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3-1ANSYS, Inc. Proprietary

2009 ANSYS, Inc. All rights reserved.April 28, 2009

Inventory #002598

Chapter 3

Domains andBoundary Conditions

Introduction to CFX

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Bound ary Condi t ions



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Training ManualDomains

Domains are regions of space in which the equations of fluid flow or heat

transfer are solved

Only the mesh components which are included in a domain are included

in the simulation

e.g. A simulation of a copper heating coil in water

will require a fluid domain and a soliddomain.

e.g. To account for rotational motion, the rotor is

placed in a rotating domain.

Rotor Stator

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Training ManualHow to Create a Domain (as shown earlier)

Define Domain Properties

Right-click on the domain and pick Edit

Or right-click on Flow Analys is 1to insert a new domain

When edit ing an item a new tab panel opens

contain ing the proper ties. You can switch

between open tabs.

Sub-tabs contain

various different

proper t ies

Comp lete the required

fields on each sub-tab

to def ine the domain

Optional f ields are

activated by enabling

a check box

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Training ManualDomain Creation

General Options panel: Basic Settings Location:Only assemblies and 3D

primitives

Domain Type:Fluid, Solid, or Porous

Coordinate Frame:select coordinate

frame from which all domain inputs will be

referenced to

Not to be confused with the reference

frame, which can be stationary or rotating

The default Coord 0frame is usually used

Fluids and Particles Definitions: select

the participating materials

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Training Manual

Ex. 2: Preference= 100,000 Pa

Domain CreationReference Pressure

General Options panel:Domain Models

Reference Pressure

Represents the absolute pressure datum from

which all relative pressures are measured

Pabs= Preference+ Prelative

Pressures specified at boundary and initial

conditions are relative to the Reference Pressure

Used to avoid problems with round-off errors which

occur when the local pressure differences in a fluid

are small compared to the absolute pressure level

PressurePressure

Ex. 1: Preference

= 0 Pa

Pref

Prel,max=100,001 Pa

Prel,min=99,999 Pa

Prel,max=1 Pa

Prel,min=-1 Pa

Pref

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Training ManualDomain Creation - Buoyancy

General Options panel: Buoyancy

When gravity acts on fluid regions with differentdensities a buoyancy force arises

When buoyancy is included, a source term is addedto the momentum equations based on the differencebetween the fluid density and a reference density

SM,buoy=(ref)g

refis the reference density. This is just the datumfrom which all densities are evaluated. Fluid withdensity other than refwill have either a positive ornegative buoyancy force applied.

See below for more on the reference density

The (ref) term is evaluated differently dependingon your chosen fluid:

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Full Buoyancy Model

Evaluates the density differences directly

Used when modeling ideal gases, real fluids, ormulticomponent fluids

A Reference Density is required

Use an approximate value of the expected domaindensity

Boussinesq Model

Used when modeling constant density fluids

Buoyancy is driven by temperature differences

(ref) = - ref(TTref)

A Reference Temperature is required

Use an approximate value of the averageexpected domain temperature

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Buoyancy Ref. Density

The Buoyancy Reference Densityis used to avoid

round-off errors by solving at an offset level

The Reference Pressure is used to offset theoperating pressure of the domain, while theBuoyancy Reference Densityshould be used tooffset the hydrostatic pressure in the domain

The pressure solution is relative to rref g h, where hisrelative to the Reference Location

If rref= the fluid density (r), then the solution becomesrelative to the hydrostatic pressure, so when visualizingPressureyou only see the pressure that is driving theflow

Ab solute Pressurealways includes both thehydrostatic and reference pressures

Pabs= Preference+ Prelative+ rref g h

For a non-buoyant flow a hydrostatic pressure does

not exist

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Training ManualPressure and Buoyancy Example

