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Mac January 17, 2008 08:10

negative pressures
 
I am using STARCD to simulate the oil flow in a domain that is rotating at 4000 rpm. I see negative pressures only at higher rotational speeds, which I would like to get rid of...how do i do this? Any comments will be appreciated. Thanks.

sandro January 19, 2008 13:18

Re: negative pressures
 
Could it be a postprocessing issue? do you use absolute or relative pressure? in computational or static frame?

James January 19, 2008 15:14

Re: negative pressures
 
Could it really be wanting to cavitate ?

Mac January 20, 2008 23:17

Re: negative pressures
 
Thanks....I don't think cavitation is applicable since the fluid is oil. I am talking about negative absolute pressures in the domain. What may be the reasons for the same??

James January 21, 2008 08:43

Re: negative pressures
 
It may not be called cavitation directly but if the static pressure drops below the saturation vapor pressure of the fluid it will boil. For example if you drop 3000 psi from a hydraulic system though a valve can you can get some pretty low pressures in the high speed zone.

Do you have a very high pressure drop/gradient. In which case it may just be an overshoot, does it vary with differencing scheme or grid density. How big is the negative pressure with relation to the overall pressure range in the the simulation? Are you sure all boundary conditions are correct and that you have all the physics/geometry in the simulation that could stop this happening 'in reality'?

Pauli January 21, 2008 16:44

Re: negative pressures
 
Sounds like you are trying to calculate pressure rise in a crankshaft cross-drill - or something similar. My guess is you have a mass flow boundary condition. If so, you found the speed at which the device can pump more mass than you are specifying. Above this point, the passage will not stay full of liguid oil. Maybe it's time to ask how the system will respond.

Cavitation does occur in lubricating oil - c.f. journal bearings. In the Reynolds equation analysis typically used for journal bearings, the absolute pressure is truncated at either vapor pressure or 0. Typically this happens in the diverging section. Otherwise for moderate to highly loaded bearings you will predict large negative absolute pressures. In an operating bearing, the large negative pressure is precluded by air pulled in through the bearing ends (small L/d) and oil vaporization (i.e. cavitation) for higher L/d.

Mac January 21, 2008 23:09

Re: negative pressures
 
Thats correct Pauli..at some point in time we see that the device pumping more oil than actually entering, in which case the domain is not completely filled with oil. I am trying to use VOF simulation, but still see the negative pressures. Is there a solution to this ??

Pauli January 22, 2008 14:00

Re: negative pressures
 
Just adding VOF should not make the negative pressure go away. You need a mechanism to generate the vapor phase. Have you activated the cavitation model? That should do the trick. Of course you then have to decide upon a bunch of properties. :(

Bfore proceeding, I would stop & ask a couple of questions. What happens in the real world? Does my model & boundary conditions represent the real world?


Mac January 22, 2008 23:32

Re: negative pressures
 
Thanks Pauli...I did try out with cavitation earlier and the negative pressures disappeared. But my concern is whether the real world situation is being simulated or not. Need to give a bit of thought into this !!

Pauli January 23, 2008 00:45

Re: negative pressures
 
If this device is part of a lubrication system, I'd expect your real world mass flow rate will increase. I say this because lube system flow is usually metered by an "orifice" at the end of the pipe. (I put that it quotes because the flow metering device might actually be say a fluid film bearing.) So adding a pump in the passage should increase mass flow. If the metering device is on the inlet, the passage will likely empty itself to the point where their will be insufficient oil to generate a low pressure.



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