Negative pressure


#1

Hi!

I am running a steady state internal flow simulation through a choke valve with inlet velocity (2 m/s), outlet pressure (0 gauge pressure) and no-slip walls as my boundary conditions. I am getting negative vales of pressure (in Pa) at the exit of the valve and i do not understand the physical significance of this. Could someone please help me out? Thanks!

Best,
Aadit

https://www.simscale.com/workbench/?pid=2839912886904346613#tab_0-0


#2

Hi @ashroff,

It’s because you are have an incompressible flow case. Don’t worry about negative pressure in this case, it’s normal to occur this kind of thing, look, when you are running an incompressible simulation and have tied one boundary to a low pressure or set the operating pressure to 0 Pa (in this case) to cut down on rounding errors, then it is entirely possible that you will have regions of negative pressure in your domain. This is because the pressure gradient enters the Navier-Stokes equations and so it is pressure differences that drive the flow. So, in regions of separated flow, the low pressure inside that region will be relative to the lowest fixed pressure in your system and may well go negative. It works subtracting the pressure in the equation, and so grossly this is the reason for the negative pressure to happen, does make sense?
Of course, in general, absolute pressure can’t be negative, but for an incompressible case (constant rho), you’re dealing with gauge pressures and not absolute ones. If, however, you were to model an ideal gas, then your operating pressure might need to be fixed at, for example, atmospheric because now a negative absolute pressure would result in negative densities and so would make things difficult to say the least.

Please, let me know if it helped you :slight_smile:

Best,

VinĂ­cius


Pressure Outlet and gauge pressure (Incompressible flow CFD)
#3

Hi @vgon_alves,

Thank you for your reply. I did understand what you are trying to say. I have a further doubt though. As my gauge pressure at the exit is 0 bar, does this mean that the max. pressure in the system (approx. 33,000 Pa in my case) is actually the difference in pressure between the inlet and outlet? Also, referring to your comment, would I be correct in not considering the sign of the pressure, only its magnitude?

Best,
Aadit


#4

Hi @ashroff

If I got what you are trying to assume, to consider only the magnitude is not correct, look: when you have, for example, -2Pa inside your fluid domain, you can not consider it as 2Pa because at the same time you have -2Pa and 2Pa acting in their fluid domain. As I said, in this case, the negative pressure is a consequence of the boundary condition, which does not mean that it is wrong. And yes, the maximum pressure in the system comes from the pressure difference at the outlet.

But please, let me know if it is still not clear!

Cheers,

VinĂ­cius


#5

Hi @vgon_alves,

Yes, I understand! Thank you so much for your help, I shall post it on this thread when I have completed the sim, if you are interested, you can have a look and perhaps give me your comments. Thanks again!

Cheers,
Aadit


#6

Hi @ashroff,

So glad to know it helped you! It will be a pleasure to help you again if another opportunity occurs!

Cheers,

VinĂ­cius