Consider the case of flow through a tank

The inlet is at 30 [psi] absolute

Buoyancy is included, therefore a

hydrostatic pressure gradient exists

The outlet pressure will be approximately

30 [psi] plus the hydrostatic pressure

given by g h

The flow field is driven by small dynamicpressure changes

NOT by the large hydrostatic pressure or the

large operating pressure

To accurately resolve the small dynamic

pressure changes, we use the Reference

Pressureto offset the operating

pressure and the Buoyanc y Reference

Density to offset the hydrostatic

pressure

30 psi

h

~30 psi + gh

Gravity, g

Small pressure

changes drive the

flow field in the tank

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Training ManualDomain Creation

General Options panel:Domain Motion

You can specify a domain that is rotating about

an axis

When a domain with a rotating frame is specified,

the CFX-Solver computes the appropriate

Coriolis and centrifugal momentum terms, and

solves a rotating frame total energy equation

Mesh Deformation Used for problems involving moving boundaries

or moving subdomains

Mesh motion could be imposed or arise as an

implicit part of the solution

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Training ManualDomain Types

The additional domain tabs/settings

depend on the Domain Type selected

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Training ManualDomain Type: Fluid Models

Heat Transfer

Specify whether a heat transfer model isused to predict the temperature throughout

the flow

Discussed in Heat Trans fer Lecture

Turbulence Specify whether a turbulence model is

used to predict the effects of turbulence in

fluid flow

Discussed in Turbulence Lecture

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Reaction or Combustion Models

CFX includes combustion models to allow thesimulation of flows in which combustion

reactions occur

Available only if Option = Material Definitionon

the Basic Settings tab

Not covered in detail in this course

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Radiation Models

For simulations when thermal radiation issignificant

See the Heat Transfer chapter for moredetails

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Training ManualDomain Type: Solid Models

Solid domains are used to model regions

that contain no fluid or porous flow (for

example, the walls of a heat exchanger)

Heat Transfer (Conjugate Heat Transfer)

Discussed in Heat Trans fer Lecture

Radiation

Only the Monte Carlo radiation model is

available in solids

Theres no radiation in solid domains if it is

opaque!

Solid Motion

Used onlywhen you need to account foradvection of heat in the solid domain

Solid motion must be tangential to its

surface everywhere (for example, an object

being extruded or rotated)

Tubular heat exchanger

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Training Manual

Images Courtesy of Babcock and Wilcox, USA

Domain Type: Porous Domains

Used to model flows where thegeometry is too complex to

resolve with a grid

Instead of including the geometric

details, their effects are

accounted for numerically

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Training ManualDomain Type: Porous Domains

Area Porosity

The area porosity (the fraction of physicalarea that is available for the flow to go

through) is assumed isotropic

Volume Porosity

The local ratio of the volume of fluid to the

total physical volume (can vary spatially)

By default, the velocity solved by the code

is the superficial fluid velocity. In a porous

region, the true fluid velocity of the fluid

will be larger because of the flow volume

reduction

Superficial Velocity = Volume Porosity * True VelocityThis setting should be

consistent with the

velocity used when

the Loss Coefficients

(next slide) were

calculated

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Training ManualDomain Type: Porous Domains

Loss Model

Isotropic: Losses equal in all directions

Directional Loss: For many applications,

different losses are induced in the streamwise

and transverse directions. (Examples:

Honeycombs and Porous plates)

Losses are applied using Darcys Law

Permeability and Loss Coefficients

Linear and Quadratic Resistance Coefficients

ilossi

permi

UKUKdx

dp

2

r

ilossi

permi

UKUKdx

dp

2

r

iRiR

i

UCUCdxdp

21

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Training ManualMaterials

Create a name for the fluid to be used

Select the material to be used in the domain Currently loaded materials are available in the drop down list

Additional Materials are available by clicking

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Training ManualMaterials

A Material can be created/edited by right clicking Materials

in the Outline Tree

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Training ManualMulticomponent/Multiphase Flow

ANSYS CFX has the capability to model fluid mixtures

(multicomponent) and multiple phases

Multicomponent(more details on next slide)One flow field for the mixture

Variations in the mixture accounted for by variable mass

fractions

Applicable when components are mixed at the molecular

level

MultiphaseEach fluid may possess its own flow field

(not available in CFD-Flo product) or all

fluids may share a common flow field

Applicable when fluids are mixed on a

macroscopic scale, with a discernibleinterface between the fluids.

