An experimental study of recirculating flow through fluid-sediment interfaces

We report here visualizations and quantitative measurements of scalar trans port, under the influence of rotation, through permeable sediments with an overlying fluid layer. The experimental set-up considered here is a station ary cylinder containing a fluid-saturated porous medium up to its midheight , with supernatant water on top. A rotating lid generates, in the upper flu id region, a flow that partially percolates into the porous layer below. Th e velocity field in the fluid layer is obtained using particle image veloci metry (PIV). Further, dye transport from the sediment is studied using two different techniques. The first one is positron emission tomography (PET), a non-invasive method which allowed us to 'see' through the opaque solid ma trix, and to obtain full three-dimensional pictures of dye transport throug h the sediment. The second one is digital photographic visualization from o utside, and subsequent image processing in order to obtain the near-wall dy e-washout depth. The experimental data suggest that the temporal evolution of washout depth for different sediments follows near-logarithmic behaviour . This finding is of importance for the a priori estimation of the transpor t of fluid and other solute substances in sandy aquatic sediments.