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How to calculate the mechanical dispersion

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Hello. I am inquiring about how to know the mechanical dispersion in the PetraSim.

When I input the diffusion coefficient and tortuosity/porosity, I just can obtain the so-called effective diffusion coefficient. If we want to get the dispersion coefficient, we must know the mechanical dispersion coefficient (dispersion = effective diffusion + mechanical dispersion). Then we can apply the dispersion coefficient to the advection-dispersion equation. So how can we get the mechanical dispersion coefficient?

Thank you.

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TOUGH2 (Pruess et al., 1999) does not simulate the mechanical dispersion. It simulates just the molecular diffusion.

Full hydrodynamic dispersion is simulated by TOUGH2 in 2D cartesian geometries by coupling the T2DM module (Oldenburg C.M e Pruess K. (1993). A two-dimensional dispersion module for the TOUGH2 simulator. Earth Sci. Div., Lawrence Berkeley National Laboratory report LBL-32505, Berkeley, CA.)

Full hydrodynamic dispersion can be simulated in 3D non structured grids by a special version of TOUGH2 called T2R3D (Wu Y-S, Pruess K. (2000). Numerical simulation of non-isothermal multiphase tracer transport in heterogeneous fractured porous media. Adv. in Water Resour., 699-723.)

For a cartesian grid, for which the IFD method is equivalent to the finite difference method, the numerical dispersion induced by the space discretization  is equivalent to the mechanical dispersion for which the longitudinal dispersion coefficient is half of grid spacing (Bear, 1979). Emulating the mechanical dispersion with grid spacing can work only for very simple cartesian grid geometries.

Alfredo