Creating multiple fluids will

allow you to specify fluid

specific and fluid pair models

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Training ManualMulticomponent Flow

Each component fluid may have a distinct set of physical

properties

The ANSYS CFX-Solver will calculate appropriate average values

of the properties for each control volume in the flow domain, for

use in calculating the fluid flow

These average values will depend both on component property

values and on the proportion of each component present in the

control volume

In multicomponent flow, the various components of a fluid share

the same mean velocity, pressure and temperature fields, and

mass transfer takes place by convection and diffusion

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Training ManualCompressible Flow Modelling

Activated by selecting an Ideal Gas, Real Fluid, or a General Fluidwhose density is a function of pressure

Can solve for subsonic, supersonic and transonic flows

Supersonic/Transonic flow problems

Set the heat transfer option to Total Energy

Generally more difficult to solve than subsonic/incompressible flow problems,especially when shocks are present

Click to load a

real gas library

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Boundary Conditions

B d C di t i

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Training ManualDefining Boundary Conditions

You must specify information on the dependent (flow) variables at the

domain boundaries

Specify fluxes of mass, momentum, energy, etc. into the domain.

Defining boundary conditions involves:

Identifying the location of the boundaries (e.g., inlets, walls, symmetry)

Supplying information at the boundaries

The data required at a boundary depends upon the boundary

condition type and the physical models employed

You must be aware of types of the boundary condition available andlocate the boundaries where the flow variables have known values or

can be reasonably approximated

Poorly defined boundary conditions can have a significant impact on your

solution

B d C di t i

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Training ManualAvailable Boundary Condition Types

Inlet Velocity Components -Static Temperature (Heat Transfer)

Normal Speed -Total Temperature (Heat Transfer)

Mass Flow Rate -Total Enthalpy (Heat Transfer) Total Pressure (stable) -Relative Static Pressure (Supersonic)

Static Pressure -Inlet Turbulent conditions

Outlet Average Static Pressure -Normal Speed

Velocity Components -Mass Flow Rate

Static Pressure

Opening Opening Pressure and Dirn -Opening Temperature (Heat Transfer)

Entrainment -Opening Static Temperature (Heat Transfer)

Static Pressure and Direction -Inflow Turbulent conditions

Velocity Components

Wall No Slip / Free Slip -Adiabatic (Heat Transfer)

Roughness Parameters -Fixed Temperature (Heat Transfer)

Heat Flux (Heat Transfer) -Heat Transfer Coefficient (Heat Transfer)

Wall Velocity (for tangential motion only)

Symmetry No details (only specify region which corresponds to the symmetry plane

Inlet

Opening

Outlet

Wall

Symmetry

B d C di t i

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Training Manual

Right-click on the domain to insert BCs

How to Create a Boundary Condition

After completing

the boundary

condition, it

appears in the

Outline tree

below its domain

B d C di t i

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Training ManualInlets and Outlets

Inlets are used predominantly for regions where inflow is expected;however, inlets also support outflow as a result of velocity specified

boundary conditions

Velocity specified inlets are intended for incompressible flows

Using velocity inlets in compressible flows can lead to non-physical results

Pressure and mass flow inlets are suitable for compressible andincompressible flows

The same concept applies to outlets

Velocity Specified Condition Pressure or Mass Flow Condition

Inlet Inlet

Inflow

allowed

Inflow

allowed

Outflow

allowed

Bound ary Cond i t ions

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Training ManualOpenings

Artificial walls are not erected with the opening type boundary, as

both inflow and outflow are allowedYou are required to specify information that is used if the flow

becomes locally inflow

Do not use opening as an excuse for a poorly placed boundary

See the following slides for examples

Pressure Specified Opening

Inlet

Inflow

allowedOutflow

allowed


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Training ManualSymmetry

Used to reduce computational effort in problem.

No inputs are required.

