Laboratory investigation of lateral dispersion within dense arrays of randomly distributed cylinders at transitional Reynolds number

Yukie Tanino, Heidi M. Nepf
2009 Physics of Fluids  
Relative ͑effective͒ lateral dispersion of a passive solute was examined at transitional Reynolds numbers within a two-dimensional array of randomly distributed circular cylinders of uniform diameter d. The present work focuses on dense arrays, for which previously developed theory ͓Y. Tanino and H. M. Nepf, J. Fluid Mech. 600, 339 ͑2008͔͒ implies that the asymptotic ͑long-time/ long-distance͒ dispersion coefficient, when normalized by the mean interstitial fluid velocity, ͗ū͘, and d, will only
more » ... ͘, and d, will only exhibit a weak dependence on Reynolds number, Re d ϵ͗ū͘d / , where is the kinematic viscosity. However, the advective distance required to reach asymptotic dispersion is expected to be controlled by pore-scale mixing, which is strongly Re d -dependent prior to the onset of full turbulence. Laser-induced fluorescence was used to measure the time-averaged lateral concentration profiles of solute released continuously from a point source in arrays of solid volume fraction = 0.20 and 0.35 at Re d = 48-120. Results are compared to previous measurements at higher Re d . Lateral dispersion reaches the same rate as asymptotic dispersion in fully turbulent flow at x Ϸ 154d at ͑ ,Re d ͒ = ͑0.20, 110-120͒ and at x Ϸ 87d at ͑ ,Re d ͒ = ͑0.35, 300-390͒. In contrast, dispersion does not reach the fully turbulent flow limit at Re d Ͻ 100 within the range of x considered. Also, concentration profiles deviate further from a Gaussian distribution at = 0.35 than at 0.20 for similar Re d and x / d. From these observations, it can be inferred that the pre-asymptotic regime extends farther downstream, in terms of the number of cylinders spanned, at lower Re d and at larger . Downloaded 12 Oct 2010 to Redistribution subject to AIP license or copyright; see 046603-10 Y. Tanino and H. M. Nepf Phys. Fluids 21, 046603 ͑2009͒ Downloaded 12 Oct 2010 to Redistribution subject to AIP license or copyright; see
doi:10.1063/1.3119862 fatcat:6enhzlcx7ngetlmvqfldtchqgi