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1-1ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights reserved.

April 28, 2009Inventory #002598

Chapter 1

Introduction to CFD

Introduction to CFX


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Introduction To CFD



Training ManualWhat is CFD?

Computational fluid dynamics (CFD) is the science of predicting fluidflow, heat and mass transfer, chemical reactions, and relatedphenomena by solving numerically the set of governing mathematicalequations

Conservation of mass, momentum, energy, species mass, etc.

The results of CFD analyses are relevant in: Conceptual studies of new designs

Detailed product development

Troubleshooting

Redesign

CFD analysis complements testing and experimentation by:

reducing total effort

reducing cost required for experimentation


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Introduction To CFD



Training ManualHow Does CFD Work?

ANSYS CFD solvers are based on the finitevolume method

The fluid region is decomposed into a finiteset of control volumes

General conservation (transport) equationsfor mass, momentum, energy, species, etc.are solved on this set of control volumes

Continuous partial differential equations (thegoverning equations) are discretized into asystem of linear algebraic equations that canbe solved on a computer

Control

Volume*

* FLUENT control volumes

are cell-centered (i.e. they

correspond directly with the

mesh) while CFX controlvolumes are node-centered

Unsteady Advection Diffusion Generation


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Training ManualCFD Modeling Overview

Problem Identification

1. Define goals

2. Identify domain

Pre-Processing

3. Geometry

4. Mesh

5. Physics

6. Solver Settings

Solve7. Compute solution

Post Processing

8. Examine results

9.

Upda

teModel


1. Define your modeling goals

2. Identify the domain you will model

PreProcessing and Solver Execution3. Create a solid model to represent the

domain

4. Design and create the mesh (grid)5. Set up the physics

Physical models, domain properties,boundary conditions,

6. Define solver settings numerical schemes, convergence

controls, 7. Compute and monitor the solution

Post-Processing

8. Examine the results

9. Consider revisions to the model


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Training Manual1. Define Your Modeling Goals

What results are you looking for (i.e. pressure drop, mass flow rate),and how will they be used?

What are your modeling options? What physical models will need to be included in your analysis (i.e. turbulence,

compressibility, radiation)?

What simplifying assumptions do you have to make?

What simplifying assumptions can you make (i.e. symmetry, periodicity)?

Do you require a unique modeling capability? User-defined functions (written in C) in FLUENT or User FORTRAN functions in CFX

What degree of accuracy is required?

How quickly do you need the results?

Is CFD an appropriate tool?


1. Define goals

2. Identify domain


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Training Manual2. Identify the Domain You Will Model

How will you isolate a piece of thecomplete physical system?

Where will the computational

domain begin and end? Do you have boundary condition

information at these boundaries?

Can the boundary condition typesaccommodate that information?

Can you extend the domain to a point

where reasonable data exists?

Can it be simplified or approximatedas a 2D or axisymmetric problem?

Cyclone Separator

Gas

Riser

Cyclone

L-valve

Gas

Domain of interest


1. Define goals

2. Identify domain


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Introduction To CFD



Training Manual3. Create a Solid Model of the Domain

How will you obtain a solid model of thefluidregion?

Make use of existing CAD models? Create from scratch?

Can you simplify the geometry?

Remove unnecessary features that wouldcomplicate meshing (fillets, bolts)?

Make use of symmetry or periodicity?

Do you need to split the model so thatboundary conditions or domains can becreated?

Solid model of a

Headlight Assembly

Pre-Processing

3. Geometry

4. Mesh

5. Physics

6. Solver Settings


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Introduction To CFD



Training Manual4. Design and Create the Mesh

Triangle Quadrilateral

Pyramid Prism/Wedge

Tetrahedron Hexahedron

What degree of mesh resolution is required ineach region of the domain?

The mesh must resolve geometric features ofinterest and capture gradients of concern

e.g. velocity, pressure, temperature gradients

Can you predict regions of high gradients?

Will you use adaption to add resolution?

What type of mesh is most appropriate? How complex is the geometry?

Can you use a quad/hex mesh or is a tri/tet orhybrid mesh suitable?

Are mesh interfaces needed?

Do you have sufficient computer resources? How many cells/nodes are required?

Which physical models will be used?

