it is necessary to

estimate the importance of chemical reactions between them for the problemssolution, and, if the reactions are important, i.e., the reactions rates are rather high

and the reacting fluids are intensely mixed with each other under the problems

conditions, then to estimate a possibility of introducing the reaction products as an

additional fluid when solving this problem, since chemical reactions are not

considered in Flow Simulation,

if the problem includes fluids of different types (for example, a gas and a liquid),

and there is an interface between them or these fluids are mixing, then it is


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necessary to estimate the importance of taking this into account, since Flow

Simulation does not consider a free fluid surface, or mixing of fluids of different

types.

We can present other examples of an clear impossibility of solving some engineering

problems with Flow Simulation, as well as of simplifying the engineering problems for

solving them with Flow Simulation, but it is impossible to envision and describe all thepossible situations in the present document, so that on each particular case you will have

to make decision by yourself.

Creating the Model and the Flow Simulation Project

If the SolidWorks model has already been created when designing the object, i.e. it is fully

adequate to the object, then, to solve the engineering problem with Flow Simulation, it

may be required:

to simplify the model by removing the parts, which do not influence the problems

solution, but consume computer resources, i.e. memory and CPU time. Forexample, a corrugated model surface which will result in an exceedingly large

number of mesh cells required to resolve it can be specified instead as smooth

surface with equivalent wall roughness. If a model has narrow fluid-filled blind

holes whose influence on the overall flow pattern is, by rough estimate, barely

perceptible, it would be better to remove these features in order to avoid the

excessive mesh splitting around them.

to add auxiliary parts to the model, e.g. inlet and outlet tubes for stabilization of the

flow, lids to cover the inlet and outlet openings, and parts to denote rotating regions,

local initial meshes or other areas where special conditions are applied.

Both these actions, being executed properly, can be very pivotal in obtaining a reliable andaccurate solution. Naturally, adding the auxiliary parts to a model will inevitably cause an

increase of the computational mesh cells and, consequently, the required computer

memory and CPU time, therefore these parts dimensions must be adequate to the stated

problem.

If a model has not been created yet, it is expedient to take all the above-mentioned factors

into account when creating it.

If all effects of these actions are not clear enough, it may be worthwhile to vary the model

parts and/or their dimensions in a series of calculations in order to determine their

influence on the obtained solution.

Then, in accordance with the problems physical features revealed and adapted to Flow

Simulation capabilities, the basic part of the Flow Simulation project is specified, i.e., the

problem type (internal or external), fluids and solids involved in the problem, physical

features taken into account (e.g. heat conduction in solids, time-dependent analysis,

gravitational effects, etc.), boundaries of the calculation domain, initial and boundary

conditions, and, if necessary, fluid subdomains, rotating regions, volume and/or surface

heat sources, fans and other features and conditions.


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The specified boundary conditions, as well as heat sources, fans, and other conditions and

features must correspond to the problems physical statement and not conflict with each

other.

Eventually, you specify the physical parameters of interest as the Flow Simulation project

goals. They can be local or integral, defined within the whole computational domain or a

certain volume, on a surface or a point. The parameters determined over some region areexpressed in the form of their minimum, or maximum, average, or bulk average values. This

allows you to increase substantially reliability and accuracy of determination of these

parameters, since their behavior is saved on each iteration during the calculation and can be

analyzed later. On the contrary, the convergence behavior of the parameters not specified as

goals can not be analyzed afterwards, as they are saved only at the last iteration and,

optionally, at the user-specified iterations.

2 Solving Engineering Tasks

As soon as you have specified the basic part of the Flow Simulation project that is unlikely

to be changed in the subsequent calculations, the next step is to select the strategy of

solving the engineering problem with Flow Simulation, i.e., obtaining the reliable and

accurate solution of the problem.

Strategy of Solving the Engineering Tasks

As it has been mentioned in Introduction, by performing a series of calculations on a set of

computational meshes ranging from coarser to finer ones, we can estimate the accuracy of

solution of the mathematical problem. As soon as the calculation on a finer mesh does not

yield a noticeably different (from the engineering problems viewpoint) solution, i.e. thesolution flattens with respect to the mesh cells number, we can conclude that the solution

of the mathematical problem has achieved mesh convergence, i.e., the required

mathematical solution accuracy is attained. Naturally, first you must determine the

threshold for a solution-vs.-mesh change, so that the change smaller than this threshold

will be considered as negligible. Since the determination of this threshold is possible only

in relation with some physical parameter, it is natural to connect it with the physical

parameters of interest of the engineering problem in question, in particular, with the

admissible determination errors of these physical parameters. Moreover, since steady-state

problems are solved with Flow Simulation through the steady-state approach, the

supervision for a behavior of the calculation goals during the calculation (i.e., in

iterations) can serve two purposes. Firstly, if these parameters oscillate during the

solution, it will allow you to determine their values and observation errors more accurately

by averaging them over a number of iterations and determining their deviation from this

average value. Secondly, you may want to intervene in the calculation process by finishing

the calculation manually if you see that either the calculation is unacceptable for you by

some reasons, or, vice versa, if the solution has actually already converged, so that there is

no reason to calculate any further.


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Therefore, the strategy of solving an engineering problem with Flow Simulation consists,

first of all, in performing several calculations on the same basic project (i.e., with the same

model, inside the same computational domain, and with similar boundary and initial

conditions) varying only the computational mesh. Since the computational mesh is built

automatically in Flow Simulation, it may be varied by varying the project parameters that

govern its design (the initial computational mesh on which the calculation starts, and

maybe its refinement during the calculation): Result Resolution Level, Minimum Gap

Size, Minimum Wall Thickness.

