General Mechanical

General Mechanical

Excessive Distortion or Excessive thickness change in Mechanical during FSI

    • hnorthrup
      Subscriber
      I am trying to run a two-way FSI using mechanical, fluent and coupled through the System Coupling GUI (using Quasi-Newton Stabilization Algorithm). I was able to get an idealized tube to run (on the same scale, same time step size, similar material properties and similar fluent and mechanical setup), but now that I am trying it with a more complex geometry, I get an error during the first or second iteration saying ?Participant MAPDL-2 encountered a fatal error. Message: Element xxxxx has excessive distortion?.nI usually receive a few warnings but I have not found anything to say that these warnings are causing the excessive distortion. Regardless, the warnings I usually encounter are: n?Some of the defined and selected elements did not require FSIN. Use SFELIST,ALL,fluid-solid interaction surfaces command to view a list of elements that did have the fluid-solid interaction surfaces surface load stored?n?Element shape checking is currently inactive. Issue SHPP,ON or SHPP,WARN to reactivate, if desired.?n?All selected contact pairs are initially open. Rigid body motion can occur. You may use auto CNOF/ICONT by seeing KEYOPT(5) to close small gaps.?n?A reference force value times the tolerance is used by the Newton-Raphson method for checking convergence. The calculated reference FORCE CONVERGENCE VALUE = 8.929315604E-06 is less than a threshold. This threshold is internally calculated. You can overwrite it by specifying MINREF on the CNVTOL command. Check results carefully.?nThe only warning I get from fluent is ?reading 4 partition grid onto 1 compute node machine. Combining every 4 partitions?. nI can successfully run the mechanical and fluent set up separately. nI have tried multiple mesh set ups for mechanical. Some of the things I have tried are:n? Leaving all default settings but changing the element size to match the outer most # of elements in the fluid mesh (roughly 75,000). n? Changed adaptive sizing to ?No?n? Changed element type from program controlled to ?quadratic?n? Changed physics from ?Mechanical? to ?Nonlinear Mechanical?n? Decreased element size to increase # of elements (186,676 elements)n? Changed element type to ?linear?.n? Decreased element size to increase # of elements (295,713 total elements)n? Decreased time step sizen? Decreased Poisson?s ration? Changed URF for both data transfers from 1 to 0.08nDepending on what I have tried the error will change to ?Participant MAPDL-2 encountered a fatal error. Message: Element xxxxx has excessive thickness change.?nThanks!n
    • hnorthrup
      Subscriber
      Follow up question: In my continued search to solve this problem I see a lot of suggestions for similar problems to try and ramp the boundary conditions. However I have no idea how to do this. Does anyone know of a good tutorial or can explain how to do this?nThanks!n
    • hnorthrup
      Subscriber
      I back tracked a bit and realized that when I was running the FEA alone I was not applying pressure so of course it was working. I took the amount of pressure from the CFD (about 7000 pascals) and applied it to my vessel. Sure enough it failed with excessive distortion and unrealistic deformation. I can get it to work by changing the wall properties and thickness to unrealistic values (excessively thick or excessively high Young's modulus). I can get it to work with realistic wall properties if I changed the pressure from 7000 to 7 pascals which of course is way too low for my application. Any help or suggestions are appreciated. I am uploading the archived file as well. nn
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