Excess Shear Stress Output

claudiadalmora · January 5, 2015, 17:16

Hello Flow3D users,
How exactly Flow3D calculates the "Excess Shear Stress" output?
Thank you for your attention.

JBurnham · January 7, 2015, 17:59

If the flow is turbulent (a turbulence model is active) then the cells that include some solid surface are assumed to have a logarithmic velocity distribution. The distribution is derived from the work of von Karman, Nikuradse, and a lot of later researchers. There's a smooth function that adjusts the shape of the profile for whether the near-solid flow is hydraulically smooth, transitionally rough, or fully rough. The "log law of the wall" can be represented as a plot with y+ on the x-axis and u+ on the y-axis. Both y+ and u+ have shear stress in them, so the solution is iterative and coupled with the velocity field, which is in turn coupled with the pressure field. The FLOW-3D Users Site has several technical notes that fill in the rest of the details.

claudiadalmora · February 4, 2015, 13:54

Thank you for your answer.
I have another doubt. Is it possible to obtain shear stress value at specific points determined by the user? So far I can only obsverve excess shear stress graphical results.

JBurnham · February 4, 2015, 14:12

Yes. Before you run, you can add History Probes from the Meshing & Geometry tab. They are x y z points that record data. Put them close to the bed (in the same cell as the bed) and they will report shear.

Or, after the run, use Analyze > Text Output and get a box-shaped region of cell-centered shear stresses.

Or, after the run, make a text Neutral file (see the User Manual for the format) with x y z coordinates for data and use Analyze > Neutral File.

You can also get average shear stress on a solid by turning on Component Properties > Output > Pressure & Shear Force output and Wetted Area output. Then run the case, find the output in Analyze > Probe > General History Data, and take (component shear stress magnitude)/(component wetted area) to find the average shear stress.

mahdi1 · May 27, 2015, 16:38

Hi,
So, How does Flow-3D compute bed shear stress in separated flows where Log-law of the wall in not valid ( for k-e model)?

Thanks in advance
Mahdi

January 5, 2015, 17:16	Excess Shear Stress Output	#1
claudiadalmora New Member Join Date: Dec 2014 Posts: 3 Rep Power: 11	Hello Flow3D users, How exactly Flow3D calculates the "Excess Shear Stress" output? Thank you for your attention.

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January 7, 2015, 17:59		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	If the flow is turbulent (a turbulence model is active) then the cells that include some solid surface are assumed to have a logarithmic velocity distribution. The distribution is derived from the work of von Karman, Nikuradse, and a lot of later researchers. There's a smooth function that adjusts the shape of the profile for whether the near-solid flow is hydraulically smooth, transitionally rough, or fully rough. The "log law of the wall" can be represented as a plot with y+ on the x-axis and u+ on the y-axis. Both y+ and u+ have shear stress in them, so the solution is iterative and coupled with the velocity field, which is in turn coupled with the pressure field. The FLOW-3D Users Site has several technical notes that fill in the rest of the details.

February 4, 2015, 13:54		#3
claudiadalmora New Member Join Date: Dec 2014 Posts: 3 Rep Power: 11	Thank you for your answer. I have another doubt. Is it possible to obtain shear stress value at specific points determined by the user? So far I can only obsverve excess shear stress graphical results.

February 4, 2015, 14:12		#4
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	Yes. Before you run, you can add History Probes from the Meshing & Geometry tab. They are x y z points that record data. Put them close to the bed (in the same cell as the bed) and they will report shear. Or, after the run, use Analyze > Text Output and get a box-shaped region of cell-centered shear stresses. Or, after the run, make a text Neutral file (see the User Manual for the format) with x y z coordinates for data and use Analyze > Neutral File. You can also get average shear stress on a solid by turning on Component Properties > Output > Pressure & Shear Force output and Wetted Area output. Then run the case, find the output in Analyze > Probe > General History Data, and take (component shear stress magnitude)/(component wetted area) to find the average shear stress.

May 27, 2015, 16:38		#5
mahdi1 Member mahdi Join Date: Jan 2013 Posts: 43 Rep Power: 13	Hi, So, How does Flow-3D compute bed shear stress in separated flows where Log-law of the wall in not valid ( for k-e model)? Thanks in advance Mahdi