An additional item of this strategy of solving an engineering problem with Flow

Simulation consists in varying the auxiliary elements added to the model as needed to

solve the problem with Flow Simulation (e.g. inlet and outlet tubes attached to the inlet

and outlet openings, for internal problems), the dimensions of which are questionable

from the viewpoint of their necessity and sufficiency. Those physical parameters of the

engineering problem whose values are not known exactly and which, in your opinion, can

influence the problem solution, must be varied also. When performing these calculations,

there is no need to investigate the solution-vs.-mesh convergence again, since it has been

already achieved before. It is enough to just perform these calculations with the project

mesh settings that provided the solution with satisfactory accuracy during the

solution-vs.-mesh convergence investigation. The same applies also to the parametric

engineering calculations while you are changing the model parts and/or flow parameters.

However, you must keep in mind the potential necessity for checking the

solution-vs.-mesh convergence, because in doubtful cases it must be checked again.

In spite of the apparent simplicity of the proposed strategy, its full realization is usually

troublesome due to the substantial difficulties including, first of all, a dramatic increase of

the requirements for computer memory and CPU time when you are substantially

increasing the number of cells in the computational mesh. Since both the computer

memory and the time for which the engineering problem must be solved are usually

restricted, the specific realization of this strategy eventually governs the accuracy of the

problem solution, whether it will be satisfactory or not. Perhaps, a further simplification of

the model and/or reducing the computational domain will be required.

Some specific description of this strategy is presented in the next sections of this

document.

Settings for Resolving the Geometrical Features of the Model and forObtaining the Required Solution Accuracy

The computational mesh variation described in Section 2.1 is the key item of the proposed

strategy of solving engineering problems with Flow Simulation.

The result resolution level specified in the Wizard governs the number of basic mesh cells,

the criteria for refinement (splitting) of the basic mesh to resolve the model geometry, i.e.,

creating the initial mesh, as well as the criteria for refinement (splitting) of the initial mesh

during the problem solution. The Result Resolution specified in the Wizard defines the

following parameters in the created project: the Level of initial mesh and the Results


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resolution level. The Level of initial mesh governs only the initial mesh and is accessible

(after the Wizard has been finished) from the Initial Mesh dialog box. The Results

resolution level is accessible from the Calculation Control Options dialog box and

controls the refinement of computational mesh during calculation and the calculation

finishing conditions. The Geometry Resolution options that also influence the initial mesh

may be changed on the Automatic Settings tab of the the Initial Mesh dialog box. Their

effects can be altered on the other tabs of the Initial Mesh dialog box or in the Local Initial

Mesh dialog box.

Before creating the initial mesh, Flow Simulation automatically determines the minimum

gap size and the minimum wall thickness for the walls which are in contact with a fluid on

both sides. That is required for resolving the geometrical features of the model with Flow

Simulation computational mesh. So, when creating the initial mesh, it is taken into

account that the number of the mesh cells along the normal to the model surface must not

be less than a certain number if the distance along this normal from this surface to the

opposite wall is not less than the minimum gap size. Depending on the mesh cell

arrangement, the model flow passages not resolved with the computational mesh either are

automatically replaced with a wall, or increased up to the mesh cell size. In the automatic

mode these mesh parameters are determined from dimensions of the surfaces on which

boundary conditions have been specified, e.g. the model inlet and outlet openings in an

internal analysis, as well as those surfaces and volumes on or in which heat sources, local

initial conditions, surface and/or volume goals and some of the other conditions and

features. Before the calculation, you can see the minimum gap size and the minimum wall

thickness that are determined in such a way. If these values cannot provide an adequate

resolution of the model geometry, you can specify them manually. At that, it is necessary

to take into account that the number of the computational mesh cells generated to resolve

the model geometrical features depends on the specified result resolution level.

Evidently, when creating a Flow Simulation project it is necessary to make sure that both

the minimum gap size and the minimum wall thickness are relevant to the model

geometry. However, if the model geometry is complicated (e.g. there are non-circular flow

passages, sharp edges protruding into the stream, etc.), it can be difficult to determine

these parameters unambiguously. In this case it may be useful to perform several

calculations by varying these parameters within a reasonable range in order to reveal their

influence on the problem solution. In accordance with the strategy of solving engineering

problems, these calculations must be performed at different result resolution levels.

The initial mesh created at result resolution levels of 35 is not changed during the

solving of a problem, i.e. is not adapted to the solution being obtained. Result resolution

levels of 57 yield the same initial mesh, but at result resolution levels of 6 and 7 the

mesh is refined during the calculations in the regions of increased physical parameters

gradients. At level 8, a finer initial mesh is generated and refinements during calculation

take place.

It makes sense to perform calculations at the result resolution level of 3 if both the model

geometry and the flow field are relatively smooth. For more complex problems we

recommend first of all to perform the calculation at the result resolution level of 4 or 5


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(naturally, with specifying explicitly the minimum gap size and minimum wall thickness).

After that, if the calculation at the result resolution level of 5 has been performed, we

recommend, in order to ascertain the mesh convergence, to perform the calculation at the

result resolution level of 7 and, if the computer resources allow you to do this, at the result

resolution level of 8.

Monitoring the Calculation

Monitoring the calculation, i.e., at least, monitoring behavior of the physical parameters

specified by you as the project goals (you can inspect also physical parameters fields at the

specified planar cross-sections) is useful for the following reasons:

you can intervene in the process of calculation, i.e., manually finish the calculation

before it finishes automatically, if you see that either the calculation is unacceptable

for you for some reasons (e.g. if Flow Simulation has generated warnings making

clear that the sequential calculation is senseless), or, vice versa, when solving a

steady-state problem (that concerns some time-dependent problems also), the

solution has already converged, so that there is no reason to continue thecalculation;

if a steady-state problem is solved, and the physical parameters specified by you as

the project goals oscillate during the iterations, then inspecting these parameters

behavior during the calculation will allow you to determine their values and

determination errors more accurately by averaging their values over the iterations

and determining their deviations from these average values;

if the physical parameters of interest do not change substantially during the

calculation, you can obtain their intermediate (preliminary) values beforehand, and

in the subsequent iterations they will be refined finally;

if you solve a time-dependent problem, you can see the calculation results obtained

at the current physical time moment before the calculation is finished.

