[Astrodan]

Hi everyone!

As I plan to write my own solver that merges several capabilities of existing solvers, right now I'm trying to understand what is going on in existing codes.
My primary interest is in settlingFoam/driftFluxFoam and interFoam right now.

But somehow I'm stuck at the pressure calculations in createFields.H (and pEqn.H):

As input the solvers use the p_rgh value, which appears to be the static pressure in my domain. Later in the code the piezometric pressure p is calculated from density and phase concentrations.
What I don't get here is the part of the hydrostatic pressure. Density rho as well as gamma (g*h) are volScalarFields, and should represent the value in the cell center (if I'm not wrong). Now, calculating

p = p_rgh + rho * gh

(L.111) it appears to me that the local cell density is used, and thus the pressure is highly dependent on alpha, and not on the type of fluid that is above the cell.

So, what I had expected was a pressure calculation by integration from the top of the domain to the cell:


Can anyone explain to me what is going on there?
I already read:

• p_rgh in OF 1.7 which explains the pressure calculations
• Hydrostatic Pressure and Gravity which calculates an additional laplacian that I don't complete comprehend yet.
• many more tabs that I closed already

Thank you in advance,
-Timm

Sidenote:
I'm aware that there is an implementation of an interSettlingFoam solver created at Chalmers in one of the courses, but for sake of clarity I'm trying to understand the solver-creation-process a bit more detailed.

[Tobi]

Hi Timm,

just for you as an hint. The calculation of "p" in the createFields.H is just an initial guess of the pressure field. While you are calculating/solving your problem, you solve other pressure equations.

Regards
Tobi


PS: Are you doing your Ph.D. in Munic ? <-- okay you profil showed me that this is true (:

[Astrodan]

Hi Tobi,

thanks for the answer. I indeed realise that the p is not used for any solving operation. But that confuses me rather more than before.
Apparently p is only used, when a reference pressure is needed, i.e. no fixedValue pressure boundary is given.
So, in many cases it will not be required at all?
And still, rho*gh is used to calculate p, when a reference cell is being used, where the rho*gh field looks kind of weird to me (also see attached pictures below).

Just for a comparison I did a small modification of the laminar damBreak case. And added a second water field floating in mid-air above the original (but now smaller) one.

From the three fields discussed p seems to be pretty reasonable to me at t=0.05s:
0.05-p.jpg

p_rgh on the other hand shows a - to me - completely confusing profile. Why can we calculate velocities with this a profile?
0.05-p_rgh.jpg

For completeness I also have rho*gh (written by modified interFoam). Here I have logical problems with the simple gradient resulting from the simple assumtion of constant density over height, which presumably is the reason for the profile in p_rgh.
0.05-rgh.jpg

Now my basic question would be: What is the "correct" pressure (the one I would physically measure)? I assume it must be p, but don't understand the why adding rho*gh should give the correct solution.

I'm still pretty new to OpenFOAM (and CFD as such), since I only started two month ago. So I'd also be happy if there was any document which describes the pressure calculation for these multiphase cases (more than the usual SIMPLE/PISO description).

[Astrodan]

Okay, I think I worked half of it out:

p_rgh is the static pressure field, which is independent from kinematics and hydrostatic. Thus it is used since it offers the easiest definition of BC and is (apperently) treated better in a numerical way.

Previous not correct thought: And since the hydrostatic pressure only depends on its gradient and not the absolute value, the calculation utilizes the ghf field and the gradient of the density in ghf*fvc::snGrad(rho) for the flux calculation.

Now, the absolute pressure field written should still be wrong? Which is what - I assume - was the original intention of the interFoamPressure tool (which seems to be not applicable to the current OF Version) to correct.

I hope this is correct now, thank you anyways.
-Timm

Small correction:
I finally found this thread, which seems to explain the matter. The use of the static pressure introduces an error due to an additional force in the momentum equation based on the derivative of rho. By removing the term from the flux field this error is being corrected.