very high courant number

dsanza · August 16, 2011, 10:16

Hi everyone,

i got a newbie question. i'm trying to simulate the turbulent(k-epsilon) transient flow in water channels with the interFoam solver. after +- 0.01 seconds i get courant numbers of up to 7*e-20 delta t 6*e-35

any idea what i am doing wrong? have i chosen the wrong solver?

thnx

kwardle · August 16, 2011, 10:27

A little more info would be helpful (BCs, general setup), but chances are that your k and/or epsilon are blowing up. Do you see "bounding k" in the output? Did you initialize them as 0 or something very small (e.g. 1e-11). Small value tends to work better--though I must say I have not worked much with k/epsilon (interFoam is transient anyway so LES has virtually no added cost) but in the little I have done I have found it to be a little unstable for some reason. Have you tried adding relaxation factors for k and epsilon? This is done in system/fvSolution.

dsanza · August 17, 2011, 03:29

i have a half full water channel, there is no water comming in, just water going out with a constant velocity. i checked what you sugested, and epsilon is grows from 0.01, as initialised to 0.7 in 0.05 seconds. however this happens in the air part of the channel, not in the water.

any sugestions as how to chose the relaxation factors?

i also tried changing the geometry of the channel, if i use a rectangle as a crosssection, everything is ok, when using a circle as a crosssection i get the time problem. is it posible that my mesh is causing it?

thank you very much for your help!

August 16, 2011, 10:16	very high courant number	#1
dsanza New Member Diego Sanz Join Date: Aug 2011 Posts: 9 Rep Power: 15	Hi everyone, i got a newbie question. i'm trying to simulate the turbulent(k-epsilon) transient flow in water channels with the interFoam solver. after +- 0.01 seconds i get courant numbers of up to 7e-20 delta t 6e-35 any idea what i am doing wrong? have i chosen the wrong solver? thnx

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August 16, 2011, 10:27		#2
kwardle Senior Member Kent Wardle Join Date: Mar 2009 Location: Illinois, USA Posts: 219 Rep Power: 21	A little more info would be helpful (BCs, general setup), but chances are that your k and/or epsilon are blowing up. Do you see "bounding k" in the output? Did you initialize them as 0 or something very small (e.g. 1e-11). Small value tends to work better--though I must say I have not worked much with k/epsilon (interFoam is transient anyway so LES has virtually no added cost) but in the little I have done I have found it to be a little unstable for some reason. Have you tried adding relaxation factors for k and epsilon? This is done in system/fvSolution.

August 17, 2011, 03:29		#3
dsanza New Member Diego Sanz Join Date: Aug 2011 Posts: 9 Rep Power: 15	i have a half full water channel, there is no water comming in, just water going out with a constant velocity. i checked what you sugested, and epsilon is grows from 0.01, as initialised to 0.7 in 0.05 seconds. however this happens in the air part of the channel, not in the water. any sugestions as how to chose the relaxation factors? i also tried changing the geometry of the channel, if i use a rectangle as a crosssection, everything is ok, when using a circle as a crosssection i get the time problem. is it posible that my mesh is causing it? thank you very much for your help!