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Old   October 26, 2005, 09:55
Default negative pressure?
  #1
Aykut
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HI all; I am making a simple unsteady 2D backwardstep flow with k-epsilon model. My question is after the convergence I get unrealistic negative pressure values when I contour the pressure. Can anyone help me where should be the mistake? Thanks in advance
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Old   October 26, 2005, 12:33
Default Re: negative pressure?
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edi
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Well, could be realistic if, for example, the flow is incompressible and you're plotting the static pressure (relative) and somewhere you're getting negative values (keep in mind the operating pressure)...

Hope it can help.

Edi.
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Old   October 26, 2005, 15:27
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aykut
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hi edi, I am simulating an air flow with a velocity of 2-6 m/s. I set the Gauge pressure to zero. There is a normal turbulence flow after the step and realistic velocity values unless these negative pressures. So you tell that I dont need to change any parameters or conditions and except these realistic static pressure values? Thanks in advance
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Old   October 27, 2005, 00:33
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mAx
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Hi, If you have negative static pressure, it is normal. If you have negative absolute (or total pressure) it is physically not possible. But I got also negative absolute pressure, if I have high drop pressure: it can show presence of cavitation. But I didn't achieved avoiding those negative domains, even with cavitation's model. Max
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Old   October 27, 2005, 06:36
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aykut
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Hi Max; did u also worked with a backward facing step or with a cavity flow? Thanks
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Old   October 27, 2005, 07:20
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mAx
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Hi, I am working in hydraulic domain. I got those negative pressure,espicially after small valve opening (nozzle).

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Old   October 27, 2005, 22:28
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Rajesh
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What is your operating pressure? is it zero? Then you have to give absolute pressures ( not the gauge pressure!) in your boundary conditions. ..and you can give finer grids near the corner of the step.

Rajesh
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Old   October 28, 2005, 03:33
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aykut
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Hi Rajesh;

I kept the operation pressure value of 101325 Pa for air. Also in the BC , i dont know to change the Gauge pressure into Absolute pressure!!! Thanks in advance
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Old   October 28, 2005, 04:08
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Rajesh
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Absolute pressure is Gauge pressure + 101325 Pa. If you have considered the ideal gas law, you should set the operating pressure to mean flow pressure. but, if you have used "constant density" in your incompressible calculations you don't have to worry about the operating pressure. It will not be considered. I would then suggest you to check the grid fineness near the corner of the step

Rajesh

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Old   October 28, 2005, 06:05
Default Re: negative pressure?
  #10
aykut
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ok I figured out the problemLast question ; under the Materials and then properties section of the gas , should I take the density as constant , ideal-gas or incompressible-ideal gas? what would you suggest about that? Thanks a lot
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Old   October 28, 2005, 06:40
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Rajesh
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Constant density should be used if you do not want density to be a function of temperature. The incompressible ideal gas law should be used when pressure variations are small enough that the flow is fully incompressible, but you wish to use the ideal gas law to express the relationship between density and temperature (e.g., for a natural convection problem). I think your case is an icompressible case with non-negligible pressure gradients. I would suggets you to use the ideal gas law. Don't forget to use the correct operating pressure since the Mach Number is very low. Operating pressure could be taken approaximately equal to the mean flow pressure

all the best

Rajesh

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