the role of operating pressure in flow simulation
 

Dear All,

I am simulating gas flow in a complex geometry to get the velocity and pressure field. I use incompressible model and simply assume density constant. Viscosity is also constant. Operaring pressue is 1 atm, and gauge pressure in the pressure outlet is 0 Pa. I check Fluent user mannual, and it says that operating pressure is not used in the density calculation. Based on assumption of constant density and viscosity, my questions are

1) Is operating pressure involved in solving N-S equation?

2) If I compare two cases, in the first one, operation pressue is 1atm, gauge pressure in the outlet is 0 Pa; in the second case, operating pressure is changed to 2 atm, the othters are the same. If velocity filed of these two cases the same?

Thanks in advance.

Sunflower,

The operating pressure, defined at a specific coordinate, simply gives a reference pressure off of which pressure differentials are calculated.

Think of it like a regular pressure gauge which is calibrated to read "psig," or pounds per square inch gauge. That pressure meter will read "0" at atmospheric (or whatever ambient is defined as) pressure.

The same goes for Fluent calculations. In a simple example, if at the top of a tank, you define the operating pressure to be 1 atm, the bottom of the tank (in a stagnant system) will simply be 1 atm + rho * g * height of tank. If I define the top of the tank operating pressure to be 2 atm, the pressure at the bottom will be 2 atm + rho * g * h. Thus, if the flow conditions are the same, and you're using a constant density fluid, there is no reason for the bottom of the tank gauge pressure to be different.

That said, if you're performing a calculation wherein the fluid density changes as a function of absolute pressure and temperature, you do want to be careful with your ambient pressure definition.

ComputerGuy