CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

cyclone separator particle tracking prob..

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 20, 2013, 07:47
Default
  #21
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
If the Smagorinsky SGS model is used in LES with a mesh which is grossly too large then the SGS viscosity goes very high (the SGS viscosity is proportional to mesh length scale squared see eqn 2-161 in the theory manual) and is therefore highly dissipative.

This means fast, easy convergence to a solution which is completely wrong.

So Sakura's comment that RSM is harder to converge and slower than LES is further evidence that the mesh size he is using is far too large for LES.
ghorrocks is offline   Reply With Quote

Old   April 20, 2013, 08:10
Default
  #22
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
Dear Glenn and OJ,
Really really thankful for helping.
I got what you meant. Yes, I can understand there is a problem with my mesh.
If RSM takes more time than LES is impossible, because it is two times slower than LES now.
I'll check mesh again.
sakurabogoda is offline   Reply With Quote

Old   April 20, 2013, 08:16
Default
  #23
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
Hi Glenn,
Of course I am using Smagorinsky LES.

If that so, the problem definitely with the mesh. Thank you, thank you very much for help me to find the wrong point, as I have very limited time to complete this.
sakurabogoda is offline   Reply With Quote

Old   May 1, 2013, 08:23
Default
  #24
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
Hi All,
I have done mesh sensitivity analysis for 5 mesh types and considered a significant point rms velocity.
M1(elements:2500000): 6.474m/s
M2(elements:3250000): 6.501m/s
M3(elements:5300000): 6.511m/s
M4(elements:6450000): 6.443m/s
M5(elements:9700000): 6.489m/s
Although there is no much variation, I used M5 for RSM as M1 is the mesh size that gave the wrong results.

Meanwhile I ran LES with mesh size M5, it runs with smaller time step than RSM, but still I can see it runs faster than RMS. (no. of time steps per a given time). Can you please tell me is this ok?

I am using adaptive time steps now and can also see LES reaches convergence quicker than RSM.
sakurabogoda is offline   Reply With Quote

Old   May 1, 2013, 12:03
Default
  #25
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 475
Rep Power: 11
oj.bulmer will become famous soon enough
As Glenn suggests, the faster convergence is probably attributed to the numerical dissipation. LES can't be trusted unless you use adequate grid size and timescales, and the "mesh independence study" you are doing doesn't seem to be an appropriate way to conclude, without taking into account if adequate lengthscales are resolved.

You should resolve the scales only to the extent that below it, the turbulence is isotropic, which is handled well by the SGS models. Often this lengthscale is considered to be close to Taylor's lengthscale. The accuracy lost in modelling the anisotropic turbulence by SGS models, when you use larger than appropriate grid size, will be decided by your physics.

Situation is aggravated when you have wall bounded flows, where the grid size requirements are still critical!

OJ
oj.bulmer is offline   Reply With Quote

Old   May 1, 2013, 20:47
Default
  #26
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
Dear OJ,
Thanks a lot.
I can see the mesh is very fine, but as you said, still it is not appropriate for LES. Then, how can I check if adequate lengthscales are resolved?
The other problem is, by this way, are results from RSM also incorrect? It takes more computational cost, like to run 0.5S, uses 5days.
sakurabogoda is offline   Reply With Quote

Old   May 1, 2013, 20:59
Default
  #27
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The key check for an LES model that your mesh and timescales are adequately resolved is to plot the turbulence power spectrum (intensity versus frequency, eg http://theeternaluniverse.blogspot.c...ber-space.html). If you get the -5/3 decay in the spectrum down to your filtering size then you know you are on the right track.
ghorrocks is offline   Reply With Quote

Old   May 2, 2013, 00:34
Default
  #28
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
Dear Glenn,
Thanks a lot.
But again I have a problem. Is there anyway to monitor Turbulence intensity(I) in CFX Pre?
I=u'/U
u'=(2k/3)^0.5 ----- k-kinetic energy relates to rms velocities
So how can I monitor rms velocities? I have already used them in trn statistics.
I can calculate U(mean velocity) by monitoring u,v,w velocity components.

Thanks.
sakurabogoda is offline   Reply With Quote

Old   May 2, 2013, 00:41
Default
  #29
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
You have to define all these variables as CEL expressions. The tricky one is the u' terms - are you doing spatial or temporal averaging to obtain them? Neither of these are easy to implement in CEL.
ghorrocks is offline   Reply With Quote

Old   May 2, 2013, 02:18
Default
  #30
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
Dear Glenn,
I am thinking about temporal averaging velocities.

Another issue is when use expressions in monitor tab, I cannot specify a location.

I heard TI can be calculated by FFT.
sakurabogoda is offline   Reply With Quote

Old   May 2, 2013, 02:23
Default
  #31
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
CEL cannot calculate anything in time, so you are either going to have to do this in post processing or write your own user fortran to do it (and that will not be easy).

When I did this previously in CFX I set monitor points at the locations I wanted the turbulence intensity at and wrote the UVW velocity components to monitor points. Then you can externally process the monitor point data to get your u'v'w' parameters. Then you do FFT on the u'v'w' to get the turbulence intensity spectrum.
ghorrocks is offline   Reply With Quote

Old   May 2, 2013, 11:55
Default
  #32
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 475
Rep Power: 11
oj.bulmer will become famous soon enough
Quote:
The other problem is, by this way, are results from RSM also incorrect?
I think RSM is tuned to handle anisotropic turbulence far better than SGS models. So if your grid is not small enough to capture the anisotropy-range lengthscales, they are passed on to SGS models and I am not sure if these are robust enough to deal with it, as they are tuned for isotropic turbulence.

OJ
oj.bulmer is offline   Reply With Quote

Old   May 2, 2013, 18:48
Default
  #33
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,824
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The SGS models in LES are generally based on the assumption (which is generally a good assumption) that all the anisotropy in turbulence resides in the largest turbulence scales and the small scale turbuelnce is isotropic. In LES the large scale turbulence is modelled by the solver and the small scale stuff is modelled by the SGS, so an isotropic SGS is adequate for LES.

But for this assumption to be correct you need to show that your mesh and filtering is in fact correctly set up for this distinction.
ghorrocks is offline   Reply With Quote

Old   May 29, 2013, 06:36
Default
  #34
Senior Member
 
S.Bogoda
Join Date: Jul 2012
Posts: 133
Rep Power: 5
sakurabogoda is on a distinguished road
I have data of u,v,w and can do the fft. but my problem is how to get the frequency?

in matlab we can write write code,
N=--; %no. of data points (2^n value)
T=--; %sample length
fc=(0:N)/T; %sample frequency

but my problem is I have omitted first half of data, so sampling time does not start from 0S, it has a value (t).

then, fc should be,
fc=(t:N)/T; ????? Am I correct?

Thanks.
sakurabogoda is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Ubuntu 12.10 + openfoam2.2.0 ==> paraview error message peteryuan OpenFOAM Installation 6 August 18, 2013 18:00
Particle tracking prob, urgent. sakurabogoda CFX 1 March 11, 2013 22:11
Prob in Particle Tracking sakurabogoda CFX 7 March 5, 2013 20:01
cyclone LES simulation prob: ERROR # 004100018 sakurabogoda CFX 15 November 6, 2012 10:07
Particle tracking in cyclone shhe FLUENT 3 April 8, 2005 14:00


All times are GMT -4. The time now is 20:41.