I have done some simulation about the gas flow, the inlet BC is 100000Pa, outlet is 50 Pa. When the results are convergent, I check the contour of static pressure and find it has negative value. And the operating pressure is 0 Pa。 I am confused.
Is there anyone can help me? Thank you!
Geometry of the system and other characteristics of the model will help to understand the problem. Check the momentum, velocities, density and temperature profiles.
Dear Pingping Gan
According to my knowledge, It should be relative pressure that shows the difference with your operating pressure.
Good Luck
Thank you all! The attached file gives the structure. The inlet and outlet BC are pressure in and out. The model calculates the gas ionizations.
A -ve gauge pressure means that your pressure is below the operating pressure, hence contour in your simulation shows -ve.
Fluent calculates & shows all pressures as relative pressure from operating pressure. If u r keeping Operating Pressure = 0, this means that gauge pressure = absolute pressure, your pressures are now expressed in absolute.
Check the velocity at negative presssure location that with the high temperature will explain it
If pressures shown by Fuent are relative pressures, it is possible that negative pressures appear in the system. Is it physically impossible? Maybe the static pressure has changed to dynamic pressure (high velocities), being the total pressure similar. In any case, energy, momentum and mass have to obey the conservative equations...
If you are running an incompressible simulation and have set the operating pressure to 0 Pa (to cut down on rounding errors) or tied one boundary to a low pressure (50 Pa in this case), then it is entirely possible that you will have regions of negative pressure in your domain. This is because the pressure gradient enters the Navier-Stokes equations and so it is pressure differences that drive the flow. So, in regions of separated flow, the low pressure inside that region will be relative to the lowest fixed pressure in your system and may well go negative.
Of course, you'll get people on here saying that (absolute) pressure can't be negative and they'd be right, but for an incompressible case (constant rho), you're dealing with gauge pressures and not absolute ones. If, however, you were to model an ideal gas, then your operating pressure might need to be fixed at, for example, atmospheric because now a negative absolute pressure would result in negative densities and so would make things difficult to say the least.
I am paying attention to the large variation of the pressure from 1e5 to -4e4 Pa, particularly the sharp drop from 8e4 to -4e4. Even an expansion fan may have difficulty achieving this. Is this pressure variation consistent with what you expect for this problem. I am asking because if the answer is profoundly no, then there may be misuse of pressure because of the way ANSYS FLUENT implements it.
Also, the temperatures are gigantic (35000 K)! This is 10 times a hot oxy-fired application.
I believe that anytime absolute pressure becomes negative in an in-compressible flow calculation (assume that the operating pressure is default i.e. 101325 ) then it signifies that cavitation is occurring in the flow domain. Since pressure-kinetic energy trade-off is governed by Bernoulli equation there is nothing in the incompressible flow formulation that limits the negative absolute pressure. In reality the flow regulates itself because cavitation will occur and compressibility will enter the physics to limit pressure from becoming more negative As observed by Hargreaves incompressible flow is driven by only pressure differences or gravity. One nominal way to avoid such negative pressure is to set the OP very high. But the solution will not be valid (though numerically it is) because cavitation is being ignored
I have one question. I am modeling cavitation using fluent. I have written UDF mass transfer from water to vapor. both being incompressible. when I am not allowing mass transfer (cavitation model is turned off) I get sinusoidal pressure profile where pressure drops below vapor pressure and then rise again. theoretically thats what I supposed to get. then turned on mass transfer so that below vapor pressure bubble forms and then when pressure increases bubble need to be collapsed. but as soon as bubble starts to grow my pressure starts to decrease and go to negative absolute pressure though I am getting converged solution. since my pressure is not increasing I don't get bubble collapse rather bubble elongate to low pressure side. my question is how can I model my case to allow pressure to follow sinusoidal profile so that after bubble grow I can also have bubble collapse? after seeing this discussion I am trying compressible EOS with energy equation on. but now I am getting diverging solution as soon as pressure reach vapor pressure.
Dear researcher, hello
To use the Ideal Gas law correctly, you must employ absolute units of temperature. Degrees Celsius and Fahrenheit won’t work because they can go to negative numbers. Negative temperatures in the Ideal Gas law give you negative pressure or volume, which can’t exist. Instead, use the Kelvin scale, which starts at absolute zero. If you work with English units and want a Fahrenheit-related scale, use the Rankine scale, which also starts at absolute.
source: https://sciencing.com/what-is-the-ideal-gas-law-13710230.html
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