Download CFD ANALYSIS 5.7

Chapter 7
COUPLED ANALYSES
Fluid-Structure Analysis
CFD ANALYSIS 5.7
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Training Manual
“One Way” Analysis
– Structural deformation effect on flow field is negligible
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“Two Way” Analysis
– Re-analysis of the flowfield is required
– Re-mesh new geometry according to deformations
– May restart from previous results if mesh pattern is unchanged
– Free meshing will require re-meshing
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In the two way analysis, care must be taken to ensure
convergence of the flow analyses. The entire process may be put
in a “macro" loop.
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Optimization may be employed if a suitable objective function can
be defined.
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“One Way” Fluid Structures
Single Database
– Create flow field and structure with element 141 (142 if 3D)
– Flowfield - material 1
– Structure - material 2 (or greater
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Converge Flow Analysis
– Change to Structural Analysis
CFD ANALYSIS 5.7
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Training Manual
– Switch element type to structural
– Unselect fluid elements
– Apply constraints
– Transfer pressures from fluid analysis
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Additional Information
ANSYS Coupled-Field Analysis Guide
– Sequential Coupled-Field Analysis
– Contains discussion of PHYSICS ENVIRONMENTS
– Provides several examples and input files
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LDREAD documentation
– Loads available for transfer
CFD ANALYSIS 5.7
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Training Manual
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Example
Input file “flustr.inp”
– Creates finite element FLOTRAN model of a wind flow over a wall.
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Do some post processing of flow results
– Velocity near the wall
– Pressure Contours
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Find the stress in the wall and the resulting displacements from
the wind load
CFD ANALYSIS 5.7
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Training Manual
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The Input File - some details
Training Manual
CFD ANALYSIS 5.7
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Complete Finite Element Mesh
Training Manual
CFD ANALYSIS 5.7
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Near the Wall...
Color elements by material number
CFD ANALYSIS 5.7
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Training Manual
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Get Ready To Plot
Select Vector Mode under Utility>Plot Ctrls>Device Options
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Select elements of material 1 (fluid)
– select associated nodes
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Get 128 contours
– /contour,1,128
CFD ANALYSIS 5.7
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Training Manual
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Pressure Contours
Training Manual
CFD ANALYSIS 5.7
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Velocity
Training Manual
CFD ANALYSIS 5.7
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Now the Structural Analysis
Return to PREP7
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Set the properties for
material 2
– Preprocessor > Materials
> Isotropic
• Material 2
CFD ANALYSIS 5.7
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Training Manual
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The Structures Model
Select elements of material 2
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Select associated nodes
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Set Element Type 1 to 42 on the Command line
– et,1,42
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Apply Zero Displacement to base
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Training Manual
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Apply the Static Pressure to Structure
Training Manual
CFD ANALYSIS 5.7
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Note...
Training Manual
Pressure loadings on a structure from a fluids analysis are from
the file jobname.rfl.
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Note that types of loading available are a function of the element
type. The CFD preferences, if turned on, will prevent viewing
structure loadings.
CFD ANALYSIS 5.7
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Von Mises Stress
Training Manual
CFD ANALYSIS 5.7
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Pretty Small Displacments !
Training Manual
CFD ANALYSIS 5.7
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Fluid Magnetics
Training Manual
Body forces in an electrically conducting fluid can be created by
electromagnetic fields.
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Electric resistively in the fluid is accounted for in magnetics
analysis.
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Body force terms are added to the momentum equations via BF
commands.
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Direct current analysis creates total force on a node.
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Harmonic analysis includes steady-state variations.
CFD ANALYSIS 5.7
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Thermal Problems
It is possible to transfer heat transfer coefficients, heat fluxes, or
temperatures from a FLOTRAN model to a more detailed model
that contains elements currently incompatible with FLOTRAN.
– This could be a thermal model or a thermal stress model
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Boundary conditions - E.G. Heat flux
LDREAD,HFLU,,,,,jobname,ext
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Temperatures
LDREAD,TEMP,LAST,,,kimg,jobname,ext
CFD ANALYSIS 5.7
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Training Manual
– These temperatures will become body loads or nodal DOF, depending
on what kind of element is being used.
