[ata]

Hi FOAMers
What is the advantage of combining P and rho*g*h in one term "p_rgh" in OpenFOAM 1.7.0 comparing to OpenFOAM 1.6?
Best regards

Ata

[LarsPT]

Have a look at Rusche's Ph.D. Thesis (pp. 155/156). The dynamic pressure is used because it simplifies the specification of the boundary conditions. For example you can use p_rgh=0 at an outlet and you dont't have to deal with the hydrostatic pressure. I think the release notes of OpenFOAM-1.7 say something about this, too.

Best regards,

Lars

[ata]

Hi Lars
Thank you very much
Best regards

[Sam-CFD]

Hi,

I was trying a simple tank draining problem and was trying to specify the right boundary conditions. I am implementing my case in 2.1.0 and it is basically a tank with water filled up to a specific height with a small outlet port at the bottom center of the tank.

At the top I gave the same boundary conditions as in the Dam_break case.

I am having difficulty with specifying appropriate boundary conditions for the outlet port. I have been basically playing around with values for alpha, p_rgh and U.

1. For p_rgh = 0 (as suggested above):
The water does not flow out of the domain. The other parameters were U = (0,0,0), zeroGradient and alpha = {InletOutlet}, 0.

2. For p_rgh - zeroGradient:
The flow is going out but still the results are quite strange. While keeping alpha = {InletOutlet} and U - zeroGradient, the water suddenly jumps up and after a few time steps, it starts draining. It seems as if, the solver detects a high pressure at the outlet port and causes flow to occur from higher to lower pressure, but then realizes actually there is gravity acting against and then the liquid flows out.

The reason I gave p_rgh to be zeroGradient is because I understood that it is the dynamic pressure and the dynamic pressure at the outlet cannot be zero but rather the normal gradient should be zero.

I would be thankful for any help since I am not able to grasp yet the right implementation. In other solvers (like FLUENT, CFX), I would just specify the static pressure to be zero at the outlet with the possibility for reverse flow of air into the domain.

[Sam-CFD]

Hi,

I was finally able to solve the problem using the second formulation but with a finer mesh. <This was finally recognized as a mistake. Please refer here for the right approach.>

I have another question with regard to the p_rgh formulation in OpenFOAM.

If I want to specify some initial velocity in the fluid (e.g., due to rigid body motion of the tank), do I only need to specify it as the internal field in the U file or also the p_rgh file?

Regards,

Sam

[kalesanjay001]

Hi all,

I am trying to simulate flow around cylinder which is partially submerged in water i.e. multiphase with water and air. Can anybody suggest p_rgh boundary condition for outlet. I tried 'fixedValue - uniform 0' but it is not converging maybe because it is multiphase.

thank you.

[mzzmrt]

zeroGradient

[kalesanjay001]

thank you mzzmrt for suggestion.
but unfortunately it is not working. Water level is increasing which is not desirable.
below figure shows the alpha of water in the simulation domain.

boundary conditions I used are as below
for velocity
inlet - type fixedValue ; value uniform (0.01 0 0);
outlet - type zeroGradient;
bottom - type slip;
up (atmosphere) - type fixedValue ; value uniform (0 0 0);
sides - slip;
cylinder - type fixedValue; value uniform 0;

for Pressure
inlet - type zeroGradient;
outlet - type zeroGradient;
bottom - type zeroGradient;
up (atmosphere) - type fixedValue ; value unifom 0;
sides - type zeroGradient;
cylinder - type zerogradient;

so is there any suggestion for boundary conditions?
thanks again.

[Traction]

Hi kalesanjay,

interFoam has some drawbacks relating to low velocity flows which lead to numerical problems. Just search the forums for artificial / parasitic or spurious currents to find some more information on this topic.

[mzzmrt]

As an addition to the comment of Traction, you can check tuturials for boundary conditions, for example;

https://github.com/OpenFOAM/OpenFOAM.../DTCHull/0.org

Computational domain must be large enough and the distance between the object and the outlet is important as well as the Co...