Flow field and geometry must be symmetric:

Zero normal velocity at symmetry plane

Zero normal gradients of all variables at symmetry plane

Must take care to correctly define symmetry boundary locations

Can be used to model slip walls in viscous flow

symmetry

planes


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Training Manual

Fuel

Air

Manifold box1Nozzle

1

23

Specifying Well Posed Boundary Conditions

1 Upstream of manifold

Can use uniform profilessince natural profiles will

develop in the supply pipes Requires more elements

2 Nozzle inlet plane

Requires accurate velocityprofile data for the air andfuel

3 Nozzle outlet plane Requires accurate velocity

profile data and accurateprofile data for the mixturefractions of air and fuel

Consider the following case in which contain separate air and fuelsupply pipes

Three possible approachesin locating inlet boundaries:


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Training ManualSpecifying Well Posed Boundary Conditions

If possible, select boundary

location and shape such that

flow either goes in or out

Not necessary, but will typically

observe better convergence

Should not observe large

gradients in direction normal to

boundary

Indicates incorrect boundarycondition location

Upper pressure boundary modified to

ensure that flow always enters domain.

This outlet is poorly located. It should

be moved further downstream


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Training Manual

Boundaries placed over recirculation zones

Poor Location: Apply an opening to allow inflow

Better Location: Apply an outlet with an accurate velocity/pressure profile

(difficult)

Ideal Location: Apply an outlet downstream of the recirculation zone to allow

the flow to develop. This will make it easier to specify accurate flow

conditions


Opening

Outlet

Outlet


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Training Manual

Turbulence at the Inlet

Nominal turbulence intensities range from 1% to 5% but will dependon your specific application.

The default turbulence intensity value of 0.037 (that is, 3.7%) is

sufficient for nominal turbulence through a circular inlet, and is a good

estimate in the absence of experimental data.

For situations where turbulence is generated by wall friction, consider

extending the domain upstream to allow the walls to generate

turbulence and the flow to become developed



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Training Manual

External Flow

In general, if the building has height H and width W, you would want your

domain to be at least 5H high, 10W wide, with at least 2H upstream of the

building and 10 H downstream of the building.

You would want to verify that there are no significant pressure gradients

normal to any of the boundaries of the computational domain. If there are,

then it would be wise to enlarge the size of your domain.


w

h

5h

10HAt least 2H

10w

Concentrate mesh in

regions of high

gradients


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Training Manual

Symmetry Plane and the Coanda Effect

Symmetric geometry does not necessarily mean symmetric flow

Example: The coanda effect. A jet entering at the center of a

symmetrical duct will tend to flow along one side above a certain

Reynolds number


No Symmetry Plane Symmetry Plane

Coanda effect

not allowed


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Training Manual

When there is 1 Inlet and 1 Outlet

Most Robust: Velocity/Mass Flow at an Inlet; Static Pressure at an Outlet.

The Inlet total pressure is an implicit result of the prediction.

Robust:Total Pressure at an Inlet; Velocity/Mass Flow at an Outlet. The

static pressure at the Outlet and the velocity at the Inlet are part of the

solution.

Sensitive to Initial Guess:Total Pressure at an Inlet; Static Pressure at an

Outlet. The system mass flow is part of the solution

Very Unreliable:Static Pressure at an Inlet; Static Pressure at an Outlet.

This combination is not recommended, as the inlet total pressure level andthe mass flow are both an implicit result of the prediction (the boundary

condition combination is a very weak constraint on the system).



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At least one boundary should specify Pressure (either Total or Static)

Unless its a closed system

Using a combination of Velocity and Mass Flow conditions at all boundaries

over constrains the system

Total Pressure cannot be set at an Outlet

It is unconditionally unstable

Outlets that vent to the atmosphere typically use a Static Pressure = 0

boundary condition

With a domain Reference Pressure of 1 [atm]

Inlets that draw flow in from the atmosphere often use a Total

Pressure = 0 boundary condition (e.g. an open window)

With a domain Reference Pressure of 1 [atm]


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Mass flow inlets result in a uniform velocity profile over the inlet

Fully developed flow is not achieved You cannot specify a mass flow profile

Mass flow outlets allow a natural velocity profile to develop based on

the upstream conditions

Pressure specified boundary conditions allow a natural velocity

profile to develop

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CFX12_03_Physics1