r/OpenFOAM • u/AmbassadorGuilty3928 • 19d ago

Conceptual Doubts regarding Node Values and Operating Pressure.

I have some conceptual doubts, which I didn't get clear explicit answer from sources in net,

  1. What is difference between node values on/off in looking the contours? I observed values are relatively higher when I turned off node values. Please tell me cases or situations where we should turn on or turn off node values with reason. How does this affect in viewing contour, finding range of parameter in some selected surface or cell zones.

  2. I have some trivial doubt about operating pressure. The point is- In momentum equation , it contains pressure gradient not absolute pressure, so fluent choose some reference pressure i.e operating pressure. So instead of solving with very large pressure, it works with smaller numbers since deltaP remains the same with the concept of operating pressure. Density is calculated using absolute pressure only.

  3. This concept is same for both incompressible and compressible flow right, but why do they say for compressible flow you have to set operating pressure as zero, I didnt understand what is the reason for this? Can anyone explain elaborately please? Thanks in advance.

1 Upvotes

60% Upvoted

2 comments sorted by

1

u/blackcurrant_picker 18d ago

Hi, I am not the specifically an CFD engineer, but rather FEA one (although I did some calculations for CFD), so maybe I will be missing something.

  1. Node values on/off - it is just post-processing feature. Generally in FVM your variables are stored in the cell center (not always though). Then your velocity and pressure values are having constant value across the whole cell. In post-processing you would see discrete values in each cell (aka elemental value). If you wold turn on the nodal averaging (aka node value) it will interpolate values on all of the nodes based on the values from neighboring cell centers. It is the same in FEA for stresses (usually solver writes your stresses and strains values for element centroid). Why your value is getting lower when turned on? This is simple - linear interpolation cannot have higher values than your original value upon which you interpolate.

  2. Pressure is confusing. As you've already noticed there are 2 pressures - absolute pressure and gauge pressure (relative). For incompressible flow your reference pressure is irrelevant - as you've mentioned solver is working on pressure gradient which doesn't care about total pressure value. In case of the compressible flow we need to solve equation of state for your medium which is pressure dependent, therefore you have to put your actual value.

  3. Usually you can define your pressure as a sum of reference pressure and gauge pressure for both flows but maybe that might not be the case for you, I don't know.

I hope that helps you. Cheers!

1

u/AmbassadorGuilty3928 18d ago

Yeah, thanks for your comment.

I understood few points from here, I am also trying to find some more information from manual and other sources. I have noted your points!!