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p_rgh in OF 1.7

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Old   June 14, 2012, 10:12
Lightbulb Hydrostatic Pressure????
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Mohammad Fereshtehpour
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Dear Friends
Hi
Here, a lot of discussion about p and p-rgh were done. But I myself couldn't get the final results from the discussions. I want to know, for a multiphase flow, which pressure should be used in order to compare with experimental data? as you know, We can have just hydrostatic pressure in the lab.
Is the p-rgh value depended on the origin of the domain?
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Old   August 16, 2012, 15:24
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Christian Fri
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I see that since p_rgh is not the dynamic pressure it means that p is not the total pressure (as we could think) and is actually the static pressure. To calculate the total pressure you can use ptot (this will calculate 1/2*rho*U^2 [dynamic pressure] + p [static pressure)]). So, to compare experimental data with the results from OpenFOAM I will use the p file which is the static pressure in OpenFOAM.
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Old   November 25, 2012, 04:27
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Dongyue Li
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Quote:
Originally Posted by christianfrias View Post
I see that since p_rgh is not the dynamic pressure it means that p is not the total pressure (as we could think) and is actually the static pressure. To calculate the total pressure you can use ptot (this will calculate 1/2*rho*U^2 [dynamic pressure] + p [static pressure)]). So, to compare experimental data with the results from OpenFOAM I will use the p file which is the static pressure in OpenFOAM.

so is the p(static pressure) plus 1/2*rho*U^2(dynamic pressure) usually measured in experiment? Why I remember usually in a tube or something ,static pressure is measured in a pressure gauge
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Old   May 24, 2013, 10:49
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Albrecht vBoetticher
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Quote:
Originally Posted by sharonyue View Post
so is the p(static pressure) plus 1/2*rho*U^2(dynamic pressure) usually measured in experiment? Why I remember usually in a tube or something ,static pressure is measured in a pressure gauge
Depending on what you measure. Water pressure level sensors (and piezometers) yust measure the equivalent hydrostatic pressure above the sensor, so they are liquid level sensors and you should compare measurements to p in OF. If you measure the pressure in flow direction to gain drag forces etc. it is more difficult to directly compare measurements and simulation.
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Old   July 8, 2013, 07:58
Default Pressure things ...
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Hi all,

now i am confused about multiphase pressure. Could anybody put the corresponding qualitative pressure profiles into the file attached?
First figure:
stationary two phase system (water and air); i think nbadano already posted the answer at December 3, 2010, via this thread
Second figure:
bubble rising to surface; snap shot
Third picture; first contact of a water drop with water surface

Thank you very much!
Attached Images
File Type: jpg pressure.jpg (31.6 KB, 134 views)
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Old   April 6, 2014, 21:51
Unhappy Cannot find patchField entry for wall?
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We set up the separator model, and stimulates the multiphase flow. After we ran the setFields, the following information occurred:

Setting field default values
Setting internal values of volScalarField alphaair
Setting internal values of volScalarField alphawater


--> FOAM FATAL IO ERROR:
Cannot find patchField entry for wall

file: /home/dell/OpenFOAM/damBreak4phaseFinelmxlmx/0/alphawater.boundaryField from line 34 to line 56.

From function GeometricField<Type, PatchField, GeoMesh>::GeometricBoundaryField::readField(const DimensionedField<Type, GeoMesh>&, const dictionary&)
in file /home/opencfd/OpenFOAM/OpenFOAM-2.2.2/src/OpenFOAM/lnInclude/GeometricBoundaryField.C at line 206.

FOAM exiting
-----------------------------------alphawater-------------------------------------------
/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.2.2 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
location "0";
object alphawater;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 0 0 0 0 0 0];

internalField uniform 1;

boundaryField
{
Wall
{
type zeroGradient;
//value uniform 0;
}
new_new_out
{
type fixedValue;//;
value uniform 1;
}
new_out
{
type fixedValue;
value uniform 1;
}
out
{
type fixedValue;//outletInlet;
value uniform 1;
}
in
{
type outletOutlet;
outletValue uniform 0;
value uniform 0;
}
}

-----------------------------
Maybe the boundary conditions were set incorrectly, but we didn't know how to revise them.
Thanks so much for help!
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Old   April 7, 2014, 02:35
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Andrea Ferrari
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Hi,

replace

Wall
{
type zeroGradient;
//value uniform 0;
}


with

wall
{
type zeroGradient;
//value uniform 0;
}

Best,

andrea
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Old   April 7, 2014, 04:04
Default
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Albrecht vBoetticher
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Quote:
Originally Posted by baedmaen View Post
Hi all,

now i am confused about multiphase pressure. Could anybody put the corresponding qualitative pressure profiles into the file attached?
First figure:
stationary two phase system (water and air); i think nbadano already posted the answer at December 3, 2010, via this thread
Second figure:
bubble rising to surface; snap shot
Third picture; first contact of a water drop with water surface

Thank you very much!
Well, p_rgh depends on your ccordinate system, boundary conditions and eventually a pRefValue and pRefPoint. However, if you have something like an atmosphere pressure = 0 Pa in the air above the surface by specifying an atmosphere boundary condition or a pRefValue = 0 Pa, and your water density and g is specified correctly, you will get a nice triangular hydrostatic pressure starting from zero at the surface for p, but not for p_rgh. The solvers I use work with p_rgh in the Navier-Stokes equations but I use p as result for comparison. I attatched two screenshots of a 3D domain starting at z = 2m at the bottom and reaching z = 3m at the top, with atmospheric pressure 0 Pa, and a bubble at z = 2.25 m and a water drop at z = 2.51 m, both with 0.01 m radius, and the free surface at 2.5 m. The screenshots are taken at T = 0.01 s after simulation start, cell size is 5mm in all directions. Note how the density affects p_rgh, 24680 Pa = 2.51m * g * rho_water
Attached Images
File Type: jpg airBubbleWaterDropAndSurface_p.jpg (14.0 KB, 70 views)
File Type: jpg airBubbleWaterDropAndSurface_p_rgh.jpg (13.6 KB, 67 views)

Last edited by vonboett; April 7, 2014 at 07:33.
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Old   April 7, 2014, 20:28
Default Thanks so much!
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Quote:
Originally Posted by Andrea_85 View Post
Hi,

replace

Wall
{
type zeroGradient;
//value uniform 0;
}


with

wall
{
type zeroGradient;
//value uniform 0;
}

Best,

andrea
thanks so much!
we have solved this problem. the reason is that we wrote the capital letter W with respect to the word wall.
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