The first above-mentioned reason is especially useful since it allows you to substantially

reduce the CPU time in some cases. For example, if you do not specify the high Mach

number gas flow in the project settings, whereas in fact the flow becomes supersonic, or if

Flow Simulation warns you about a vortex at the model outlet, that substantially reduces

the calculation accuracy, making it necessary to change some of the problem settings (i.e.

specify high Mach number flow for the first case or lengthen the model outlet tube for the

second one). If you solve a steady-state problem at the result resolution level of 7 or 8 and

you see that the computational mesh refinements performed during the calculation do notincrease the number of cells in the mesh and, therefore, do not noticeably improve the

problem solution (the values of the project goals does not change), you can finish the

calculation relatively early (say, after 12 travels have been performed).


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Viewing and Analyzing the Obtained Solution

When viewing and analyzing the obtained solution after finishing the calculation, it is

recommended to plot the evolutions of the project goals during the calculation, if you did

not monitor them directly as the calculation went on. If a steady-state problem is solved,

and you have specified the physical parameter of interest as the project goal, then, if this

parameter has oscillated during the calculation, you can determine its value moreaccurately by averaging it over the last iterations interval, in which its steady-state

oscillation is seen. This wayt you also determine the variance of this goal, i.e., its

deviation from the average value, that characterizes the goal determination error in the

obtained solution.

It is also useful to check for vortices at the model outlet, as well as to see the flow pattern

in the model and, if heat transfer in solids has been calculated, the temperature distribution

over the solid parts of the model. Naturally, first of all it is expedient to see the obtained

field of the physical parameter you are interesting in, not only in the region of interest, but

also in a broader area, in order to check this field for apparently incosistent results.

It is also worthwhile to examine the obtained fields of other physical parameters related to

the one you are interested in. For example, if you are interested in the total pressure loss,

you may want to see the velocity field, whereas if you are interested in the temperature of

solid, a picture of the fluid-to-solid heat flux field is also useful.

Estimating the Reliability and Adequacy of the Obtained Solution

In accordance with the general approach to estimating reliability and accuracy of the

engineering problem solution obtained with a computational code, this estimation consists

of the following two parts: an estimation of how accurate is the solution of the

mathematical problem corresponding to the mathematical model of the physical process,and an estimation of accuracy of simulating the physical process with the given

mathematical model.

The accuracy of solution of the mathematical problem is determined by mathematical

methods, independently of the consistency of the model to the physical process under

consideration. In our case, this accuracy estimation is based on analyzing the mesh

convergence of the problem solutions obtained on different computational meshes (See

Section 2.2). Then, since steady-state problems are solved with Flow Simulation via a

steady-state approach by employing local time steps, it is useful to verify additionally the

accuracy of the obtained solution by solving the similar time-dependent problem not

employing local time steps.

As soon as the mathematical problem solution of a satisfactory accuracy has been

obtained, the next step consists of estimating the accuracy of simulating the physical

process under consideration with the mathematical model employed in the computational

code. To do this, the obtained solution is compared with the available experimental data

(taking into account their errors which consist of measurement errors and experimental

errors arising from possible spurious influences). Naturally, since experimental data are

always restricted, for this validation it is desirable to select the data which are as close to


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the engineering problem being solved as possible. To validate the computational code on

the available experimental data, you have to solve the corresponding test problem in

addition to the engineering problem being solved (preferably before you start to solve the

engineering problem following the above-mentioned strategy), but this operation

increases the reliability of estimating the obtained solution of the engineering problem so

substantially that the required additional time and efforts will be fully paid back later on,

in particular when solving similar engineering problems.

If after solving the test problem you see that accuracy of its solution obtained with Flow

Simulation is not satisfactory from your viewpoint, check to see that you have properly

specified the Flow Simulation project, that all substantial features of the engineering

problem have been taken into account, and, finally, that Flow Simulation restrictions do

not impede solving this engineering problem.

3 Frequent Errors and Improper Actions

Let us consider errors and improper actions frequently done when solving engineering

problems with Flow Simulation.

When Specifying Initial Data:

not taking into account physical features which are important for the engineering

problem under consideration: e.g. high Mach number gas flow (should be taken

into account if M>3 for steady-state or M>1 for transient tasks or supersonic flow

occurs in about a half of the computational domain or greater), gravitational

effects (must be taken into account if either the fluid velocity is small, the fluid

density is temperature-dependent, and a heat source is considered, or several

fluids having substantially different densities are considered in a gravitationalfield), necessity of the time-dependent analysis (e.g. at the moderate Reynolds

numbers, when unsteady vortices are generated);

incorrectly specifying symmetry planes as the computational domain boundaries

(e.g. at the moderate Reynolds numbers, when unsteady vortices are generated;

you should keep in mind that the symmetry of model geometry and initial and

boundary conditions does not guarantee you the symmetry of flow field);

if symmetry planes have been specified and you click Resetat the Sizetab of the

Computational Domaindialog box, please do not forget to replace Symmetry

by Defaultat the Boundary Conditiontab;

if you have specified symmetry planes and intend to specify a mass or volumeflow rate at a model inlet or outlet opening, please do not forget to specify only

the fraction of total flow rate proportional to the fraction of the opening area

laying inside the computational domain, instead of specifying the total flow rate;

if you specify integral boundary or volume conditions (heat transfer rates, heat

generation rates, etc.), please remember that their values specified in the Flow

Simulation dialog boxes correspond to the fraction of area or volume laying

inside the computational domain;


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if you specify a flow swirl on a model inlet or outlet openings (in the Fansor