• Kimg = 0 - Temperatures are body loads
• Kimg = 1 - Temperatures are nodal loads
• Kimg = 2 - Temperatures are initial conditions on nodes
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Multiphysics Load Transfer - Thermal
The heat transfer coefficients at the boundary of a flow field can
be calculated by FLOTRAN.
– The flow and energy equations are solved
– FLOTRAN calculates surface temperature and heat flux
– Heat transfer coefficients are calculated based on the surface
temperatures and the Bulk Temperature
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The non-fluid region producing the heat may be of considerable
complexity and require thermal options (such as conduction links)
not available in FLOTRAN.
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In such cases, the heat transfer coefficients and associated bulk
temperature can be transferred from FLOTRAN to an ANSYS
thermal model….
CFD ANALYSIS 5.7
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Training Manual
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Overall Procedure (Multiphysics)
A data base is constructed that will contain the detailed ANSYS
thermal model as well as the FLOTRAN fluid model.
– A fluid-solid interface in the FLOTRAN Model will be the boundary of
the Thermal Model
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Consider the fluid portion of the model with an adjoining solid as
the FLOTRAN model.
– Place the specified heat sources or temperatures at some appropriate
boundary in the non-fluid regions
CFD ANALYSIS 5.7
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Training Manual
– If the heat sources are close to the fluid region, it is not likely that this
procedure is necessary, since the non-fluid regions can be modeled
with FLOTRAN...
– Solve fluid-thermal FLOTRAN analysis
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Overall Procedure (continued)
Training Manual
The solid-fluid interface of the FLOTRAN model forms the
boundary of the detailed ANSYS thermal model.
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FLOTRAN will calculate the heat flux at the surface along with the
surface temperature.
– Heat transfer coefficient is calculated based on the Bulk Temperature
specified in FLOTRAN
– Activate only the elements which will receive the heat transfer
coefficient before load transfer
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Remove any superfluous boundary conditions left over from the
FLOTRAN analysis
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Activate thermal elements
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Load Transfer
Training Manual
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Load transfer
Training Manual
CFD ANALYSIS 5.7
Specify .rfl file to get
FLOTRAN values...
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Data Transferred
Bulk Temperature
CFD ANALYSIS 5.7
Film Coefficient
Training Manual
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Notes on Data Transfer
FLOTRAN has calculated the heat transfer coefficients based on a
positive heat flow into the fluid and the use of the FLOTRAN
specified Bulk Temperature….
CFD ANALYSIS 5.7
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Training Manual
Flow Environment > Reference Conditions
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Heat Transfer Coefficient
Training Manual
If the heat flow is from the fluid to the wall, FLOTRAN calculates a
MODIFIED Bulk temperature, which when used with the surface
temperature, produces the correct heat flux…
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This is to avoid the specification of negative values of the heat
transfer coefficient.
CFD ANALYSIS 5.7
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Load Transfers - Sample Problem
Training Manual
Introduction to Physics Environments
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It is now possible to transfer temperatures between ANSYS
thermal and FLOTRAN.
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In this example, we will use FLOTRAN to calculate the heat
transfer coefficients at a surface and then pass them to a thermal
model.
– This procedure is used when the thermal model requires more detail
than FLOTRAN allows.
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CFD ANALYSIS 5.7
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Physics Files
– Contain everything except the geometry/mesh
– One for the FLOTRAN model, one for the thermal model
– Both will use the same finite element model database.
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Sample Problem...
We want the heat transfer
coefficients at this surface
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Inlet velocity of 1 in/sec..
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Symmetry boundary condition
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Solid is Carbon Steel
Temp
400F
along
solid
edge
CFD ANALYSIS 5.7
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Training Manual
Inlet water Temp 70F
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Setup Steps for the Student
CFD ANALYSIS 5.7
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Training Manual
Read in the input file “flwthrm.inp”
– Creates the finite element mesh using FLOTRAN elements
• Assigns type number 1 to Water, type number 2 to Steel
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Establish FLOTRAN boundary conditions, properties and
operational parameters
–
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Create Fluid Physics file
Establish ANSYS/THERMAL boundary conditions, properties and
operational parameters
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Create Thermal Physics file
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Execution
Read in Fluid physics and execute FLOTRAN
– Look at the temperature at the fluid-solid interface
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Read the heat transfer coefficients from the FLOTRAN results file
onto the thermal model.
