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Chapter 7 COUPLED ANALYSES Fluid-Structure Analysis CFD ANALYSIS 5.7 • Training Manual “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. May 15, 2001 Inventory #001477 7-2 “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 CFD ANALYSIS 5.7 • Training Manual – Switch element type to structural – Unselect fluid elements – Apply constraints – Transfer pressures from fluid analysis May 15, 2001 Inventory #001477 7-3 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 CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-4 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 CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-5 The Input File - some details Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-6 Complete Finite Element Mesh Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-7 Near the Wall... Color elements by material number CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-8 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 CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-9 Pressure Contours Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-10 Velocity Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-11 Now the Structural Analysis Return to PREP7 • Set the properties for material 2 – Preprocessor > Materials > Isotropic • Material 2 CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-12 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 CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-13 Apply the Static Pressure to Structure Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-14 Note... Training Manual 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. CFD ANALYSIS 5.7 • May 15, 2001 Inventory #001477 7-15 Von Mises Stress Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-16 Pretty Small Displacments ! Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-17 Fluid Magnetics Training Manual 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. CFD ANALYSIS 5.7 • May 15, 2001 Inventory #001477 7-18 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 CFD ANALYSIS 5.7 • 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 May 15, 2001 Inventory #001477 7-19 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…. CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-20 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 CFD ANALYSIS 5.7 • 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 May 15, 2001 Inventory #001477 7-21 Overall Procedure (continued) Training Manual 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 CFD ANALYSIS 5.7 • May 15, 2001 Inventory #001477 7-22 Load Transfer Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-23 Load transfer Training Manual CFD ANALYSIS 5.7 Specify .rfl file to get FLOTRAN values... May 15, 2001 Inventory #001477 7-24 Data Transferred Bulk Temperature CFD ANALYSIS 5.7 Film Coefficient Training Manual May 15, 2001 Inventory #001477 7-25 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 • Training Manual Flow Environment > Reference Conditions May 15, 2001 Inventory #001477 7-26 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… • This is to avoid the specification of negative values of the heat transfer coefficient. CFD ANALYSIS 5.7 • May 15, 2001 Inventory #001477 7-27 Load Transfers - Sample Problem Training Manual 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. • CFD ANALYSIS 5.7 • 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. May 15, 2001 Inventory #001477 7-28 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 CFD ANALYSIS 5.7 • Training Manual Inlet water Temp 70F May 15, 2001 Inventory #001477 7-29 Setup Steps for the Student CFD ANALYSIS 5.7 • 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 • 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 May 15, 2001 Inventory #001477 7-30 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 CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-31 FLWTHRM.INP Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-32 Relevant Properties • • 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 • • Training Manual – Then tell FLOTRAN you are using MP tables for the fluid May 15, 2001 Inventory #001477 7-33 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 CFD ANALYSIS 5.7 • Training Manual – 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 May 15, 2001 Inventory #001477 7-34 The FLOTRAN Model Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-35 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. • Physics > Write • SOLVE the FLOTRAN model for the flow solution • Convergence is obtain for flow in about 50 global iterations CFD ANALYSIS 5.7 • May 15, 2001 Inventory #001477 7-36 Convergence Monitors Training Manual CFD ANALYSIS 5.7 May 15, 2001 Inventory #001477 7-37 The FLOTRAN Thermal Solution Turn on TEMP solution • SOLVE - Model will converge in 2 global iterations CFD ANALYSIS 5.7 • Training Manual Temperature at Solid - Fluid Interface May 15, 2001 Inventory #001477 7-38 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 • Training Manual Heat Transfer Coefficient May 15, 2001 Inventory #001477 7-39 Next : The Thermal Model In SOLUTION: Physics > Clear • Now set up the thermal model…. CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-40 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 May 15, 2001 Inventory #001477 7-41 Thermal Model Training Manual 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 CFD ANALYSIS 5.7 • May 15, 2001 Inventory #001477 7-42 Final Setup…. • Verify their existence and check the bulk temperature CFD ANALYSIS 5.7 – Look at the fluid/solid boundary temperatures Training Manual May 15, 2001 Inventory #001477 7-43 Thermal results The ANSYS/THERMAL result for temperature... CFD ANALYSIS 5.7 • Training Manual May 15, 2001 Inventory #001477 7-44 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. ANSYS Thermal CFD ANALYSIS 5.7 • Training Manual FLOTRAN May 15, 2001 Inventory #001477 7-45