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Hello everybody,

I'm an environmental health scientist working on a review of modeling approaches to track the fate and transport of pathogens during flood events. Telemac 2D is among the list of models discussed, and I am trying to understand how the diffusion coefficients are calculated for the transport equation. I understand there are several available methods for estimating turbulent viscosity, but, since I am fairly new to hydrology/fluid science, it's not clear to me what the relationship between turbulent viscosity and the diffusion terms is.

Thanks in advance!

Philip Collender

Hello,

The diffusion coefficient taken into account in diffusion is the sum of the molecular diffusivity and the turbulent diffusion. Turbulent diffusion is unique but may act slightly differently on velocities and on tracers like temperature and concentrations of a pollutant, so there should be also a coefficient depending on the advected variable (Prandtl coefficient, etc., close to 1). With Telemac-2D, which is a depth-averaged model, the concept of turbulent viscosity also includes what we call dispersion, which is an artificial diffusion acting on the depth-averaged tracer, due to the fact that in reality the velocity varies on the vertical.

With best regards,

Jean-Michel Hervouet

Thanks very much Jean-Michel! Just did some more background reading and realized the relationship between turbulent viscosity, diffusivity, and the Prandtl number. To recap, diffusivity is turbulent viscosity divided by the Prandtl number, so turbulent viscosity and diffusivity are roughly equivalent.

Actually, should be the Schmidt number.

Yes, exactly, I could not recall this name on the spot, thus my vague etc...

JMH

Gotcha. Thanks again!