Boundary Conditionsdialog boxes), please do not forget to properly specify

their swirl axes and the coordinate system in the Definition tab;

if you specify a Unidirectionalor Orthotropicporous medium, please do not

forget to specify their directions;

please do not forget that the specified boundary conditions must not conflict witheach other. For example, if you deal with gas flows and the model inlet flow is

subsonic, whereas the flow inside the model becomes supersonic, it is incorrect

to specify flow velocity or volume flow rate as a boundary condition at the model

inlet, since they are fully determined by the geometry of the model flow passage

and the fluids specific heat ratio;

if you solve a time-dependent problem, and this problem has cyclic-in-time

boundary conditions, thus leading to a steady-state cyclic-in-time solution, to

obtain which you have to calculate the flow several times in cycle, every time

specifying the solution from the previous calculation as the initial condition for

the next calculation, there is no need to specify the boundary conditions forseveral cycles. Instead it is more convenient to specify them for a cycle and

perform a series of calculations, running each calculation with selected Take

previous resultscheck box in the Rundialog box;

when specifying Surface Goals, Volume Goals, Point Goals or Equation

Goals, it is better to give them sensible names to identify these goals

unambiguously, instead of selecting them in the tree and looking for the

respective places at the model in the SolidWorks graphics area;

if you want to monitor the intermediate calculation results at certain sections of

the model during the calculation, it is better to determine these sections positions

in the Global coordinate system beforehand, i.e. before actually running thecalculation, since during the calculation it is a bit more difficult and you may be

literally late in terms of the problems physical time;

When Monitoring a Calculation:

when monitoring intermediate calculation results during a calculation, please do

not forget the spatial nature of the problem being solved (of course, if the

problem itself is not 2D). To take a look at the full pattern it is expedient to see

the results at least in 2 or 3 intersecting planes;

When Viewing the Obtained Solution after Finishing a Calculation:

please take into account that all settings made in the View Settingsdialog boxconcern all Cut Plots, 3D Plots, Surface Plots, Flow Trajectories, Isosurfaces,

which are active in the SolidWorks graphicsarea, therefore:

your will not be able to open the Flow Trajectoriesdialog box if a parameter

defined only on wall surfaces or in solid has been selected on the Contours

tab and the Use from contoursoption has been selected at the Flow

Trajectoriestab of the View Settingsdialog box;


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to view different result features in different panes simultaneously, you can

split the SolidWorks graphics area into 2 or 4 panes and build different result

features in different graphical areas through their individual Cut Plots, 3D

Plots, Surface Plots, Flow Trajectories, Isosurfacesdefined in these areas;

if you intend to see integral physical parameters (e.g. area, mass or volume flow

rates, heat generation rates, forces, etc.) with the Surface Parametersdialogbox, please remember that:

their shown values are determined over the parts of the surface that belong to

the computational domain;

their determination errors include errors of representing these surfaces in

SolidWorks and Flow Simulation, the latter depends on the computational

mesh;

if you want to see the computational mesh in Cut Plotsand/or Surface Plots,

make sure that the Display meshis enabled under Tools, Options, Third Party

Options, otherwise the Meshbutton in the Cut Plotsand Surface Plots

PropertyManagers will be unavailable.


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21/90Solving Engineering Problems with Flow Simulation 2012 2-1

2

Advanced Knowledge

Introduction

The present document supplies you with our experience of employing the advanced Flow

Simulation capabilities, organized in the following topics:

Manual adjustment of the initial computational mesh settings

Mesh-associated tools (viewing the mesh before and after the calculation and

advanced post-processing tools)

Calculation control options (refinement of the computational mesh during calculation,

conditions of finishing the calculation)

Flow freezing

1 Mesh - Introduction

This chapter provides the fundamentals of working with the Flow Simulation

computational mesh, describes the mesh generation procedure and explains the use of

parameters governing both automatically and manually controlled meshes.

First, let us introduce a set of definitions.

Types of Cells

Any Flow Simulation calculation is performed in a rectangular parallelepiped-shaped

computational domain which boundaries are orthogonal to the axes of the Cartesian

Global Coordinate System. A computational mesh splits the computational domain with a

set of planes orthogonal to the Cartesian Global Coordinate System's axes to form

rectangular parallelepipeds called cells. The resulting computational mesh consists of

cells of the following four types:


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Advanced Knowledge

2-2

Fluid cells are the cells located entirely in the fluid.

Solid cells are the cells located entirely in the solid.

Partial cells are the cells which are partly in the solid and partly in the fluid. For

each partial cells the following information is kept: coordinates of intersections of

the cell edges with the solid surface and normal to the solid surface within the cell.

As an illustration let us look at the original model (Fig.1.1) and the generated

computational mesh (Fig.1.2).

Fig.1.1 The original model.

Fig.1.2 The computational mesh cells of different types

Zero level cell (basic cell)

Solid cell

Partial cell

First level cellFluid cell

Partial cell


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Mesh Construction Stages

Refinement is a process of splitting a rectangular computational mesh cell into eight cells

by three orthogonal planes that divide the cell's edges in halves. The non-split initial cells

that compose the basic mesh are called basic cells or zero level cells. Cells obtained by the

first splitting of the basic cells are called first level cells, the next splitting produces

second level cells, and so on. The maximum level of splitting is seven. A seventh level cellis 8

7times smaller in volume than the basic cell.

The following rule is applied to the processes of refinement and merging: the levels of two

neighboring cells can only be the same or differ by one, so that, say, a fifth level cell can

have only neighboring cells of fourth, fifth, or sixth level.

The mesh is constructed in the following steps: Construction of the basic mesh taking into account the Control Planes and the

respective values of cells number and cell size ratios.

Resolving of the interface between substances, including refinement of the basic mesh

at the solid/fluid and solid/solid boundaries to resolve the relatively small solid features

and solid/solid interface, tolerance and curvature refinement of the mesh at a

solid/fluid, solid/porous and a fluid/porous boundaries to resolve the interface

curvature (e.g. small-radius surfaces of revolution, etc).

Narrow channels refinement, that is the refinement of the mesh in narrow channels

taking into account the respective user-specified settings.

Refinement of all fluid, and/or solid, and/or partial mesh cells up to the user-specified

level.

Mesh conservation, i.e. a set of control procedures, including check for the difference

in area of cell facets common for the adjacent cells of different levels.

After each of these stages is passed, the number of cells is increased to some extent.

In Flow Simulation you can control the following parameters and options which governthe computational mesh:

1 Nx, the number of basic mesh cells (zero level cells) along the X axis of the Global

Coordinate System. 1 Nx1000

2 Ny, the number of basic mesh cells (zero level cells) along the Y axis of the Global

Coordinate System. 1 Ny1000.

