Getting started on LES

gareth__it_power · September 20, 2010, 04:55

Hi,
I have been using a RANS based simulation on a 2D wind turbine model for quite a while now, and I am interested to look at the difference in results with DES and LES. I was wondering if someone could be kind enough to give me some suggestions on how best to approach this.

Obviously a 3D mesh is required. However my problem (flow over an aerofoil) is essentially 2D. Will in be acceptable to form a pseudo-3D mesh, only 1 cell thick, with Symmetry conditions on the front and back of the mesh? This will create some very high aspect ratios, if the mesh is of uniform thickness.

DES and LES both require a fine mesh. Can anyone quantify this? Is a Y+ of 1 low enough? Is y+ this only way to define mesh quality?

Also, timestep size seems to have an important influence. Is there a rule of thumb for setting this?

I'd be very grateful for any tips.

Many thanks

Gareth

sreerao · September 21, 2010, 05:09

Hi Gareth

Y+ of about 13 is ok in LES. Werner and Wengle wall functions will be used in FLUENT while using LES. You need to initialise it from GUI.

To decide what grid can be used in LES, you can check turbulent kinetic energy from your RANS solution and can judge the requirements % of TKE to be resolved on LES grid.

Hope this helps to start with.

Sree

September 20, 2010, 04:55	Getting started on LES	#1
gareth__it_power New Member Gareth Join Date: Mar 2009 Posts: 24 Rep Power: 17	Hi, I have been using a RANS based simulation on a 2D wind turbine model for quite a while now, and I am interested to look at the difference in results with DES and LES. I was wondering if someone could be kind enough to give me some suggestions on how best to approach this. Obviously a 3D mesh is required. However my problem (flow over an aerofoil) is essentially 2D. Will in be acceptable to form a pseudo-3D mesh, only 1 cell thick, with Symmetry conditions on the front and back of the mesh? This will create some very high aspect ratios, if the mesh is of uniform thickness. DES and LES both require a fine mesh. Can anyone quantify this? Is a Y+ of 1 low enough? Is y+ this only way to define mesh quality? Also, timestep size seems to have an important influence. Is there a rule of thumb for setting this? I'd be very grateful for any tips. Many thanks Gareth

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September 21, 2010, 05:09		#2
sreerao New Member sreenivasa rao gubba Join Date: Sep 2010 Location: Leeds University, UK Posts: 28 Rep Power: 15	Hi Gareth Y+ of about 13 is ok in LES. Werner and Wengle wall functions will be used in FLUENT while using LES. You need to initialise it from GUI. To decide what grid can be used in LES, you can check turbulent kinetic energy from your RANS solution and can judge the requirements % of TKE to be resolved on LES grid. Hope this helps to start with. Sree