CFD Online Discussion Forums

CFD Online Discussion Forums (
-   Fluent UDF and Scheme Programming (
-   -   Outlet Pressure-flow coupled boundary condition (Windkessel) (

cfdfsi November 16, 2010 18:45

Outlet Pressure-flow coupled boundary condition (Windkessel)

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


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 11:01

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

All times are GMT -4. The time now is 18:15.