[MOHAMMAD67]

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?

[christianfrias]

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.

[sharonyue]

Originally Posted by christianfrias

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

[vonboett]

Originally Posted by sharonyue

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.

[baedmaen]

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!

[Dream]

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!

[Andrea_85]

Hi,

replace

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


with

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

Best,

andrea

[vonboett]

Originally Posted by baedmaen

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

[Dream]

Originally Posted by Andrea_85

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.

[John Handel Kennedy]

Hi,
I am trying to simulate Flow in a Straight Pipe with Heat transfer.
I am using the buoyantBoussinesqSimpleFoam solver.
I have made g and beta to be zero.
The temperature of the wall is 373K and inlet fluid temperature is 293K.
The inlet velocity is 1m/s.
The diameter of the pipe is 1m and the nu value is 0.01 which makes a Reynolds number to be 100.
The laminar Prandtl number is 1.5.

I got fully developed flow in simpleFoam i.e. the velocity jumped to 2m/s.
However I am not able to get the same velocity profile in buoyantBoussinesqSimpleFoam, The velocity is decreasing towards the outlet.

How do we solve this problem?
What should I specify in the alpha_t and p_rgh files?

Regards
John

[SRKR]

P_rgh doesn't indicate 'dynamic pressure'. 'P' indicates static pressure which usually contains 2 components pressure of state and hydrostatic pressure. So, p_rgh indicates state pressure. The reason why do we need to use this pressure is in dealing with multiphase flows along with continuity, momentum and energy equations eqn. Of state is also required.
This in my opinion. Please, Correct me if I am wrong.

[Bdew8556]

Morning. I can't seem to access what I'm sure is a fantastic figure.

[YUGU]

Originally Posted by nbadano

Hi Kerim,

p_rgh is not really dynamic pressure, specially in multiphase flow where rho changes throughout the domain. Is just de difference between real pressure and the rho*g*z field. I think that's one of the reasons the field is no longer called pd, as in 1.5 version of OF.

Just to add to the confusion, bear in mind thart rho*g*z is not the real hydrostatic pressure either!! Even if rho is constant it differes from hydrostastic component by a constant (the distance between the z=0 plane and the atmosphere p=0 plane times rho*g).

Here's a quick sketch of the relation between p_rgh, rgh and p for a hydrostatic condition (no movement at all). Hope this helps



Best regards!

Nico

Hi,



can someone show me the sketch? It's not visible already.


Best regards,

[Harish Selvam]

Dear all,
I am new to OpenFOAM. I am using interFoam, a multiphase solver for my research work. As far as I understood, 'p_rgh' is not a dynamic pressure. So, it is better to think of using 'p' which incorporates all the pressure term (static and dynamic) for your measurement.

Suppose, if you take a numerical tank of 0.5m*0.5m of water depth 0.4m and grid size of 0.05m*0.05m and measure p_rgh and p at different points (say (0.25,0), (0.25,0.05), (0.25,0.1)), the p_rgh gives same value (i.e., 3924 pa) whereas p gives (3678.75 pa, 3188.25pa, 2697.75pa). This was my experience with the p_rgh and p when I checked simply for the hydrostatic condition. If p_rgh is dynamic pressure, it should be zero practically. However, it is not the case.

p_rgh is simply the pressure measured about the boundary incorporating dynamic pressure about that cell in which it is solving while p is the pressure corrected for the cell centers after solving. Maybe you can think p_rgh as a reference pressure with dynamic pressure incorporated.

Kindly note that I have not checked this case with rotational flows.
Solvers incorporate p_rgh in calculations. Since pressure difference is the driving force for any fluid motion, it would not affect the results I think

All the above discussions are based on my experience in this short term. Please correct me if I am wrong

Regards
Harish

[granzer]

It can be called relative pressure. Total_pressure-static_pressure=dynamic_pressure; Total_pressure-(hydrostaic_pressure+Reference_pressure-etc) = Relative_pressure

[finn_amann]

Originally Posted by The King

To understand the different pressures, look at Bernoulli:

Dynamic pressure --> 1/2*rho*v^2
Hydraulic pressure--> rho*g*h
Static pressure --> p

1/2*rho*v^2 + rho*g*h + p = Constant

From the openFoam site, p_rgh = p - rho*g*h.

So, p_rgh is the static pressure minus the hydraulic pressure, based on a arbitrary height.

I do not understand where the dynamic pressure came into this discussion. I think it has nothing to do p_rgh. Dynamic pressure is the pressure of the moving fluid and it will convert into static pressure if you bring the velocity of the fluid to zero. Conservation of energy, back to Bernoulli.

Good to know:
To get my VOF model working, I placed in the fvSolutions file under the PISO solver
pRefPoint (0.0 0.0 0.0);
pRefValue 1e5;

Succes!

So it is therefore impossible to compute the real water depth from a multiphase simulation via the Bernoulli equation, correct?

We have

0.5*rho*u^2 + p + rho*g*h = p_total

In this equation, p would also be the p in our results folders of our interFoam simulation.

p can be rewritten p = p_rgh + rho*g*h. Plugging this in will cancel out rho*g*h

0.5*rho*u^2 + p_rgh = p_total.

and p_total is dependent on a reference height.


In general, we know that

p_rgh = p - rho*g*h

However, h is not really the water depth, its just a reference height given by the user. It's zero by default, but can be given in an hRef file in the constant folder.

Unfortunately, this also means, that if you have a deformed water surface, which is the big feature of interFoam (imo), your hRef will definitely not be the water surface elevation. This effectively prevents us from directly computing the water depth via the Bernoulli equation.

This is really annoying, if you want to compare pressure results from multiphase simulations with other solvers.

Please correct me if I got some stuff wrong.