We investigate experimentally the bouncing motion of solid spheres onto a solid plate in an ambient fluid which is either a gas or a liquid. In particular, we measure the coefficient of restitution e as a function of the Stokes number, St, ratio of the particle inertia to the viscous forces. The coefficient e is zero at small St, increases monotonically with St above the critical value Stc and reaches an asymptotic value at high St corresponding to the classical “dry” value emax measured in air or vacuum. This behavior is observed for a large range of materials and a master curve e/emax=f(St) is obtained. If gravity is sufficient to describe the rebound trajectory (after the collision) in a gas, this is not the case in a liquid where drag and added-mass effect are important but not sufficient: History forces are shown to be non-negligible even at large Reynolds number.

Three regimes of granular avalanches in fluids are put in light depending on the Stokes number St which prescribes the relative importance of grain inertia and fluid viscous effects, and on the grain/fluid density ratio r. In gas (r ≫ 1 and St > 1, e.g., the dry case), the amplitude and time duration of avalanches do not depend on any fluid effect. In liquids (r ∼ 1), for decreasing St, the amplitude decreases and the time duration increases, exploring an inertial regime and a viscous regime. These regimes are described by the analysis of the elementary motion of one grain. Granular matter has received much attention from physicists over the past few years [1]. Beyond the fundamental interest in the physics of granular systems which can present some features of either solids, liquids or even gases, the understanding of granular materials is essential in many industrial activities such as pharmacology, chemical engineering, food, agriculture, and so on. Many studies concern the avalanches that may arise on the slope of a granular pile in air. Such granular avalanches occur in various places in Nature, from small scale, as for the building of any sand pile, to large scale, as the event observed after the Mont St-Helen eruption in 1980. Two angles can be defined when building a pile: the maximum angle of stability θ m at which an avalanche starts and the angle of repose θ r at which the avalanche stops. Between these two angles is a region of bistability where the grains can either be flowing ("liquid state") or at rest ("solid state"). Many experiments performed with dry grains in a rotating cylinder [2,3,4,5,6] showed clearly the existence of these two angles.To date, no detailed study has focused on the influence of the interstitial fluid for a totally immersed grain assembly. This influence is certainly important in granular avalanche processes, as evidenced by the marked differences observed by geologists between subaqueous and eolian cross strata [7]. As a matter of fact, the propagation of subaqueous dunes differs in general from the propagation of eolian dunes even if the slope angles are quite similar: When the transport rate of sand particles is large enough, the flow is continuous in the lee side of the structure in the immersed case, but occurs by successive avalanches in the dry case [7]. This observation prompted geologists to accumulate data on avalanches of sand or beads in rotating drums filled with air or water [8] or even with glycerol mixtures [9], that seemed to show that the amplitude of avalanches decreases and the time duration increases with the fluid viscosity. We have performed an extensive series of experiments to investigate the influence of the interstitial fluid on the packing stability and the avalanche dynamics. The analysis of our results obtained with a rotating drum set-up indicate the existence of three regimes: (i) a free-fall regime for which there is no fluid influence and that corresponds to the classical dry regime, and two regimes where the interstitial fluid governs the a...

We study experimentally the parallel flow in a Hele-Shaw cell of two immiscible fluids, a gas and a viscous liquid, driven by a given pressure gradient. We observe that the interface is destabilized above a critical value of the gas flow and that waves grow and propagate along the cell. The experimental threshold corresponds to a velocity difference of the two fluids in good agreement with the inviscid Kelvin-Helmholtz instability, while the wave velocity corresponds to a pure viscous theory deriving from Darcy's law. We report our experimental results and analyze this instability by the study of a new equation where the viscous effects are added to the Euler equation through a unique drag term. The predictions made from the linear stability analysis of this equation agree with the experimental measurements.