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-   -   Outlet Pressure-flow coupled boundary condition (Windkessel) (https://www.cfd-online.com/Forums/fluent-udf/82101-outlet-pressure-flow-coupled-boundary-condition-windkessel.html)

cfdfsi November 16, 2010 18:45

Outlet Pressure-flow coupled boundary condition (Windkessel)
 
Hi,

I'm trying to apply a windkessel type boundary condition at the outlet, which is of the form:

P(t) + k*P'(t) = Q(t), where k is a known constant

To do this, I model the outlet as a 'pressure-outlet' and use a UDF to impose P(t) on the outlet faces.
To calculate P(t), I approximate P'(t) by [P(t) - P(t-1)]/delta_t, and arrive at the closed form expression
P(t) = Function[P(t-1), Q(t), k, delta_t]

My question is:
how do I specify this equation in the UDF ?

At present, I'm using the DEFINE_PROFILE(pressure, t, i) macro,
calculating Q(t) by

Code:

density = 100;
sumFlow = 0;
face_t f;
  begin_f_loop(f, t)
      {
      sumFlow = sumFlow + F_FLUX(f,t);
      }
  end_f_loop(f, t)
sumFlow = sumFlow/density;

Now, what is the best way to find P(t-1) on all the faces of the outlet ?
Is there a macro like F_P_M1(f,t) ?

nacaairfoil October 12, 2011 10:01

I am also facing the same problem. Does anyone has answer for this. Also since Q is evaluated at current time step. I need to iterate to get p. How should I do the iteration using UDF?
Thanks a lot

PranjalNewton December 11, 2015 23:48

I'm also stuck on this. I made UDM for storing P(t-1) and initialised it. So, it should work but in the results, I see that the compliance phenomenon isn't observed. I've modelled wall material as elastic. Does it need to be hyperelastic or something to get FSI coupling behavior?

behest July 21, 2017 12:30

Dear all,
did you manage the problem? I have aslo the same issue, I need previous time step preesure on the wall, please let me know if you found any solutions


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