Introduction to CFD Training Manual

Chapter 1
Chapter 1
Introduction to CFD
Introduction to
Introduction to
ANSYS Polyflow
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ANSYS, Inc. Proprietary
© 2010 ANSYS, Inc. All rights reserved.
Release 12.1
June 2010

Introduction to POLYFLOW training
Training Manual
Agenda Day 1
8:30 Introduction to CFD 14:00 Polyflow B
8:30 Introduction to CFD 14:00 Polyflow B
9:30 Polyflow A 14:30 Tutorial 5
11:00 Example 15:30 Evolution
11:30 Tutorial 1 16:30 Tutorial 8
12:30 Lunch 17:00 Break for the day
12:30 Lunch 17:00 Break for the day
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Training Manual
8 30 Ti d d t Fl 14 00 I t d ti t Rh l
Agenda Day 2
8:30 Time dependent Flows 14:00 Introduction to Rheology
9:30 Tutorial 7 15:00 CFD Post
10:30 Adaptive meshing 16:00 Workshop
11:30 Tutorial 9 17:00 Break for the day
12:30 Lunch
12:30 Lunch
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Training Manual
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 mass
– Conservation of momentum
– Conservation of energy
– Conservation of species
p
– 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
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• CFD analysis complements testing and experimentation by reducing total
effort and cost required for experimentation and data acquisition.

Training Manual
How Does CFD Work?
• ANSYS Polyflow solver is based on the
finite element method
– Domain is discretized into a finite set of Control
elements
– General conservation (transport) equations
for mass, momentum, energy, species, etc.
are solved on this set of control volumes
Volume*
are solved on this set of control volumes
Fluid region of pipe flow is
discretized into a finite set
of control volumes.
– Partial differential equations are
Equation Variable
Continuity 1
X momentum u
Unsteady Convection Diffusion Generation
discretized into a system of algebraic
equations
– All algebraic equations are then solved
numerically to render the solution field
Y momentum v
Z momentum w
Energy h
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numerically to render the solution field

Training Manual
CFD Modeling Overview
• Problem Identification
1. Define your modeling goals
2. Identify the domain you will model
Problem Identification
1. Define goals
• PreProcessing and Solver Execution
3. Create a solid model to represent the
domain
2. Identify domain
Pre-Processing
domain
4. Design and create the mesh (grid)
5. Set up the physics (physical models,
material properties, domain properties,
3. Geometry
4. Mesh
5. Physics p p p p
boundary conditions, …)
6. Define solver settings (numerical
schemes, convergence controls, …)
7 Compute and monitor the solution
6. Solver Settings
Solve
e
Model
7. Compute and monitor the solution
• Post-Processing
8 Examine the results
7. Compute solution
Post Processing
9.
Update
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8. Examine the results.
9. Consider revisions to the model.
Post Processing
8. Examine results

Training Manual
1. Define Your Modeling Goals
• What results are you looking for (i e pressure drop mass flow rate
1. Define goals
2. Identify domain
• 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?
M h l ti
– Mesh resolution
• How quickly do you need the results?
2d 3d h l ti t
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– 2d vs. 3d, mesh resolution, etc.

Training Manual
2. Identify the Domain You Will Model
• How will you isolate a piece of the
1. Define goals
2. Identify domain
• How will you isolate a piece of the
complete physical system?
• Where will the computational
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?
y ( )
point where reasonable data exists?
• Can it be simplified or approximated
as a 2D or axisymmetric problem?
Domain of interest
isolated and meshed
for CFD simulation.
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y p

Training Manual
3. Create a Solid Model of the Domain
Pre-Processing
3. Geometry
4. Mesh
5. Physics
Solid “steel”
extrusion die
geometry
• How will you obtain a solid model of the
fluid region?
y
6. Solver Settings
g
– 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?
Extracted
fluid “cavity”
– Make use of symmetry or periodicity?
• Are both the solution and boundary conditions
symmetric / periodic?
• Do you need to split the model so that
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boundary conditions or domains can be
created?

