Delwaq: ambient shear stress? - Home - Delft3D
Delwaq: ambient shear stress?
Could you tell me the difference between ambient shear stress and bed shear stress in Delwaq? How the ambient shear stress will be calculated for IM(Inorganic Matter)? Is it the shear stress of momentum equations (i.e. Tij=(v+vt)(dui/dxj+duj/dxi))? If yes, is the ambient shear stress the value of the shear stress Tensor? How is the ambient shear stress for each cell in a 3D-mesh by Delwaq calculated?
I had to search the manual for "ambient shear stress" and it seems I am responsible for this text. A slip of the pen, I am afraid - bottom shear stress is meant, nothing else. In most cases the shear stress can be picked up directly from the hydrodynamics. Otherwise it is calculated from the flow velocity and a user-defined value of the Chézy coefficient, as described in the manual. For DELWAQ the shear stress is not treated as a tensor, it is simply considered a scalar (or an isotropic tensor if you like).
Thank a lot Arjen. This term (ambient shear stress) is written in new Manual (published on December 22, 2018) in page 358. I have still two questions about IM :):
1- How can I use IM for materials with smaller density than water (Floating or colloide material)? If not, do you know a better substance for Floating/sub-Floating (Floating in water columne) contaminants in water? How can I consider their densities (apart from settling velocity)?
2-How can I increase the number of IM from 3 to 6 or larger?
Truly floating material is a bit tricky - but you can specify a negative settling velocity. Then the material will rise to the surface instead of sink to the bottom. As it is still under the influence of vertical diffusion/mixing, it will be spread over the vertical, but as long as the mixing due to vertical diffusion is small in comparison with the rising due to the negative settling velocity, it shoudl concentrate at the surface.
The thing that will be missing is the influence of the wind at the surface. Most floating materials will feel the wind. It is possible to define an additional velocity per substance, but with a curvilinear or even unstructured grid that is a lot of work.
You may try to do that with so-called fractions. The manual is fairly terse about it, but check section 1.5.1 in the description of the input file. Mind you: it has limitations. The three ordinary sediment types IM1, IM2 and IM3 (and IM1S1, IM2S1, IM3S1) are subject to "processes" like composition of sediment in the bottom - necessary for properly taking care of resuspension and the like - and the use of fractions may give slightly wrong results (I have never tried it in this way myself).
The safer way to do this is by splitting up the calculation into two parts.