Transcient & Relaxation factor (negative densities)

sheintz · April 19, 2010, 08:17

Hello,
I'm trying to simulation a injection spray of diesel in a chamber, also I have a transcient time domain, lagrangian multi-phase model (Reitz breakup and Huh atomization).
I have a problem of "negative densities", and I would like to solve this problem with a change of the relaxation factors of the pressure, of the density (and maybe of the temperature). But I can change the value of the relation factor (in Analysis Controls>Solution Controls>Equation Behavior>Primary Values, in the Solver Parameters Panel),why ? (the values are in grey).
Why can not I change this relaxation factors ?
Some help would be greatly appreciated.
Kind regards
Simon

sdwijeya · April 29, 2010, 10:04

When you are using the PISO algorithm, StarCD doesnot let you change these parameters. If you are getting Negative densities, that means your time step is too large and the pressure correction equation is outputting negative values for densities. The time step you need to give can be calculated as foolows:
1. Identify the region which has relatively higer velocities and the smallest dimension.
2. You should be able to resolve a turbulent eddy in that region which is as big as your smallest dimension
3. Using this dimension and velocity in that region, calculate a angular velocity and hence a frequency
4. Get your timestep such that you will resolve this frequency in 100 time steps.

Sameera

April 19, 2010, 08:17	Transcient & Relaxation factor (negative densities)	#1
sheintz Member Heintz Simon Join Date: Sep 2009 Location: Germany Posts: 36 Rep Power: 16	Hello, I'm trying to simulation a injection spray of diesel in a chamber, also I have a transcient time domain, lagrangian multi-phase model (Reitz breakup and Huh atomization). I have a problem of "negative densities", and I would like to solve this problem with a change of the relaxation factors of the pressure, of the density (and maybe of the temperature). But I can change the value of the relation factor (in Analysis Controls>Solution Controls>Equation Behavior>Primary Values, in the Solver Parameters Panel),why ? (the values are in grey). Why can not I change this relaxation factors ? Some help would be greatly appreciated. Kind regards Simon

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April 29, 2010, 10:04		#2
sdwijeya New Member Sameera Wijeyakulasuriya Join Date: Aug 2009 Posts: 12 Rep Power: 16	When you are using the PISO algorithm, StarCD doesnot let you change these parameters. If you are getting Negative densities, that means your time step is too large and the pressure correction equation is outputting negative values for densities. The time step you need to give can be calculated as foolows: 1. Identify the region which has relatively higer velocities and the smallest dimension. 2. You should be able to resolve a turbulent eddy in that region which is as big as your smallest dimension 3. Using this dimension and velocity in that region, calculate a angular velocity and hence a frequency 4. Get your timestep such that you will resolve this frequency in 100 time steps. Sameera