open TELEMAC-MASCARET The mathematically superior suite of solvers

I am modelling an estuarine waterbody using the vertical mixing model in non-hydrostatic mode for salinity with Munk Anderson damping specified. In terms of salinity the results are not perfect but promising. The k-e model keeps crashing and requires extremely short times steps given the complexity of the geometry in question and thus is not an option for me from a computer resources point of view.

There is a narrow sea channel entry to a shallow tidal embayment for which measurement data of tide levels inside and outside indicate significant resistance exerted by the entry channel. I have found that the model results appear to be quite insensitive to the value of the roughness coefficient specified. I have artifically roughened the entry channel to extreme values (eg mannings n =0.4) so as to constrict inflow, But to little avail as the vertical mixing model (prandtl or any of the other choices) seems insensitive to roughness). (I have specified this as a FRICTION COEFFICIENT field in the geometry input file.

1/ This lack of insensitivity to the friction factor Is this an attribute of the vertical mixing length model?

2/ In calculating the horizontal viscosity does Telemac use the information from the vertical viscosity or does it use a constant viscosity term as defined by the horizontal diffusion coefficient?

3/ "Treatment on tidal flats for tracers" the default is 0 but surely this represents a potential sink for tracer and should retain the previous concentration before drying and thus the default should be set to 1 (particularly for a conservative tracer such as salinity)?

I would appreciate any comments/insight on the above queries

Regards
Tony

Hello Tony,

To answer your questions:
1) For me, the vertical mixing length model deals with the diffusion whereas the friction coefficient is a source term. They must be decorrelated.
2/ In TELEMAC, the diffusion is split for the two "directions" (horizontal and vertical), so when calculating the horizontal viscosity, TELEMAC uses the COEFFICIENT FOR HORIZONTAL DIFFUSION OF VELOCITIES if HORIZONTAL TURBULENCE MODEL = 1.
3/ I think you are right. I will discuss this point with Jean-Michel when he comes back and this default value may be changed for the next version. Anyway, you can manually activate value 1 in your case.

Hope this helps,

Chi-Tuan

Thanks Chi-Tuan,

I not clear as to you meaning regarding your reply to my first query. I assume the bottom friction applies a shear stress/force to the bottom layer face which is transferred through the water column via the viscous term. In a 2-dim simulation I can artificially roughen the inlet channel such that the inflow is severly restricted and the water levels within the tidal lough do not reach those in the open sea (as is the case from measurements). However when running the 3-d Baroclinic model with prandtl vertical mixing, increasing the bed roughness and lateral wall roughness along the inlet channel would apppear to have minimal effect. I also note that the horizontal turbulence model when selecting a mixing length model is the constant viscosity model which ignores roughness coefficient.

Is there anyway of roughening the inlet channel so as to introduce significant restriction when using the vertical mixing length model option.

Regards
Tony C.