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2-1ANSYS, Inc. Proprietary
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Introductory FLUENT
Training
Chapter 2
Introduction to CFD
www.ptecgroup.ir
Introduction to CFD
2-2ANSYS, Inc. Proprietary
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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.
Introduction to CFD
2-3ANSYS, Inc. Proprietary
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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
Introduction to CFD
2-4ANSYS, Inc. Proprietary
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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
2-5ANSYS, Inc. Proprietary
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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?
Problem Identification
1. Define goals
2. Identify domain
Introduction to CFD
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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?
Problem Identification
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
Introduction to CFD
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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
Introduction to CFD
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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.
Introduction to CFD
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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
Introduction to CFD
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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
2-12ANSYS, Inc. Proprietary
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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
Introduction to CFD
2-13ANSYS, Inc. Proprietary
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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
Introduction to CFD
2-14ANSYS, Inc. Proprietary
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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
Introduction to CFD
2-15ANSYS, Inc. Proprietary
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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
Introduction to CFD
2-16ANSYS, Inc. Proprietary
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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
2-17ANSYS, Inc. Proprietary
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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
Introduction to CFD
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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
2-19ANSYS, Inc. Proprietary
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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.
Introduction to CFD
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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.
Introduction to CFD
2-21ANSYS, Inc. Proprietary
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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.
Introduction to CFD
2-22ANSYS, Inc. Proprietary
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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
Introduction to CFD
2-23ANSYS, Inc. Proprietary
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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.
Introduction to CFD
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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.