3 Nz, the number of basic mesh cells (zero level cells) along the Z axis of the Global

Coordinate System. 1 Nz1000.

During the solution-adaptive meshing the cells can be refined andmerged. See Refinement of the Computational Mesh DuringCalculation on page 36.

If you switch on or off heat conduction in solids, or add/moveinsulators, you should rebuild the mesh.


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Advanced Knowledge

2-4

4 Control planes. By adding and relocating them you can contract and/or stretch the

basic mesh in the specified directions and regions. Six control planes coincident with

the computational domain's boundaries are always present in any project.

5 Small solid features refinement level (Lb). 0 Lb 7.

6 Curvature refinement level (Lcur). 0 Lcur7.

7 Curvature refinement criterion (Ccur). 0 Ccur.

8 Tolerance refinement level (Ltol). 0 Ltol7.

9 Tolerance refinement criterion (Ctol). 0 Ctol.

10 Narrow channels refinement: Characteristic number of cells across a narrow channel,

Narrow channels refinement level, The minimum and maximum height of narrow

channels to be refined.

These options are described in more detail below in this chapter.

Basic MeshThe basic mesh is a mesh of zero level cells. In case of 2D calculation (i.e. if you select the

2D plane flowoption in the Computational Domaindialog box) only one basic mesh cell

is generated automatically along the eliminated direction. By default Flow Simulation

constructs each cell as close to cubic shape as possible.

The number of basic mesh cells could be one or two less than theuser-defined number (Nx, Ny, Nz). There is no limitation on a celloblongness or aspect ratio, but you should carefully check thecalculation results in all cases for the absence of too oblong orstretched cells.

Fig.1.3 Basic mesh examples.

a) 10x12x1 b) 40x36x1


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Control Planes

The Control Planes option is a powerful tool for creating an optimal computational mesh,

and the user should certainly become acquainted with this tool if he is interested in

optimal meshes resulting in higher accuracy and decreasing the CPU time and required

computer memory. Control planes allow you to resolve small features, contract the basic

mesh locally to resolve a particular region by a denser mesh and stretch the basic mesh toavoid excessively dense meshes.

Resolving Small Features by Using the Control Planes

If the level of splitting is not high enough, small solid features may be not resolved

properly. In this case, two methods can be used to improve the mesh:

increase the level of splitting. However, this may result in unnecessary increase of

the number of cells in other regions, creating a non-optimal mesh, or

set a control plane crossing the relevant small feature (e.g. a solid's sharp edge).

This will allow you to resolve this feature better without creating an excessivelydense mesh elsewhere. It is especially convenient in cases of sharp edges oriented

along the Global Coordinate System axes.

Contracting the Basic Mesh

Using control planes you may contract the basic mesh in the regions of interest. To do this,

you need to set control planes surrounding the region and assign the proper Ratiovalues tothe respective intervals. The cell sizes on the interval are changed gradually so that the

proportion between the first and the last cells of the interval is close (but not necessarily

equal) to the entered Ratiovalue. Negative values of the ratio correspond to the reverse

order of cell size increase. Alternatively, you may explicitly set the Number of cellsfor

each interval, in which case the Ratiovalue becomes mandatory. For example, assume

that there are two control planes Plane1 and Plane2 (see Fig.1.4) and the ratio on the

interval between them is set to 2. Then the basic mesh cells adjacent to the Plane1 will be

approximately two times longer than the basic mesh cells adjacent to the Plane2.

It is recommended that you place a control plane slightly submergedinto the solid, and avoid placing it coincident with the solid surface.


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Advanced Knowledge

2-6

Use of control planes is especially recommended for external analyses, where the

computational domain may be substantially larger than the model.

In the Fig.1.6two custom control planes are set through the center of the body with the

ratio set to 5 and -5, respectively, on the intervals to the both sides of each plane.

Fig.1.5 Default control planes. Fig.1.6 Two custom control planes.

Fig.1.4 Specifying custom control planes.

Default control plane

Default control plane

Custom control plane

Custom control plane

Interval 3:number of cells=3 (automatic)ratio=1

Interval 2:number of cells=12 (manual)ratio=1

Interval 1:number of cells=12 (automatic)ratio=2

Plane 4

Plane 3

Plane 2

Plane 1


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Resolving Small Solid Features

The procedure of resolving small solid features refines only the cells where the solid/fluid

(solid/solid, solid/porous as well as fluid/porous) interface curvature is too high: the

maximum angle between the normals to a solid surface inside the cell exceeds 120, i.e.

the solid surface has a protrusion within the cell.

Such cells are split until the the Small solid features refinement levelof splitting mesh

cells is achieved.

Curvature Refinement

The curvature refinement level is the maximum level to which the cells will be split during

refinement of the computational mesh until the curvature of the solid/fluid or fluid/porous

interface within the cell becomes lower than the specified curvature criterion (Ccur).

The curvature refinement procedure has the following stages:

1 Each solid surface is triangulated: Flow Simulation gets triangles that make up theSolidWorks surfaces.

2 A local (for each cell) interface curvature is determined as the maximum angle

between the normals to the triangles within the cell.

3 If this angle exceeds the specified Ccur, and the curvature refinement level is not

reached then the cell is split.

The performance settings do not govern the triangulationperformance.


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Advanced Knowledge

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The curvature refinement is a powerful tool, so that the competent usage of it allows you

to obtain proper and optimal computational mesh. Look at the following illustrations to

the curvature refinement by the example of a sphere.

Fig.1.7 Curvature refinement level is 0;Total number of cells is 64.

Fig.1.8 Curvature refinement level is 1;Total number of cells is 120.

Fig.1.9 Curvature refinement level is 2;Curvature criterion is 0.317;

Total number of cells is 120.

Fig.1.10 Curvature refinement level is 2;Curvature criterion is 0.1;

Total number of cells is 148.


