# Pressure inlet boundary condition?

Hi,
Is pressure inlet boundary condition for internal flow have to be a positive value? can the pressure inlet be negative?
Could you explain what is the difference between the 2?
Thanks,

Vinh

Wecome to the forum @vnguyen! I think @psosnowski, @Ali_Arafat, @dylan, @TobiasHolzmann might be able to help you out here

Incompressible single phase solvers use kinematic gauge pressure, so yes it can be negative. A negative value means smaller than pressure reference.

Compressible single phase solvers use absolute dynamic pressure, which is usually >= 0, unless you are looking at cosmology.

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Hi,

I just want to complete the answer from dylan. Based on the mathematics, the incompressible solvers have one characteristic - we assume that the density is constant and hence we can put it out of all derivatives. Doing that, we are able to divide everything by the density (even the pressure). Hence, we end up with the pressure as p’ = p/rho and further more as already said, its a reference pressure (not an absolute value). For drag and force lifts you should multiply the calculated stuff by rho again (why? because everything we calculate is phi/rho).

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Hi Tobias and Dylan,
So is the reference pressure in Simscale the atmospheric pressure? in other way, does it mean that if I put pressure inlet =0, Simscale will assume it’s the same as atmospheric pressure?
Thank you very much.

No it is not necessarily atmospheric pressure. The reference is usually of no interest when dealing with kinematic gauge pressure. The only physical transport property specified in this case is kinematic viscosity nu. An inlet pressure of value zero means it is the reference pressure ( zero gauge is the reference ) - whatever absolute value it holds.