[sinatahmooresi]

Hello Dear Foamers,

I am intended to provide a useful guideline on pressure and everything about it in OF. Pressure boundary condition and the struggles associated with it is of the highest frequently asked and discussed about in CFD online. Thus, I think it is good to provide a very comprehensive discussion on it for whom are interested in using OF. To be honest, sometimes I feel confused with the variety of the B.C.s in OF.



First of all, it is very helpful to mention that, for incompressible cases, it is only the gradient of the pressure that matter. In contrast, most of the importance of the pressure and the relevant problems come up with compressible cases. Keeping the above-mentioned deceleration in mind, we can start with incompressible case. I list some features that I think comes handy for OF users dealing with pressure. I would appreciate your comments to correct this thread based on your knowledge.



Based on what we can see in source code definitions (https://www.openfoam.com/documentati...-pressure.html), we can rewrite our interpretation of pressure in openfoam:


P_total = P_ref + p + 0.5*U^2;
p_total = P Gauge.

p = static pressure = rho*p_k;
p_k = kinematic pressure (m2/s2), this is what we read and write in openfoam, dealing with incompressible flow.

p_ref = Is a referenced value used by incompressible solvers. Since they hold the relative values instead of absolute, it is just a value for starting the calculation.


A full complete relation for pressure regardless of the solver, software, … in whole fluid dynamics can be written as:
p_abs = P_atm + p_Gauge (relative pressure)

p_abs = absolute pressure.

p_atm = atmospheric pressure.

To turn the result of an icompressibel solver (in m2/s2) output of pressure to real relative values in Pa, one can write:
P = rho*(p + 0.5*U^2 + g*h)

[rishik686]

Originally Posted by sinatahmooresi

Hello Dear Foamers,

I am intended to provide a useful guideline on pressure and everything about it in OF. Pressure boundary condition and the struggles associated with it is of the highest frequently asked and discussed about in CFD online. Thus, I think it is good to provide a very comprehensive discussion on it for whom are interested in using OF. To be honest, sometimes I feel confused with the variety of the B.C.s in OF.



First of all, it is very helpful to mention that, for incompressible cases, it is only the gradient of the pressure that matter. In contrast, most of the importance of the pressure and the relevant problems come up with compressible cases. Keeping the above-mentioned deceleration in mind, we can start with incompressible case. I list some features that I think comes handy for OF users dealing with pressure. I would appreciate your comments to correct this thread based on your knowledge.



Based on what we can see in source code definitions (https://www.openfoam.com/documentati...-pressure.html), we can rewrite our interpretation of pressure in openfoam:


P_total = P_ref + p + 0.5*U^2;
p_total = P Gauge.

p = static pressure = rho*p_k;
p_k = kinematic pressure (m2/s2), this is what we read and write in openfoam, dealing with incompressible flow.

p_ref = Is a referenced value used by incompressible solvers. Since they hold the relative values instead of absolute, it is just a value for starting the calculation.


A full complete relation for pressure regardless of the solver, software, … in whole fluid dynamics can be written as:
p_abs = P_atm + p_Gauge (relative pressure)

p_abs = absolute pressure.

p_atm = atmospheric pressure.

To turn the result of an icompressibel solver (in m2/s2) output of pressure to real relative values in Pa, one can write:
P = rho*(p + 0.5*U^2 + g*h)

Thanks for this important information @sinatahmooresi

[vv123]

Hello,

I am trying to translate pressure that is calculated to a water level deviation from the imposed rigid lid.

I am using the solver pimpleFoam, which calculates the kinematic pressure. I impose a rigid lid on the surface, where a free surface is the real case. It seems like the calculation h = pk/g - 0.5u^2/g gives accurate results. However, I am not able to get the right derivation to this solution.

Furthemore I tried another derivation with Bernoulli, however I am still a bit confused by the different pressure terms in e.g. Bernoulli compared to the model.

P_model +rho*g*h_model +0.5*u_model^2 = P_realistic + rho*g*h_realistic + 0.5*u_realistic^2

Than this P_model, would it be rho*p_k, or would it P_tot described above.

And is this method possible at all? to derive water level fluctuations from the pressure?

Thanks in advance!