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Chapter 1
Introduction to CFD
Introduction to CFX
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Training ManualWhat is CFD?
Computational fluid dynamics (CFD) is the science of predicting fluidflow, heat and mass transfer, chemical reactions, and relatedphenomena by solving numerically the set of governing mathematicalequations
Conservation of mass, momentum, energy, species mass, 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
reducing cost required for experimentation
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Training ManualHow Does CFD Work?
ANSYS CFD solvers are based on the finitevolume method
The fluid region is decomposed into a finiteset of control volumes
General conservation (transport) equationsfor mass, momentum, energy, species, etc.are solved on this set of control volumes
Continuous partial differential equations (thegoverning equations) are discretized into asystem of linear algebraic equations that canbe solved on a computer
Control
Volume*
* FLUENT control volumes
are cell-centered (i.e. they
correspond directly with the
mesh) while CFX controlvolumes are node-centered
Unsteady Advection Diffusion Generation
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Training ManualCFD Modeling Overview
Problem Identification
1. Define goals
2. Identify domain
Pre-Processing
3. Geometry
4. Mesh
5. Physics
6. Solver Settings
Solve7. Compute solution
Post Processing
8. Examine results
9.
Upda
teModel
Problem Identification
1. Define your modeling goals
2. Identify the domain you will model
PreProcessing and Solver Execution3. Create a solid model to represent the
domain
4. Design and create the mesh (grid)5. Set up the physics
Physical models, 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
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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
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Training Manual2. Identify the Domain You Will Model
How will you isolate a piece of thecomplete physical system?
Where will the computational
domain begin and end? Do you have boundary condition
information at these boundaries?
Can the boundary condition typesaccommodate that information?
Can you extend the domain to a point
where reasonable data exists?
Can it be simplified or approximatedas a 2D or axisymmetric problem?
Cyclone Separator
Gas
Riser
Cyclone
L-valve
Gas
Domain of interest
Problem Identification
1. Define goals
2. Identify domain
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Training Manual3. Create a Solid Model of the Domain
How will you obtain a solid model of thefluidregion?
Make use of existing CAD models? Create from scratch?
Can you simplify the geometry?
Remove unnecessary features that wouldcomplicate meshing (fillets, bolts)?
Make use of symmetry or periodicity?
Do you need to split the model so thatboundary conditions or domains can becreated?
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
Triangle Quadrilateral
Pyramid Prism/Wedge
Tetrahedron Hexahedron
What degree of mesh resolution is required ineach region of the domain?
The mesh must resolve geometric features ofinterest 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 orhybrid mesh suitable?
Are mesh interfaces needed?
Do you have sufficient computer resources? How many cells/nodes are required?
Which physical models will be used?
Pre-Processing
3. Geometry
4. Meshing
5. Physics
6. Solver Settings
A mesh divides a geometry into many elements. Theseare 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 providehigher-quality solutions with fewercells/nodes than a comparable tri/tetmesh
Quad/Hex meshes show reduced false
diffusion when the mesh is aligned withthe flow.
It does require more effort to generate aquad/hex mesh
Meshing tools designed for aspecific application can streamlinethe process of creating a quad/hexmesh for some geometries.
E.g. TurboGrid, IcePak,AirPak
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Training ManualTri/Tet vs. Quad/Hex Meshes
For complex geometries, quad/hex meshesshow no numerical advantage, and youcan save meshing effort by using a tri/tetmesh or hybrid mesh
Quick to generate
Flow is generally not aligned with the mesh
Hybrid meshes typically combine tri/tetelements with other elements in selectedregions
For example, use wedge/prism elementsto 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 differentregions, e.g:
Hex mesh for fan and heat sink
Tet/prism mesh elsewhere
Multizone meshes yield a goodcombination of accuracy, efficientcalculation time and meshing effort.
When the nodes do not match across
the regions, a General Grid Interface(GGI) can be used.
Model courtesy of ROI Engineering
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Training ManualNon-Matching Meshes
Non matching meshes are usefulfor meshing complex geometries
Mesh each part then join together
Non matching mesh interfaces arealso used in other situations
Change in reference frames
Moving mesh applications
Non-matching
interface
3D Film Cooling
Coolant is injected into a duct from
a plenum. The plenum is meshed
with tetrahedral cells while the ductis meshed with hexahedral cells
Compressor and Scroll
The compressor and scroll are joined through a General
Grid 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 complex problemssolving a simplified or 2Dproblem will provide
valuable experience with themodels and solver settingsfor your problem in a shortamount of time.
Pre-Processing
3. Geometry
4. Mesh
5. Physics
6. 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 allboundary zones
Provide initial values or a previous solution
Set up solver controls
Set up convergence monitors
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Training ManualCompute the Solution
The discretized conservation equations aresolved iteratively; some number of iterations is
required to reach a converged solution.
Parallel processing can provide faster solutionsand access to more memory (solve larger cases)
Convergence is reached when: Changes in solution variables from one iteration
to the next are negligible
Overall property conservation is achieved
Quantities of interest (e.g. drag, pressure drop)have reach steady values
The accuracy of a convergedsolution isdependent upon: Appropriateness and accuracy of physical models
Mesh resolution and independence
Numerical errors
A converged and mesh-independent solution ona well-posed problem
will provide usefulengineering 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 usedto calculate quantitative results:
Forces and Moments
Average heat transfer coefficients
Surface and Volume integrated quantities
Flux BalancesExamine results to ensureproperty conservation and
correct physical behavior. Highresiduals may be attributable toonly a few cells of poor quality.
Post Processing
8. Examine results 9.
Upd
ateModel
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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 refinedmesh, or is the solution mesh independent?
Does the mesh resolution of the geometry need to beimproved?
Post Processing
8. Examine results9.
Upd
ateModel