[aja1345]

Hi,
I am simulating flow around a model. This model have an inlet and an outlet. Flow is incompressible. I know that value of pressure at the outlet is very much different from ambient pressure(Operating pressure). But, I don't know real value of pressure at the outlet. Now, I applied value of zero gauge pressure at the outlet (Is this OK? Why?) and this means that pressure at the outlet is equal to operating pressure while this is not. How do i find correct boundary condition for outlet?


But some people say that when flow is incopressible and boundary condition at inlet is velocity, we can use pressure-outlet as a boundary condition at the outlet because flow is not dependent on outlet pressure (so, we can use zero gauge pressure at the outlet). Why?




Thank.

[LuckyTran]

You can always use a pressure outlet BC. The question is whether you can use a velocity inlet. In general you can't always use a velocity inlet. But when the flow is incompressible you can use a pressure outlet w/ a velocity inlet.

[aja1345]

Originally Posted by LuckyTran

You can always use a pressure outlet BC. The question is whether you can use a velocity inlet. In general you can't always use a velocity inlet. But when the flow is incompressible you can use a pressure outlet w/ a velocity inlet.

Thanks. You are right. Therefore, Is it OK to use zero gauge pressure at the outlet in this case? You can prove your statement computationally? why does flow is not dependent on outlet pressure for incompressible flow?


According the Bernoulli equation,


P_1+1/2(Ro)*V_1^2+Ro*g*h_1=P_2+1/2(Ro)&V_2^2+Ro*g*h_2

Ro=density


Now,

P_1-P_2=Ro*g*(h_2-h_1)+1/2(Ro)*(V_2^2-V_1^2)

It means:

P_1-P_2=DP_static+DP_dynamic

Now according to above computations, How to understand that flow is not dependent on pressure at the outlet for incompressible flow?


Thanks.

[LuckyTran]

For incompressible flow the pressure is no longer the thermodynamic pressure and its absolute value is meaningless. You can specify any pressure anywhere to be any number. Only pressure gradients have any meaning.

But, in Fluent, there is a minimum threshold setting on the value of the absolute pressure and by default it's greater than 0 Pa. So if you did set your BC to zero gauge pressure, you'd get an error. But you can easily change this threshold.

In the Navier-Stokes equations, only the pressure gradient appears so it should be apparent that the momentum transport has little to do with absolute pressure. The only reason you need the any hard number for pressure is because you usually have an equation of state (e.g. density). In Fluent you have no way of directly imposing the divergence free velocity constraint (Fluent is a compressible solver). When you say incompressible flow in Fluent, it is implied you are talking about an incompressible EOS which means either a constant density, temperature dependent density and these have no dependence on pressure.