How to define the tortuosity in TOUGHREACT
Hi. I am confused by the concept of the tortuosity. I found it was defined as is the ratio of the average length of the flow paths through the medium to the straight-line length across the medium (greater than unit) in some papers; sometimes it's defined as the inverse of the above definition (less than unit); another literature defined the tortuosity coefficient as the square of the inverse of that ratio and it's sometimes defined as the diffusion coefficient of diffusing species in a porous medium, De, relative to its value in free fluid, D0.
I was wondering what its definition in TOUGH/PetraSim. Thank you!
Best regards,
Lang
2 replies
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Hi Lang,
There are numerous definitions floating around. One of the most detailed discussions is in Jacob Bear's book (Dynamics of Fluid Flow in Porous Media). I recall that he said it was not the ratio of the path length to the straight line length. Regardless of the preferred mathematical derivation it is usually estimated or fitted from tracer data, and is basically a "fudge" factor, unless you have an idealized system and you are testing actual mathematical relationships knowing the porosity and pore geometry. In TOUGHREACT, we assume that it is a number from 0 to 1, and to obtain the diffusivity in the porous medium (D_pm) the equation is D_pm = tort*porosity*D_m, where D_m is the molecular diffusion coefficient in the free fluid.
regards,
Eric
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Dear Dr. Sonnenthal,
Thank you for your helpful reply.
Best regards,
Lang