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Ideal gas density solver

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Old   January 7, 2021, 16:18
Default Ideal gas density solver
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Antonio Camberos
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Gods of fluent i humbly come to you seeking for an answer to my query.
Im simulating a compressible fluid inside a piston. I read somewhere that the best solver for compressible simulations is the density based solver. But after running the simulation i do not see any changes in density nor temperature but there IS a change in pressure. Furthermore i ran the simulation with the pressure based solver and it does show changes in density and temperature. What am i missing??. Should there be a difference in the results whe using density based solver? Also the contours results for the density based solver look weird.



The setup of the simulation is the same for both simulations being the solver the only thing that changes. The setup is as follows:
  • Rectangular domain of 30 mm x 27.5 mm
  • Quad elements for domain
  • Transient simulation
  • Energy ON
  • Viscous model SST Omega
  • Fluid material set to helium using ideal gas density
  • Dynamic mesh for horizontal top edge moving downwards
  • Time step of 0.001 s (also used 0.00001 but the results did not changed)
  • Initial temperature of 300 K
  • Pressure patched to 303975 Pa
  • Total time 1.5 s.
  • Rest of setup is as default.
I attach images for the mesh, the end result for temperature and density for both solvers.


Thnaks in advance to anyone willing to help a humble peasant.
Attached Images
File Type: jpg densitysolv_density.JPG (67.3 KB, 10 views)
File Type: jpg densitysolv_temperature.JPG (72.6 KB, 8 views)
File Type: jpg domain.JPG (136.6 KB, 8 views)
File Type: jpg pressuresolv_density.jpg (40.1 KB, 8 views)
File Type: jpg pressuresolv_temperature.jpg (41.6 KB, 5 views)
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Old   January 7, 2021, 18:31
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Lorenzo Galieti
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What is weird is that if your pressure changes but your density is fixed, there has to be a change in temperature, no matter what.

Yes i expect changes of solution when using different solvers, but not to the point where the density solver outputs unphysical results

Tbh, unless at very high mach number, the pressure solver is perfectly fine. If it gives you reasonable results just stick with the pressure solver and go ahead with your project. Then at one point if you feel like it, come back and try again with density one
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Old   January 8, 2021, 11:39
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Antonio Camberos
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Thank you so much for your quick response god of Fluent. I will use the pressure solver to finish my project.

Allow me to offer this pile of berries as a tribute: oo88o
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Old   January 8, 2021, 13:50
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Dear worshiper, I suggest that you anyways keep in check the mach number during the simulation, by setting some report definitions. E.g monitor mean and max mach number in the cylinder.

So if somebody asks you about this, you show them plots
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Old   January 9, 2021, 11:59
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Quote:
Originally Posted by LoGaL View Post
Dear worshiper, I suggest that you anyways keep in check the mach number during the simulation, by setting some report definitions. E.g monitor mean and max mach number in the cylinder.

So if somebody asks you about this, you show them plots

Will do, Thanks!
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compression, density based solver, ideal gas, piston, pressure based solver

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