Charles Dubois

Rotation of melting ice disks due to melt fluid flow.

We report experiments concerning the melting of ice disks (85 mm in diameter and 14 mm in height) at the surface of a thermalized water bath. During the melting, the ice disks undergo translational and rotational motions. In particular, the disks rotate. The rotation speed has been found to increase with the bath temperature. We investigated the flow under the bottom face of the ice disks by a particle image velocimetry technique. We find that the flow goes downwards and also rotates horizontally, so that a vertical vortex is generated under the ice disk. The proposed mechanism is the following. In the vicinity of the bottom face of the disk, the water eventually reaches the temperature of 4 °C for which the water density is maximum. The 4 °C water sinks and generates a downwards plume. The observed vertical vorticity results from the flow in the plume. Finally, by viscous entrainment, the horizontal rotation of the flow induces the solid rotation of the ice block. This mechanism seems generic: any vertical flow that generates a vortex will induce the rotation of a floating object.Large circular ice blocks up to 80 m of diameter have been observed on frozen river around the world. This rare event has been reported in a publication in 1993. This fascinating self-fashioned object slowly rotates at about 1 per second. In this paper, we report a model experiment consisting in a 85 mm of diameter ice disc at the surface of a thermalised pool. The rotation speed has been found to increase with the bath temperature. Using particle image velocimetry technique, we evidence the presence of a vortex below the ice block. This vortex results from the descending flow of high density water at 4C. The vorticity of the vortex induces the rotation of the ice block. This mechanism is generic of any vertical flow that generates a vortex which induces the rotation of a floating object.

Between inertia and viscous effects: Sliding bubbles beneath an inclined plane

The ascent motion of an air bubble beneath an inclined plane is experimentally studied. The effects of the surrounding liquid viscosity and surface tension, the bubble radius and the tilt angle are investigated. A dynamical model is proposed. It opposes the buoyant driving force to the hydrodynamical pressure arising from the bubble motion and the capillary meniscus generated in front of the bubble in order to create a lubrication film between the bubble and the plate. This model is compared to experimental data and discussed.

STUDIES ON CONVECTIVE COOLING OF CRYOGENIC FLUIDS TOWARDS SUPERCONDUCTING APPLICATIONS

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form of N-S equations is solved to obtain the detailed flow features through path line profiles, isotherm contours and velocity vectors. The effect of heater aspect ratio (x/L) on the rate of heat transfer is studied in terms of the average Nusselt number (Nuave). The results indicate that effective heat transfer enhancement occurs for a small heater length, resulting in an efficient cooling. Increasing the heater length will favor heat transfer through conduction over convection. The maximum temperature difference across the fluid and the velocity magnitude are found to decrease with heater length. 3D and 2D results are in agreement for short heater lengths, but vary for higher heater lengths, presumably due to the essential effect of the heater width. Further analysis on different types of coolant reveals a constant correlation between Nuave and the Rayleigh number (Ra), with Nuave ~ Ra0.374. Benchmark validation for natural convection in a square enclosure is found to be satisfactory against the reported results.

Locally induced laminar convection in liquid nitrogen and silicone oils

Abstract.We present an experimental study of a laminar convective phenomenon induced by a centimetric heater totally immersed in a liquid pool (Rayleigh number ranging from 104 to 107). This local heating is observed to induce a laminar convection that differs from the classical Rayleigh-Bénard cells created by heating the whole bottom of the fluid: the convection pattern is no more periodic. In order to obtain a complete map of the velocity field, we use Particle Image Velocimetry technique. The vertical velocity between the counter-rotating convective cells is used as the relevant physical parameter to describe the phenomenon. The potential cooling applications of this problem lead us to choose liquid nitrogen as an experimental fluid. We thus compare the results obtained for various temperature gradients in liquid nitrogen with experiments performed at room temperature with silicone oils of various viscosities. The theoretical law for the maximal vertical velocity from classical Rayleigh-Bénard experiments is adapted to the specific geometry investigated by using a new definition for the characteristic wavelength. This length is studied and appears to be dependent on the liquid properties. We finally obtain a remarkable agreement between theory and experimental data.Graphical abstract