Pre-Processing

3. Geometry

4. Meshing

5. Physics

6. Solver Settings

A mesh divides a geometry into many elements. Theseare used by the CFD solver to construct control volumes


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Training ManualTri/Tet vs. Quad/Hex Meshes

For flow-aligned geometries,

quad/hex meshes can providehigher-quality solutions with fewercells/nodes than a comparable tri/tetmesh

Quad/Hex meshes show reduced false

diffusion when the mesh is aligned withthe flow.

It does require more effort to generate aquad/hex mesh

Meshing tools designed for aspecific application can streamlinethe process of creating a quad/hexmesh for some geometries.

E.g. TurboGrid, IcePak,AirPak


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Training ManualTri/Tet vs. Quad/Hex Meshes

For complex geometries, quad/hex meshesshow no numerical advantage, and youcan save meshing effort by using a tri/tetmesh or hybrid mesh

Quick to generate

Flow is generally not aligned with the mesh

Hybrid meshes typically combine tri/tetelements with other elements in selectedregions

For example, use wedge/prism elementsto resolve boundary layers

More efficient and accurate

than tri/tet alone

Tetrahedral mesh

Wedge (prism) mesh


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Training ManualMultizone (or Hybrid) Meshes

A multizone or hybrid mesh uses

different meshing methods in differentregions, e.g:

Hex mesh for fan and heat sink

Tet/prism mesh elsewhere

Multizone meshes yield a goodcombination of accuracy, efficientcalculation time and meshing effort.

When the nodes do not match across

the regions, a General Grid Interface(GGI) can be used.

Model courtesy of ROI Engineering


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Training ManualNon-Matching Meshes

Non matching meshes are usefulfor meshing complex geometries

Mesh each part then join together

Non matching mesh interfaces arealso used in other situations

Change in reference frames

Moving mesh applications

Non-matching

interface

3D Film Cooling

Coolant is injected into a duct from

a plenum. The plenum is meshed

with tetrahedral cells while the ductis meshed with hexahedral cells

Compressor and Scroll

The compressor and scroll are joined through a General

Grid Interface. This serves to connect the hex and tet

meshes and also allows a change in reference frame


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Training ManualSet Up the Physics and Solver Settings

For complex problemssolving a simplified or 2Dproblem will provide

valuable experience with themodels and solver settingsfor your problem in a shortamount of time.

Pre-Processing

3. Geometry

4. Mesh

5. Physics

6. Solver Settings

For a given problem, you will need to:

Define material properties

Fluid

Solid

Mixture

Select appropriate physical models

Turbulence, combustion, multiphase, etc.

Prescribe operating conditions

Prescribe boundary conditions at allboundary zones

Provide initial values or a previous solution

Set up solver controls

Set up convergence monitors


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Training ManualCompute the Solution

The discretized conservation equations aresolved iteratively; some number of iterations is

required to reach a converged solution.

Parallel processing can provide faster solutionsand access to more memory (solve larger cases)

Convergence is reached when: Changes in solution variables from one iteration

to the next are negligible

Overall property conservation is achieved

Quantities of interest (e.g. drag, pressure drop)have reach steady values

The accuracy of a convergedsolution isdependent upon: Appropriateness and accuracy of physical models

Mesh resolution and independence

Numerical errors

A converged and mesh-independent solution ona well-posed problem

will provide usefulengineering results!

Solve

7. Compute solution


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Training ManualExamine the Results

Examine the results to review solution

and extract useful data Visualization tools 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?

Numerical Reporting Tools can be usedto calculate quantitative results:

Forces and Moments

Average heat transfer coefficients

Surface and Volume integrated quantities

Flux BalancesExamine results to ensureproperty conservation and

correct physical behavior. Highresiduals may be attributable toonly a few cells of poor quality.

Post Processing

8. Examine results 9.

Upd

ateModel


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1 16ANSYS, Inc. Proprietary 2009 ANSYS I All i h d

April 28, 2009I

Training ManualConsider Revisions to the Model

Are the physical models appropriate?

Is the flow turbulent?

Is the flow unsteady?

Are there compressibility effects?

Are there 3D effects?

Are the boundary conditions correct? Is the computational domain large enough?

Are boundary conditions appropriate?

Are boundary values reasonable?

Is the mesh adequate?

Can the mesh be refined to improve results?

Does the solution change significantly with a refinedmesh, or is the solution mesh independent?

Does the mesh resolution of the geometry need to beimproved?

Post Processing

8. Examine results9.

Upd

ateModel

cfx12 01 intro cfd

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