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Turbulent September 8, 2012 06:27

Negative absolute pressure
 
Hello all,

I've a question regarding negative absolute pressure values I get in the flow field and their meaning.

I ran an incompressible transient laminar flow past a circular cylinder with zero reference pressure. When I check the variation of absolute pressure in certain points close to the cylinder, I get negative values. I should add the problem is not physically well defined in means of density, visocisty etc., but rather theoretic.

Although negative values of abolute pressure are not physical as far as I know, can I trust those values?

Thanks,
Jack
:)

ghorrocks September 8, 2012 07:17

Negative absolute pressure usually means that the liquid will cavitate and this will eliminate the negative pressure.

However negative absolute pressure is physically possible - cavitation takes time to happen. Not much time, but for that short period of time a fluid sustains a negative absolute pressure. This is an effect of subcool/superheats and other non-equilibrium phase change.

But for your analysis, it is up to you. If you are doing a theoretical analysis you probably don't care about cavitation. But if you are designing a real system you might and in that case you better see if that cavitation is going to be a problem. Alternately you might have a rare case where you really do have negative absolute pressures.

vga67 December 31, 2014 02:44

1 Attachment(s)
Dear Glenn

Hi,

in my problem i apply a sinusoidal pressure inlet and let the operating pressure zero. my pressure inlet is of the form of : 101325 - 131788*sin(omega*t) but my pressure never goes below zero and I think it is becuase of limitation of absolute pressure(min 1 pa) what should I do to have also negative values of pressure??? in the figure i have attached the pressure at the inlet versus time is shown.
thank you
Hamid

ghorrocks December 31, 2014 06:44

If this simulation is incompressible then simply raise the reference pressure so you do not have negative pressures. If the simulation is compressible (or has something the function of absolute pressure) then you better think about what is really happening.

vga67 December 31, 2014 07:30

Dear Glenn,
Thank you

my simulation is compressible and i want to have negative pressures but the pressure never goes below 1 pa. I have made operating pressure equal to zero (i.e. absolute pressure = static pressure) here my static pressure is sinusoidal but may be because it is equal to absolute pressure and the absolute pressure minimum limit is 1 pa the pressure never goes below 1 pa. my simulation needs to experience negative values of pressure as well as positive values.
thanks again

ghorrocks December 31, 2014 18:09

You cannot have negative pressures in CFX for compressible flow. The whole compressible flow mathematical framework breaks down - it implies negative density as well.

It is also quite likely that your model which requires negative pressures is wrong. You need very specific conditions to get negative absolute pressures. What is generating these negative pressures?

vga67 January 1, 2015 01:42

Dear Glenn

I had discussed about my model in some other place in the forum. I have a air bubble in water both water and air are compressible. I apply sinusoidal pressure at the boundary of the water. when the pressure goes negative the bubble starts to expand until the pressure goes positive at this time the bubble collapses dramatically. in the expansion period of the bubble there should be a negative pressure outside which causes the bubble to expand. I simulate my this case in CFX with mixture model although there also there was no negative pressure, my supervisor did not accept the mixture method for this case. I tried this with Fluent and VOF method.but I need large negative pressure to make the bubble expand 10 times the initial state and then collapse with
Mach ~ 4. Is it clear to you Glenn??? i need ti have negative pressure to somehow suck the bubble from outside.


thank you for your patience
Hamid

ghorrocks January 1, 2015 05:02

I cannot keep track of every conversation on the forum. If another thread is relevant then put a link to it in your post.

Can you explain in your case why the water does not cavitate when the absolute pressure is approaching zero, and therefore stop the negative pressure from occurring?


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