Maintaining Static Pressure at Fluid Flow Inlet
I know that there are a ton of older posts on this topic, but I would like to raise the issue again. If there is another post which I overlooked which answers this question, please let me know.
I am validating Darcy's law for flow through a porous medium, which states (for the 1D) case that the permeability is equal to the superficial_velocity*viscosity*length/static_pressure_drop. I set up a simple 2D model in Fluent as a rectangle which has the left face as an inlet and the right face as an outlet. The top and bottom faces are symmetry lines. Fluent works great if I specify the inlet as a Velocity BC and the outlet as a Pressure BC. I can calculate the pressure drop and the superficial velocity from the postprocessing menu and validate the permeability value. However, I would now like to prescribe the static pressure drop across the subdomain, where the Velocity Inlet BC would be replaced with the Pressure Inlet BC. Within the Pressure Inlet BC, I realize that the total pressure is a sum of the gauge pressure and the dynamic pressure, and the dynamic pressure depends on the velocity of the fluid. But what if the velocity of the fluid is totally unknown at the onset of the problem? From the postprocessing menu, Fluent is calculating this value (static pressure). It doesn't seem straightforward to define this as a pressuredrivenflow condition. So my question boils down to: how could one define a pure static pressure drop over the length of a subdomain within fluent using the Pressure Inlet BC? Is there a UDF or something else I am overlooking? 
I think there is no way to study this kind of driving force directly. you may use velocity inlet condition with a trayanderror method to obtain a specified pressure drop. to be onest, I have lack of information about this problem and I would be happy if anyone suggest a method to overcome this problem.

Also the intake fan boundary may help you in this situation. does anyone have an idea about that?

This was originally an old post, but since new posts have been added:
The pressure drop or driving force can easily be implemented in Fluent by activating body forces (turning on gravity). Specify the accelerate cause by gravity to be the antiflow direction and choose the magnitude of the accelerate such that it will cause a linear decrease in static pressure. This method works great if the static pressure drop desired is known beforehand. If it needs to be iterated, a UDF may be needed. 
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