Timescale convergence?!

Schwimmy · April 15, 2016, 08:45

Hi guys,

I'm running a NACA0015 profile at low reynolds numbers (10000-1000000) with steady sst including gamma-theta transition model. I varied my timescale to become a converged (steady state) solution. But I found out, that in some cases both decreasing and increasing the timescale from my converged timestep gives me an oscillating (transient) result.

My question is, does the "good" timescale maybe damp transient effects so the solution can convergate?

Cheers

turbo · April 15, 2016, 14:57

at what AOA?

Schwimmy · April 16, 2016, 05:44

AoA=0, critical Re from 10000 til 100000. above 100000 it's ok.

ghorrocks · April 16, 2016, 07:42

If the flow is not in the gross separated flow regime (which I suppose it is not at AOA=0), then this is probably a laminar separation bubble. They tend to be unstable and jiggle about quite a bit. If the bubble is small it probably does not affect things much but at low-moderate Re (50k-200k for many airfoils) the separation bubble can be large. It might be important in this Re range.

April 15, 2016, 08:45	Timescale convergence?!	#1
Schwimmy New Member Tim Join Date: May 2014 Posts: 8 Rep Power: 12	Hi guys, I'm running a NACA0015 profile at low reynolds numbers (10000-1000000) with steady sst including gamma-theta transition model. I varied my timescale to become a converged (steady state) solution. But I found out, that in some cases both decreasing and increasing the timescale from my converged timestep gives me an oscillating (transient) result. My question is, does the "good" timescale maybe damp transient effects so the solution can convergate? Cheers

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April 15, 2016, 14:57		#2
turbo Senior Member Join Date: Jun 2009 Posts: 174 Rep Power: 17	at what AOA?

April 16, 2016, 05:44		#3
Schwimmy New Member Tim Join Date: May 2014 Posts: 8 Rep Power: 12	AoA=0, critical Re from 10000 til 100000. above 100000 it's ok.

April 16, 2016, 07:42		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	If the flow is not in the gross separated flow regime (which I suppose it is not at AOA=0), then this is probably a laminar separation bubble. They tend to be unstable and jiggle about quite a bit. If the bubble is small it probably does not affect things much but at low-moderate Re (50k-200k for many airfoils) the separation bubble can be large. It might be important in this Re range.