Discussion Forum

Dear Elen T,

are your initial values in your domains maybe inconsistent with your boundary conditions? E.g. velocity in domain = 0 at t=0, but velocity at a boundary is set to some non-zero value? In that case it is helpful to ramp your inlet velocites to the value you want by using e.g. a step function. Check out the following knowledgebase entry: https://www.comsol.com/support/knowledgebase/1172/

And our tutorial applications that use that kind of modeling: https://www.comsol.com/models?q=fsi

Also please note current version 5.3a has some helpful updates on FSI and time-dependent CFD: https://www.comsol.com/release/5.3a/cfd-module

Thankyou for you answer, Erik.

My inlet velocity is time dependent whith a sinusoidal function u_in=Usin(2pit[1/s]/0.5)(t>0)(t<=0.25)+0(t>0.25)*(t<0.83), (wich works in 2D). So i think a ramp up is not needed.

I've also tried the stationary as step1 and timedependent as Step 2 but i get this error: Segregated Step 2 Undefined variable. - Variable: t - Geometry: globalscope - Domain: 1

If you already have a ramp you could try a less steep one first. But maybe the issue is something else here and other conditions are not consistent at t=0 - which is hard to give a general advise on except you can try to build up your model starting with a more simple setup to see when the problem occurs: https://www.comsol.de/blogs/the-optimal-workflow-for-complex-modeling-projects-on-a-deadline/

For your approach using a stationary solver first you have to keep in mind "t" (the time variable) is not defined for a stationay solver, but it is used in your boundary conditions. You can add a "t" parameter within the parameter list though, e.g. t = 0 or whatever fits to your boundary functions. It will be overridden by the time-dependent study-step 2. Another way would be to add two alternative boundary conditions and switch them in the study-step settings.