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System Coupling: Two way FSI:RMS convergence target and Under relaxation factor value?

    • harishkamath86
      Subscriber
      Hi everyone. I am doing a two-way FSI analysis of bearing (Fluent + Structural). Can anyone tell me how to decide the values for RMS convergence target and Under relaxation factor in the system coupling? On what basis these values should be decided,Kindly help
    • Stephen Orlando
      Ansys Employee
      The default convergence target is 0.01 and this value can be lowered if you need more accuracy. Note that you'll also need to ensure the participants are converged at the end of the coupling step. The under relaxation factor needed is case dependent. Sometimes you need a small URF if more stability is needed. nThe following information should also help. nWhen running System Coupling and it stops running, the first thing to do is examine the output files for each solver to look for clues to what happened. For Fluent, look at the transcript .trn file. For Mechanical, look at the .out file. For CFX, look at the .out file. Also, look at the System Coupling log file, .scl.nnI recommend going over this tutorial in the Ansys documentation that shows a 2-way FSI simulation with Fluent and Mechanical.https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v202/en/sysc_tut/sysc_tut_oscplate_wb_fluent.htmlnSystem Coupling Tutorials \\ Tutorials with Workbench-Setup Workflows \\ Tutorials with Workbench Setup and Execution \\ Oscillating Plate FSI with Fluent and MechanicalnnYou can also look at the following for a similar tutorial but run with the System Coupling GUI outside of Workbench. The new System Coupling GUI (run outside of Workbench) is available by searching for System Coupling 2019R3 (or newer) in the Windows Start menu.https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/sysc_tut/sysc_tut_reedvalve_fluent.htmlnnFSI simulations with very soft materials or membranes are prone to numerical instabilities. In 2020R1 we have introduced a stabilization method in System Coupling called the Quasi-Newton Stabilization Algorithm. Note that this has to be used with the new System Coupling GUI or Command Line Interface that is run outside of Workbench. More information here:https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/sysc_ug/sysc_gen_scservice_dt_supplemental_iqnils.htmlnSystem Coupling User's Guide \\ System Coupling Data Transfers \\ Supplemental Processing Algorithms \\ Quasi-Newton Stabilization AlgorithmnnIt is very important to build up the FSI simulation in stages as opposed to setting up the 2-way FSI right at the start. This document Best Practices for Coupled Fluid-Structure Interaction (FSI) describes this process and is available here:https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/sysc_ug/sysc_bestpractices_fsi.htmlnSystem Coupling User's Guide \\ Best Practices for System Coupling \\ Best Practices for Coupled Fluid-Structure Interaction (FSI)n
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