On the decrease of intermittency in decaying rotating turbulence

The scaling of the longitudinal velocity structure functions, $S_q(r) = < | \delta u (r) |^q > \sim r^{\zeta_q}$, is analyzed up to order $q=8$ in a decaying rotating turbulence experiment from a large Particle Image Velocimetry (PIV) dataset. The exponent of the second-order structure function, $\zeta_2$, increases throughout the self-similar decay regime, up to the Ekman time scale. The normalized higher-order exponents, $\zeta_q / \zeta_2$, are close to those of the intermittent non-rotating case at small times, but show a marked departure at larger times, on a time scale $\Omega^{-1}$ ($\Omega$ is the rotation rate), although a strictly non-intermittent linear law $\zeta_q / \zeta_2 = q/2$ is not reached.Comment: 5 pages, 5 figures. In revision for Phys. Fluids Letter

Competitive dynamics of two erosion patterns around a cylinder

We investigate experimentally the local erosion of a granular bed near a fixed vertical cylinder that emerges from the bed. The onset of erosion arising at the base of the cylinder and usually ascribed to the wrapping horseshoe vortex is determined and rationalized by a flow contraction effect. We report a new erosion pattern visible downstream of the cylinder that consists of two side-by-side elongated holes. This pattern is observed for flow regimes close to the horseshoe scour onset, whose growth usually inhibits its spatiotemporal development.Comment: 6 pages, 6 figure

Experimental investigation of tsunami waves generated by granular collapse into water

The generation of a tsunami wave by an aerial landslide is investigated through model laboratory experiments. We examine the collapse of an initially dry column of grains into a shallow water layer and the subsequent generation of waves. The experiments show that the collective entry of the granular material into water governs the wave generation process. We observe that the amplitude of the wave relative to the water height scales linearly with the Froude number based on the horizontal velocity of the moving granular front relative to the wave velocity. For all the different parameters considered here, the aspect ratio and the volume of the column, the diameter and density of the grains, and the height of the water, the granular collapse acts like a moving piston displacing the water. We also highlight that the density of the falling grains has a negligible influence on the wave amplitude, which suggests that the volume of grains entering the water is the relevant parameter in the wave generation.Comment: 23 pages, 16 figure

On two-dimensionalization of three-dimensional turbulence in shell models

Applying a modified version of the Gledzer-Ohkitani-Yamada (GOY) shell model, the signatures of so-called two-dimensionalization effect of three-dimensional incompressible, homogeneous, isotropic fully developed unforced turbulence have been studied and reproduced. Within the framework of shell models we have obtained the following results: (i) progressive steepening of the energy spectrum with increased strength of the rotation, and, (ii) depletion in the energy flux of the forward forward cascade, sometimes leading to an inverse cascade. The presence of extended self-similarity and self-similar PDFs for longitudinal velocity differences are also presented for the rotating 3D turbulence case

Resuspension threshold of a granular bed by localized heating

International audienceThe resuspension and dispersion of particles occur in industrial fluid dynamic processes as well as environmental and geophysical situations. In this paper, we experimentally investigate the ability to fluidize a granular bed with a vertical gradient of temperature. Using laboratory experiments with a localized heat source, we observe a large entrainment of particles into the fluid volume beyond a threshold temperature. The buoyancy-driven fluidized bed then leads to the transport of solid particles through the generation of particle-laden plumes. We show that the destabilization process is driven by the thermal conductivity inside the granular bed and demonstrate that the threshold temperature depends on the thickness of the granular bed and the buoyancy number, i.e., the ratio of the stabilizing density contrast to the destabilizing thermal density contrast

Decay laws, anisotropy and cyclone–anticyclone asymmetry in decaying rotating turbulence

International audienceThe effect of a background rotation on the decay of grid-generated turbulence is investigated from experiments using the large-scale ‘Coriolis’ rotating platform. A first transition occurs at 0.4 tank rotation (instantaneous Rossby number Ro ≃ 0.25), characterized by a t −6/5 → t −3/5 transition of the energy-decay law. After this transition, anisotropy develops in the form of vertical layers, where the initial vertical velocity fluctuations remain trapped. The vertical vorticity field develops a cyclone–anticyclone asymmetry, reproducing the growth law of the vorticity skewness, S ω ( t ) ≃ (Ω t ) 0.7 , reported by Morize, Moisy & Rabaud ( Phys. Fluids , vol. 17 (9), 2005, 095105). A second transition is observed at larger time, characterized by a return to vorticity symmetry. In this regime, the layers of nearly constant vertical velocity become thinner as they are advected and stretched by the large-scale horizontal flow, and eventually become unstable. The present results indicate that the shear instability of the vertical layers contributes significantly to the re-symmetrization of the vertical vorticity at large time, by re-injecting vorticity fluctuations of random sign at small scales. These results emphasize the importance of the nature of the initial conditions in the decay of rotating turbulence

Growth of the blue mackerel Scomber scombrus in Tunisia using the otolith microstructure

This paper studies the age and the growth of male and female of the Tunisian blue mackerel Scomber scombrus during 2004-2006. The age was determined by interpreting and counting daily increments on the otoliths sagitta of 320 individuals sampled from landings caught by different gears specially by purse seine. The parameters of the Von Bertalanffy growth curve were estimated for sexes combined: L-f = 31.46(1 - e(-0.228) ((t+3.034))) (L-infinity = 31.46 cm, k = 0.228 per year, t(o) = -3.034 years); for male L-f = 27.43(1 - e (-0.422) ((t+1.747))) (L-infinity = 27.43 cm, k = 0.422 per year, t(o) = -1.747 years) and for female L-f = 31.70(1 - e(-0.227) ((t+2.196))) (L-infinity = 31.70 cm, k = 0.227 per year, t(o) = 22.916 years) for the whole study period. The difference between the growth of male and female is not significant. The relationship between length and weight shows a length growth faster than of the weight with parameters of the equation W-t = a*L-b; a = 9.89*10(-5) and b = 2.37 for the sexes combined