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Strange Velocity Gradient in the Viscous Sublayer

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Old   June 24, 2022, 11:05
Default Strange Velocity Gradient in the Viscous Sublayer
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Will Crawford-Jones
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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
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File Type: jpg VEL_GRAD.jpg (40.1 KB, 9 views)
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Old   June 25, 2022, 03:44
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Lefteris
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Out of curiosity, are you using wall functions?
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Old   June 27, 2022, 06:18
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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?
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Old   June 27, 2022, 06:23
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Quote:
Originally Posted by wcj1n18 View Post
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.
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Old   June 27, 2022, 06:25
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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?
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Old   June 27, 2022, 06:35
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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!
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