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Tolerance Refinement

Tolerance refinement allows you to control how well (with what tolerance) mesh polygons

approximate the real interface. The tolerance refinement may affect the same cells that

were affected by the small solid features refinement and the curvature refinement. It

resolves the interface's curvature more effectively than the small solid features refinement,

and, in contrast to the curvature refinement, discerns small and large features of equalcurvature, thus avoiding refinements in regions of less importance (see images below).

Any surface is approximated by a set of polygons which vertices are surface's intersection

points with the cells' edges. This approach accurately represents flat faces though

curvature surfaces are approximated with some deviations (e.g. a circle can be

approximated by a polygon). The tolerance refinement criterion controls this deviation. A

cell will be split if the distance (h, see below) between the outermost interface's pointwithin the cell and the polygon approximating this interface is larger than the specified

criterion value.

Fig.1.11 Curvature refinement level is 3; Curvature criterion is 0.1;

Fig.1.12 Tolerance refinement level is 3; Tolerance criterion is 0.1 mm;


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Narrow Channel Refinement

The narrow channel refinement is applied to each flow passage within the computational

domain (or a region, in case that local mesh settings are specified) unless you specify for

Flow Simulation to ignore passages of a specified height. TheNarrow Channels term is

conventional and used for the definition of the flow passages of the model in the direction

normal to the solid/fluid interface.

The basic concept of narrow channel refinement is to resolve the narrow channels with a

sufficient number of cells to provide a reasonable level of solution accuracy. It is

especially important to have narrow channels resolved in analyses of low Reynolds

numbers or analyses with long channels, i.e. in such analyses where the boundary layer

thickness becomes comparable to the size of the partial cells where the layer is developed.

The narrow channel settings available in Flow Simulation are the following:

Narrow channels refinement level the maximum level of cells refinement in

narrow channels with respect to the basic mesh cell.

Characteristic number of cell across a narrow channel the number of cells

(including partial cells) that Flow Simulation will attempt to set across the model

flow passages in the direction normal to the solid/fluid interface. If possible, the

number of cells across narrow channels will be equal to the specified characteristic

number, otherwise it will be as close to it as possible. The Characteristic number

of cells across a narrow channel (let us denote it asNc) and the Narrow channels

refinement level (let us denote it asL) both influence the mesh in narrow channels

in the following manner: the basic mesh in narrow channels will be split to have Nc

number per channel, if the resulting cells satisfy the specified L. In other words,

whatever the specifiedNc, the smallest possible cell in a narrow channel is 8L times

smaller in volume (or2L

times smaller in each linear dimension) than the basicmesh cell. This is necessary to avoid undesirable mesh splitting in very fine

channels that may cause the number of cells to increase to an unreasonable value.

The minimum height of narrow channels, The maximum height of narrow

channels the minimum and maximum bounds for the height outside of which a

flow passage will not be considered as a narrow channel and thus will not be refined

by the narrow channel resolution procedure.

For example, if you specify the minimum and maximum height of narrow channels, the

cells will be split only in those fluid regions where the distance between the opposite walls

of the flow passage in the direction normal to wall lies between the specified minimum

and maximum heights.

The narrow channel refinement operates as follows: the normal to the solid surface for

each partial cell is extended up to the next solid surface, which will be considered to be the

opposite wall of the flow passage. If the number of cells per this normal-to-wall direction

is less than the specifiedNc,the cells will be split to satisfy the narrow channel settings as

described above.


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Although the settings that produce an optimal mesh depends on a particular task, here are

some rule-of-thumb recommendations for narrow channel settings:

1 Set the number of cells across narrow channel to a minimum of 5.

2 Use the minimum and maximum heights of narrow channels to concentrate on the

regions of interest.

3 If possible, avoid setting high values for the narrow channels refinement level, since it

may cause a significant increase in the number of cells where it is not necessary.

Fig.1.13 Small solid features refinement level is 3; Narrow channel refinement is disabled.

Fig.1.14 Small solid features refinement level is 3; Narrow channel refinement is on: 5 cells acrossnarrow channels, Narrow channels refinement level is 2.


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Local Mesh Settings

The local mesh settings option is one more tool to help create an optimal mesh. Use of

local mesh settings is especially beneficial if you are interested in resolving a particular

region within a complex model.

The local mesh settings can be applied to a component, face, edge or vertex. You can

apply local mesh settings to fluid regions and solid bodies. To apply the local mesh

settings to a fluid region you need to specify this region as a solid part or subassembly and

then disable this component in the Component Control dialog box. The local mesh settingsare applied to the cells intersected with the selected component, face, edge, or a cell

enclosing the selected vertex. However, cells adjacent to the cell of the local region may

be also affected due to the refinement rules described in the Mesh Construction Stages

chapter.

Fig.1.15 Small solid features refinement level is 3; Narrow channel refinement is on: 5 cells across

narrow channels, Narrow channels refinement level is 5.


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Recommendations for Creating the Computational Mesh

1 At the beginning create the mesh using the default (automatic) mesh settings. Start

with the Level of initial meshof 3. On this stage it is important to recognize the

appropriate values of the minimum gap sizeand minimum wall thicknesswhich will

provide you with the suitable mesh. The default values of the minimum gap sizeand

minimum wall thicknessare calculated using information about the overall model

dimensions, the Computational Domain size, and area of surfaces where conditions

(boundary conditions, sources, etc.) and goals are specified. Don't switch off the

Optimize thin walls resolutionoption, since it allows you to resolve the model's thinwalls without the excessive mesh refinement.

2 Closely analyze the obtained automatic mesh, paying attention to the total numbers of

cells, resolution of the regions of interest and narrow channels. If the automatic mesh

does not satisfy you and changing of the minimum gap sizeand minimum wall

thicknessvaluesdo not give the desired effect you can proceed with the custom mesh.

3 Start to create your custom mesh with the disabled narrow channel refinement, while

the Small solid features refinement leveland the Curvature refinement levelare

both set to 0. This will produce only zero level cells (basic mesh only). Use control

planes to optimize the basic mesh.

