RMS Fluctuation

Cafard · July 21, 2014, 05:03

Hi all,
when I caculate my turbine model I found that when the RMS Accumulated Time Step bigger then 400(sometimes 500),fluctuation will happen progressively.
I tried to solve this question,then I found some setting may improve this phenomenon:

1. increase mesh quality
I used turbogrid to mesh my turbine blade,but the Max.volume ratio is 7.7,
(in turbomachine the value seems to be acceptable ?? )
and I thought the mesh size is small enought.

2. use upwind scheme

3. use mass outlet boundary

I have done all these methods，but the flictuation don't reduce.
May someone give me some recommends,thx a lot.

ghorrocks · July 21, 2014, 05:58

This FAQ gives some tips: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

Cafard · July 21, 2014, 23:01

I have read the FAQ,but I don't find something about residual fluctation??

ghorrocks · July 21, 2014, 23:05

Try a larger physical time step. You want a physical time step large than the oscillation period.

July 21, 2014, 05:03	RMS Fluctuation	#1
Cafard New Member Cafard Join Date: Jun 2014 Posts: 29 Rep Power: 11	Hi all, when I caculate my turbine model I found that when the RMS Accumulated Time Step bigger then 400(sometimes 500),fluctuation will happen progressively. I tried to solve this question,then I found some setting may improve this phenomenon: 1. increase mesh quality I used turbogrid to mesh my turbine blade,but the Max.volume ratio is 7.7, (in turbomachine the value seems to be acceptable ?? ) and I thought the mesh size is small enought. 2. use upwind scheme 3. use mass outlet boundary I have done all these methods，but the flictuation don't reduce. May someone give me some recommends,thx a lot.

July 21, 2014, 23:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Try a larger physical time step. You want a physical time step large than the oscillation period. Cafard likes this.

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July 21, 2014, 05:58		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	This FAQ gives some tips: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

July 21, 2014, 23:01		#3
Cafard New Member Cafard Join Date: Jun 2014 Posts: 29 Rep Power: 11	I have read the FAQ,but I don't find something about residual fluctation??