Specified massflow rate in an outlet
Hy there!
I'm currently trying to implement a boundary condition into a 3D Finite Volume method. This bc should handle an outlet in which the user know the massflow rate. The bc actually modifies the exit static pressure in each iteration in order to reach the prescribed massflow rate. I have a doubt, however: What's the equation that relates massflow rate and the pressure? Does any one now it? I know some commercial codes do it, but I couldn't have access to the proper formulation. Does anyone have any idea? Thanx a lot in advance. Biga 
Re: Specified massflow rate in an outlet
The equation you are looking for is the NavierStokes equation, relating velocity to pressure and therfore massflow to pressure.

Re: Specified massflow rate in an outlet
In general the pressure gradient is given by irrotational part of (u.grad(u) + 1/Re*del^2(u)) + conservative forces, which cannot be expressed in terms of flow.
For the special case of fullydeveloped internal flow, a relation of the form you described does exist. It is of the form: grad(p)=c_p/Re*phi, where phi is the net internal flow, Re the Reynolds number, and c_p is known as the "Poiseuille constant". c_p depends only on the flow geometry, and is 3 for flow in a 2D duct. 
Re: Specified massflow rate in an outlet
Thanks, Jonas
That's more "implementable"... =D However, that gradient would make it a bit harder... :\ I've been looking in the Fluent UDFs and found one equation, which is delta p = MFR_desired * delta MFR / (rho * area^2), where MFR = mass flow rate. Do you where this equation comes from??? :) I've tried the implementaion with a pressure "driver" of my own, written as: delta p = p * delta MFR / MFR_desired, and it seems to work, what makes me think that I don't really need a physical equation but only a driver to the desired pressure. Weel, thanks again anyway, Biga 
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