4 Next, adjust the basic mesh by step-by-step increase of the Small solid features

refinement leveland the Curvature refinement level. Then, enable the narrow

channels refinement.

5 Finally, try to use the local mesh settings.

Fig.1.16 The local mesh settings used: Two narrow channels are refined to have 10 cells across them.


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2 Mesh-associated Tools

Introduction

Since the mesh settings is an indirect way of constructing the computational mesh, to

better visualize the resulting mesh various post-processing tools are offered by FlowSimulation. In particular, these tools allow to visualize the mesh in detail before the

calculation, substantially reducing the CPU and user time.

The computational mesh constructed by Flow Simulation or other CFD codes cannot

resolve the model geometry at the mesh cell level exactly. A discrepancy can lead to

prediction errors. To facilitate an analysis of these errors and/or to avoid their appearance,

Flow Simulation offers various options for visualizing the real computational geometry

corresponding to the computational mesh used in the analysis.

Since the numerical solution is obtained inevitably in the discrete form, i.e., in the centers

of computational mesh cells, it is interpolated and extrapolated by the post-processor to

present the results in a smooth form, which is typically more convenient to the user. As a

result, some prediction errors can stem from these interpolations and extrapolations. To

facilitate an analysis of such errors and/or to prevent their appearance, Flow Simulation

offers an option to visualize the physical parameters values calculated at the centers of

computational mesh cells, so that when presenting results by coloring an area with a

palette, the results are considered constant within each cell.

Visualizing the Basic Mesh Before Constructing the Initial Mesh

Using this option the user can inspect the Basic mesh and its Control planes corresponding

to the mesh settings, which can be made manually or retained by default. The plot appearsas soon as these settings have been made or changed, so you immediately see the resulting

mesh. (See Help or Users Guide defining the Basic mesh and its Control planes).

To enable this option, select theShow basic meshoption in the Flow Simulation,

Project menu, or in the Initial Mesh dialog box. The option is accessible both before and

after the calculation.

Using this option, you may shifting the Control planes to desired positions to assure that

certain features of the model geometry are captured by the computational mesh.

Enhanced Capabilities of the Results LoadingFlow Simulation allows to view not only the calculation results and the current

computational mesh, which they have been obtained on, but also the initial computational

mesh (i.e., which the calculation begins on). The latter can be viewed either before or after

the calculation, allowing the user to compare the initial and current (i.e., refined during the

calculation) computational meshes.


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To view various meshes, you must open the corresponding file via the Load resultsdialogbox. The calculation results, including the current computational mesh, are saved in the

.fldfiles, whereas the initial computational mesh is saved separately in the .cptfile. All

these files are saved in the project folder, which name (a numeric string) is formed by

Flow Simulation and must not be changed. The .cptfiles and the final (i.e., with the

solution obtained at the last iteration) .fldfiles have the name similar to that of the project

folder, whereas the solutions obtained during the calculation at the previous iterations

(corresponding to certain physical time moments, if the problem is time-dependent) are

saved in the .fldfiles with names r_, e.g. the project initial data are

saved in the r_000000.fld file.

Do not try to load the calculation results obtained in another projectwith a different geometry; the effect will be unpredictable.

Fig.2.1 The Basic mesh (left) and the Initial mesh (right).

Fig.2.2 The Load Results dialog box.


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Viewing the Initial Computational Mesh Saved in the .cpt Files

To optimize the process of solving an engineering problem and to save time, in some cases

it may be useful to view the initial computational mesh before performing the calculation,

particularly to be sure that the model features are resolved well by this mesh. To view the

initial computational mesh after loading the .cptfile, Flow Simulation offers you Cut

Plots, Surface Plots, and the Meshoption (see below), which are also used for viewingthe calculation results.

Viewing the Computational Mesh Cells with the Mesh Option

To view fluid cells of the computational mesh cells (i.e. the cells lying fully in the fluid),

solid cells (lying fully in the solid), and partial cells lying partly in the fluid and partly in

the solid, Flow Simulation offers you the Mesh option.

Different colors can be used to better differentiate between the computational mesh cells

of each of the above-mentioned types. To see the cells in a certain parallelepiped region,

the user must specify the coordinates of the region boundaries in the Global CoordinateSystem.

Using the Meshoption, you can also save the information concerning the mesh cells,

including the physical parameters values obtained in their centers, in ASCII or

Microsoft Excel files.

Visualization of a large amount of computational mesh cells (e.g. allfluid cells in the whole computational domain) may be impractical,since it could require substantial time and memory, and even thenyou might not be able to see all the visualized cells because themajority of them will likely be screened from view by other cells.


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Visualizing the Real Computational Geometry

Since the SolidWorks model geometry, especially its high-curvature parts, cannot be

resolved exactly at the cell level by the rectangular (parallelepiped) computational mesh,

the real computational geometry corresponding to the computational mesh used in the

analysis can be viewed after the calculation to avoid or estimate the prediction errors

stemming from this discrepancy. If no solution-adaptive meshing occurs during thecalculation, the real computational geometry can be viewed just after the mesh generation.

This option is employed by clearing the Use CAD geometrycheck box in Cut Plots, 3D

Plots, Surface Plots, Flow Trajectories, Point Parameters and XY Plots. The result is

especially clear when colored Contours are used to visualize a physical parameter values

(see Fig.2.3).

This capability is especially useful for revealing important surface regions in the model,

which are inadequately resolved by the computational mesh.

On the other hand, this option may be useful when creating Surface Plots for SolidWorksmodels containing rippled surfaces, where ripples, which are supposed to be not essential

from the problem solution viewpoint, were not resolved by the computational mesh. In

this case, coloring of the simplified solid/fluid interface instead of coloring the actual

SolidWorks model faces can lead to substantial reduction of the CPU time and memory

requirements.

Fig.2.3 Cut Plots around the SolidWorks model outer surface (left) and on its computationalrealization (right).


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If the computational mesh has resolved the SolidWorks model well,so the obtained computational results are adequate, then enable theUse CAD geometryoption before performing the final Cut Plots andSurface Plots to obtain smooth pictures which are more convenientfor the analysis.

