[paka]

Did anyone create a tool which helps obtain the full pressure including both hydrostatic and dynamic pressures?

Current rasInterFoam solver gives only the modified pressures output in 'pd' file, which does not include hydrostatic terms.

Thanks,
Krystian

[hjasak]

Yup, I wrote one of those a while back. It works in a post-processing mode, but if you wish you can build it into the top-level solver as well. Depends what you want to do with the real pressure...

Hrv

[paka]

Aloha Hrvoje,

Would you mind posting a tool over here?

First I just need a real pressure data plot, than I would need to calculate surface forces using calcPressureForce tool.

Does your tool give an output file to something like p or pd file?
Based on one of these files the other tool which I use calculate forces.

Thanks,
Krystian

[hjasak]

Well, actually, this was neither easy to implement nor trivial to work out. Is there anything you are prepared to offer in return?

Hrv

[paka]

Well, keeping in mind I'm still green, I can only promise my current commitment to OpenFOAM and my input, in-between, like tools development (I had some small input to foamToTecplot postprocessing).

What else could I offer you?

Regards,
Krystian

[richard_kenny]

Could someone please help me to understand the "p" and "pd" fields in "rasInterFoam"?

I am a civil engineer and would like to use OpenFOAM for the design of dam spillways. Specifically, I am trying to calculate the static pressure over a spillway crest. I am new to CFD and have just begun experimenting with OpenFOAM. I have no C++ programming experience. I have built and run a model but am having difficulty interpreting the pressure results.

The following four points summarise my understanding of the "p" and "pd" fields (based on my reading of the message board, OpenFOAM manuals and the rasInterFoam.C file).
1) "p" is the total pressure in Pascals [1 -1 -2 0 0 0]
2) p = pd + rho*gh (from rasInterFoam.C file)
3) rho*gh = static pressure in Pascals [1 -1 -2 0 0 0]
4) "pd" = dynamic pressure in Pascals [1 -1 -2 0 0 0] = 0.5*rho*U^2

I have built a test case with static water (i.e. water level at the crest of the spillway but not flowing over the spillway). The static water depth is 38.63 metres. The total and static pressures should both be 378,960.3 Pascals (p = rho*g*h = 1000*9.81*38.63). The solver is giving me a result of 369,834 Pa which is reasonably close but not exactly the same as the analytical solution (within about 2.4%).

The solver velocities (U) are very close to zero (eg. -0.000287, -0.000288, 0). Therefore, the dynamic pressure (pd) should be very small. However, the model results indicate a "pd" of 1,528,580 Pa. I don't understand this result. I suspect that I don't really understand what the "p" and "pd" fields are.

Any advice you can provide would be greatly appreciated.

[maruthamuthu_venkatraman]

Hello Kenny,
my experience with OpenFoam and Free surface flows is not much , however i would like to mentions some comments.

1. Have you succeed with InterFoam solver? usually RAS modeling needs quite fine grids to predict the mean pressure. I would suggest lets just run interFoam and look at your pressure.

2. Also you shall refine the grid in your test case, Mostly discretisation errors are the main sources in CFD.

May be other Foamers may give better suggestions...

[daniels]

Hi Richard,

find attached a extended interFoam, which computes the pressure including hydrostatic effects. It requires 0/p defining boundary conditions for the pressure. I recommend to use fixedValue at top of your domain, and zeroGradient for all remaining boundaries.

Daniel

[daniels]

here is the attachment again

interFoamGL.tgz

[richard_kenny]

Maruthamuthu & Daniel,

Thanks for your assistance.

I have tried the interFoam solver and a fine mesh but have the same issue with the "pd" field. I have not yet tried the interFoamGL solver but hope to get time shortly.

In the meantime, could you please confirm whether my understanding of the "p" and "pd" fields is correct? In other words, are the four points listed in my post correct?

Thanks in advance,
Richard

[daniels]

Richard,

in principle you are right, but the p = pd + rho*gh is only valid for simple free surface flows. Have a look at this discussion:

http://www.cfd-online.com/cgi-bin/Op...3280#POST23280

the solver posted above follows this proposal. It is similar to the tool Prof. Jasak mentioned further above.

Daniel

[kwardle]

Daniel,
Thank you for posting your solver here. I have read many of the posts mentioning this issue between static pressure and pd, etc. so your solver is helpful. I have a question about boundary conditions for p, pd, and U for an outlet boundary. For example, say you want to solve for the liquid height of an asymmetrically shaped u-tube manometer with the short side 'submerged', like:

p1
---
| |
| |
| |
|x|
|x| p2
|x| ---
|x| |x|
|x| |x|
|x|_|x|
|x-x-x|

where p1 is atmospheric pressure and p2 is 0 *static pressure*. The initial gamma condition (marked with 'x' in crude diagram) shows the left column has a higher liquid height. Since a zero static p is specified for p2, the equilibrium gamma position in the left column should be equal to the height of the right side of the tube. What are the boundary conditions one would use for p, pd, and U to make this happen? For U, it would seem like you cannot use a pressureInletOutletVelocity BC in this case--one would like to have a BC for U that keeps the total *static pressure* equivalent (i.e. U from bernoulli). Do you (or anyone) have any suggestions on this?
THANK YOU.

[hamcer]

hi foamers!

I work on the injector (nozzel) of hydraulic turbin , my case is axisymetrique with a RANS turbulence model ,
Can I find a similar case in OpenFoam?, I m already investigated the LesInterfoam, and Rasinterfoam, but, I have a problemes with those cases, some advices will be welcome.