# [OpenFOAM] High pressure range using totalpressure BC

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February 5, 2019, 11:59
High pressure range using totalpressure BC
#1
New Member

Join Date: Aug 2018
Posts: 12
Rep Power: 5
Dear foamers;
I have been trying to understand how pressure works in incompressible flow solvers (PISO in my case) and i found out that pressure in Navier Stokes has no physical meaning but just the gradient.
So i tried to find a good BC to couple velocity and pressure and gives me a good results for my project but the pressure in Parafoam makes no sense to me.

My BC are as follow;
Pressure:
Outlet: totalpressure
P0 1.5e5
Gamma 0.0
Value uniform 0

Velocity:
Inlet: fixedvalue
Value uniform (25 0 0)
Outletressureinletoutletvelocity
Value uniform(0 0 0)

My project is only on an open test section that is symmetric and cst area, you can consider it as a simple jet of air, inflow and outflow.

I can't get the pressure to give me a good result, the velocity in the other hand works perfectly if i can say so!
The pressure range is so high and has negative values.
Since this is incompressible, there are a lot of pressure fluctuations.

I have been trying for months to get this to work.
Here are some pictures of what it looks like. One for velocity and the rest pressure.
Attached Images
 Resized_20190130_135024.jpg (68.4 KB, 11 views) Resized_20190130_135021.jpg (91.8 KB, 10 views) Resized_20190130_135017.jpg (94.4 KB, 7 views) Resized_20190130_135014.jpg (90.7 KB, 9 views) Resized_20190130_135105.jpg (79.1 KB, 7 views)

 September 10, 2020, 08:54 #2 New Member   Tobias Kienzler Join Date: Mar 2020 Posts: 13 Rep Power: 4 As you mentioned only the pressure gradient is relevant for incompressible solvers, so could just as well set p0 to zero. But more importantly (and what screwed me up for a while) the solver uses kinematic pressure as described at https://www.openfoam.com/documentati...-pressure.html. This means p is not in Pascal but in mē/sē and must be rescaled by rho to actually make sense (and your p0 would have to be divided by rho in return, so 1.5e5 really only makes sense for a density of ca. 1). This rescaling is also relevant in the transportProperties, where nu = mu/rho is used instead of mu!

 Tags #fluctuation, #piso, #pressure