# Proper use of LES

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

 September 6, 2001, 17:57 Proper use of LES #1 Karl Kevala Guest   Posts: n/a Sponsored Links I am doing three-dimensional sliding mesh simulation and have used several RANS models. I want to use LES. Based on relation in Temkes and Lumley for the Taylor microscale, L, L ~ l (15 nu) / (TKE l) where nu is kienematic viscosity, TKE is turbulent kinetic energy, and l is characteristic length. Using this formula and experimental data for the TKE, L is about 0.1 mm throughout significant regions for my problem. There is no way to make my mesh so small. Right now, my cell sizes are about 0.25 mm length on all sides. This is about the smallest I can make my mesh and still solve in a reasonable amount of time (i.e. on the order of > 2 months). Might I get useful information from LES simulation using mesh cells larger than Taylor microscale? Thanks

 September 6, 2001, 18:02 Re: Proper use of LES #2 Karl Kevala Guest   Posts: n/a For L, that should have been written: L ~ l (15 nu) / (TKE l)^ (1/2)

 September 7, 2001, 19:22 Re: Proper use of LES #3 Chetan Kadakia Guest   Posts: n/a I am not sure of the Taylor microscales, but I am familiar with the Kolmogrov scales. If you are attempting DNS, then your grid needs to be smaller than the Kolmogrov scales. But if you are using LES, for many flows, the grid size can be one or two magnitudes greater than the Kolmogrov scales and still obtain valid results that are close to DNS and still better than a RANS solution.

 September 8, 2001, 12:38 Re: Proper use of LES #4 Mike Henneke Guest   Posts: n/a LES models don't require you to model the Taylor microscale. The idea behind LES is to separate the turbulent motions into 'resolved' scales (which are larger than your mesh) and 'unresolved' scales (which are smaller than your mesh). Done correctly, the resolved scales are the geometry dependent scales and the unresolved scales are universal to all turbulent flows (at least that's the hope). This is a big reason that LES is problematic for combusting flows, the small scales aren't universal, but depend on the chemistry. The subgrid scale filter is necessarily grid dependent, so you cannot have grid independent results. You should get better than RANS results so long as your grid resolves the integral length scales well, i.e., the motions that are on the order of your geometric length scales. I'm not an LES expert, this is just what I've heard and understood. Also, I have heard that rapid changes in grid sizes and aspect ratios are problematic. YMMV, Mike

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post tianrenshui311 FLUENT 0 November 15, 2010 14:21 eelcovv OpenFOAM 0 June 8, 2010 11:35 Margherita Cadorin CFX 0 October 29, 2008 06:24 ben Main CFD Forum 9 February 17, 2005 00:40 Bin Li Main CFD Forum 2 February 20, 2004 10:58