July 10, 2023 at 8:46 amamkumSubscriber
In a few multiphase simulations, the absolute pressure falls below zero and goes negative near the throat.
I am using the Zwart-Gerber-Belamri cavitation model. Fluid is nitrogen.
The reference pressure is 0 Pa.
I also checked the pressure limiter and set the minimum absolute pressure to 1000Pa. No change was observed even after this.
Thanks in advance!
July 10, 2023 at 9:06 amRobAnsys Employee
Turn off node values to check it's a true result and not due to contour smoothing. The multiphase models (including cavitation) don't always use the pressure limiter so if convergence isn't great a nonphysical pressure can be calculated.
July 10, 2023 at 9:18 am
July 10, 2023 at 9:26 amRobAnsys Employee
What gas density did you use?
July 10, 2023 at 9:39 am
July 10, 2023 at 10:14 amRobAnsys Employee
And both give a negative pressure? If you take the gas density what pressure would that be at?
July 10, 2023 at 11:10 amamkumSubscriber
Both gives the negative pressure.
The constant fluid properties of Nitrogen was imported from the Fluent Database. I am using the default values in case of constant fluid properties case. (Ref Tem 298.15K).
However, changing the evaporation coefficient for Cavitation model changes it. If I change the evaporation coefficient from 0.25 to 0.7, the Absolute pressure value changes to positive. However, the mass flow reduces from the 0.22kg/s to 0.16kg/s, which is not desirable. The experimental mass flow rate is 0.28kg/s. Therefore, the mass flow error increases if I use evaporation coefficient of 0.7.
If I reduce the evaporation coefficient from 0.25 to lower values, the absolute pressure near the throat becomes more and more negative.
July 10, 2023 at 1:50 pmRobAnsys Employee
The default is 50, why did you drop it that far? https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_th/flu_th_sec_multiphase_cavitation.html They're factors so may need some tuning.
July 11, 2023 at 6:11 amamkumSubscriber
Thanks for reply.
Yes! The default value of Fevp is 50. However, This needs to be reduced to have a better match between experimental data ( mass flow , pressure profile, and vapor fraction).
Incorrect values of evaporation and condensation coefficients also resulted in a divereged solution.
July 11, 2023 at 7:57 amRobAnsys Employee
OK, another possible is the rate of phase change for the cavitation. They're generally a pain to converge well, so also look at time step. If the pressure goes negative it means you should have either lower density gas (it expands) or more gas: if the mass exchange is unstable you may need to switch to transient. By unstable I mean bubbles form & collapse quickly: the bulk mass fraction may remain relatively unchanged.
July 11, 2023 at 8:20 amamkumSubscriber
Thanks for the reply!
I will set up the transient run and keep you posted.
Thanks a lot for your suggestions!
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