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-   -   Large Eddy Simulation Question (http://www.cfd-online.com/Forums/flow-3d/97217-large-eddy-simulation-question.html)

Jing_min February 12, 2012 06:31

Large Eddy Simulation Question
 
Dear all
I have a question about Large Eddy Simulation:
How does the roughness take a role in the governing equations of LES?

JBurnham February 13, 2012 11:23

Answer: it varies by LES model. The model in FLOW-3D uses the logarithmic law-of-the-wall velocity profile in cells that are adjacent to or contain a solid surface. In these cells, the turbulent production and dissipation are calculated along with the velocity and shear stress. With the LES model, these calculations are only done in the near-wall cells to get the near-wall velocity. In FLOW-3D's RANS models (i.e., k-e and RNG), the turbulent energy is a transported quantity. The roughness value (ROUGH) affects the shear stress at the wall as part of the law-of-the-wall equations.

Jing_min February 14, 2012 04:43

Thanks alot for your answer. You said that it aplies the log low to the cells near the wall. So what is the distance between these cells to the wall?
All the best

JBurnham February 14, 2012 11:53

The distance (call it 'y') used for the log-law equation is the normal from the wall to the centroid of the fluid in the cell that contains the wall or abuts the wall on a cell-face. You might estimate it as 1/2 the cell length.

Jing_min February 18, 2012 03:33

Les
 
Hi
I have some questions about LES. I am modeling wave condition for a coastal structure. Is LES suitable for this?
What is the kernel of the filter used in FLOW-3D? There are little information in the manual for LES.

Thanks,

JBurnham February 21, 2012 10:36

The LES model in FLOW-3D is based on the work of Smagorinsky. The filter is the cell size, and the sub-grid model is also from Smagorinsky. From the manual: "In the LES model, the effects of turbulence too small to compute are represented by an eddy viscosity, which is proportional to a length scale times a measure of velocity fluctuations on that scale. For the length scale, (Smagorinsky, 1963) uses a geometric mean of the grid cell dimensions, and scales velocity fluctuations by the magnitude of L times the mean shear stress. These quantities are combined into the LES kinematic eddy viscosity. This kinematic eddy viscosity is incorporated into the dynamic viscosity used throughout FLOW-3D in exactly the same way as it is for the turbulence transport models."

The LES model would be fine for coastal engineering application, as long as you recognize that the mesh will be very fine relative to RNG or k-e models, and will therefore be slow to run and require a lot of memory. The only really valid way to find the correct mesh resolution is to do a mesh-dependency study, and refine the mesh by a factor of 2 in each direction (factor of 8 for cell volume) until the LES solution converges. It may help to use a smaller domain for the mesh study.

amin_veysi May 23, 2013 02:36

large eddy simulation
 
Hi
Dear all
I have some study about large eddy simulation (LES),and now i would like to know how i can do
an esay example of LES ?



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