Maxime Nicolas

Submarine granular flows down inclined planes

Submarine flows of granular material down a rough inclined plane are experimentally investigated. We focus on the dense flow regime when the whole sediment layer is flowing down the slope and when no deposition nor entrainment occurs. In this regime, steady uniform flows are observed for which we systematically measure the depth-averaged velocity, the thickness, and the excess pore pressure for different inclinations and different flow rates. The experimental measurements are analyzed within a theoretical approach inspired by recent results obtained for dry granular flows. The basic assumption of the model is that the constitutive law obtained in the dry case still holds for submarine flow, if one substitutes the inertial time scale coming into play in the rheology by a viscous time scale. The agreement between the measurements and the theory supports this assumption.

Falling clouds of particles in viscous fluids

We have investigated both experimentally and numerically the time evolution of clouds of particles settling under the action of gravity in an otherwise pure liquid at low Reynolds numbers. We have found that an initially spherical cloud containing enough particles is unstable. It slowly evolves into a torus which breaks up into secondary droplets which deform into tori themselves in a repeating cascade. Owing to the fluctuations in velocity of the interacting particles, some particles escape from the cloud toroidal circulation and form a vertical tail. This creates a particle deficit near the vertical axis of the cloud and helps in producing the torus which eventually expands. The rate at which particles leak from the cloud is influenced by this change of shape. The evolution toward the torus shape and the subsequent evolution is a robust feature. The nature of the breakup of the torus is found to be intrinsic to the flow created by the particles when the torus aspect ratio reaches a critical value. Movies are available with the online version of the paper.

Flow of dense granular material: towards simple constitutive laws

Recent experiments and numerical simulations of dry granular flows suggest that a simple rheological description in terms of a friction coefficient varying both with shear rate and pressure through a dimensionless inertial number may be sufficient to capture the major properties of granular flows. In this paper we first present the empirical constitutive laws and their interpretation using dimensional analysis, before analysing the prediction for different flow configurations. The successes and limits of the approach are discussed based on comparison with recent studies.

Initiation of underwater granular avalanches: Influence of the initial volume fraction

We experimentally investigate how a layer of granular material fully immersed in a liquid starts to flow when suddenly inclined from a horizontal position. The flow is shown to strongly depend on the initial volume fraction, its initiation being dramatically delayed by a slight initial compaction. A model, which takes into account the dilatant behavior of the granular material and the coupling with the interstitial fluid, captures the main experimental features.

Compaction of a granular material under cyclic shear

Abstract:In this paper we present experimental results concerning the compaction of a granular assembly of spheres under periodic shear deformation. The dynamics of the system is slow and continuous when the amplitude of the shear is constant, but exhibits rapid evolution of the volume fraction when a sudden change in shear amplitude is imposed. This rapid response is shown to be uncorrelated with the slow compaction process.

Constitutive laws in liquid-fluidized beds

The objective of the present work is to test experimentally the two-phase modelling approach which is widely used in fluidization. A diculty of this way of modelling fluidized beds is the use of empirical relations in order to close the system of equations describing the fluidized bed as a two-phase continuum, especially concerning the description of the solid phase. We performed an experimental investigation of the primary wavy instability of liquid-fluidized beds. Experiments demonstrate that the wave amplitude saturates up the bed and we were able to measure the precise shape of this voidage wave. We then related this shape to the unknown solid phase viscosity and pressure functions of a simple two-phase model with a Newtonian stress-tensor for the solid phase. We found the scaling laws and the particle concentration dependence for these two quantities. It appears that this simplest model is quite satisfactory to describe the one-dimensional voidage waves in the limited range of parameters that we have studied. In our experimental conditions, the drag on the particles nearly balances their weight corrected for buoyancy, the small imbalance being mostly accounted for by solid phase viscous stress with a much smaller contribution from the solid phase pressure.

A falling cloud of particles at a small but finite Reynolds number

Through a comparison between experiments and numerical simulations, we have examined the dynamics of a cloud of spheres at a small but finite Reynolds number. The cloud is seen to flatten and to transition into a torus, which further widens and eventually breaks up into droplets. While this behaviour bears some similarity to that observed at zero inertia, the underlying physical mechanisms differ. Moreover, the evolution of the cloud deformation is accelerated as inertia is increased. Two inertial regimes in which macro-scale inertia and micro-scale inertia become successively dominant are clearly identified.

Spreading of a drop of neutrally buoyant suspension

The spreading of suspension drops on a flat surface is studied using mixtures of liquid and density-matched particles. As expected from an energy balance model, the spreading factor is reduced with increasing particle volume fraction. This decrease is quantitatively understood through an effective viscosity coefficient. However, for large drop Reynolds number, the particles are not uniformly distributed into a spread drop but form an annulus. For higher impact velocities, a particle-induced break-up of the drop is observed.

Experimental study of gravity-driven dense suspension jets

This article presents experimental results from a study of a jet of dense suspension falling under gravity in a quiescent liquid bath of miscible liquid. The initial jet velocity v0 scales with the square of the initial jet diameter. Four different flow behaviors are observed. The jet remains cylindrical and stable when viscous forces are dominant. A capillary-like instability with formation of blobs occurs when a Reynolds number based on the particle diameter and a free-falling velocity is over unity. The blobs are stable and settle without changing shape only for a blob Reynolds number below a critical number. Dispersion of the jet particles is observed when the particle Reynolds number is over 1, and an atomization behavior occurs when particle inertia is large compared to viscous forces, i.e., when the Stokes number of the particles is large compared to unity.

Experimental investigations on the nature of the first wavy instability in liquid‐fluidized beds

Experiments are described which suggest that the first wavy instability of fluidized beds is convective in nature. In particular, this instability is shown to be sensitive to a harmonic forcing localized at the bottom of the bed.