Notice that when creating a Surface Plot with theUse CADgeometryoption switched off, only the solid/fluid interfaces ofpartial cells within the computational mesh will be colored. When aSurface Plot is created in the Use all facesmode, solid/fluidinterfaces of all partial cells will be colored. However, when aSurface Plot is created on a selected surface, the solid/fluid interfacesare colored only in the partial cells intersected by the SolidWorksmodel surface approximated by triangles inside SolidWorks, whichmay differ from the mesh-approximated surface of the model.Depending on the problem considered, there may be such cases whencertain partial cells are not intersected by the triangulated surface andtherefore the corresponding solid/fluid interfaces would not becolored. Naturally, this circumstance concerns visualization only anddoes not affect the calculation results.


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Switching off the Interpolation and Extrapolation of CalculationResults

Since the numerical solution is obtained inevitably in the discrete form, i.e., in the form of

values in the centers of the computational mesh cells in Flow Simulation, it is interpolated

and extrapolated by the post-processor to present the results in a smooth form, which is

typically more convenient to the user. As a result, prediction errors can stem from and/orbe hidden by such interpolation and extrapolation that smoothens the calculation results.

To facilitate the revealing, analysis, and elimination of such errors, Flow Simulation offers

an option to visualize the physical parameter values as is, i.e. without interpolation,

when presenting calculation results in Cut Plots and Surface Plots (other result features,

namely, isolines, isosurfaces, flow streamlines and particle trajectories can not be built at

all without interpolation), so when coloring a surface with a palette, the results are

considered constant within the mesh cells (see Fig.2.4).

Since the mesh cells centers used in coloring the surface can lie at

different distances from the surface, this can introduce an additionalvariegation into the picture, if the value of the displayed parameterdepends noticeably on this distance (see Fig.2.4).

Fig.2.4 The fluid velocity Surface Plots in the near-wall region created with the interpolation of thecalculation results (left) and without interolation (right).


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Conclusion

The presented mesh-associated tools of Flow Simulation are additional tools for obtaining

reliable and accurate results with this code. These tools are summarized in the table:

3 Meshing - Additional Insight

Flow Simulation considers the real model created in SolidWorks and generates a

rectangular computational mesh automatically distinguishing the fluid and solid domains.

The corresponding computational domain is generated in the form of a rectangular

parallelepiped enclosing the model. In the mesh generation process, the computational

domain is divided into uniform rectangular parallelepiped-shaped cells, which form a

so-called basic mesh. Then, using information about the model geometry, Flow

Simulation further constructs the mesh by means of various refinements, i.e. splitting of

the basic mesh cells into smaller rectangular parallelepiped-shaped cells, thus better

representing the model and fluid regions. The mesh, which the calculation starts from,

so-called initial mesh,is fully defined by the generated basic mesh and the refinement

settings.

Option

Application

ReasonBasicmesh

Initialmesh

After thecalculation

Visualizing theBasic mesh

+ + + To inspect the Basic mesh andsetting its Control planes

Widenedcapabilities ofloading the results

+ + To view the Initial mesh andthe calculation results

Viewing the Initial

mesh+ + To analyze the Initial mesh

Viewing meshcells of differenttype

+ + To view mesh cells and savethe respective physicalparameters values

Visualizing thecomputationalgeometry

+ + For analysis of inadequateresults and quickpost-processing of the resultsof complicated models

Switching off the

interpolation ofresults

+ For analysis of inadequate

results


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Each refinement has its criterionand level. The refinement criterion denotes which cells

have to be split, and the refinement level denotes the smallest size, which the cells can be

split to. Regardless of the refinement considered, the smallest cell size is always defined

with respect to the basic mesh cell size so the constructed basic mesh is of great

importance for the resulting computational mesh.

The main types of refinements are:

Small Solid Features Refinement

Curvature Refinement

Tolerance Refinement

Narrow Channel Refinement

Square Difference Refinement

In addition, the following two types of refinements can be invoked locally:

Cell Type Refinement

Solid Boundary Refinement

During the calculation, the initial mesh can be refined further using the

Solution-Adaptive Refinement.

Though it depends on a refinement which criterion or level is available for user control,

we will consider all of them (except for the Solution-Adaptive Refinement) to give you a

comprehensive understanding of how the Flow Simulation meshing works.

In the chapter below the most important conclusions are marked with the blue italic font.

For abbreviation list refer to the Glossaryparagraph.

Initial Mesh Generation Stages

Basic Mesh Generation and Resolving the Interface

1 Create basic mesh cells which sizes are governed by the computational domain size,

the user-specified Control Planes and the number of the basic mesh cells. [Nx, Ny, Nz,

Control Planes. Parameters which act on each stage are summarized in square

bracketsat the end of the stage.]

2 Analyze triangulation in each basic mesh cell at the interfaces between differentsubstances (such as solid/fluid, solid/porous, solid/solid and porous/fluid interfaces) in

order to find the maximum angle between normals to the triangles which compose the

interface within the cell.

3 Depending on the maximum angle found, the decision whether to split the cell or not is

made in accordance with the specified Small solid features refinement level (SSFRL),

Narrow channel refinement level (NCRL), Curvature refinement level (CRL) and

Curvature criterion (CRC), Tolerance refinement level (TRL) and Tolerance


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Refinement Criterion (TRC) (see the Refinements at Interfaces Between

Substancesparagraph). [SSFRL, NCRL, CRL and CRC].

4 If a basic mesh cell is split, the resulting child cells are analyzed as described in items 2and 3, and split further, if necessary. The cell splitting will proceed until the interface

resolution satisfies the specified SSFR criterion, CRC and TRC, or the corresponding

level of splitting reaches its specified value.

5 The operations 2 to 4 are applied for the next basic mesh cell and so on, taking into

account the following Cell Matingrule: two neighboring cells (i.e. cells having a

common face) can be only cells which levels are similar or differ by one .This rule has

the highest priority

Documents

SolidWorks Flow Simulation 2012