Eddy viscosity ratio and LES
Hi,
I've read a couple of papers where people have computed the eddyviscosity ratio and used it to motivate the use of LES in their lowRe models. For example, if the eddyviscosity ratio becomes 0.30, some authors claim that the subgrid scale dissipates 30% of the energy of the flow, which justifies their use of LES compared to a laminar solution. Is this really true? And what happens when the eddy viscosity ratio becomes more than 1? Does that mean that the subgrid scale dissipate more energy than there is in the flow? edit: on second thought, energy is also created in the subgrid scale. Is that why the ratio can become more than 1? 
The sub grid model is not limited (in my knowledge anyway) to a ratio of 0.3. These low ratioes of dissipation simply mean the turbulence dissipation is small relative to the viscous dissipation.
I think a better way of validating an LES model is by showing that you are in the right length and time scales, or by showing that the turbulence spectrum is correct. Quote:
Quote:
By the way, for the mathematically picky  It has not been proven mathematically that the Navier Stokes equations are dissipative (ie that flow always dies out in time and does not perpetually flow). This is such a physically obvious thing but diabolical to prove formally that it has been made a millenium problem (http://www.claymath.org/millennium/N...kes_Equations/) and anybody who can prove it will be award $1 million. 
Quote:
Quote:

Quote:

Related query
Dear All
I have been reading very similar papers I suspect, one of them suggests that a simple grid size estimator for an LES grid would be the SGS viscosity/ molecular viscosity, you would be looking to achieve approx 20 for a high Re flow. Could this be applied to an SASSST model? Such as obtaining the eddy/turbulent viscosity in the fine near wall region where the SST model would have resolved the value? I can't see a way of extracting an SST specific term from Ansys CFX. I have played with extracting the kolmogorov length scale from a previous SST simulation and using that as a measure for the required transient grid length but I'm not convinced by the results although it is early days Mark 
The Kolmogorov length scale has little relevance to a LES simulation.
The Taylor length scale is a useful starting point for working out the mesh size required for LES: http://en.wikipedia.org/wiki/Taylor_microscale 
Energy is not created in the subgrid scale. In a turbulence model the turbulence just increases viscosity, so this is dissipative and cannot create energy at least not in this universe, but if you can prove this you will win the $1 million Millenium prize (http://planetmath.org/millenniumproblems see number 3).
Energy can move about on the turbulence frequency spectrum, but not be generated. As I said previously the ratio being less than 1 simply means there is more viscous dissipation than turbulent dissipation. This is true for all laminar flows and turbulent flows at low Re number. There is nothing special about it. 
Just goes to prove you shouldn't believe everything you read even if it is a journal! Hehe
Will look into the Taylor length scale Many Thanks 
I am not saying the journal is wrong  you suggest it was saying you can estimate LES mesh size by when the viscous to turbulent dissipation ratio is one. This also sounds like a reasonable starting point for a LES mesh size. In most flows the turbulent dissipation is much bigger than the viscous, so if the ratio is 1 it means the mesh size has been reduced to the size where viscous dissipation is becoming important.
This is another independant way of estimating mesh size for LES. This is one of the really tricky things about LES. The results are dependant on mesh size and the mesh size you can use depends on the SGS model you are using. 
Hello
Sorry for taking so long to thank you I have been chasing a bug in some simulations and hadn't got round to having a think. It had struck me that as the Kolmogorov length scale relates to the smallest features it was going in the direction of a DNS simulation hence my initial post. The results look really good, more inline with the experimental data, can i ask despite it being a slightly different subject  is there a way of exporting location settings from CFX post? I have 60 odd points/lines in a post file and I want to use them in another one, typing it in would be very dull. I export pre settings a lot as CCL files but I've never needed to do this before. Thank you again Mark 
If you intend to do DNS style simulations you need to consider things like numerical dissipation and related issues. It is critical that the mesh defines the dissipation, not the numerics.
You can both import and export CCL in CFXPre and when you run the solver on the command line. In CFXPre you can also edit the CCL of individual objects in the tree (right click on the tree item). 
Hi OJ
Yes my venturing into the realms of Kolmogorov length scale produced some rather interesting meshes and were essentially beyond the practical computational power I had available for any reasonable time step. I'm currently attempting to emulate some experimental results for a backstep and I got some quite interesting results using the Taylor microscale which I calculated based on some semiempirical formulas based on the RANS SST simulation I was using the trigger the transient simulation. The result do not however show any BrownRoshko structures in the shear layer region between the recirculating fluid and the main flow which I was expecting to see at the Reynolds number I was considering. Clearly a bit more fiddling is need on my part! Does anyone have an opinion on what type of plot would show these structures most effectively, I was using contour plots but unsurprisingly they have a rather defined contoured nature to them which I don't think is helping. Many thanks 
First of all, is your model sufficiently accurate that you will get the structures you expect? You will need a low dissipation numerical scheme  use the LES settings with a central differencing advection scheme. You will also need second order time stepping. Are you using these?
There are some vortex visualisation tools in CFDPost. Have a look at the additional variables available under the puzzlingly labelled "..." button. 
All times are GMT 4. The time now is 03:33. 