Sediment bed destabilization by an isolated vortex: an experimental approach

Abstract

The destabilization of an erodible sediment bed by a reproducible impulsive phenomenon is studied experimentally. For this, a specific setup is designed to produce a well-controlled isolated vortex, advected over a uniform sand bed. Particle image velocimetry (PIV) measurements are performed to get fluid flow information; suspended particles images are also recorded. A new measurement technique, based on a stereo-correlation method, provides a precise reconstruction of the bedform morphology over time. Strong turbulent structures (namely sweep events) are evidenced in a near-wall region, where particles are mainly dislodged. The sediment plume follows the vortex: in particular, the upward velocity inside maintains the suspension phenomenon. However, the dispersion process seems to be not dependent on the vortex parameters. Then, the bed morphology is investigated: a scour hole is dug as the vortex hits the granular layer. Its dynamics is described by a typical scour law, even if the initial perturbation is transient. Another area, where the particles settled, can be observed: it behaves like a ripple, with a logarithmic growth of its crest. This deposition region has a specific geometry, studied here by a Fourier contour approach. Finally, a pickup rate is also derived from the morphological data: it appears independent of the vortex and bedform features.