Very small wall shear values (Bottom of a channel)

- August 22, 2018 at 10:12 am
  
  shaheen wahab
  Subscriber
  
  Hello,
  
  I have a rectangular channel with 200m*25m*2 m dimension. I want to calculate the bottom wall shear in order to calculate the erosion later. I have used k-w; SST model with Bottom B.C as "No-slip" with Sand grain roughness as 200 micrometers and Roughness constant as 1. The wall shear values exported from CFD Post are always found to be very less (in the order of -3).
  
  I tried varying the Sand grain roughness to 500 micrometers and even 1 cm and 10 cm. Still, the values are in the order of -3. What could be done to increase the Wall shear stress?
  
  Thank you
- August 22, 2018 at 10:39 am
  
  peteroznewman
  Subscriber
  
  Hello,
  
  You don't mention the mesh and specifically, the inflation layers and the first layer thickness at the bottom wall. That can have an effect on the wall shear. Please reply with details.
  
  Regards,
  
  Peter
- August 22, 2018 at 10:52 am
  
  shaheen wahab
  Subscriber
  
  Hi,
  
  For my meshing, I have used the Sweep method with 20 sweep divisions and the thickness of the first layer of the bottom wall as 0.010 m (by default, I haven't assigned any value to the first layer thickness though). I have attached screenshot of the Sweep method detail.
- August 22, 2018 at 11:47 am
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  Can you check the values in Fluent and tell us with release you are working?
- August 22, 2018 at 12:13 pm
  
  Ananth Narayan
  Subscriber
  
  Try changing roughness constant
- August 22, 2018 at 12:22 pm
  
  shaheen wahab
  Subscriber
  
  Can you check the values in Fluent and tell us with release you are working?
  
  Hi
  
  I am using Fluent version 19.0
- August 22, 2018 at 12:22 pm
  
  shaheen wahab
  Subscriber
  
  Hi,
  
  I changed the roughness constant to 1 (Default value was 0.5)
- August 22, 2018 at 2:24 pm
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  Have you checked the values in Fluent? Are they the same reported in Post?
- August 22, 2018 at 4:03 pm
  
  shaheen wahab
  Subscriber
  
  Yes, the values in Fluent and in CFD Post are both of the order of -3.
  
  * Will changing the near wall treatments make an effect o the Wall shear stress?
  
  *Is Wall shear actually the shear or is it u* (Shear velocity)? Seeing such small values of Wall shear (which should have been originally for u*) makes me doubt about it.
- August 22, 2018 at 5:26 pm
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  If it is a real rough wall then a log-wall mesh is sufficient. With the automatic wall treatment of SST you are on the safe side as the default treatment is shifting the wall with Kplus_s/2 and the viscous sublayer is completely disturbed Check the theory there.
  
  Wall shear is the wall shear stress and ustar. What you can check is the grid density on the wall (not the inflation layer): . Moreover please verify whether you have fully turbulent flow or you are experiencing a transitional flow where other input (geometric roughness correlation) is required.
- August 22, 2018 at 6:27 pm
  
  shaheen wahab
  Subscriber
  
  Thank you. I will check this and get back to you asap.
- September 4, 2018 at 9:05 am
  
  shaheen wahab
  Subscriber
  
  Hi,
  
  I found out the solution to this problem
  
  Everything remains the same except "Reference values" (which should have been Water instead of Air) and Check the Cell Zone which sometimes might not take the desired material for which we want to model. That's the reason that I was getting such small values.
  
  Thanks and Regards
  
  Shaheen