Temperature anisotropy in a driven granular gas

When smooth granular material is fluidized by vertically shaking a container, we find that the temperature in the direction of energy input always exceeds the temperature in the other directions. An analytical model is presented which shows how the anisotropy can be traced back to the inelasticity of the interparticle collisions and the collisions with the wall. For a dilute granular gas, the model compares very well with molecular-dynamics simulations. It is concluded that any non-isotropic driving of a granular gas in a steady state necessarily causes anisotropy of the granular temperature

Breaking waves on a dynamic Hele-Shaw beach

We report the formation of quasi-steady beaches and dunes via breaking waves in our tabletop ‘Hele-Shaw’ beach experiment. Breaking waves are generated by a wave maker, and zeolite particles act as sand. The tank is narrow, just over one-particle diameter wide, creating a quasi-2D set-up. Classical breaker types are observed on a time-scale of about a second. Beach formation under breakers occurs on a longer time-scale, and is a matter of minutes for a range of mono-chromatic wave frequencies. Alternating the wave maker motion between two frequencies generally leads to beach formation but occasionally to formation of a stable dune with water on either side. Finally, the Hele-Shaw configuration explored here experimentally lends itself to multi-scale modeling of beach dynamics

Revisiting Hele-Shaw dynamics to better understand beach evolution

Wave action, particularly during storms, drives the evo lution of beaches. Beach evolution by non-linear break ing waves is poorly understood due to its three-dimensional character, the range of scales involved, and our limited understanding of particle-wave interactions. We show how a novel, three-phase extension to the classic “Hele-Shaw” laboratory experiment can be designed that creates beach morphologies with breaking waves in a quasi-two-dimensional setting. Our thin Hele-Shaw cell simplifies the inherent complexity of three-phase dynamics: all dynamics become clearly visible and measurable. We show that beaches can be created in tens of minutes by several types of breaking waves, with about one-second periods. Quasi-steady beach morphologies emerge as function of initial water depth, at-rest bed level and wave-maker frequency. These are classified mathematically and lead to beaches, berms and sand bars

Giant bubble pinch-off

Self-similarity has been the paradigmatic picture for the pinch-off of a drop. Here we will show through high-speed imaging and boundary integral simulations that the inverse problem, the pinch-off of an air bubble in water, is not self-similar in a strict sense: A disk is quickly pulled through a water surface, leading to a giant, cylindrical void which after collapse creates an upward and a downward jet. Only in the limiting case of large Froude number the neck radius $h$ scales as $h(-\log h)^{1/4} \propto \tau^{1/2}$, the purely inertial scaling. For any finite Froude number the collapse is slower, and a second length-scale, the curvature of the void, comes into play. Both length-scales are found to exhibit power-law scaling in time, but with different exponents depending on the Froude number, signaling the non-universality of the bubble pinch-off.Comment: 5 pages, 2 figures. Figure quality was reduced considerably and converted to greyscale to decrease file siz

Air-induced inverse Chladni patterns

When very light particles are sprinkled on a resonating horizontal plate, inverse Chladni patterns are formed. Instead of going to the nodal lines of the plate, where they would form a standard Chladni pattern, the particles are dragged to the antinodes by the air currents induced by the vibration of the plate. Here we present a detailed picture of the mechanism using numerical simulations involving both the particles and the air. Surprisingly, the time-averaged Eulerian velocity, commonly used in these type of problems, does not explain the motion of the particles: it even has the opposite direction, towards the nodal lines. The key to the inverse Chladni patterning is found in the averaged velocity of a tracer particle moving along with the air: this Lagrangian velocity, averaged over a vibration cycle, is directed toward the antinodes. The Chladni plate thus provides a unique example of a system in which the Eulerian and Lagrangian velocities point in opposite direction

From the granular Leidenfrost state to buoyancy-driven convection

Grains inside a vertically vibrated box undergo a transition from a density-inverted and horizontally homogeneous state, referred to as the granular Leidenfrost state, to a buoyancy-driven convective state. We perform a simulational study of the precursors of such a transition and quantify their dynamics as the bed of grains is progressively fluidized. The transition is preceded by transient convective states, which increase their correlation time as the transition point is approached. Increasingly correlated convective flows lead to density fluctuations, as quantified by the structure factor, that also shows critical behavior near the transition point. The amplitude of the modulations in the vertical velocity field are seen to be best described by a quintic supercritical amplitude equation with an additive noise term. The validity of such an amplitude equation, and previously observed collective semiperiodic oscillations of the bed of grains, suggests a new interpretation of the transition analogous to a coupled chain of vertically vibrated damped oscillators. Increasing the size of the container shows metastability of convective states, as well as an overall invariant critical behavior close to the transition

Suction of splash after impact on dry quick sand

It is well known that a splash occurs when an object impacts at high velocity on a liquid¿s surface. If the impact is fast enough, surface tension and air pressure gradients pull the crown-shape splash all the way towards the axis of symmetry, making it to collapse and seal the surface. In this paper we show that splash and surface sealing are also observed in impacts on soft, dry sand. We observe influence of air pressure and grains size on the shape of the splash. By tracking individual grains using high-speed imaging we calculate their acceleration, which results from gravity and drag forces. Assuming friction drag parallel, and pressure drag perpendicular to the direction of motion of grains we estimate the friction and pressure drag contributions to the drag force. Our results support the idea that pressure drag from Bernoulli effect is at the origin of the surface seal

Role of Air in Granular Jet Formation

A steel ball impacting on a bed of very loose, fine sand results in a surprisingly vigorous jet which shoots up from the surface of the sand [D. Lohse et al., Phys. Rev. Lett. 93, 198003 (2004)]. When the ambient pressure p is reduced, the jet is found to be less vigorous [R. Royer et al., Nature Phys. 1, 164 (2005)]. In this Letter we show that p also affects the rate of penetration of the ball: Higher pressure increases the rate of penetration, which makes the cavity created by the ball close deeper into the sand bed, where the hydrostatic pressure is stronger, thereby producing a more energetic collapse and jetting. The origin of the deeper penetration under normal ambient pressure is found to lie in the extra sand fluidization caused by the air flow induced by the falling ball