Rotational dispersion in porous media due to fluctuating flows

We suggest that dispersion in a saturated porous medium subject to a fluctuating pressure gradient can occur as a result of the processes of shear (Taylor) dispersion and what we will label rotational dispersion. In contrast to shear dispersion, rotational dispersion does not rely on molecular diffusion to be effective but requires that the direction of the pressure gradient rotates with time. Such rotational gradients are ubiquitous in nature, occurring whenever a pressure wave propagates across the surface of a porous medium such as soil or marine sediments. The efficiency and character of rotational dispersion is investigated using Monte Carlo simulations of the dispersal of clouds of particles through a highly idealized porous medium. These simulations demonstrate that rotational dispersion behaves as a diffusive process and that it can be many times more effective than molecular diffusion or shear dispersion as a transport mechanism. The results of the theory were tested experimentally using a wave tank with a bed of sand as the porous medium. These experiments demonstrate that passing waves can greatly enhance solute transfer between the bed and the overlying water. Furthermore, the measured increases in solute transfer rates are quantitatively consistent with the predictions obtained from the theory of rotational dispersion presented herein.