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nino July 26, 2007 07:25

negative static pressure at inlet
 
hi, I've got a simple geometry: 2 pipes are connected on the same axis. the first one has 32mm the second one has 40mm. Massflow is about 1900L/min. (incompressible/ steady state / hydraulic oil) When my solution converged, I compute the drop pressure as the the total pressure difference between inlet (32) and outlet (40). drop pressure is ok with theoretical solution. BUT when I take a look at he static pressure distribution, the static pressure at the inlet is negative. (and positive at the outlet)

I cannot understand this phenomenon (and I don t trust this solution). Anyone could me explain that?


Joe July 26, 2007 08:06

Re: negative static pressure at inlet
 
There is nothing wrong, in principle, with a negative static pressure at a boundary. Your absolute pressure is probably still positive. If it isnt you may have cavitation.

bashu July 26, 2007 11:01

Re: negative static pressure at inlet
 
If you don't like it that way you can change the pressure reference point in define --> Opertaing Conditions panel. But the solution is correct the way you have it too.

mAx July 26, 2007 12:07

Re: negative static pressure at inlet
 
thanks for your replies. Unfortunately, the operating pressure condition doesn t have any influence on the solution

bashu July 26, 2007 13:12

Re: negative static pressure at inlet
 
That's exactly what should happen if the density is constant (not a function of pressure at least) , right?

Razvan July 27, 2007 02:31

Re: negative static pressure at inlet
 
I cannot believe how far from the subject we can go sometimes, trying to post a decent reply, but actually confusing the poor user who put the question even more.

People, NINO asked a simple question, that has a simple answer too: the static pressure at inlet is negative because of three reasons:

- first, all pressures are calculated as differences between the operating pressure and the absolute pressure;

- second, the diameter variation between inlet and outlet is small, so the total pressure drop is small, maybe even less then 100Pa;

- third, the inlet diameter is smaller then the outlet's, so the velocity of the fluid is higher at the inlet, consequently the static pressure (in the case of non-viscous flow, at least) is smaller.

All these effects superimposed could easily generate this situation, especially if the total pressure drop is small.

Am I wright NINO?

All the best,

Razvan

nino July 27, 2007 03:03

Re: negative static pressure at inlet
 
hello, the drop pressure is small (about 1.7 bar by 1900L/min, which means a inlet velocity about 38.7m/s and of course a smaller outlet velocity) I just had problem for understanding the negative static pressure at inlet. The compression along the pipe (with elevation of diameter up to 40) is for me ok (on the understanding side) I was expecting a small static pressure at inlet, BUT positive.

I have at the inlet (static pressure: -1.9 bar and dynamic pressure: +6.3 bar (ok with 0.5 * rho *U**2)

At the outlet: (static: 0 bar and dynamic: +2.7 bar (also ok)

so drop pressure is: (6.3-1.9) - 2.7 = 1.7bar

But I think it's just a question of operating system. I shoudn't check the static, but the absolute pressure

bashu July 27, 2007 08:22

Re: negative static pressure at inlet
 
Sorry for the not well thought through answer.


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