open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello,

I'm running a simulation in a laboratory flume. In calibrating the roughness, I found that the Nikuradse roughness ks=0.01m produces good results. However, based on how telemac computes friction velocity (attached), a mesh with vertical mesh size smaller than the roughness will result in a negative u*, assuming u_plane1 is a positive value (In my case, the vertical grid size, i.e. dz, is about 0.003m). I thus wonder how and in which routine u* is computed & is there any recommendation for vertical resolution with respect to roughness height?

A related question is: how is the velocity at the bed (a solid boundary) computed? I noticed that it's non-zero and it's assumed to be at the height of about 1/10 of the vertical grid size. I'm wondering if it's computed by fitting a log curve with u* obtained by knowing u_plane1 (attached screenshot).

Any input would be welcome and very much appreciated!

Thank you,
Jason

Hello Jason,

Friction velocity^2 is computed in TFOND subroutine, see the test KFROT.EQ.5 for Nikuradse law.
You can see in the implementation that there is a threshold below which what is inside the log operation cannot below 1.001 which ensures no negative value.

By default, the keyword BOUNDARY CONDITION ON THE BOTTOM is equal to 1 (i.e. log law for velocities on bottom). It would be 0 if setting to 2 (see the TELEMAC-3D user or reference manual).

Hope this helps,

Chi-Tuan

Thanks a lot, Chi-Tuan!

Hi Chi-Tuan and everyone,

Do you know how ks (Nikuradse roughness height for LAW OF BOTTOM
FRICTION = 5) comes into play if the boundary is set to no-slip (BOUNDARY CONDITION ON THE BOTTOM = 2 with a rather fine mesh)?

I understand that for a slip condition (BOUNDARY CONDITION ON THE BOTTOM = 1), ks modulates the shear velocity like the attached screenshot in my first post above. But I couldn't find a clue as to how ks is related to the no-slip condition.

I'd appreciate any hints/insights. Thank you!

Jason