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Polyflow_L03_Ex_Introduction_to_CFD Flipbook PDF
Polyflow_L03_Ex_Introduction_to_CFD
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Lecture 3: Introduction to CFD Introduction to ANSYS Polyflow
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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 (e.g. gravity) • 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. 2
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How Does CFD Work? ANSYS Polyflow solver is based on the finite element method • Domain is discretized into a finite set of elements • General conservation (transport) equations for mass, momentum, energy, species, etc. are solved on this set of control volumes
Control Volume*
Fluid region of pipe flow is discretized into a finite set of control volumes. Unsteady
Convection
Diffusion
Generation
• Partial differential equations are discretized into a system of algebraic equations • All algebraic equations are then solved numerically to render the solution field
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Equation Continuity X momentum Y momentum Z momentum Energy
Variable 1 u v w h
CFD Modeling Overview Problem Identification
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
Pre-Processing 3. 4. 5. 6.
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Geometry Mesh Physics Solver Settings
Solve 7.
Post-Processing 8. Examine the results. 9. Consider revisions to the model.
Define goals Identify domain
Compute solution
Update Model
1. Define your modeling goals 2. Identify the domain you will model
1. 2.
9.
Problem Identification
Post Processing 8.
Examine results
1. Define Your Modeling Goals What results are you looking for (i.e. pressure drop, mass flow rate, extrudate shape, flow balance, ..), and how will they be used? • What are your modeling options? ‐ What physical models will need to be included in your analysis? ➢ temperature, gravity, viscous heating, free surface)? ‐ What simplifying assumptions do you have to make? ➢ E.g. simplify the geometry ➢ 2d vs. 3d ➢ Symmetry
What degree of accuracy is required? • Mesh resolution
How quickly do you need the results? • 2d vs. 3d, mesh resolution, etc. 5
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Problem Identification 1. Define goals 2. Identify domain
2. Identify the Domain You Will Model Problem Identification 1. Define goals 2. 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?
Domain of Interest as Part of a Larger System (not modeled)
• 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? 6
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Domain of interest isolated and meshed for CFD simulation.
3. Create a Solid Model of the Domain
Solid “steel” extrusion die geometry
Pre-Processing 3. 4. 5. 6.
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? 7
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Extracted fluid “cavity”
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 3. 4. 5. 6.
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?
Triangle
What type of mesh is most appropriate?
Hexahedron
• 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? 8
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Quadrilateral
Pyramid
Prism/Wedge
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
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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 Wedge (prism) mesh boundary layers. • More efficient and accurate than tri/tet alone. Tetrahedral mesh 10
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Hybrid Meshes Hybrid mesh uses different meshing methods in different regions. For example, ‐ Hex mesh for fan and heat sink ‐ Tet/prism mesh elsewhere Hybrid 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.
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TET
HEX
Non-Conformal Meshes Non conformal meshes are useful for meshing complex geometries • Mesh each part then join together
Non-conformal interface 12
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Set Up the Physics and Solver Settings For a given problem, you will need to: • Define material properties ‐ Fluid ‐ Solid
Pre-Processing 3. 4. 5. 6.
Geometry Mesh Physics Solver Settings
• Select appropriate physical models ‐ Isothermal, non-isothermal, gravity, viscous heating, etc.
• Prescribe operating conditions • Prescribe boundary conditions at all boundary zones ‐ Inlets, outlets, wall temperatures, heat fluxes, etc. • Provide initial values or a previous solution • Set up solver controls • Set up convergence monitors 13
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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.
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
The accuracy of a converged solution is dependent upon: • Appropriateness and accuracy of physical models.
• Mesh resolution • Numerical errors
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Solve 7. Compute solution
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? ‐ Are there recirculation zones? ‐ What is the free surface shape? ➢ Is a blow molded part fully formed? ‐ 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 ‐ Flow Balance ‐ Contact time 15
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Post Processing
9. Update Model
Examine the Results 8. Examine results
Examine results to ensure property conservation and correct physical behavior. High residuals may be caused by just a few poor quality cells.
Post Processing
Are the physical models appropriate? • Is the flow unsteady? • 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? Effect of mesh resolution on exit velocity variation at a die exit 16
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8.
Examine results
9. Update Model
Consider Revisions to the Model
POLYFLOW Workflow under Workbench 2 Start ANSYS Workbench Drag the Fluid Flow (POLYFLOW) system from Analysis Systems group in the Toolbox onto preview drop target shown in the Project Schematic. • Same for the extrusion or blow molding
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Import the Geometry Right-click on Geometry cell A2 and select Import Geometry Import the geometry file (CAD model or a previous DesignModeler .agdb file)
You can also link the POLYFLOW simulation to an existing DesignModeler session.
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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 POLYFLOW, because Meshing was opened from a POLYFLOW system.
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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. ‐ bs.1, bs.2, etc…
die exit
You will also need to define the regions of the flow containing fluid and solid (if any). • Fluid, solid regions • Different regions for different remeshing
• More details will be presented later ‐ sd.1, sd.2, sd.3, etc… 20
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Set Up and Run POLYFLOW Edit the Setup cell to set up the model options • Boundary conditions • Solver settings • Solution • Post processing Once run, the solution will be post processed in CFD-Post for post processing • Contour and vector plots • Profile plots • Calculation of forces and moments • Animation of unsteady flow results • Free surface shape •… 21
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Demonstration of POLYFLOW Software Start POLYFLOW (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 Input files for all tutorials are at:
http://support.ansys.com/training
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