Discussion Forum

For those interested in pressure definitions

As I read often comments on the Forum, and I heard many questions on the last workshop at the Milan conference on CFD pressure, gauge, absolute, reference pressure and how and when to use them ...
I took some time today to see if I could find the description in the COMSOL doc and physics nodes entries. Yes it's all there (4.3a), somewhat scattered in all those pages, but I noticed that I got confused myself, but finally I believe I got some order on it, so why not comment it here too for other interested (and then I know where I can find it back too ;)

CFD and pressure:
This is so obvious for CFD experts that they do not think over it, but for most newcomers, deciding on which pressure to set in a CDF model is not that obvious. We are all used to work on earth at 1[atm] absolute pressure (about), but when we simulate some flow, often we are interested in cases with "p" of only a few [Pa] pressure drop, "p" compared to the "pA" 1[atm] or about 1E5 [Pa] that is a large difference. And, for those having studied numerical computations, making differences of large numbers that are similar in amplitude induces large relative error on the result. Therefore, CFD people mostly work with gauge pressures with p close to "0". The physics equations remain unconcerned if we add a constant value here, it solves as well, but with a small gauge pressure we are sure the results are more precise. Using an absolute outlet pressure of 1[atm] is certainly possible in COMSOL, but then do not forget o change also the initial value pressure to at least 1[atm] ... etc

Using a gauge pressure close to "0" [Pa] works well until we enter some material property that depends on the pressure, these will depend on the Absolute pressure pA, and we will get wrong results if we used simply "p" the gauge pressure.
Therefore: 1) when we define materials in the materials library, and these depend on the pressure, we should use the variable name pA (similar to T[1/K] that should be entered in, and arranged for, T in Kelvin and not °C, as K is COMSOL's default).
From the moment we call in a material property that is dependent on T or on pA, one will notice that COMSOL will populate, the "Model input" tab of the main physics sub-node, and here it will define pA as "pA = p_ref+p" (note this is for 4.3a I believe it has been adapted lately), where "p_ref" is a reference pressure, by default 1[atm]. In this way COMSOL calls for material properties with the correct absolute pressure and our results appear correctly ;)

You can find these explanations in the doc, partly in the CFD, partly in the materials doc. But also in the HT manual, as for thermal properties, as well as for ideal gas modelling (p*V=R*T), pA and T are required, in correct units and values, for all energy estimations

The gauge pressure, just as magnetic field gauge settings, voltage GND (gauge) settings ... are values one must often control, as else our PDE's are left in an unknown state with INF true and valid solutions

I hope these lines will help some of you too

And if I have missed something, or said something wrong pls update !

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Have fun COMSOLing
Ivar