© Fluent Inc. 5/24/2015B1 Fluent Software Training TRN-98-006 Review: CFD Step by Step

© Fluent Inc. 04/18/23B1

Fluent Software TrainingTRN-98-006

Review: CFD Step by Step



CFD Analysis: Basic Steps

1. Define your modeling goals.

2. Identify the domain you will model.

3. Choose an appropriate solver.

4. Design and create the grid.

5. Set up the numerical model.

6. Compute the solution.

7. Examine the results.

8. Adapt the grid (optional).

9. Consider revisions to the model.



Define Your Modeling Goals

What results are you looking for, and how will they be used? What degree of accuracy is required? How quickly do you need the results? Do you require a unique modeling capability?

User-defined subroutines (written in FORTRAN) in FLUENT 4.5 User-defined functions (written in C) in FLUENT 5



Identify the Domain You Will Model

How will you isolate a piece of the complete physical system? Where will the computational domain begin and end? What boundary conditions are needed? Can the problem be simplified to 2D?



Choose an Appropriate Solver (1)

FLUENT 5 Incompressible or compressible flow Complex geometry and physics Applications: external aerodynamics, underhood flows, fans, pumps,

cyclones, reactor vessels, furnaces, power plants, aircraft, turbomachinery, nozzles, other aerospace problems

FLUENT 4.5 Incompressible (or mildly compressible) flow Less complicated geometry, complex physics Applications: risers, fluidized beds, other multiphase and VOF (free

surface) problems



FIDAP Incompressible (or mildly compressible) flow Complex geometry and physics Applications: biomedical flows; metal casting, solidification and

extrusion; extruders; complex die flows; other materials processing flows




POLYFLOW Viscous, laminar flow Complex rheology (including viscoelasticity) Applications: extrusion, coextrusion, die design, blow molding,

thermoforming, film casting, fiber drawing NEKTON

Viscous, laminar flow Applications: thin film coating flows

IcePak Electronics cooling

MixSim Mixing tank analysis




Design and Create the Grid

Should you use a quad/hex grid, a tri/tet grid, or a hybrid grid? What degree of grid resolution is required in each region of the

domain? Will you use adaption to add resolution? How many cells are required for the problem? Do you have sufficient computer memory?



Tri/Tet vs. Quad/Hex Meshes

For simple geometries, quad/hex meshes can provide high-quality solutions with fewer cells than a comparable tri/tet mesh.

For complex geometries, quad/hex meshes show no numerical advantage, and you can save time by using a tri/tet mesh.



Quad/Hex Mesh Example

RAE 2822 airfoil grid Flow gradients predominantly normal to airfoil surface (stretched mesh

required). Easy to grid with quads. Quad grid is more efficient than comparable tri grid.

Stretched quadrilateral grid for the RAE 2822 airfoil



Tri/Tet Mesh Example

Centrifugal blower grid Geometry is complicated. Flow field is complex. Difficult to grid with quads; easy

to grid with triangles!

Triangular grid for a 2D model of a centrifugal blower



Hybrid Mesh Example

Valve port grid Specific regions can be meshed with different cell types. Both efficiency and accuracy are enhanced relative to a hexahedral or

tetrahedral mesh alone. Tools for hybrid mesh generation are now available (Gambit and TGrid).

Hybrid mesh for an IC engine valve port

tet mesh

hex mesh

wedge mesh



Set Up the Numerical Model

For a given solver, you will need to: Select appropriate physical models. Define material properties.

Fluid Solid Mixture

Prescribe boundary conditions at all boundary zones. Provide an initial solution. Set solver controls. Set up solution convergence monitors (residuals and surface or force

monitors).



Compute the Solution: Steady-State Calculation normally requires many iterations before converged solution

obtained. Solution considered converged when changes in key solution parameters

(convergence monitors) become small. Possible convergence monitors include:

Residuals Point values Integrated flux balances Integrated forces

Converged solution is not necessarily accurate due to: Grid resolution Accuracy of numerics (discretization error) Accuracy of physical models (e.g., turbulence)



Compute the Solution: Unsteady

Unsteady calculations computed by subiterating from one physical time level to next.

Subiteration process should be well-converged before proceeding to next time level.

Choose time step that can resolve unsteady physics of problem. Determine characteristic time scale T of unsteady physics of problem. Choose time step that is some fraction of time scale.

e.g., t = T /100. Generally, adjust time step so calculation requires about 10 - 20

subiterations to converge. For problems with fast “startup” transient, may need to decrease time

step during initial phase of solution.



Examine the Results

Visualization can be used to answer such questions as: What is the overall flow pattern? Is there separation? Where do shocks, shear layers, etc. form? Are key flow features being resolved? Are physical models and boundary conditions appropriate? Are there local convergence problems?

Numerical reporting tools can be used to calculate quantitative results: Lift and drag Average heat transfer coefficients Surface-averaged quantities



Tools to Examine the Results

Graphical tools Grid, contour, and vector plots Pathline and particle trajectory plots XY plots Animations

Numerical reporting tools Flux balances Surface and volume integrals and averages Forces and moments



Adapt the Grid Locally increase resolution of grid exactly where needed. You can adapt to:

Gradients of flow or user-defined variables Isovalues of flow or user-defined variables All cells on a boundary All cells in a region Cell volumes or volume changes y+ in cells adjacent to walls Combinations of the above

To assist adaption process, you can: Draw contours of adaption function Display cells marked for adaption Limit adaption based on cell size and number of cells



Adaption Example: Grid

2D planar shell - initial grid 2D planar shell - final grid



Adaption Example: Solution

2D planar shell - contours of pressure initial grid

2D planar shell - contours of pressure final grid



Consider Revisions to the Model Are physical models appropriate?

Is flow turbulent? Is flow unsteady? Are there compressibility effects? Are there 3D effects?

Are boundary conditions correct? Is the computational domain large enough? Are boundary conditions appropriate? Are boundary values reasonable?

Is grid adequate? Can grid be adapted to improve results? Does solution change significantly with adaption, or is the solution grid independent? Does boundary resolution need to be improved? Would another mesh type be more suitable (quad vs. tri, hex vs. tet)?



The Analyst's Paradox

Everyone believes a test except the tester. No one believes an analysis except the analyst.

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© Fluent Inc. 5/24/2015B1 Fluent Software Training TRN-98-006 Review: CFD Step by Step