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

Validation of the accuracy of LES simulation

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

Like Tree1Likes
  • 1 Post By FMDenaro

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 19, 2017, 17:32
Default Validation of the accuracy of LES simulation
  #1
Member
 
Sidharath
Join Date: May 2015
Location: UK
Posts: 65
Rep Power: 9
sidharath is on a distinguished road
I am modelling the acoustic spectrum at near surge operation of a small high-speed centrifugal compressor numerically using LES.

I've previously modelled the same operation using DDES (SBES) and now trying to understand the impact of using explicit WALE LES model (CFX) on the acoustic spectrum and infer if there are any substantial changes in the spectra or hybrid RANS -LES models are good enough.

I've experimentally measured performance data but measured acoustic data isn't available at this point in time.
I was wondering if someone can share their experiences on quantifying the credibility of the LES results while the model is still running.

I've done timestep and grid independence study (a crude form in which 3 kinds for each spatial and temporal discretization were used) on DDES model but can't afford to do the same on LES.

The convergence is judged from the residual (rms) reaching upto to the fourth order of magnitude (10^-4) and courant number (rms) is around 10.

I was wondering if someone can introduce me to methods to check the credibility of the LES results may be like Reynolds stress (used for experimental or DNS results) or turbulent kinetic energy curve. Any ideas to check if the LES is working fine or the way it should be, would be a great help since grid and timepsteps are not studied
sidharath is offline   Reply With Quote

Old   November 20, 2017, 03:23
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,034
Rep Power: 64
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
I suggest to have a reading to the books of Sagaut.

Consider that in the implicit filtering formulation, there is no grid independent LES solution.
For complex geometries and flows, it is not possible to give you only one general route to assess the results. The quality of the grid is important if you want to use a wall-resolved LES. A dynamic SGS model is reccommended. If possible, energy spectra are useful to see how behaves the energy transfer.
A further check I always suggest is to perform the no-model simulation on the same grid.
FMDenaro is online now   Reply With Quote

Old   January 27, 2018, 16:45
Default
  #3
Member
 
Sidharath
Join Date: May 2015
Location: UK
Posts: 65
Rep Power: 9
sidharath is on a distinguished road
Thank you for the directions. I somehow missed the notification to this reply and saw it only today; my apologies.

I am following the books by Sagaut and also, I've been following the presentations made by you on LES.

I was wondering if you could please elaborate on the points of 'energy spectra' from the perspective of numerical work.

Also, I don't think I understand when you meant 'no-model' in your last line. No model as in no SGS at all? If yes, I am not using any SGS and relying on numerical dissipation.

Once again, thank you.
sidharath is offline   Reply With Quote

Old   January 27, 2018, 16:59
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,034
Rep Power: 64
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Energy spectra are generally used to check LES and DNS in many test cases (but I don't know if your flow problems has some direction of homogeneity) as well as RMS, correlations, etc.
They can show if you get an artificial energy pile-up at the level of the filter cut-off caused by the numerical method that does not drain efficiently kinetic energy in the inertial range. On the other hand, they can show you also the opposite, that is an excess of dissipation introduced by the numerical method (and SGS model).
In your case you are performing an ILES and the local truncation error of your discretization should mimic the effect of an SGS model.
Have you first tested your formulation in a well known test-case?
Have you used a grid that resolve the BL or you are using wall-model bc.s?
Have you averaged the solutions over a sufficient number of samples?

However, more details about your flow problems would help to understand what variables you could you to assess the quality of the simulation.
sidharath likes this.
FMDenaro is online now   Reply With Quote

Old   January 27, 2018, 17:27
Default
  #5
Member
 
Sidharath
Join Date: May 2015
Location: UK
Posts: 65
Rep Power: 9
sidharath is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Energy spectra are generally used to check LES and DNS in many test cases (but I don't know if your flow problems has some direction of homogeneity) as well as RMS, correlations, etc.
They can show if you get an artificial energy pile-up at the level of the filter cut-off caused by the numerical method that does not drain efficiently kinetic energy in the inertial range. On the other hand, they can show you also the opposite, that is an excess of dissipation introduced by the numerical method (and SGS model).
In your case you are performing an ILES and the local truncation error of your discretization should mimic the effect of an SGS model.
Have you first tested your formulation in a well known test-case?
Have you used a grid that resolve the BL or you are using wall-model bc.s?
Have you averaged the solutions over a sufficient number of samples?

