open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi,
I've set up a z-layer model with fixed vertical spacing of 2m. Using the Nikuradse law for bottom friction, I am getting abnormal bed shear stress output.

This seems to have to do with the distance between the bottom two layers. My model has 16 layers. Layer 10 has assigned elevation of -12 m. If a model node has elevation of -12.5 m, the elevation of my bottom layer (1) is at -12.5 m, but the layers above it have fine interval at elevations at -12.49 m, -12.48 m, -12.47 m..etc. until Layer 10. If I use evenly spaced sigma layer, the distance between the bottom two layers would be say 12.5/16 = 0.78 m. Any help would be greatly appreciated.

Thanks.

Hello,

If using a Z layer transformation and you have fixed several planes below bottom elevation, by default, TELEMAC-3D shifts the different planes with a minimum distance so that the planes do not cross. You then get small prisms over the vertical following the bottom.

You can change the keyword MINIMUM DISTANCE BETWEEN PLANES CLOSE TO THE BOTTOM.
You also have the possibility to consider crushed elements (volume = 0) by giving a value to THRESHOLD HEIGHT BEFORE CRUSHED ELEMENTS not equal to 0.
Please read the TELEMAC-3D user manual.

If you are not OK with that, you can also try MESH TRANFORMATION = 2 with a finer discretisation close to the bottom.

Hope this helps,

Chi-Tuan

Thanks. I was not aware of those two commands. I will have a look and see if I get better results.