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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?

ndabir September 5, 2015 15:24

Hi Vahid,

Were you able to solve your problem either in CFX or Fluent? I am doing similar simulation in Fluent and I am facing some problems with that. My problem is when I use VOF model, the oscillating pressure does not go below 1 Pascal (The 1 Pascal is the limit of Fluent for minimum pressure value and it cannot be set to a lower value). However, I need the negative pressure to force the bubble to expand and grow.

Thanks,

ghorrocks September 5, 2015 18:46

The numerics of every CFD code I am aware of does not allow negative pressures. The physical model breaks down.

I am aware that in some very specific circumstances negative absolute pressures do occur (for instance non-equilibrium cavitation).

So if you can define a equation of state which can handle this situation you might be able to get a Nobel Prize out of it.

ndabir September 6, 2015 01:55

Thanks Glenn for your response. In Fluent, If I do not activate a multiphase model (such as VOF) and the only fluid is water, then the oscillating pressure (the pressures are generated from an oscillating boundary and they will propagate inside the domain) will generate negative absolute pressures. I think the areas with negative pressure will suggest that cavitation might happen in those areas. Any ideas?

ghorrocks September 6, 2015 05:57

Incompressible flows can have negative pressure as the pressure does not contribute to the equation of state. The comments I made are referring to compressible flows.

You last comment seem to suggest you are getting negative pressures in a single phase flow, and that this suggests the flow probably cavitates. This is correct.

But then your previous comment does not make sense. If your fluid has a vapour pressure greater than 1Pa then you will not reach it so the 1Pa limit is irrelevant. If your fluid's vapour pressure is less than 1Pa then you can just assume it to be 1Pa and in most cases the assumption will have insignificant effects on the result.

ndabir September 8, 2015 19:39

Glenn here is what I have done:

- set a single phase fluid (water) to be compressible using Tait EOS.

- set a pressure boundary using udf that has the form -pb*sin(w*t) where pb=2 bar. This means while the pressure oscillates the absolute pressure will be negative at some times using P=P0 - pb*sin(w*t)=100000- 200000*sin(w*t).

This kind of simulation is performed with no problem in, for example, Fluent. And the density will not be negative due to the Tait EOS. However, I do not understand why you mention the density will be negative. Could you explain that please?

ghorrocks September 8, 2015 19:54

I was referring to simpler EOS relations such as ideal gas.

I am no expert on these types of EOS, but this link (http://www.soest.hawaii.edu/oceanogr...yhli/1967a.pdf) suggests the Tait EOS is only applicable from 1 bar pressure upwards. It appears to be an EOS to give sea water properties to very high pressures seen in the deep ocean. So using it below atmospheric pressure does not sound wise.

Applying a non-physical boundary condition (which creates negative pressures) and hoping that the solver can handle it does not sound like a wise approach to CFD to me. I would reconsider your boundary conditions and use something which is physically possible.


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