However, more details about your flow problems would help to understand what variables you could you to assess the quality of the simulation.

Thank you for the information and such a prompt response.
1. I'll read up more on how to compute the energy spectra from the numerical results although I vaguely remember CFX storing it for each iteration. Based on that i can see if the decay rate is 'right' and what energy spectrum is implying.

2. Yes, I am using the implicit wall modelled les, as you rightly pointed out and relying on numerical dissipation to act as sgs.

3. Unfortunately, I haven't due to the lack of time and computational resources. Although the formulation should be credible, not so sure about my grid and timestep sizes.

4. I am positive that grid captures boundary layer as I've performed a study on first cell height, number of inflation/prism as well as the rate of growth. The y+ as well as the results from Hybrid RANS-LES simulations are good. I don't think i have any reason to beleive that the bourndary layer isn't captured.

5. I think i've run the simulation long enough, around 200 revolutions of compressor wheel. Surge cycle is around 40 revolution as per the previous results and same is seen here.

I don't see a huge difference between the LES and hybrid model's results, which is causing me to suspect the integrity of les results. It could be case that hybrid model (IDDES/SBES) is doing a good job or otherwise. I should have experimental noise spectrum by next week or so and I would be in a place to better answer my question.

One other thing is for les i am using second order scheme instead of higher order which could be a source of major errors.
sidharath is offline   Reply With Quote

Old   January 27, 2018, 17:41
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,034
Rep Power: 64
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Well, the problems I see:
1) if you can compute the 1D energy spectra alomng some spatial direction, I don't know exactly your details about geometry, inflow/outflow;
2) even if you compute the spectra, it is hard to compare it to typical spectra, for example we don't know if you can expect an inertial region in your problem.

I immagine you have some references to use as comparison. What variables are controlled?
What about the h+ value where the filter acts?
FMDenaro is online now   Reply With Quote

Old   January 27, 2018, 18:11
Default
  #7
Member
 
Sidharath
Join Date: May 2015
Location: UK
Posts: 65
Rep Power: 9
sidharath is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Well, the problems I see:
1) if you can compute the 1D energy spectra alomng some spatial direction, I don't know exactly your details about geometry, inflow/outflow;
2) even if you compute the spectra, it is hard to compare it to typical spectra, for example we don't know if you can expect an inertial region in your problem.

I immagine you have some references to use as comparison. What variables are controlled?
What about the h+ value where the filter acts?
1. I've attached a plot showing the geometry of the compressor being modelled.

2. I do have some work published by KTH wherein they've modelled a similar compressor using LES. The energy spectrum is also published and I can see if the spectra computed in my case is any similar.

3. I am sorry, I am not sure what you mean by 'contrrled varriables'. I am using pressure inlet and mass flow outlet as boundary conditions.

4. I don't think i ever looked at roughness number values till now. Are they of importance in les or current scenario? I can find out the h+ values on monday and let you know.
Attached Images
File Type: jpg M9_full_1.jpg (25.5 KB, 14 views)

Last edited by sidharath; January 27, 2018 at 18:21. Reason: image attach
sidharath is offline   Reply With Quote

Old   January 27, 2018, 18:54
Default
  #8
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,034
Rep Power: 64
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
I have a doubt: you wrote are using a second order discretization, does that mean a central or upwind discretization? I wonder what is the basis of your ILES. The formulation is based on FV or FD method?
FMDenaro is online now   Reply With Quote

Reply

Tags
les model, surge in compressor

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
Validation: Turbulent Channel Flow, pisoFoam, LES Xiaoyu Yang OpenFOAM Running, Solving & CFD 23 December 19, 2019 11:51
Validation of LES results smog OpenFOAM Running, Solving & CFD 0 January 8, 2016 08:52
Divergence with Simulation using Embedded LES CarlosGRR FLUENT 3 October 2, 2014 17:52
[OpenFOAM 1.6] How to run a simulation based on either DES or LES? imranazizahmed OpenFOAM Running, Solving & CFD 1 January 29, 2013 01:27
Transient statistics for LES simulation jonathan CFX 0 March 5, 2008 15:51


All times are GMT -4. The time now is 06:25.