Large Eddy Simulation Question

Jing_min · February 12, 2012, 06:31

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

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

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 ?

February 12, 2012, 06:31	Large Eddy Simulation Question	#1
Jing_min New Member Jing Join Date: Dec 2011 Posts: 20 Rep Power: 14	Dear all I have a question about Large Eddy Simulation: How does the roughness take a role in the governing equations of LES?

February 18, 2012, 03:33	Les	#5
Jing_min New Member Jing Join Date: Dec 2011 Posts: 20 Rep Power: 14	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,

May 23, 2013, 02:36	large eddy simulation	#7
amin_veysi New Member amin veysi Join Date: May 2013 Posts: 5 Rep Power: 12	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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February 13, 2012, 11:23		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	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.

February 14, 2012, 04:43		#3
Jing_min New Member Jing Join Date: Dec 2011 Posts: 20 Rep Power: 14	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

February 14, 2012, 11:53		#4
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	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.

February 21, 2012, 10:36		#6
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	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.