Wavelength scaling of spiral patterns formed by granular media underneath a rotating fluid

Chain formation of spheres in oscillatory fluid flows

A collection of spherical particles subjected to horizontal oscillatory fluid flow is known to form chains perpendicular to the direction of the oscillation. We have developed computer simulations to model such a system and have validated them against experiments carried out in a small fluid-filled cell. In both experiment and simulation we find that the particles go through the same stages of evolution from a dispersed initial configuration to an ordered chain structure. We then use our computer simulations to investigate in detail the interactions responsible for chain formation and the interaction between fully formed chains.

Oscillation-induced sand ripples in a circular geometry

This study deals with the observation of sand ripples in a circular geometry under oscillatory flow. We characterize the observed patterns as a function of the excitation parameters. We report the time evolution of the corrugated front invading the flat bed. These experiments reveal unambiguously, because of the gradient of shear stress, the existence of two separated thresholds: one for grain motion and the other for the appearance of ripples. In addition, we display the phase diagram of this instability as a function of the Froude number and a Reynolds number.

Micrometric granular ripple patterns in a capillary tube

The oscillatory motion of a fluid carrying micron-sized particles inside a capillary tube is investigated experimentally. It is found that initially uniformly distributed particles can segregate and accumulate to form regularly spaced micron-sized particle clusters. The wavelength of the microclusters is compared to data for macroscale sand-ripple patterns and found to obey the same universal scaling as these. A dimensional analysis is performed that confirms the universality of the experimentally observed scaling. The experimental data for the microripple clusters further suggest the existence of a minimum particle length scale for which patterns can form and below which the Brownian motion associated with the molecules of the matrix fluid inhibits pattern formation.

Granular ripples under rotating flow: a new experimental technique for studying ripples in non-rotating, geophysical applications?

A review of our research investigating a new pattern formation process in granular material underlying a rotating fluid is given. The purpose of this summary is to introduce the phenomenon to the geophysical research community and to draw attention to the potential practical benefits of our new experimental method. To this end, the applied and scientific advantages of the technique over traditional studies employing, for instance, water channels, are discussed for the first time. It is shown here that the system rotation in our new technique does not appear to affect the scaling law expressing the dependence of the ripple-pattern wavelength on the governing independent experimental parameters. This suggests that it may become possible to extrapolate appropriate results from rotating to non-rotating systems and, hence, to geophysical environments. Consequently, our new technique may find applications in the context of geophysical research on the formation of sedimentary granular ripple structures.

Granular spirals on erodible sand bed submitted to a circular fluid motion

An experimental study of a granular surface submitted to a circular fluid motion is presented. The appearance of an instability along the sand-water interface is observed beyond a critical radius r(c). This creates ripples with a spiral shape on the granular surface. A phase diagram of such patterns is constructed and discussed as a function of the rotation speed omega of the flow and as a function of the height of water h above the surface. The study of r(c) as a function of h, omega, and r parameters is reported. Thereafter, r(c) is shown to depend on the rotation speed according to a power law. The ripple wavelength is found to decrease when the rotation speed increases and is proportional to the radial distance r. The azimuthal angle epsilon of the spiral arms is studied. It is found that epsilon scales with homegar. This lead to the conclusion that epsilon depends on the fluid momentum. Comparison with experiments performed with fluids allows us to state that the spiral patterns are not the signature of an instability of the boundary layer.

Universal scaling for ripple formation in granular media

The wavelength scaling of ripple patterns formed by granular materials underneath flowing fluids is investigated. Experimental results from five systems involving substantially different experimental conditions are compared to each other. The data analysis reveals that all systems display a common, global scaling behavior for the onset of ripple formation on short time scales. This suggests the existence of common physical mechanisms governing ripple formation in these systems.