COUPLED ANALYSES Chapter 7

Training Manual May 15, 2001 Inventory # Fluid-Structure Analysis “One Way” Analysis –Structural deformation effect on flow field is negligible “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 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. Optimization may be employed if a suitable objective function can be defined.

Training Manual May 15, 2001 Inventory # “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 Converge Flow Analysis –Change to Structural Analysis –Switch element type to structural –Unselect fluid elements –Apply constraints –Transfer pressures from fluid analysis

Training Manual May 15, 2001 Inventory # Additional Information ANSYS Coupled-Field Analysis Guide –Sequential Coupled-Field Analysis –Contains discussion of PHYSICS ENVIRONMENTS –Provides several examples and input files LDREAD documentation –Loads available for transfer

Training Manual May 15, 2001 Inventory # Example Input file “flustr.inp” –Creates finite element FLOTRAN model of a wind flow over a wall. Do some post processing of flow results –Velocity near the wall –Pressure Contours Find the stress in the wall and the resulting displacements from the wind load

Training Manual May 15, 2001 Inventory # The Input File - some details

Training Manual May 15, 2001 Inventory # Complete Finite Element Mesh

Training Manual May 15, 2001 Inventory # Near the Wall... Color elements by material number

Training Manual May 15, 2001 Inventory # Get Ready To Plot Select Vector Mode under Utility>Plot Ctrls>Device Options Select elements of material 1 (fluid) –select associated nodes Get 128 contours –/contour,1,128

Training Manual May 15, 2001 Inventory # Pressure Contours

Training Manual May 15, 2001 Inventory # Velocity

Training Manual May 15, 2001 Inventory # Now the Structural Analysis Return to PREP7 Set the properties for material 2 –Preprocessor > Materials > Isotropic Material 2

Training Manual May 15, 2001 Inventory # The Structures Model Select elements of material 2 Select associated nodes Set Element Type 1 to 42 on the Command line –et,1,42 Apply Zero Displacement to base

Training Manual May 15, 2001 Inventory # Apply the Static Pressure to Structure

Training Manual May 15, 2001 Inventory # Note... Pressure loadings on a structure from a fluids analysis are from the file jobname.rfl. Note that types of loading available are a function of the element type. The CFD preferences, if turned on, will prevent viewing structure loadings.

Training Manual May 15, 2001 Inventory # Von Mises Stress

Training Manual May 15, 2001 Inventory # Pretty Small Displacments !

Training Manual May 15, 2001 Inventory # Fluid Magnetics Body forces in an electrically conducting fluid can be created by electromagnetic fields. Electric resistively in the fluid is accounted for in magnetics analysis. Body force terms are added to the momentum equations via BF commands. Direct current analysis creates total force on a node. Harmonic analysis includes steady-state variations.

Training Manual May 15, 2001 Inventory # 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 Boundary conditions - E.G. Heat flux LDREAD,HFLU,,,,,jobname,ext Temperatures LDREAD,TEMP,LAST,,,kimg,jobname,ext –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

Training Manual May 15, 2001 Inventory # 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 The non-fluid region producing the heat may be of considerable complexity and require thermal options (such as conduction links) not available in FLOTRAN. In such cases, the heat transfer coefficients and associated bulk temperature can be transferred from FLOTRAN to an ANSYS thermal model….

Training Manual May 15, 2001 Inventory # 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 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 –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

Training Manual May 15, 2001 Inventory # Overall Procedure (continued) The solid-fluid interface of the FLOTRAN model forms the boundary of the detailed ANSYS thermal model. 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 Remove any superfluous boundary conditions left over from the FLOTRAN analysis Activate thermal elements

Training Manual May 15, 2001 Inventory # Load Transfer

Training Manual May 15, 2001 Inventory # Load transfer Specify.rfl file to get FLOTRAN values...

Training Manual May 15, 2001 Inventory # Data Transferred Bulk Temperature Film Coefficient

Training Manual May 15, 2001 Inventory # Flow Environment > Reference Conditions 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….

Training Manual May 15, 2001 Inventory # Heat Transfer Coefficient 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… This is to avoid the specification of negative values of the heat transfer coefficient.

