Effects on the free surface of a turbulent flow

Abstract : We study surface manifestations of a turbulent flow from an experimental point of view. Specifically we study a turbulent flow in a thin layer of fluid (a liquid metal) with free surface. The flow is generated with an electromagnetic force. It exhibits interacting vortices, shear bands and waves, depending on the forcing conditions. We explored three consequences of the horizontal turbulent motion as observed on the surface: Surface deformation itself; the effects on propagating waves; and the effects on floating particles. Concerning the surface deformation: when the forcing strength is increased, we observe a linear increase of the surface level r.m.s. fluctuations up to 10% of the liquid layer thickness. Largest deformations, however, can reach a half of the layer thickness. Surface deformation is mainly produced by vortices, thus it is asymmetric through values under the mean. This contrast with observations in random sea waves and wave turbulence, were an asymmetry appears as well --this time through values above the mean--, as a consequence of sharp crests in steep gravity waves. The frequency spectrum of the deformation follows a power-law with an exponent close to -5, similar to the singularity spectrum of Phillips. Thus, we presented the statistical signature of vortical motion. We considered another aspect of the wave-turbulence relation: we mechanically induced a monochromatic wave over the turbulent flow. We measure a reduction and widening of the wave spectral peak that happens when turbulent motion is increased. Also, we computed coherent averages to emphasize the wavy part of the signal. We observe a spatial decay in wave content when turbulent motion is increased. Therefore, we quantified the enhancement of wave attenuation due to turbulence, and we observed its non trivial dependence on the wave frequency. Concerning dynamics of floating particles: We observe that particles have the tendency to form clusters, and we confirm this observation by developing a statistical method based on the areas defined by the position of three nearest neighbors. This tool allows us to clearly identify particles belonging to a cluster. Indeed, clustered particles exhibit much stronger velocity and angular correlations than the unconditioned case. Several mechanisms are susceptible to induce clustering of floating particles. We identify (i) particles' inertia, (ii) upwelling/downwelling flows and (iii) surface tension. For each mechanism we construct suitable quantities, which we correlate with the cumulated concentration of particles. These correlations suggest upwelling and downwelling motions as responsible for particles clustering.
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Submitted on : Monday, December 23, 2013 - 4:27:27 AM
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  • HAL Id : pastel-00921954, version 1

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Pablo Gutiérrez-Matus. Effects on the free surface of a turbulent flow. Fluid Dynamics [physics.flu-dyn]. Ecole Polytechnique X, 2013. English. ⟨pastel-00921954⟩

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