Erosion of cohesive grains by an impinging turbulent jet

A common method to clean surfaces is to use a turbulent jet of a gas to trigger the erosion and transport of surface contaminants. The onset of erosion, the subsequent transport and the resulting morphology of the eroded region are all modified by the presence of interparticle cohesive forces or surface adhesion. We perform experiments of a turbulent round jet of air impinging a granular bed. We finely tune the cohesion force between the grains and report the effects of cohesion in such an erosion process. First, we show that results for the onset of erosion can be rationalized by a cohesive Shields number that accounts for the interparticle cohesion force. Despite the complex nature of a turbulent jet, we provide a scaling law to correlate the erosion threshold, based on the outlet velocity at the nozzle, to a local cohesive Shields number. Beyond the onset of erosion, a crater is observed in the granular bed where erosion has locally removed the grains. The morphology of the observed craters is analyzed in detail as dependent on the local cohesive Shields number, and different types of craters are compared to similar experiments for cohesionless grains. Finally, we also consider the effects of the angle of impingement and the thickness of the cohesive granular bed on the mass removal.