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Run the ANSYS thermal model
– Look at the temperatures at the boundary
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Training Manual
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FLWTHRM.INP
Training Manual
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Relevant Properties
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Use PSI system of units
Water (material 1)
– Density
9.3x10-5 (lbf-sec2/in4)
– Viscosity 1.38x10-7 (lbf-sec/in2)
– Conductivity
8.2x10-6 (Btu/sec-in-F)
– Specific Heat
378.0 (Btu-in/lbf-sec2-F)
Steel (material 2)
– Density
7.3x10-4 (lbf-sec2/in4)
– Conductivity
3.47x10-3(Btu/sec-in-F)
– Specific Heat
43.6 (Btu-in/lbf-sec2-F)
Input these through Preproc > Material Props > Material Models
CFD ANALYSIS 5.7
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Training Manual
– Then tell FLOTRAN you are using MP tables for the fluid
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FLOTRAN Operational Parameters
Execution Controls - Set for 100 global iterations
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Fluid Properties - Set to “MP Table”
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Flow Environment – Set bulk temperature to 100F (estimate)
– Set offset temperature to 460R
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Relax/Stab/Cap
CFD ANALYSIS 5.7
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Training Manual
– Set temperature relaxation factor to 1.0
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CFD Solver Control
– Set the temperature solver to the Sparse Direct
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Advection
– Designate the SUPG method to be used for temperature
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The FLOTRAN Model
Training Manual
CFD ANALYSIS 5.7
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Physics File and Execution
Training Manual
Actually, for this case we would not need a physics file since we
will have repeated execution of FLOTRAN, but it is convenient to
have for Post-Processing.
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Physics > Write
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SOLVE the FLOTRAN model for the flow solution
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Convergence is obtain for flow in about 50 global iterations
CFD ANALYSIS 5.7
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Convergence Monitors
Training Manual
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The FLOTRAN Thermal Solution
Turn on TEMP solution
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SOLVE - Model will converge in 2 global
iterations
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Training Manual
Temperature
at Solid - Fluid
Interface
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Heat Transfer Coefficient (HTC)
With a bulk temperature of 100F and an Inlet temperature of 70F,
the HTC will calculate to be a negative number near the inlet.
Heat Flux
CFD ANALYSIS 5.7
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Training Manual
Heat Transfer Coefficient
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Next : The Thermal Model
In SOLUTION: Physics > Clear
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Now set up the thermal model….
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Training Manual
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ANSYS/Thermal problem - Physics file
• ET,1,0
• ET,2,55
– Re-Input solid properties for material 2
• mp,kxx,2,3.47E-3
• mp,c,2,43.6
CFD ANALYSIS 5.7
– Change element types on the command line
Training Manual
• mp,dens,2,7.3E-4
– Reset outside temperature boundary condition
• dl,6,,temp,400.,1
– Write physics file with title “thermal”
• physics,write,thermal
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Thermal Model
Training Manual
Apply heat transfer coefficients from the FLOTRAN results to the
“fluid/solid” boundary
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Loads - > Apply -> Convection:
– Choose appropriate “jobname”.rfl file, OK
• ldread,hflm,last,,,,,rfl
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Final Setup….
• Verify their existence and check the bulk temperature
CFD ANALYSIS 5.7
– Look at the fluid/solid boundary temperatures
Training Manual
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Thermal results
The ANSYS/THERMAL result for temperature...
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Training Manual
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Temperature results
Note that in the FLOTRAN
problem, the corner node
wasspecified as 70F by the
flow inlet condition.
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This was not enforced by
the thermal solution.
Hence the difference.
ANSYS
Thermal
CFD ANALYSIS 5.7
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Training Manual
FLOTRAN
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