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Strange Velocity Gradient in the Viscous Sublayer |
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June 24, 2022, 11:05 |
Strange Velocity Gradient in the Viscous Sublayer
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#1 |
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Will Crawford-Jones
Join Date: Jan 2021
Posts: 43
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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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June 25, 2022, 03:44 |
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#2 |
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Lefteris
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Out of curiosity, are you using wall functions?
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Lefteris |
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June 27, 2022, 06:18 |
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#3 |
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Will Crawford-Jones
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Posts: 43
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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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June 27, 2022, 06:23 |
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#4 |
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Lefteris
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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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Lefteris |
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June 27, 2022, 06:25 |
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#5 |
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Will Crawford-Jones
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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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June 27, 2022, 06:35 |
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#6 |
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Will Crawford-Jones
Join Date: Jan 2021
Posts: 43
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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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dns, open channel flow, velocity gradient, viscous sublayer |
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