# Coupled Pressure-Flow Outlet Boundary Condition (Windkessel)

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 January 23, 2012, 10:34 Coupled Pressure-Flow Outlet Boundary Condition (Windkessel) #1 New Member   Join Date: Jan 2012 Posts: 1 Rep Power: 0 Sponsored Links Hi there, is it possible to use a Windkessel type outlet boundary condition with OpenFoam? Here, P(t) depends on P(t-1) and Phi(t). How can this be done as a UDF? The question is similar to an unsolved thread http://www.cfd-online.com/Forums/flu...indkessel.html. Cheers

 February 29, 2012, 06:51 #2 New Member   Michael Bruckner Join Date: Apr 2009 Location: France Posts: 27 Rep Power: 10 Hi, have you find a way of implementing the Windkessel boundary conditions in OpenFoam ? I'd be very interested in knowing how you proceeded. Cheers, Michael

 March 2, 2012, 12:12 #3 Member   Vitaly Join Date: Jan 2012 Posts: 32 Rep Power: 7 Hi, the previous thread read: 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) ? ----- I am also starting to work on this issue. I think that you can find P(t-1) using a memory macro, and I think you can find Q(t) using the code supplied. If you only have one outlet for an incompressible fluid, then Q(t) should not be a problem. In my case, I have multiple outlets. But, I think that if each outlet has the same R and C values, then the flow split should stay the same.

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