Training Manual May 15, 2001 Inventory # Load Transfers - Sample Problem Introduction to Physics Environments It is now possible to transfer temperatures between ANSYS thermal and FLOTRAN. 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. 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.

Training Manual May 15, 2001 Inventory # Sample Problem... We want the heat transfer coefficients at this surface Inlet velocity of 1 in/sec.. Symmetry boundary condition Solid is Carbon Steel Temp 400F along solid edge Inlet water Temp 70F

Training Manual May 15, 2001 Inventory # Setup Steps for the Student 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 Establish FLOTRAN boundary conditions, properties and operational parameters – Create Fluid Physics file Establish ANSYS/THERMAL boundary conditions, properties and operational parameters – Create Thermal Physics file

Training Manual May 15, 2001 Inventory # Execution Read in Fluid physics and execute FLOTRAN –Look at the temperature at the fluid-solid interface Read the heat transfer coefficients from the FLOTRAN results file onto the thermal model. Run the ANSYS thermal model –Look at the temperatures at the boundary

Training Manual May 15, 2001 Inventory # FLWTHRM.INP

Training Manual May 15, 2001 Inventory # Relevant Properties Use PSI system of units Water (material 1) –Density9.3x10 -5 (lbf-sec 2 /in 4 ) –Viscosity1.38x10 -7 (lbf-sec/in 2 ) –Conductivity8.2x10 -6 (Btu/sec-in-F) –Specific Heat378.0 (Btu-in/lbf-sec 2 -F) Steel (material 2) –Density7.3x10 -4 (lbf-sec 2 /in 4 ) –Conductivity3.47x10 -3 (Btu/sec-in-F) –Specific Heat43.6 (Btu-in/lbf-sec 2 -F) Input these through Preproc > Material Props > Material Models –Then tell FLOTRAN you are using MP tables for the fluid

Training Manual May 15, 2001 Inventory # FLOTRAN Operational Parameters Execution Controls - Set for 100 global iterations Fluid Properties - Set to “MP Table” Flow Environment - –Set bulk temperature to 100F (estimate) –Set offset temperature to 460R Relax/Stab/Cap –Set temperature relaxation factor to 1.0 CFD Solver Control –Set the temperature solver to the Sparse Direct Advection –Designate the SUPG method to be used for temperature

Training Manual May 15, 2001 Inventory # The FLOTRAN Model

Training Manual May 15, 2001 Inventory # Physics File and Execution 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. Physics > Write SOLVE the FLOTRAN model for the flow solution Convergence is obtain for flow in about 50 global iterations

Training Manual May 15, 2001 Inventory # Convergence Monitors

Training Manual May 15, 2001 Inventory # The FLOTRAN Thermal Solution Turn on TEMP solution SOLVE - Model will converge in 2 global iterations Temperature at Solid - Fluid Interface

Training Manual May 15, 2001 Inventory # 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 FluxHeat Transfer Coefficient

Training Manual May 15, 2001 Inventory # Next : The Thermal Model In SOLUTION: Physics > Clear Now set up the thermal model….

Training Manual May 15, 2001 Inventory # ANSYS/Thermal problem - Physics file –Change element types on the command line ET,1,0 ET,2,55 –Re-Input solid properties for material 2 mp,kxx,2,3.47E-3 mp,c,2,43.6 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

Training Manual May 15, 2001 Inventory # Thermal Model Apply heat transfer coefficients from the FLOTRAN results to the “fluid/solid” boundary Loads - > Apply -> Convection: –Choose appropriate “jobname”.rfl file, OK ldread,hflm,last,,,,,rfl

Training Manual May 15, 2001 Inventory # Final Setup…. –Look at the fluid/solid boundary temperatures Verify their existence and check the bulk temperature

Training Manual May 15, 2001 Inventory # Thermal results The ANSYS/THERMAL result for temperature...

Training Manual May 15, 2001 Inventory # FLOTRAN ANSYS Thermal Temperature results Note that in the FLOTRAN problem, the corner node wasspecified as 70F by the flow inlet condition. This was not enforced by the thermal solution. Hence the difference.