Lectures Fluent Lecture

© Fluent Inc. 17-Sep-05A1

Fluent Software TrainingTRN-99-003

Introduction to CFD Analysis



Background

FLUENT solvers are based on thefinite volume method.

Domain is discretized into a finite set of control volumes or cells.General conservation (transport) equation for mass, momentum, energy, etc.,

are discretized into algebraic equations.

All equations are solved to render flow field.

∫∫∫∫ +⋅∇Γ=⋅+∂∂

VAAV

dVSdddVt φφρφρφ AAV

unsteady convection diffusion generation

Eqn.continuity 1

x-mom. uy-mom. venergy h

φ

Fluid region of pipe flow discretized into finite set of control volumes (mesh).

control volume



CFD Analysis: Basic Steps

Problem Identification and Pre-Processing1. Define your modeling goals.2. Identify the domain you will model.3. Design and create the grid.Solver Execution4. Set up the numerical model.5. Compute and monitor the solution.Post-Processing6. Examine the results.7. Consider revisions to the model.



Define Your Modeling Goals

What results are you looking for, and how will they be used?What physical models will need to be included in your analysis?

Multiphase?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 4.5User-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?



Design and Create the GridShould you use a quad/hex grid, a tri/tet grid, a hybrid grid, or a non-conformal grid?What degree of grid resolution is required in each region of thedomain?Can you take advantage of MixSim or IcePak?Will you use adaption to add resolution?How many cells are required for the problem?Do you have sufficient computer memory?

triangle

quadrilateraltetrahedron pyramid

prism or wedgehexahedron



Set Up the Numerical Model

For a given problem, you will need to:Select appropriate physical models.

Turbulence, combustion, multiphase, etc.Define material properties.

Fluid SolidMixture

Prescribe operating conditions.Prescribe boundary conditions at all boundary zones.Provide an initial solution.Set up solver controls.Set up convergence monitors.



Compute the SolutionThe discretized conservation equations are solved iteratively.

A number of iterations are usually required to reach a convergedsolution.

Convergence is reached when:Changes in solution variables from one iteration to the next arenegligible.

Residuals provide a mechanism to help monitor this trend.Overall property conservation is achieved.

The accuracy of a converged solution is dependent upon:Appropriateness and accuracy of the physical models.Grid resolution and independenceProblem setup

A converged and grid-independent solution on a well-posed problem will provide useful engineering results!



Examine the ResultsExamine the results to review solution and to extract useful engineering data.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 dragAverage heat transfer coefficientsSurface-averaged quantities



Tools to Examine the Results

Graphical toolsGrid, contour, and vector plotsPathline and particle trajectory plotsXY plotsAnimations

Numerical reporting toolsFlux balancesSurface and volume integrals and averagesForces and moments



Consider Revisions to the ModelAre 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?



Review for Demo

Problem Identification and Pre-Processing1. Define your modeling goals.2. Identify the domain you will model.3. Design and create the grid.Solver Execution4. Set up the numerical model.5. Compute and monitor the solution.Post-Processing6. Examine the results.7. Consider revisions to the model.



FLUENT DEMOStartup Gambit

load databasedefine boundary zonesexport mesh

Startup FluentGUIProblem SetupSolvePost-Processing



Operating System Basics: UnixBasic Unix commands:

pwd - prints the name current working directoryYour home directory is home/fluent/.

ls - lists the files in the current directorycd - change working directories (cd .. to go up one directory).

The environment variable $TRAINPATH contains a shortcut to the directory where training files are stored. For example:cp $TRAINPATH/fluent5.x/tut/elbow/elbow.msh .

will copy the mesh file for the first example problem into your current working directory.To start Fluent 5: % fluent 2d &To start Fluent 4.5: % fluent -r4.5 &



Operating System Basics: Windows NT

PC users will find tutorials under c:\Fluent.Inc\fluent5.x\tut\. This directory is write-protected.Save files to your home directory, c:\users\fluent\.Fluent can be started from the command prompt or from the start menu:

Command Promptfluent 2d

Start MenuStart⇒ Programs⇒ Fluent Inc⇒ Fluent 5.x

!Note: It is recommended that you restart Fluent for each tutorial for both Unix and NT systems to avoid mixing solver settings from different tutorials.

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