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-   -   LESDES deltas (https://www.cfd-online.com/Forums/openfoam-solving/57904-lesdes-deltas.html)

 cfdmarkus December 8, 2008 05:11

Hi all I had a look at the

Hi all

I had a look at the DES model currently implemented in OF1.5. For computing the DES length scale one needs a delta defined as delta()=C*max(dx,dy,dz). This definition is a crucial point in DES and should be used.

I could only find definitions of delta() which are based on the cuberoot delta()=C*(dx*dy*dz)^1/3.

Is the definition of delta based on the maximum grid spacing readily available in OF, or do I have to implement it?

Markus

 lakeat January 11, 2009 07:10

Hi, Markus, how did you do tha

Hi, Markus, how did you do that now? I am interested!

Daniel

 braennstroem January 12, 2009 01:56

Hi, did you take a look at:

Hi,

did you take a look at: src/turbulenceModels/LES/LESdeltas/PrandtlDelta/PrandtlDelta.C
or
src/turbulenceModels/LES/LESdeltas/cubeRootVolDelta/cubeRootVolDelta.C

I think this could be a good start to implement this!?

Fabian

 lakeat January 12, 2009 03:06

Well, I am not poor in using C

Well, I am not poor in using C++, so I need help.

PrandtlDelta.C and cubeRootVolDelta.C does not give me much infomation, since it seems the length of each cell is needed, But in fvMesh.H, only mesh().V() etc. is provided, so I'm not sure how to get mesh().maxLength() in a easiest way, could someone give me a hint? Many thanks.

Daniel

 lakeat January 12, 2009 04:11

Sorry, I mean I'm good at C++

Sorry, I mean I'm good at C++ programing.

 lakeat January 12, 2009 04:13

Sorry, I mean I'm NOT good at

Sorry, I mean I'm NOT good at C++ programing.
I hate my keyBoard

 cfdmarkus January 12, 2009 04:41

Hi Daniel Sorry I haven't h

Hi Daniel

Sorry I haven't had enough time to work on this issue.
But I still believe that a delta based on the maximum grid spacing is crucial for the old DES versions. Therefore, sooner or later I will have to implement it as well.

I am also wondering if there is a command to return the maximum grid spacing of a cell !?

Markus

 eugene January 12, 2009 09:03

Considering that FOAM cells ar

Considering that FOAM cells are represented as arbitrary polyhedra, what would the definition of "grid spacing" be?

 lakeat January 12, 2009 10:09

Yes, but isn't that most of th

Yes, but isn't that most of the DES Simulations now are carried on structured hexahedra?

 eugene January 12, 2009 10:20

The cells might look like hexa

The cells might look like hexahedra, but the internal representation is 100% face based. FOAM meshes are completely unstructured. The best you could do would be to ask for a maximum edge length, which could be misleading in some situations, but would do what you need provided all cells were hex shaped.

I do not know of any unambiguous definitions that will comply to the Spalart method for getting max(dx,dy,dx) on arbitrary meshes.

 cfdmarkus January 12, 2009 10:30

Ok I get your point. I jus

I just don't believe that the cubeRootVolDelta is an appropriate length scale for high aspect ratio hexahedral cells.
Also, the only way to ensure the correct behaviour of DES for attached flows is the design of "DES-grids" (with high aspect ratio cells inside the boundary layer) combined with the "MaxDelta" definition.

Markus

 lakeat January 12, 2009 10:30

Fully Agree! And have you t

Fully Agree!

And have you tried DDES, can you give some comments? Thanks http://www.cfd-online.com/OpenFOAM_D...part/happy.gif

 lakeat January 12, 2009 10:42

To Markus, For me, maxDelt

To Markus,

For me, maxDelta is just a method, and an easy way to control the appropriate length scale, to control the proper RANS region.
1. "Easy" but not so easy in preprocessing, that is "DES-grids" design process.
2. "Easy" but not best, why do you think cubeRootVolDelta will violate the "DES-law"?

 eugene January 12, 2009 11:19

We will be trying DDES this ye

We will be trying DDES this year, so I have no personal experience of it yet. Initial indications are that it is a definite improvement.

Assuming you have a hex dominant mesh with surface layers, then the only place that cuberoot vol will be different from max xyz, will be in the transition zone between wall distance and cell size length scales. Near the wall the wall distance remains unchanged and far from the wall, the cuberoot is equal to the max length. In any case, it doesn't seem to make a massive difference to the results. I did a study in my thesis (which is available for download from Hrv's foamcfd website), so you can follow my logic and make up your own mind.

 cfdmarkus January 12, 2009 11:20

I am not sure what you mean by

I am not sure what you mean by "DES-law".

Using the cubeRootVolDelta makes it much more difficult to maintain RANS in attached flows and consequently, the switch from RANS to LES can occur well inside the boundary layer. A switch inside the boundary layer usually has negative consequences (drop in skin friction). A length scale defined by delta=max(dx,dy,dx) combined with anisotropic cells will help to prevent a switch from RANS to LES inside the boundary layer. Thats why I want to use delta=max(dx,dy,dx).

Markus

 cfdmarkus January 12, 2009 11:37

I came across an upgraded vers

I came across an upgraded version of DDES called IDDES. If you are interested:

A hybrid RANS-LES approach with delayed-DES and wall-modelled LES capabilities
International Journal of Heat and Fluid Flow, Volume 29, Issue 6, December 2008, Pages 1638-1649
Mikhail L. Shur, Philippe R. Spalart, Mikhail Kh. Strelets, Andrey K. Travin

Markus

 lakeat January 12, 2009 12:44

I know this article. But according to Wilcox，i won't use it for its GREAT complexity, for i think its improvement is limited both in engineering-requirement results and basic theory.
It's too complex to have a model like that. :-) just my thought.

Good night!

 lakeat January 12, 2009 23:42

Hi, Markus! Do you have the

Hi, Markus!

Do you have the original paper of DES by Spalart. May I have a copy of it? http://www.cfd-online.com/OpenFOAM_D...part/happy.gif Thank you.
My Email is lakeat@gmail.com

Daniel

---------------------------
Spalart, P.R., Jou, W.-H., Strelets, M., Allmaras, S.R.: Comments on the feasibility of LES for wings, and on a hybrid
RANS/LES approach. In: Proceedings of first AFOSR international conference on DNS/LES, Ruston, Louisiana. Greyden
Press, 4&ndash;8 Aug (1997)

 braennstroem January 13, 2009 04:36

Hi, could it possible to us

Hi,

could it possible to use the gradient of a virtual field, which then could be used to determine the max value!?
For attached flows it might be better to use a different approach with a hard, kind of 'predefined', interface position. As the name suggests, DES is more suitable for detached flows. There is a review about hybrid LES/RANS methods of Froehlich and von Terzi, which is quite interesting.

By the way, it would be quite nice, if you guys support the Turbulence Special Interest Group!?

Fabian

 cfdmarkus January 13, 2009 08:04

Daniel: yes I should have a h

Daniel:
yes I should have a hardcopy of the original DES97 paper. If I find a scanner I can send it to you (might take some time).

Fabian:
I am really not sure about your first point. However, constructing a MaxDelta for hexahedral cells shouldn't be too difficult.
I agree that the definition of a proper interface location is clearly superior to any global-hybrid method. However, these zonal-hybrid methods bring up a lot of other problems and therefore, global-hybrid methods are at least a good starting point.

Do you have any experience in running hybrid simulations in OF wich use a predefined interface location? I am very interested to hear about your experiences regarding this issue!

Markus

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