Erosion by an oscillating disc in the vicinity of a granular bed
Résumé
Flatfish, such as flounders, bury themselves in the sand to hide from predators. They
are able to hide very quickly by oscillating their fins close to the seabed leading to the
suspension of sand, which then fall onto their bodies. Our experimental set-up aims at
reproducing while simplifying this natural behaviour. A rigid disc of diameter D and at an
average distance H from a flat granular bed is set to oscillate at a frequency f (or period
T) and an amplitude A such that the position of the disc is given by H + Asin(2πft).
We study experimentally the erosion thresholds in the simple case of a half-oscillation,
which means that the disc only moves downwards or upwards with smooth acceleration
and deceleration. The influence of the relevant parameters, i.e. A, D and H as well as the
initial position of the disc, on the erosion threshold is systematically characterized by finding
the critical half-period τc = Tc/2 below which beads are set into motion. In the range of
parameters considered in this study, the roll-up of the shear layer produced at the edge
of the disc generates a vortex ring that is one of the main mechanisms responsible for the
erosion of the granular bed. The key features of the vortex generated are then characterized
by PIV (Particle Image Velocimetry) and related to the parameters of the experiments. The
characterization of the fluid flow generated provides a better understanding of the erosion
thresholds.