## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Understanding the Post CFD Pressure

• skdubey
Subscriber

As per the definination in the ansys manual

Abs. Pressure = Operting Press + Guage Pressure but in the case of incompressible fluid or fluid with constant density, must not used the operting pressure. In that case, the results we see on the Post CFD results must be Guage pressure = abs pressure.

I am not sure, I am getting negative pressure at the outlet, whereas my outlet is at the 0 psig  and my inlet is at 25psi gauge.

What are the possible reason for getting the negative pressure in the system? Meshing issue, boundary condition, or fluid properties (using water).

• C N
Ansys Employee

Hello Dubey,

To clear your problem. I will explain things in detail.

1. Absolute Pressure = Static Pressure + operating pressure + any missing gravitational head
2. Total Pressure = Static Pressure + 0.5*density*velocity^2

For constant density or bousinesq there is never any gravitational head in the static pressure field,

For a full density model the solver subtracts the operating_density*g*height from the static pressure field, but always adds it back into the absolute pressure field

If you set the operating density to zero you will see the gravitational head in the static pressure field with a full density model.

But then any pressure boundaries would need to use an expression to set the pressure as a function of height.

For fully compressible flows the pressure is strictly an exponential function of height not a linear function of height .

It only approximates as linear over small distances ( a few meters or less).

I hope now you have an idea of How cfx calculates pressure.

I recommend you to check the boundary conditions and appropriate reference temperature and reference density values.

I hope this helps

Thanks,

Chaitanya natraj