Eric Lajeunesse

Spreading of a granular mass on a horizontal plane

The transient surface flow occurring when a cylindrical pile of dry granular material is suddenly allowed to spread on a horizontal plane is investigated experimentally as a function of the released mass M, the initial aspect ratio a of the granular cylinder pile, the properties of the underlying substrate (smooth or rough, rigid or erodible) and the bead size. Two different flow regimes leading to three different deposit morphologies are observed as a function of the initial aspect ratio a, whatever the substrate properties and the bead size. For a≲3, the granular mass spreads through an avalanche on its flanks producing either truncated cone or conical deposits. For a≳3, the upper part of the column descends conserving its shape while the foot of the pile propagates radially outward. The obtained deposit looks like a “Mexican hat” and the slope angle at the foot of the deposit is observed to saturate at a value of the order of 5°. For a given ground and bead size, the flow dynamics and the deposit morph...

Granular slumping on a horizontal surface

We report the results of an experimental investigation of the flow induced by the collapse of a column of granular material (glass beads of diameter d) over a horizontal surface. Two different setups are used, namely, a rectangular channel and a semicircular tube, allowing us to compare two-dimensional and axisymmetric flows, with particular focus on the internal flow structure. In both geometries the flow dynamics and the deposit morphologies are observed to depend primarily on the initial aspect ratio of the granular column a=Hi∕Li, where Hi is the height of the initial granular column and Li its length along the flow direction. Two distinct regimes are observed depending on a: an avalanche of the column flanks producing truncated deposits for small a and a column free fall leading to conical deposits for large a. In both geometries the characteristic time scale is the free fall of the granular column τc=Hi∕g. The flow initiated by Coulomb-like failure never involves the whole granular heap but remains ...

Miscible displacement in a Hele-Shaw cell at high rates

We study experimentally and theoretically the downward vertical displacement of one miscible fluid by another lighter one in the gap of a Hele-Shaw cell at suciently high velocities for diusive eects to be negligible. Under certain conditions on the viscosity ratio, M, and the normalized flow rate, U, this results in the formation of a two-dimensional tongue of the injected fluid, which is symmetric with respect to the midplane. Thresholds in flow rate and viscosity ratio exist above which the twodimensional flow destabilizes, giving rise to a three-dimensional pattern. We describe in detail the two-dimensional regime using a kinematic wave theory similar to Yang & Yortsos (1997) and we delineate in the (M;U)-plane three dierent domains, characterized respectively by the absence of a shock, the presence of an internal shock and the presence of a frontal shock. Theoretical and experimental results are compared and found to be in good agreement for the rst two domains, but not for the third domain, where the frontal shock is not of the contact type. An analogous treatment is also applied to the case of axisymmetric displacement in a cylindrical tube.

Interface instabilities during displacements of two miscible fluids in a vertical pipe

We study experimentally the downward vertical displacement of one miscible fluid by another in a vertical pipe at sufficiently high velocities for diffusive effects to be negligible. For certain viscosity ratios and flow rates, the interface between the two fluids can destabilize. We determine the dimensionless flow rate Uc above which the instability is triggered and its dependence on the viscous ratio M, resulting in a stability map Uc=Uc(M). Two different instability modes have been observed: an asymmetric “corkscrew” mode and an axisymmetric one. We remark that the latter is always eventually disturbed by “corkscrew” type instabilities. We speculate that these instabilities are driven by the viscosity stratification and are analogous to those already observed in core annular flows of immiscible fluids.

Cross-stream diffusion in bedload transport

We investigate experimentally the statistical properties of bedload transport induced by a steady, uniform, and laminar flow. We focus chiefly on lateral transport. The analysis is restricted to experiments where the flow-induced shear stress is just above the threshold for sediment transport. We find that, in this regime, the concentration of moving particles is low enough to neglect interactions between themselves. We can therefore represent bedload as a thin layer of independent walkers travelling over the bed surface. In addition to their downstream motion, the particles show significant fluctuations of their cross-stream velocity, likely due to the roughness of the underlying sediment bed. This causes particles to disperse laterally. Based on thousands of individual trajectories, we show that this lateral spreading is the manifestation of a random walk. The experiments are entirely consistent with Fickian diffusion.