Training Manual
4. Design and Create the Mesh
Pre-Processing
3. Geometry
4. Meshing
5. Physics
A mesh divides a geometry into
many elements. These are used by
the CFD solver to construct control
volumes
• What degree of mesh resolution is required in
each region of the domain?
– The mesh must resolve geometric features of
i t t d t di t f
y
6. Solver Settings
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 Quadrilateral
• 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?
Hexahedron
Tetrahedron y
– Are non-conformal interfaces needed?
• Do you have sufficient computer resources?
– How many cells/nodes are required?
H h i l d l ill b d?
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– How many physical models will be used?
Pyramid Prism/Wedge

Training Manual
Tri/Tet vs. Quad/Hex Meshes
• For flow-aligned geometries,
quad/hex meshes can provide
higher-quality solutions with fewer
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
t d/h h
generate a quad/hex mesh
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Training Manual
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
b d l
boundary layers.
– More efficient and accurate
than tri/tet alone.
W d ( i ) h
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Tetrahedral mesh
Wedge (prism) mesh

Training Manual
Multizone (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
TET
• Multizone meshes yield a good
combination of accuracy, efficient
calculation time and meshing effort.
HEX
g
• When the nodes do not match across
the regions a non-conformal interface
the regions, a non conformal interface
can be used.
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Training Manual
Non-Conformal Meshes
• Non conformal meshes are useful
for meshing complex geometries
– Mesh each part then join together
Non-conformal
interface
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Training Manual
Set Up the Physics and Solver Settings
• For a given problem, you will need to:
– Define material properties
• Fluid
Pre-Processing
3. Geometry
4. Mesh
5. Physics
• Solid
– Select appropriate physical models
• Isothermal, non-isothermal, gravity, viscous
heating etc
For complex problems
solving a simplified or 2D
y
6. Solver Settings
heating, etc.
– Prescribe operating conditions
– Prescribe boundary conditions at all
boundary zones
problem will provide
valuable experience with the
models and solver settings
for your problem in a short
boundary zones
• Inlets, outlets, wall temperatures, heat fluxes, etc.
– Provide initial values or a previous solution
– Set up solver controls
amount of time.
p
– Set up convergence monitors
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Training Manual
Compute the Solution
• The discretized conservation equations are
solved iteratively until convergence.
C i h d h
Solve
7. Compute solution
• 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
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
Mesh resolution
– Numerical errors
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Training Manual
Examine the Results
• Examine the results to review solution
and extract useful data
– Visualization Tools can be used to
Post Processing
9.
Update
Model
answer such questions as:
• What is the overall flow pattern?
• Are there recirculation zones?
• What is the free surface shape?
8. Examine results
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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
– 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
Examine results to ensure property conservation
and correct physical behavior. High residuals
may be caused by just a few poor quality cells
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Contact time
may be caused by just a few poor quality cells.

Training Manual
Consider Revisions to the Model
• Are the physical models appropriate?
– Is the flow unsteady?
– Are there 3D effects?
Post Processing
9.
Update
Model
Are there 3D effects?
• Are the boundary conditions correct?
– Is the computational domain large enough?
8. Examine results
9
p g g
– 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?
mesh, or is the solution mesh independent?
– Does the mesh resolution of the geometry need to be
improved?
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Effect of mesh resolution on exit
velocity variation at a die exit

Training Manual
POLYFLOW Workflow under Workbench 2
• Start ANSYS Workbench
• Drag the Fluid Flow (POLYFLOW)
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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Training Manual
Import the Geometry
• Right-click on Geometry cell A2 and select Import Geometry
• Import the geometry file (CAD model or a previous DesignModeler
db fil )
.agdb file)
• You can also link the POLYFLOW simulation to an existing
DesignModeler session.
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Training Manual
Generate a Mesh
• Right-click on Mesh cell and select Edit.
– Meshing opens and loads geometry
• Select Mesh under Model in Outline
• Select Mesh under Model in Outline
– Note that Preferences are automatically set
for POLYFLOW, because Meshing was opened
from a POLYFLOW system.
from a POLYFLOW system.
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Training Manual
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
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)
and solid (if any).
– Fluid, solid regions
– Different regions for different remeshing
More details will be presented later
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– More details will be presented later
• sd.1, sd.2, sd.3, etc…

Training Manual
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
– …
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Training Manual
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
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Training Manual
Navigating the PC at Fluent
• Log in to your workstation
– Login name: ansys
– Password: ansys
• Directories
– Tutorial mesh/case/data files can be found in

c:Student Filesfluenttut
– We recommend that you save your work into a central working folder:
c:users
W ki f ld h th d kt i h t t t
– Working folder shown on the desktop is a shortcut to c:users
• To start POLYFLOW 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
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each tutorial to avoid mixing solver settings from different
workshops.

Introduction to CFD Training Manual

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