1. Chapter 2 Introduction to CFD
Introductory FLUENT Training
Sharif University of Technology
Lecturer: Ehsan Saadati

2. What 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.

3. How 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
Control
Volume*
Fluid region of pipe flow is discretized into a finite set of control volumes.
Unsteady
Convection
Diffusion
Generation
Equation Variable
Continuity 1
X momentum u
Y momentum v
Z momentum w
Energy h
This background page is to introduce the concept of FVM to the user. We will assume that the majority of users will be relatively new to CFD. This page allows us to introduce the concept of spatial discretization and equation discretization which leads into the next slide that summarizes the steps involved in cfd analysis- meshing and solving.
* FLUENT control volumes are cell-centered (i.e. they correspond directly with the mesh) while CFX control volumes are node-centered

4. Problem Identification
CFD Modeling Overview
Problem Identification
Define your modeling goals
Identify the domain you will model
PreProcessing and Solver Execution
Create a solid model to represent the domain
Design and create the mesh (grid)
Set up the physics (physical models, material properties, domain properties, boundary conditions, …)
Define solver settings (numerical schemes, convergence controls, …)
Compute and monitor the solution
Post-Processing
Examine the results.
Consider revisions to the model.
Problem Identification
Define goals
Identify domain
Pre-Processing
Geometry
Mesh
Physics
Solver Settings
Solve
Compute solution
Turbulence models:
Spalart-Allmaras
k-e, RNG k-e, Realizable k-e
Non-equilibrium wall functions; compressibility and transpiration effects
Two-layer/zonal models
Low-Re models
Suite of damping function models
V2F model
k-w (Wilcox, SST) – FLUENT 6
Reynolds Stress Model (RSM) – (LRR, SSG)
Large Eddy Simulation (LES)
Heat transfer
Conduction, Convection, Radiation
Multiphase and free surface models
Discrete Phase, VOF, ASM; Eulerian
(multifluid) models in FLUENT 6
Porous media and lumped parameter models
Fan, heat exchangers, porous jump, porous media
Multiple reference frames, sliding mesh and mixing plane model
Inviscid, laminar or turbulent
Update Model
Post Processing
Examine results

5. 1. Define Your Modeling Goals
Problem Identification
Define goals
Identify domain
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?

6. 2. Identify the Domain You Will Model
Problem Identification
Define goals
Identify domain
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?
Domain of Interest as Part of a Larger System (not modeled)
Domain of interest isolated and meshed for CFD simulation.

7. 3. Create a Solid Model of the Domain
Pre-Processing
Geometry
Mesh
Physics
Solver Settings
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

8. 4. Design and Create the Mesh
A mesh divides a geometry into many elements. These are used by the CFD solver to construct control volumes
Pre-Processing
Geometry
Meshing
Physics
Solver Settings
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?
Triangle
Quadrilateral
Tetrahedron
Hexahedron
Pyramid
Prism/Wedge

9. Tri/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.

10. Tri/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.
Wedge (prism) mesh
Tetrahedral mesh

11. Multizone (or Hybrid) Meshes
Model courtesy of ROI Engineering
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.

12. Non-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

13. Set 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
Pre-Processing
Geometry
Mesh
Physics
Solver Settings
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.

14. Compute 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
Solve
Compute solution
A converged and mesh-independent solution on a well-posed problem will provide useful engineering results!

15. Examine the Results
Post Processing
Examine results
Update Model
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.

16. Consider Revisions to the Model
Post Processing
Examine results
Update 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?

17. Models 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
This slide and the next introduce a summary of all models available in fluent. I don’t think it is necessary to give examples of each model in particular, but to generalize by stating that reacting flows and multiple phase flows can be studied.
‘Can fluent do this…?’ type questions should be deferred to 1) after lecture discussion, 2) tutorial, or 3) ask an expert. If we don’t take this approach, this lecture could go on for a very long time.
Temperature Contours for Kiln Burner Retrofit

18. Models 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
Gas
outlet
Oil
Three- Phase
Inlet
Water
Contours of Oil Volume Fraction
in a Three-Phase Separator
Pressure Contours in a Squirrel Cage Blower (Courtesy Ford Motor Co.)

19. FLUENT 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.

20. Import 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.

21. Generate 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.

22. Define 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.
velocity inlet

23. Set 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

24. Demonstration 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.
The demo should include a very quick gambit session. This session will involve the import of a database file, the setting of boundary zone names, and the export of a mesh. We do not want to give the trainees enough knowledge at this point on how to generate geometry or mesh. The purpose of this step is to provide a connection between the names of the boundaries assigned in gambit to the names of the boundaries in fluent.
Discuss items 2 and 3 on the previous slide when in gambit. Discuss items when in fluent.
Assume the customers are relatively new to CFD- try not to refer to terminology that has not yet been introduced. Remember to:
1. Discuss layout of gui (working from left to right), 2. Show the grid early in the demo, 3. Scale grid, 4. Show boundary names, 5. Discuss default settings of models and properties, 6. Show several post-processing features.
It is not necessary to point out all models or capabilities at this time. It will be brought up in later lectures.

25. Navigating the PC at Fluent
Log in to your workstation
Login name: fluent
Password: fluent
Directories
Tutorial mesh/case/data files can be found in c:\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.