introductory fluent trainingdl.ptecgroup.ir/.../fluent12-lecture02-intro-to-cfd.pdf · ·...

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

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Introductory FLUENT

Training

Chapter 2

Introduction to CFD

www.ptecgroup.ir

Introduction to CFD



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Training ManualWhat is CFD?

• Computational fluid dynamics (CFD) is the science of predicting fluid flow,

heat and mass transfer, chemical reactions, and related phenomena by

solving numerically the set of governing mathematical equations

– Conservation of mass

– Conservation of momentum

– Conservation of energy

– Conservation of species

– Effects of body forces

– 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 and cost required for experimentation and data acquisition.

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Training ManualHow Does CFD Work?

• ANSYS CFD solvers are based on the

finite volume method

– Domain is discretized into a finite set of

control volumes

– General conservation (transport) equations

for mass, momentum, energy, species, etc.

are solved on this set of control volumes

– Partial differential equations are

discretized into a system of algebraic

equations

– All algebraic equations are then solved

numerically to render the solution field

Fluid region of pipe flow is

discretized into a finite set

of control volumes.

Equation Variable

Continuity 1

X momentum u

Y momentum v

Z momentum w

Energy h

Control

Volume*

* FLUENT control volumes are cell-centered (i.e. they correspond

directly with the mesh) while CFX control volumes are node-centered

Unsteady Convection Diffusion Generation

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Training ManualCFD Modeling Overview

• Problem Identification

1. Define your modeling goals

2. Identify the domain you will model

• PreProcessing and Solver Execution

3. Create a solid model to represent the domain

4. Design and create the mesh (grid)

5. Set up the physics (physical models, material properties, 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.

Problem Identification

1. Define goals

2. Identify domain

Pre-Processing

3. Geometry

4. Mesh

5. Physics

6. Solver Settings

Solve

7. Compute solution

Post Processing

8. Examine results

9.

Up

da

te M

od

el

Introduction to CFD



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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 the complete physical system?

• Where will the computational domain begin and end?

– Do you have boundary condition information at these boundaries?

– Can the boundary condition types accommodate that information?

– Can you extend the domain to a point where reasonable data exists?

• Can it be simplified or approximated as a 2D or axisymmetric problem?


1. Define goals

2. Identify domain

Domain of Interest

as Part of a Larger

System (not modeled)

Domain of interest

isolated and meshed

for CFD simulation.

Introduction to CFD



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Training Manual3. Create a Solid Model of the Domain

• How will you obtain a solid model of the fluid region?

– Make use of existing CAD models?

• Extract the fluid region from a solid part?

– Create from scratch?

• Can you simplify the geometry?

– Remove unnecessary features that would complicate meshing (fillets, bolts…)?

– Make use of symmetry or periodicity?

• Are both the solution and boundary conditions symmetric / periodic?

• Do you need to split the model so that boundary conditions or domains can be created?

Solid model of a

Headlight Assembly

Pre-Processing

3. Geometry

4. Mesh

5. Physics

6. Solver Settings

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Training Manual4. Design and Create the Mesh

• What degree of mesh resolution is required in each region of the domain?

– The mesh must resolve geometric features of interest 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 or hybrid mesh suitable?

– Are non-conformal interfaces needed?

• Do you have sufficient computer resources?

– How many cells/nodes are required?

– How many physical models will be used?Pyramid Prism/Wedge

Hexahedron

Pre-Processing

3. Geometry

4. Meshing

5. Physics

6. Solver Settings

Triangle Quadrilateral

Tetrahedron

A mesh divides a geometry into

many elements. These are 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 provide

higher-quality solutions with fewer

cells/nodes than a comparable tri/tet

mesh

– Quad/Hex meshes show reduced

numerical diffusion when the mesh is

aligned with the flow.

– It does require more effort to

generate a quad/hex mesh

• Meshing tools designed for a

specific application can streamline

the process of creating a quad/hex

mesh for some geometries.

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Training ManualTri/Tet vs. Quad/Hex Meshes

• For complex geometries, quad/hex meshes

show no numerical advantage, and you

can save meshing effort by using a tri/tet

mesh or hybrid mesh

– Quick to generate

– Flow is generally not aligned with the mesh

• Hybrid meshes typically combine tri/tet

elements with other elements in selected

regions

– For example, use wedge/

prism elements to 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 different

regions. For example,

– Hex mesh for fan and heat sink

– Tet/prism mesh elsewhere

• Multizone meshes yield a good

combination of accuracy, efficient

calculation time and meshing effort.

• When the nodes do not match across

the regions, a non-conformal interface

can be used.

Model courtesy of ROI Engineering

Introduction to CFD



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Training ManualNon-Conformal Meshes

• Non conformal meshes are useful

for meshing complex geometries

– Mesh each part then join together

• Non conformal interfaces are also

used in other situations

– Change in reference frames

– Moving mesh applications

Non-conformal

interface

3D Film Cooling

Coolant is injected into a duct from a

plenum. The plenum is meshed with

tetrahedral cells while the duct is

meshed with hexahedral cells

Compressor and Scroll

The compressor and scroll are joined through a

non conformal 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 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 all

boundary zones

– Provide initial values or a previous solution

– Set up solver controls

– Set up convergence monitors

For complex problems

solving a simplified or 2D

problem will provide

valuable experience with the

models and solver settings

for your problem in a short

amount of time.

