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How do i run Compressible Simulation?

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Old   November 23, 2007, 22:46
Default How do i run Compressible Simulation?
  #1
Jerker
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Hi,

Im running a simulation on air that is converging in a channel, reaching velocities of 1000m/s. The channel exits into a big domain where i have used opening outlet.

According to the CFX modelling manual activating Total Energy will take into account the compressible effects. However running a simulation with and without Total Energy shows no difference in temperature, velocity etc...

Inlet Conditions: Subsonic velocity 50m/s, Air at 25C, 10% Turbulence intensity.

When i try to use Ideal Gas instead of Air at 25C as in the supersonic tutorial the solver crashes.

Any ideas and hints are welcome.

Thanks

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Old   November 24, 2007, 16:37
Default Re: How do i run Compressible Simulation?
  #2
Mohammad Faridul Alam
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You have to choose right fluid. Air at 25C or Ideal Gas is just compatible to the incompressible flow. Coz, the density set for these fluids is constant.

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Old   November 25, 2007, 03:11
Default Re: How do i run Compressible Simulation?
  #3
Omer
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First find out why your solution is crashing. It could be a limited amount of memory on your PC that could be doing that.

When you say crashing, what doesn't CFX solver say about it? It usually tells you why it has stopped a particular solution.

Ideal gas, is compatible for compressible flow, I don't know why Faridul would think it is not. What it does is it activates the ideal gas equation to solve for density and tempature variations.

There are quite a few reasons your problem can crash , try to learn it yourself. These broad questions with no specifications can get you no real help.

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Old   November 25, 2007, 17:51
Default Re: How do i run Compressible Simulation?
  #4
Glenn Horrocks
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Hi,

The "Air at 25C" does not invoke the ideal gas law and therefore is an incompressible fluid. The "Air Ideal Gas" fluid does invoke the ideal gas law and is the one you need to use.

If it is not converging then that is because you have not set the compressible simulation up correctly. Work through the tips for convergence in the CFX documentation.

Glenn Horrocks
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Old   November 25, 2007, 23:22
Default Re: How do i run Compressible Simulation?
  #5
Mohammad Faridul Alam
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Thanks Omar. I am sorry for my mistake abt Ideal Gas.
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Old   November 26, 2007, 13:09
Default Re: How do i run Compressible Simulation?
  #6
Jerker
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I found out that i had to set a pressure as well with the Ideal gas to be able to perform a compressible simulation. However when i try to run the simulations now i get these messages after every iteration.

Notice: The maximum Mach number is 3.501E+02. Notice: The maximum Mach number is 7.988E+01.

And after some hundred iteration i get this error message. Does anyone have an idea why this happens?

ERROR #004100018 has occurred in subroutine FINMES. | Message: Fatal overflow in linear solver.
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Old   November 26, 2007, 17:28
Default Re: How do i run Compressible Simulation?
  #7
Glenn Horrocks
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Hi,

"Had to set a pressure as well" - of course. The ideal gas law is density=pressure*R*Temperature, where pressure and temperature are absolute quantities.

It sounds like you have a fundamental error in your physics for this simulation. Check the pressure, density and temperatures at the inlet are correct.

Glenn Horrocks
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