CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Wrong Unsteady Statistics by Fluent for rotating zone

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree4Likes
  • 1 Post By LuckyTran
  • 1 Post By LuckyTran
  • 1 Post By Светлана
  • 1 Post By Светлана

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 8, 2020, 23:34
Exclamation Wrong Unsteady Statistics by Fluent for rotating zone
  #1
New Member
 
Shakeel
Join Date: Dec 2016
Location: Sydney, Australia
Posts: 2
Rep Power: 0
Engineer2k1 is on a distinguished road
Dear All,
Good Day.
I am using LES and Fluent sliding mesh technique to rotate a small zone.
The small rotating zone (SRZ) contains a rotor while the rest of the domain is stationary. For SRZ I specify a rotational velocity, rotation axis direction (1 0 0) and use "absolute" relative to cell zone specification. To rotate the rotor with SRZ; I use "Moving Wall" with no-slip, "Relative to adjacent cell zone" and "Rotational" as motion with rotation axis direction same as SRZ (1 0 0). The velocity formulation under "General" and under "Solution Initialization">Reference Frame is "Absolute".

Everything runs fine; Except the unsteady statistics; where e.g., Mean X-Velocity is correctly time-averaged in the stationary zone but wrongly averaged (some sort of Phase-Averaging is done instead) in SRZ.

The problem is exactly like, if you plot circumferential velocity (through turbo option) in CFD-Post. However, in CFD-Post I can correctly display by simply selecting the circumferential velocity in stationary Frame available in CFD-Post. Unfortunately, no such thing as Mean Velocity (under Unsteady Statistics) in Stn Frame is available in Fluent. The problem persists even if I export Unsteady statistics to CFD-Post as CFD-Post doesn't give any Stn Frame thing apart from the regular Mean Velocity. Even exporting the Mean-Velocity at every time-step on a plane and importing and then carrying out the mean gives the same problem. Only hack is to export Instantaneous Velocity on every Time-step and then carry out averaging RMSE etc. in Matlab. But why?
Why Fluent has this bug/issue?
Is there a solution?
Please find picture attached; comparing both correct X-Velocity and it's wrong mean in the rotating zone.
Thanks n kind regards,
Shakeel
Attached Images
File Type: png Correct X-Velocity vs Wrong Mean.png (181.1 KB, 23 views)
Engineer2k1 is offline   Reply With Quote

Old   March 9, 2020, 08:23
Default
  #2
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,008
Rep Power: 60
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
The unsteady statistics are done not by absolute position but by cell in the mesh. When the SRZ moves, the cell values moves with the moving mesh so that at the end of each time-step when you calculate the statistics, each new sample is done at the new cell positions. This isn't a bug, it's how moving meshes work.


If you wanted the mesh to not move and get averages by absolute position... then don't use a rotating zone / sliding mesh. You can move the rotor still the same way without using a sliding mesh.
Engineer2k1 likes this.
LuckyTran is offline   Reply With Quote

Old   March 9, 2020, 21:28
Default
  #3
New Member
 
Shakeel
Join Date: Dec 2016
Location: Sydney, Australia
Posts: 2
Rep Power: 0
Engineer2k1 is on a distinguished road
Thanks Lucky, not just for clarifying this; but your valuable comments at CFD-online have always helped me a lot and for years.

Regarding your kind advice;

I tried rotating frame of reference for the SRZ with the same setup as sliding mesh (SM) but it gives same unsteady statistics problem as SM; worse it started eating away the vortical structures (as I ran from an earlier interpolated converged case) in the stationary wake zone region (pic attached).

I also tried rotating frame for the whole domain (SRZ + all the stationary zones) i.e., single reference frame (SRF); though just for a few time-steps as the initial results didn't look good. But now I am thinking to run it full for a couple of revolutions; and may be it converges eventually and solve the problem? If not, can you please suggest me the direction. Thanks.
Attached Images
File Type: png Wake Issue.png (172.2 KB, 14 views)

Last edited by Engineer2k1; March 9, 2020 at 21:30. Reason: Typo
Engineer2k1 is offline   Reply With Quote

Old   March 9, 2020, 21:55
Default
  #4
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,008
Rep Power: 60
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
When you use a single rotating reference frame (as opposed to multiple), Fluent does use the relative velocity formulation of the N-S equations. This should allow you to get the statistics that you want.

Off the top of my head, I can't recall how the unsteady statistics are calculated when using a SRF and whether you need to do any additional post processing or not. I think you are on the right track though. If the statistics are calculated using the relative velocities (and I think they should be) you are in good shape. If not, you might have some trouble.
Engineer2k1 likes this.
LuckyTran is offline   Reply With Quote

Old   March 10, 2020, 19:11
Default
  #5
Senior Member
 
Svetlana Tkachenko
Join Date: Oct 2013
Location: Australia, Sydney
Posts: 405
Rep Power: 12
Светлана is on a distinguished road
There is some discussion of the different models (single reference frame, multiple reference frame, sliding mesh - and the post processing) in these files:

https://www.afs.enea.it/fluent/Publi.../PDF/chp09.pdf
https://www.afs.enea.it/fluent/Publi.../PDF/chp26.pdf

Would suggest to run a simplified model with less than a million mesh points at first, as this may help to choose the appropriate model and the appropriate post-processing technique before using it for the full simulation.
Engineer2k1 likes this.
Светлана is offline   Reply With Quote

Old   March 10, 2020, 19:43
Default
  #6
Senior Member
 
Svetlana Tkachenko
Join Date: Oct 2013
Location: Australia, Sydney
Posts: 405
Rep Power: 12
Светлана is on a distinguished road
Perhaps also this one.

http://depts.washington.edu/fluidlab...la_rapport.pdf
Engineer2k1 likes this.
Светлана is offline   Reply With Quote

Reply

Tags
fluent, rmse, rotor, sliding mesh, unsteady statistics

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[Commercial meshers] Mesh conversion problem (fluent3DMeshToFoam) Aadhavan OpenFOAM Meshing & Mesh Conversion 2 March 8, 2018 01:47
polyhedral cells destroy FLUENT 0 January 18, 2018 05:14
[General] Animating unsteady data from Fluent in Paraview.. frodooon ParaView 10 May 9, 2014 10:39
[Commercial meshers] fluentMeshToFoam multidomain mesh conversion problem Attesz OpenFOAM Meshing & Mesh Conversion 12 May 2, 2013 10:52
Fluent incident radiation problem Michael Schwarz Main CFD Forum 0 October 21, 1999 05:56


All times are GMT -4. The time now is 18:14.