[Trev0r]

Hey everyone, this is my first post so I'll try to be as concise as possible for you but I apologise if it's a bit long winded as I'm not too sure what to include so you can help out.

I'm trying to model artery bifurcations using idealised geometries and so far the simulation is steady state with rigid walls and I have prescribed a fully developed parabolic velocity profile at the inlet using a CEL expression. I'm going to be adding progressively more complexity (transient inlet flow waveforms and non-Newtonian viscosity models) to the model to make it more representative but for now I've hit a snag in describing the outlet boundary conditions.

In general the outlet static pressures can be treated as linear functions of the flow rates through them (accounting for the resistance of the arteries downstream and outside the domain of the simulation). I tried to model this using the following CEL expressions applied to the pressure outlets:

Pressure1=resistance1*areaInt_z_Coord1(w)@Outlet1
Pressure2=resistance2*areaInt_z_Coord2(w)@Outlet2

resistance1 and resistance2 are predefined constants. Coord1 and Coord2 are local coordinate systems centred at the outlet faces with the z axes perpendicular to them. Basically static pressure=resistance*flowrate

I set up monitors to watch these expressions converge during the solution process. I'm under the impression that at the end of each iteration the static pressures at the outlets are recalculated according to these formulas and updated ready for the next iteration. However it quickly became apparent that the values for static pressure at the outlets in the converging and converged solution do not equal the flow rate x resistance like in equations I set above.

In fact checking the flow rate through the outlets with the function calculator in CFD post and then multiplying that value by the predefined resistance that I set gives a vastly different static pressure to the one CFD post is reporting in the solution???

For example in one trial run shown in the attachments i've provided I set both resistances to and the resultant flow rates through outlet1 and outlet2 were converged at and respectively. This should have therefore resulted in the static pressures at the outlets being and . The actual static pressures reported at the outlets in CFD post are and . When I evaluate my two expressions in CFD post they give me the values I expect yet these are not the values applied to the outlets?

In summary my outlet CEL expressions are having an affect on the outlet pressures but they are not behaving as expected. Does anybody have any idea what I am doing wrong and why my static pressures at the outlets do not equal the values set in my expressions?

Many thanks

Trev0r