|
[Sponsors] |
![]() |
![]() |
#1 |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
Good evening,
I have come across two conflicting definitions of the length scale: L = k^1.5 / e (from the CFX documentation) L = 0.09 * k^1.5 / e (from Wilcox and the wiki) where L is the length scale, k is the turbulence kinetic energy, and e is the turbulence dissipation rate (epsilon). The turbulence kinetic energy transport equation (including model coefficients) is identical between the two sources, and therefore an absorption of the constant "0.09" into epsilon seems unlikely (as has been suggested here for conflicting length scale definitions using k and omega). Wilcox explicitly states that L is the integral length scale (determined via the autocorrelation method). This would suggest that we can use experimentally-measured length scales (following Wilcox's definition above) to determine boundary conditions of k and e. This seems to conflict with the CFX definition, however, unless the CFX length scale is not equivalent to the integral length scale (having been divided by the constant 0.09). This would be equally confusing, because specifying length scale directly (in CFX) based on experimentally-measured length scales would then be incorrect. How do experimentally-measured length scales relate to the CFX length scale? Does the constant 0.09 get absorbed into the length scale or is there something else I missed? Thanks in advance for any insights! Last edited by jonasa97; January 21, 2023 at 14:43. |
|
![]() |
![]() |
![]() |
![]() |
#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,816
Rep Power: 144 ![]() ![]() ![]() ![]() |
Doesn't that just mean the Wilcox/wiki definition of length scale is different to the CFX definition of length scale by the 0.09 factor? But as for the question of how you relate an experimentally measured length scale to a turbulence model length scale - you would have to have a close read of the literature to work that out I fear.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
![]() |
![]() |
![]() |
![]() |
#3 | |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
Thank you for taking the time to engage with my question.
Quote:
|
||
![]() |
![]() |
![]() |
![]() |
#4 |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
The DES time series was unfortunately too short to determine the integral time scale accurately.
It occurred to me, however, that the integral time/length scale is not necessarily proportional to the local eddy size. One such example is a flapping shear layer, which may have a natural frequency several times lower than the passing frequency of eddies. I suppose this highlights the complexities of translating integral length scales into turbulence length scales for RANS modeling. |
|
![]() |
![]() |
![]() |
![]() |
#5 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,816
Rep Power: 144 ![]() ![]() ![]() ![]() |
Your posts show you are doing some good research into this issue and are working out some of the finer details. So you know more about this issue than I do. In fact your posts are a good summary so we can keep up with you
![]() Why can't you run the DES simulation longer to see if you can get an integral time scale?
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
![]() |
![]() |
![]() |
![]() |
#6 |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
The DES took approximately 3 months of wall time to get where it is now, so keeping it running is unfortunately not an option.
For clarification: it is technically possible to determine an integral time scale from the DES time series (i.e. auto-correlation crosses zero). The resultant timescales are, however, not well converged (I have assessed this by calculating the timescale for subsets of the DES time series). Overall, the unconverged integral time scale data is extremely noisy, and unfortunately looks nothing like the corresponding RANS time scales (entirely different distribution in space). Therefore - even if provided with converged DES integral time/length scales - I don't think that comparing these to RANS length scales would offer any useful insights. In truth, I think that this actually demonstrates that the proportionality constant in question does not exist. |
|
![]() |
![]() |
![]() |
![]() |
#7 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,816
Rep Power: 144 ![]() ![]() ![]() ![]() |
Out of interest, how long did the RANS simulation take?
How isotropic is your turbulence? Also, how long has it had to develop - is it close to the source/trigger, or has it had a while to form a energy cascade?
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
![]() |
![]() |
![]() |
![]() |
#8 | |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
The DES has 31 million cells and models the unresolved turbulence with k-omega SST. The test case ran on a server with two Xeon E5-2697 v3 CPUs (28 cores total). The DES took 3 months (~2200 hours) for approximately 5 flow-through periods (including startup transient). The RANS simulation which was used to initialize the DES (i.e. same mesh) converged in less than 60 hours.
Quote:
|
||
![]() |
![]() |
![]() |
![]() |
#9 |
Senior Member
Join Date: Jun 2009
Posts: 1,850
Rep Power: 33 ![]() |
By any chance, have you read the following paper
https://public.lanl.gov/livescu/fram...etal_FTC15.pdf Pay attention to their definition of length scale, and how it relates to your concerns. It is all about context, and interpretation.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
![]() |
![]() |
![]() |
![]() |
#10 | |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
Quote:
|
||
![]() |
![]() |
![]() |
![]() |
#11 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,816
Rep Power: 144 ![]() ![]() ![]() ![]() |
Correct me if I am wrong here, but all the definitions of length scale you have quoted so far in this thread have assumed isotropic turbulence. You have just stated your flow is strongly swirling, which means it will be strongly anisotropic. So you will have problems comparing length scale from any isotropic turbulence model (eg k-e, SST) or any length scale definition which assumes isotopy to simulations which are anisotropic (eg your DES model)
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
|
![]() |
![]() |
![]() |
![]() |
#12 | |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
Quote:
|
||
![]() |
![]() |
![]() |
![]() |
#13 |
New Member
Jonas
Join Date: Dec 2019
Posts: 15
Rep Power: 6 ![]() |
I found the answer by chance today. This paper (see Appendix A) seems to suggest that the correct relation is:
L = 0.32*k^1.5 / e Naturally, the applicability will depend on how the integral length scale is defined. |
|
![]() |
![]() |
![]() |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
DES Relative Length Scale in Fluent never positive, why? | ChezLeon | FLUENT | 1 | October 17, 2014 07:34 |
length scale option | rashmi | CFX | 2 | July 24, 2005 23:39 |
LES inlet length scale | f.neilson | Main CFD Forum | 0 | January 31, 2005 13:26 |
Length Scale vs. Hydraulic Diameter? | Will | FLUENT | 1 | April 13, 2004 06:09 |
Characteristic Length Scale | Thomas P. Abraham | Main CFD Forum | 2 | April 20, 1999 17:16 |