Recirculation cells in a wide channel

International audienceSecondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the entire channel. They are arranged in pairs of counter-rotating vortices aligned with the primary flow. Their existence away from the river banks contradicts the usual interpretation of these secondary flow structures, which invokes the influence of boundaries. Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer

Diffusive Evolution of Experimental Braided Rivers

Water flowing over a loose granular bed organizes into a braided river, a network of ephemeral and interacting channels. The temporal and spatial evolution of this network of braided channels is not yet quantitatively understood. In ∼1 m-scale experiments, we found that individual channels exhibit a self-similar geometry and near-threshold transport conditions. Measurements of the rate of growth of topographic correlation length scales, the time scale of system-slope establishment, and the random spatial decorrelation of channel locations indicate together that the evolution of the braided river system may be diffusive in nature. This diffusion is due to the separation of scales between channel formation and network evolution, and the random motion of interacting channels when viewed at a coarse-grained scale

Starting laminar plumes: Comparison of laboratory and numerical modeling

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94953/1/ggge1631.pd

Fully determined three-dimensional velocity field in a divergence-free convection experiment with rigid boundary conditions

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Birth and Decline of Magma Oceans in Planetesimals: 2. Structure and Thermal History of Early Accreted Small Planetary Bodies

International audienceThis is the second of two companion papers that present a theoretical and experimental study ofthe thermal history of planetesimals in which heating by short-lived radioactive isotopes generates an internalmagma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to formand preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetarybodies. Our model predicts that planetesimals larger than 30 km can reach 1300°C and a melt fraction of 40vol%, producing a solid-like to liquid-like rheological transition that triggers an internal magma ocean. In themagma ocean regime core-mantle differentiation occurs very quickly and the mantle convects under a relicof chondritic material whose thickness is controlled by the temperature of rheological transition. We showthat the magma ocean episode is associated with time-dependent crystal segregation and no re-entrainment.Segregation of crystals is essentially constrained by their size and by their density difference with respect to themelt, the latter being fully determined by the planetesimal's initial composition. Olivine cumulates are likely toform at the core-mantle boundary. Under certain particular conditions, a flotation crust can also form, whichreduces the efficiency of heat evacuation by convection, thereby enhancing the magma ocean's lifetime and theefficiency of crystal segregation. Two types of large-scale mantle structure are possible outcomes: a well-mixedupper mantle above an olivine cumulate, or a more finely layered “onion-shell” structure

The fate of particles in a volumetrically heated convective fluid at high Prandtl number

International audienceThe dynamics of suspensions plays a crucial role in the evolution of geophysical systems such as lava lakes, magma chambers and magma oceans. During their cooling and solidification, these magmatic bodies involve convective viscous fluids and dispersed solid crystals that can form either a cumulate or a floating lid by sedimentation. We study such systems based on internal heating convection experiments in high Prandtl fluids bearing plastic beads. We aim to determine the conditions required to produce a floating lid or a sedimented deposit. We show that, although the sign of particles buoyancy is the key parameter, it is not sufficient to predict the particles fate. To complement the model we introduce the Shields formalism and couple it with scaling laws describing convection. We propose a generalized Shields number that enables a self-consistent description of the fate of particles in the system, especially the possibility to segregate from the convective bulk. We provide a quantification of the partition of the mass of particles in the different potential reservoirs (bulk suspension, floating lid, settled cumulate) through reconciling the suspension stability framework with the Shields formalism. We illustrate the geophysical implications of the model by revisiting the problem of the stability of flotation crusts on solidifying rocky bodies

Birth and Decline of Magma Oceans in Planetesimals: 1. Experimental Study of Erosion and Deposition of Particles in an Internally Heated Convecting Fluid

International audienceThis paper is the first of two companion papers presenting a theoretical and experimental studyof the evolution of crystallizing magma oceans in planetesimals. We aim to understand the behavior of crystalsformed in a convective magma ocean, and the implications of crystal segregation for the thermal and structuralevolution of the convective system. In particular, we wish to constrain the possibility to form and preservecumulates and/or flotation crusts by sedimentation or flotation of crystals respectively. We use lab-scaleanalog experiments to study the stability and the erosion of a floating lid composed of plastics beads lyingover a convective viscous fluid volumetrically heated by microwave absorption. We propose a law for erosionand re-entrainment that depends only on two dimensionless numbers that govern these phenomena: (a) theRayleigh-Roberts number, characterizing the vigor of convection and (b) the Shields number, that encompassesthe physics of the flow-particle interaction. We further consider the formation of a cumulate at the base ofthe convective layer by sedimentation of beads that are denser than the fluid. We find that particle depositionoccurs at a velocity that scales with the Stokes velocity, a result consistent with previous experimental studies.We build up a model that describes the transient evolution of the convective system's thermal state and thefraction of particles that segregated from the flow or that remain in suspension

Early accretion of planetesimals unraveled by the thermal evolution of the parent bodies of magmatic iron meteorites

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