Orography-driven chemical denudation in the Lesser Antilles: Evidence for a new feed-back mechanism stabilizing atmospheric CO2

In this paper we present chemical composition data for major elements in rivers from three islands of the Lesser Antilles. The Lesser Antilles are a tropical volcanic subduction arc and are characterized by steep gradients of relief, bedrock age and precipitation. They constitute a natural laboratory where the response of the weathering engine to large variations of runoff can be understood. Data indicate that the Lesser Antilles are characterized by extremely variable chemical weathering (40-430 t/km2/a) and CO2 consumption (300-3500.103 mol/km2/a) rates, amongst the highest found on Earth and consistent with the previous studies on the weathering of volcanic rock. A noteworthy observation is that, along the runoff gradient, concentrations of rock-derived solutes do not follow a pure dilution law and that a buffering mechanism exists stabilizing solute concentrations. As a result concentrations vary much less than runoff and chemical weathering rates are mainly controlled by runoff. Precipitation patterns in the Lesser Antilles are essentially orographic and controlled by the adiabatic decompression of the water-saturated Atlantic air masses. The production of acidity by volcanic degassing is an additional factor that modulates the runoff effect. Two main conclusions can be drawn from this study. First, chemical weathering fluxes of oceanic islands are strongly dependent upon relief repartition, which cautions the use of regional mean values to compare volcanic islands. Second, volcanic activity in the Lesser Antilles subduction arc, by creating relief, promotes high orographic precipitation and/or infiltration regimes, that in turn results in elevated chemical weathering and atmospheric CO2 consumption fluxes. This feedback mechanism, implying mainly precipitation and relief, is proposed to act in complement to the temperature-related feedback proposed by previous authors for stabilizing the atmospheric CO2 content of the atmosphere in response to volcanic CO2 degassing. This study highlights the importance of the water cycle in controlling chemical weathering of volcanic arc islands and associated CO2 consumption rates.

New insights on the runout of large landslides in the Valles‐Marineris canyons, Mars

[1] Analogy with lab-scale dry granular flow experiments demonstrates that runouts and deposits heights of Valles-Marineris (VM) landslides can be scaled on a curve varying primarily with the initial aspect ratio of the mobilized rock mass (before slope failure). This results suggests both that any interstitial fluid played a negligible part in the VM landslides dynamics and that mobility is not an appropriate tool to characterize their dynamics.

Thermocapillary migration of long bubbles in polygonal tubes. II. Experiments

We study experimentally the thermocapillary migration of a long gas bubble in a horizontal pipe of rectangular cross section. An imposed axial temperature gradient produces a gradient of surface tension leading to a steady migration of the bubble towards the hotter region. The bubble velocity is found to be independent of bubble length for sufficiently long bubbles, and to vary linearly with the temperature gradient. For pipes of small vertical dimension, gravity is negligible and the measurements of the bubble velocity are in good agreement with the zero gravity theory of Mazouchi and Homsy [Phys. Fluids 13, 1594 (2001)]. In larger pipes, gravity is no longer negligible and the bubble is observed to move faster than that theory predicts. A simple calculation taking into account the combined effect of buoyant rise and thermocapillary stress accounts for these results.

The threshold of the instability in miscible displacements in a Hele–Shaw cell at high rates

For sufficiently large viscosity ratios and injection rates, miscible displacements in a vertical Hele–Shaw cell at high rates become unstable, leading to three-dimensional (3D) fingering patterns. Below the instability threshold, the base state is 2D in the form of a “tongue” of constant thickness. We apply the long wave Saffman–Taylor stability analysis to find an expression for the threshold of instability as a function of the viscosity ratio and the injection rate. The results are in agreement with the experimental data.

Flow Rule, Self-Channelization, and Levees in Unconfined Granular Flows

Unconfined granular flows along an inclined plane are investigated experimentally. During a long transient, the flow gets confined by quasistatic banks but still spreads laterally towards a well-defined asymptotic state following a nontrivial process. Far enough from the banks a scaling for the depth averaged velocity is obtained, which extends the one obtained for homogeneous steady flows. Close to jamming it exhibits a crossover towards a nonlocal rheology. We show that the levees, commonly observed along the sides of the deposit upon interruption of the flow, disappear for long flow durations. We demonstrate that the morphology of the deposit builds up during the flow, in the form of an underlying static layer, which can be deduced from surface velocity profiles, by imposing the same flow rule everywhere in the flow.