# Negative densities!!!

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 August 25, 2005, 15:29 Negative densities!!! #1 Nalleli Guest   Posts: n/a Hi I have an compressible flow. I have specified all my boundary conditions and thermophysical properties and there is no problem, But after 20 iterations the program stops and it tells me i have negative densities. Can anyone tell me what is happening? How can i solve it? Thanks

 August 25, 2005, 16:05 Re: Negative densities!!! #2 William Blake Guest   Posts: n/a Please use the search function of this forum.

 August 30, 2005, 05:22 Re: Negative densities!!! #3 Ahmed Guest   Posts: n/a Reduce your underrelaxation factors, especially the pressure, run 50-100 iterations and increase them etc

 August 30, 2005, 11:41 Re: Negative densities!!! #4 4xF Guest   Posts: n/a Thsi situation often happens when you have some starting value for the pressure field (typically 0.) which is very different from the solution of the compressible Navier-Stokes equations. A good trick is to underrelax the continuity equation a lot (e.g in the range of 0.01~0.025 or less) or to run the simulation using the pseudo-transient approach (which corresponds to using very low underrelaxation factors but which will be different in each cell). With this method, I was able to capture chocked flow with local Mach numbers exceeding 2.5... Hope this helps...

 August 31, 2005, 07:38 Re: Negative densities!!! #6 4xF Guest   Posts: n/a By the way, what also helps a lot is to make use of the total enthalpy (hey, that is the quantity that should be conserved in a compressible flow, so we have to solve for it) instead of the standard energy equation formulation (roughly speaking you use H=Cp*T + 1/2*p*V^2 instead of H=Cp*T) Hope this helps...

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