Velocity U and Pressure p

Linda Lee · January 14, 2019, 16:06

Hi all,

I am a beginner of using openFOAM and I use it to simulate the outdoor airflow CFD simulation.

I learnt that the resulting pressure in the simulation is relative. Is the resulting velocity also relative? I have a case that the wind vector generated on three openings (measurement surfaces) of a shelter were all pointing inwards. If the resulting velocity is not relative, the case will not be making sense at all.

Thank you.
Linda

simrego · January 16, 2019, 05:37

Hi!

First of all the pressure is not always the "real" pressure. Incompressible solvers are using kinematic pressure which is pressure/rho, that's why the dimension of the pressure in incompressible solvers are [m^2/s^2].
If you are using relative or absolute pressure is up to you in incompressible solvers. Only the change of the pressure is important in incompressible solvers (so you can shift the pressure, for example set 0 on outlet instead of 1bar, but again carefully!!! you set pressure/rho in the p file in incompressible solvers!!!), but if you are using compressible solvers, you have to use the absolute pressure since your material properties can depends on the pressure.

Relative velocity to what? Sorry I don't understand exactly what do you mean.

Linda Lee · January 19, 2019, 06:05

Hi simrego,

Many thanks for your help.
The velocity I concerned is the result I got from the post-process file (with a file of U and a file of p). The pressure (p) obtained (to my understand) would be the case as you explained. Then, how about the U? Is it the same case (relative or absolute) as p?

simrego · January 24, 2019, 08:59

Let's say it should be relative to your stationary wall.

Linda Lee · January 24, 2019, 13:32

Many thanks.

January 14, 2019, 16:06	Velocity U and Pressure p	#1
Linda Lee New Member Join Date: Jan 2019 Posts: 3 Rep Power: 7	Hi all, I am a beginner of using openFOAM and I use it to simulate the outdoor airflow CFD simulation. I learnt that the resulting pressure in the simulation is relative. Is the resulting velocity also relative? I have a case that the wind vector generated on three openings (measurement surfaces) of a shelter were all pointing inwards. If the resulting velocity is not relative, the case will not be making sense at all. Thank you. Linda

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January 16, 2019, 05:37		#2
simrego Senior Member anonymous Join Date: Jan 2016 Posts: 416 Rep Power: 14	Hi! First of all the pressure is not always the "real" pressure. Incompressible solvers are using kinematic pressure which is pressure/rho, that's why the dimension of the pressure in incompressible solvers are [m^2/s^2]. If you are using relative or absolute pressure is up to you in incompressible solvers. Only the change of the pressure is important in incompressible solvers (so you can shift the pressure, for example set 0 on outlet instead of 1bar, but again carefully!!! you set pressure/rho in the p file in incompressible solvers!!!), but if you are using compressible solvers, you have to use the absolute pressure since your material properties can depends on the pressure. Relative velocity to what? Sorry I don't understand exactly what do you mean.

January 19, 2019, 06:05		#3
Linda Lee New Member Join Date: Jan 2019 Posts: 3 Rep Power: 7	Hi simrego, Many thanks for your help. The velocity I concerned is the result I got from the post-process file (with a file of U and a file of p). The pressure (p) obtained (to my understand) would be the case as you explained. Then, how about the U? Is it the same case (relative or absolute) as p?

January 24, 2019, 08:59		#4
simrego Senior Member anonymous Join Date: Jan 2016 Posts: 416 Rep Power: 14	Let's say it should be relative to your stationary wall.

January 24, 2019, 13:32		#5
Linda Lee New Member Join Date: Jan 2019 Posts: 3 Rep Power: 7	Many thanks.