Pre-Processing

3. Geometry

4. Mesh

5. Physics

6. Solver Settings

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Training ManualCompute the Solution

• The discretized conservation equations are solved iteratively until convergence.

• Convergence is reached when:

– Changes in solution variables from one iteration to the next are negligible.

• Residuals provide a mechanism to help monitor this trend.

– Overall property conservation is achieved

• Imbalances measure global conservation

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

• Monitor points track quantities of interest.

• The accuracy of a converged solution is dependent upon:

– Appropriateness and accuracy of physical models.

– Mesh resolution and independence

– Numerical errors

A converged and mesh-

independent solution on a well-

posed problem will provide useful

engineering 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 used to calculate quantitative results:

• Forces and Moments

• Average heat transfer coefficients

• Surface and Volume integrated quantities

• Flux Balances

Examine results to ensure property conservation

and correct physical behavior. High residuals

may be caused by just a few poor quality cells.

Post Processing

8. Examine results

9.

Update

Model

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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 refined

mesh, or is the solution mesh independent?

– Does the mesh resolution of the geometry need to be

improved?

Post Processing

8. Examine results

9.

Update

Model

Introduction to CFD



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Training ManualModels Available in FLUENT 12

• Fluid flow and heat transfer

– Momentum, continuity, energy equations

– Radiation

• Turbulence

– RANS-based models (Spalart-Allmaras, k–ε, k–ω, Reynolds stress)

– Large-eddy simulation (LES) and detached eddy simulation (DES)

• Species transport

• Volumetric reactions

– Arrhenius finite-rate chemistry

– Turbulent fast chemistry

• Eddy Dissipation, non-Premixed, premixed, partially premixed

– Turbulent finite-rate chemistry

• EDC, laminar flamelet, composition PDF transport

– Surface Reactions

Pressure Contours in Near-Ground Flight

Temperature Contours for Kiln Burner Retrofit

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Training ManualModels Available in FLUENT 12

• Multiphase flows

– Discrete Phase Model (DPM)

– Volume of Fluid (VOF) model for immiscible fluids

– Mixtures

– Eulerian-Eulerian and Eulerian-granular

– Liquid/Solid and cavitation phase change

• Moving and deforming mesh

– Moving zones

• Single and multiple reference frames (MRF)

• Mixing plane model

• Sliding mesh model

– Moving and deforming (dynamic) mesh (MDM)

• User-defined scalar transport equations Pressure Contours in a Squirrel Cage

Blower (Courtesy Ford Motor Co.)

Gas

outlet

Oil

outlet

Three-

Phase

Inlet

Water

outletContours of Oil Volume Fraction

in a Three-Phase Separator

Introduction to CFD



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Training ManualFLUENT CFD Workflow under Workbench 2

• Start ANSYS Workbench

• Drag the Fluid Flow (FLUENT)

system from Analysis Systems

group in the Toolbox onto

preview drop target shown in

the Project Schematic.

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Training ManualImport the Geometry

• Right-click on Geometry cell A2 and select Import Geometry

• Import the geometry file (CAD model or DesignModeler .agdb file)

• You can also link the FLUENT simulation to an existing

DesignModeler session.

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Training ManualGenerate a Mesh

• Right-click on Mesh cell and select Edit.

– Meshing opens and loads geometry

• Select Mesh under Model in Outline

– Note that Preferences are automatically set

for FLUENT, because Meshing was opened

from a FLUENT system.

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Training ManualDefine Boundary and Cell Zones

• Create boundary zones using Named

selections.

– Select the surface which will

represent the boundary you wish

to set.

– Right-click the selection and select

Create Named Selection.

– Name the selection and click OK.

• You will also need to define the

regions of the flow containing fluid

and solid (if any).

– Solids are required for conjugate

heat transfer calculations only.

– More details will be presented

later.

velocit

y inlet

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Training ManualSet Up and Run FLUENT

• Edit the Setup cell to set up the model options

– Boundary conditions

– Solver settings

– Solution

– Post processing

• Once run, the solution can then be either post processed in FLUENT

or data exported to CFD-Post for post processing

– Contour and vector plots

– Profile plots

– Calculation of forces and moments

– Animation of unsteady flow results

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Training ManualDemonstration of FLUENT Software

• Start FLUENT (assume the mesh has

already been generated).

– Set up a simple problem.

– Solve the flow field.

– Postprocess the results.

• Online help and documentation is

available on each panel by pressing

the help button

– Requires that you have the

documentation installed and properly

connected to your web browser.

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Training ManualNavigating the PC at Fluent

• Log in to your workstation

– Login name: fluent

– Password: fluent

• Directories

– Tutorial mesh/case/data files can be found inc:\Student Files\fluent\tut\

– We recommend that you save your work into a central working folder:c:\users

– Working folder shown on the desktop is a shortcut to c:\users

• To start FLUENT and/or Workbench, use the desktop icons.

• Your support engineer will save your work at the end of the week.

• It is recommended that you restart FLUENT and/or Workbench for

each tutorial to avoid mixing solver settings from different

workshops.

introductory fluent trainingdl.ptecgroup.ir/.../fluent12-lecture02-intro-to-cfd.pdf · ·...

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