Basics of CFD and Operations in Fluent

 Basic concepts of Numerical Calculations

 Pre-processing in Fluent
 Solutions
 Post processing in Fluent
 Programming: Journaling, Scripting, UDF
Analytical vs. Numerical Approach
Analytical Calculation vs. Numerical Calculation

• Analytical Results

• Available as explicit or implicit form of an equation such as a quadratic equation.

• It is a continuous equation, available at each point is space (Infinite Unknowns)

• Numerical Calculation

• Available at discrete location in space / time dimensions (Finite Unknowns)

• It is an inherently discontinuous approach with some averaging / blending to ensure

physical correctness

• Examples

• Simply supported beam - Analytical

• Plate temperature distribution - Numerical

Analytical Calculation vs. Numerical Calculation

Numerical solution:
Approximate solution of the
actual problem

Numerical solution:
Approximate solution of the
approximate problem

P
x 3
 3 xL2  2 L3  x
6 EI

Analytical solution: Exact

solution of the approximate
problem
Traditional vs Virtual Design Evaluations

Traditional Design Methodology Modern Design Methodology

Product
Product Requirement
Requirement

Virtual Prototypes

Physical Prototypes
Virtual Testing

Design Changes
Physical Testing

Physical Prototype
CAX:
CAD, CAE,
Design Changes CFD, CAA
Physical Testing

• Long Product Development Cycle • Minimal Prototypes and Cost Effective

• Many prototypes • Product Development Cycle Drastically Reduced
• Very Costly • Better Insight into various Design Aspects
Typical Flow Chart for Numerical Analysis

Define Input / Output - Assumptions based on Engg. Judgment

Prepare CAD Model, De-featuring & Simplification for CFD meshing

Pre-processing
Generate Mesh and Define Zones / Patches – inlet, outlet, wall, symmetry …

Define Thermodynamic and Transport Properties of Material

Specify Boundary and Initial Condition

Select flow model: Laminar, Turbulent, Single-phase, Multi-phase …

Select appropriate turbulence model based on flow type, wall function

Solver Setting &
Solution Specify discretization schemes, time steps …

Analysis Run: Batch Mode (command line), GUI, Remote Server

Contour Plots, Velocity Vectors, Streamlines, Pathlines, Isolines …

Post-processing
Animations for Streamlines, Particle Tracks, Coefficients of Pressure …

Report Preparation
Local Server: GUI-operations and their meaning
Starting Fluent

1. 2D or 3D, single precision or double

precision (dp -> double precision)

2. Pre-processing, Solution and Post Process

[Full Simulation] or just post-processing
Setting of Mouse Button for PAN (MOVE) – ZOOM – ROTATE

Basic Settings

The probe option with print the information

when clicked inside mesh
Reading Mesh: File  Import Mesh, Read Case …

File GUI Menu

Select as appropriate
Reading Mesh: File  Import Mesh, Read Case …

Top panel describes the summary of case file

GUI Menu

TUI Menu

Software operation summary. Note ‘done’ at

the end! Any error will be reported here.
Check Mesh: Grid  Check

Check at the bottom for

error message
Check Mesh: Repair Shadow Zones in Periodic Mesh

WARNING: node on face thread 2 has multiple shadows.

This warning message appears only in case of periodic (translational of rotational)

faces!

These faces can be repaired only through the Text User Terminal (TUI)

TUI: grid  modify-zones  repair-periodic

1. The program will automatically try to detect the

periodic distance or angles though will ask to user
inputs as well
2. The command can be shortened as: grid  mz  rp
Manipulate Mesh: Optional for ease of simulation

1. Merging Zones: combining multiple zones of similar type – process not fully

reversible (de-merging to previous state not possible): keep back-ups

2. Separating Zones: Opposite of “Merging Zones” – required if say there are

multiple outlets and all grouped into single zone in the meshing software.

3. Creating Periodic Zones, Slitting Periodic Zones: For periodic zones

4. Scaling the Grid – FLUENT is a metric solver. Scale the mesh appropriately to

convert into meters. E.g. if mesh was generated in inch, scale factor = 0.0254

5. Translating the Grid: Move the grid in required to move near origin

6. Rotating the Grid: Rotate the mesh to orient to particular axis

Manipulate Mesh: Optional for ease of simulation

Some other options for the sake of completeness

1. Fusing Face Zones: fuse boundaries (and merge duplicate nodes and faces)

created by assembling multiple mesh regions.

2. Slitting Face Zones: Not same as separating a face zone! Slit an internal wall or

coupled wall zone into two distinct uncoupled zones.

3. Extruding Face Zones: A face can be extruded to increase the domain size say

changing location of the outlet to prevent reverse flow.

4. Replacing, Deleting, Deactivating, and Activating Zones

5. Reordering the Domain and Zones

Manipulate Mesh: Separate Face Zones

This feature is most used among all the options described earlier
Apply Boundary Conditions

Boundaries are representation of physical state of the computational domain!

Zones defined by CFD Analyst

Boundary condition type

available in FLUENT
Define Material Properties

Thermodynamic and transport properties of all the phases

Thermodynamics properties

Transport properties
Define Turbulence Model

RANS: Reynolds-Averaged Navier Stokes and k-e are the workhorse of industry!

Turbulence Models

Wall Function
Solver running, monitor and convergence
parammeters
Define Solution Limits

If during computation value exceed these limits, solver will clip to range defined.

Limits of mean values

Limits of fluctuating values

Define Initial Values: solve  Initialize  Initialize …

A better guess helps improve the convergence sometimes!

Based on selected boundary

Calculated value
Define Convergence Criteria: Solve  Monitors  Residual …

When the solver should stop running? Either criterion-1 or criterion-2 is met!

Criterion-1

Print in the console and plot

as graphical chart

Criterion-2
Qualitative plots, quantitative integration and
averaging
Display Contour Plots: Display  Contours …

Contour: coloured representation of field variables on a plane or surface

Criterion-1

How to show the plot and

interpolate the results?

Number of sub-divisions in
legend
Display Vectors: Display  Vectors …

Vectors field represented as arrows

Criterion-1

Range of plot and scale,

whether to show mesh?

Vector scale and spatial

distribution
Volumetric heat source, temperature dependent
material properties …
UDF: User-Defined Function, Journals and Transcripts

1. FLUENT uses programming languages SCHEME (TUI), FORTRAN (back-end

mathematics) and Tcl/Tk (GUI)

2. UDF: FLUENT is a general-purpose CFD simulation program and cannot

address all the physical variations. UDF fills this gap.

3. Journals and Transcripts are similar recording of VBA scripts in EXCEL.

4. The details of this feature is covered under advance topic once you get

mastery of the topics covered so far!

 Please visit http://www.cfdyna.com for explore
more about CFD and related stuff.

 You may write to me at amod.cfd@gmail.com to

get help on any advance topic!