Hi , I’m trying to model air enter high velocity from a pipe into the big rectanngle box and outlet is small nozzles from the top side of the retangle box.
My boundary conditions for velocity as below
Inlet
{
type flowRateInletVelocity;
volumetricFlowRate 0.1;
value uniform (0 0 0);
}
Outlet
{
type inletOutlet;
inletValue uniform (0 0 0);
value uniform (0 0 0);
}
and for Pressure
internalField uniform 25000;
boundaryField
{
Inlet
{
type fixedFluxPressure;
value uniform 45000;
}
Outlet
{
type totalPressure;
p0 uniform 25000;
U U;
phi phi;
rho none;
psi none;
gamma 1.4;
value uniform 25000;
}
1st doubt
1)Expected velocity is at outlet of nozzle is 200 m/s (because the diameter is nozzle is 12.5mm), I’m acheiving close to this velocity but the issue is while checking in paraview p shows 25000. According to total pressure boundary condition p0 should be fixed right, then pressure at the outlet should be p=p0-(0.5*density*U2)=25000-(0.5*1*200^2)=20000, instead in paraview the value is fixed to 25000. Can you explain on it?
2nd doubt
2) For Pressure at inlet I’m using fixedfluxpressure, the pressure at inlet raises 2e5. I’m not sure how the pressure raises to 2e5 and velocity is only 22 m/s. If I try to balance based on Bernoulli equation, then also it is not balancing, Can you please explain it?
The flow is only dependent of the pressure gradient. As you have provided the the values at the outlet it will calculate the pressure of the inlet according to the flow.
I agree with you, but pls refer my doubts below. My question is why hyrostatic pressure is not changing according to velocity and total pressure at outlet
Usage
The condition sets the static pressure at the patch pp based on a specification of the total pressure, p0. The mode of operation is determined via the input entries and the dimensions of the convective flux, phi ( ϕ).
Subsonic incompressible
Patch pressure described by subtracting the dynamic pressure from the total pressure, where the flux has dimensions of m3/s.
(https://www.openfoam.com/documentation/cpp-guide/html/guide-bcs-inlet-outlet-total-pressure.html)
why the static pressure is maintaining the same value of total pressure, inspite change in dynamic pressure.
Dear Ahmed,
Let me help you regarding total pressure concept. In the case of incompressible fluid flow the total pressure (p0) is the sum of static pressure (p) and dynamic pressure: p0=p+0.5*rho*v^2. Let consider fluid flow from one big vessel to another big one through the pipe of small diameter (when compared with the characteristic dimension of the vessel). In that case we can neglect fluid flow velocity within vessels, so the total pressure in the vessels corresponds to the static pressure. For the p01 greater than p02, the fluid will flow from the first to second vessel. At the pipe inlet total pressure will be p01, and static pressure in the pipe will be p_inlet=p01-0.5*rho*v^2, where v is the velocity in the pipe. Experiments show that the static pressure in the jet at the pipe outlet is equal to the ambient pressure. Here the ambient pressure at the pipe outlet corresponds to the total pressure p02. So, the total pressure at the pipe outlet is p0_outlet=p02+0.5*rho*v^2.
I hope that this will help you. Best regards, Zdravko.
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