# Strange Velocity Gradient in the Viscous Sublayer

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June 24, 2022, 11:05
Strange Velocity Gradient in the Viscous Sublayer
#1
Member

Will Crawford-Jones
Join Date: Jan 2021
Posts: 43
Rep Power: 4
Hello,

I am simulating the viscous sublayer of open channel flow with a bed of spheres on the bottom wall with a steady, periodic, RANS (k-w) simulation. Now, I have made the cells very small, e.g. perhaps 20 times smaller than than the viscous sublayer. When I run the simulation for a given number of iterations the residuals drop and I take the streamwise velocity along a vertically-aligned probe. The velocity gradient between cells is about what I would expect it to be, however, within the cell, it seems the velocity gradient is constant- as in, it is constant to 15 significant figures. Is there something I need to change to make Star-CCM+ interpolate values within cells?

Domain and Velocity gradient images attached

Thanks for the help in advance

Will
Attached Images
 DOMAIN.jpg (197.2 KB, 12 views) VEL_GRAD.jpg (40.1 KB, 9 views)

 June 25, 2022, 03:44 #2 Senior Member   Lefteris Join Date: Oct 2011 Location: UK Posts: 305 Rep Power: 14 Out of curiosity, are you using wall functions? __________________ Lefteris

 June 27, 2022, 06:18 #3 Member   Will Crawford-Jones Join Date: Jan 2021 Posts: 43 Rep Power: 4 Yes, I have the all y+ wall treatment turned on. Perhaps I should set this to low y+ wall treatment seeing as my first cell is much less than y+? I notice I can compute u* iteratively so perhaps I should turn this on too? Any other ideas?

June 27, 2022, 06:23
#4
Senior Member

Lefteris
Join Date: Oct 2011
Location: UK
Posts: 305
Rep Power: 14
Quote:
 Originally Posted by wcj1n18 Yes, I have the all y+ wall treatment turned on. Perhaps I should set this to low y+ wall treatment seeing as my first cell is much less than y+? I notice I can compute u* iteratively so perhaps I should turn this on too? Any other ideas?
Assuming you have some time, I'd be curious to try without any wall functions. If your y+ is so low you're actually solving the viscous sublayer so no need to model it.
__________________
Lefteris

 June 27, 2022, 06:25 #5 Member   Will Crawford-Jones Join Date: Jan 2021 Posts: 43 Rep Power: 4 I see what you're saying, although I'm also interested in obtaining a varying velocity within each cell so I think without wall functions turned on it probably wouldn't give that?

 June 27, 2022, 06:35 #6 Member   Will Crawford-Jones Join Date: Jan 2021 Posts: 43 Rep Power: 4 OK, i'd forgotten I'd run another simulation using, what I thought, was the exact same set up. In this case i've noticed that u* i set to be found iteratively and this seems to have solved the problem! It's also SST (Menter) K-omega rather than the earlier version of the model, although I doubt this would affect it based on my limited understanding of the model Thank you for the help!

 Tags dns, open channel flow, velocity gradient, viscous sublayer