Learning Forum

[shaheen wahab]

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 

[peteroznewman]

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

[shaheen wahab]

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.

[DrAmine]

Can you check the values in Fluent and tell us with release you are working?

[Ananth Narayan]

Try changing roughness constant

[shaheen wahab]

Can you check the values in Fluent and tell us with release you are working?

Hi 

I am using Fluent version 19.0

[shaheen wahab]

Hi,

I changed the roughness constant to 1 (Default value was 0.5)

[DrAmine]

Have you checked the values in Fluent? Are they the same reported in Post?

[shaheen wahab]

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. 

[DrAmine]

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. 

[shaheen wahab]

Thank you. I will check this and get back to you asap. 

[shaheen wahab]

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