Effect of mesh grids on the turbulent mixing layer of an axisymmetric jet

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.This paper focuses on the effect that two different mesh grids have on the structure of the mixing layer of an axisymmetric jet. Detailed measurements of mean velocity and turbulent velocity fluctuations are made with an X hot-wire probe in the range 0.5 ≤ x/d ≤ 10, where x is the longitudinal distance from the nozzle exit plane and d is the nozzle diameter. The grids are introduced just downstream of the nozzle exit plane: one completely covers the nozzle (full mesh or FM), the other covers the central, high speed zone (disk mesh or DM). With reference to the undisturbed jet, FM yields a significant reduction in the turbulence intensity and width of the shear layer whereas DM enhances the turbulence intensity and increases the width of the shear layer. Both grids suppress the formation of the Kelvin-Helmholtz instability in the mixing layer. Results are presented, mainly at x/d = 5, both in the spectral domain and in physical space. In the latter context, second and third-order structure functions associated with u (the longitudinal velocity fluctuation) and v (the lateral or radial velocity fluctuation) are presented. All mesh geometries have a more significant effect on the second-order structure function of u than on that of v. The third-order energy transfer term is affected in such a way that, relative to the undisturbed jet, its peak location is shifted to a smaller scale with FM is used and to a larger scale with DM. This is consistent with our observations that FM reduces the turbulence in the shear layer whilst DM enhances it. It is suggested that the large scale vortices that are formed at the edge of the grids play a significant role in the transfer of energy.mp201

Modelling the general circulation of shelf seas by 3D k-epsilon models

One examines the modifications which must be made--and the limitations which must be set--to classical k- epsilon models to extend their application to the simulation of marine mesoscale, synopticscale and macroscale processes which compose the weather-like and general circulations of the sea. The case of the general circulation--for which sub-grid scale fluctuations include such semi-organized motions as tides and storm surges--is discussed in more detail. A 3D k- epsilon model appropriate to the study of the general circulation in a shallow stratified sea is presented and illustrated with the results of a simulation of the general summer circulation in the Northern Bering Sea, made in the scope of the NSF ISHTAR ("Inner Shelf Transfer and Recycling") Program

Diderot, l’Encyclopédie & autres études, sillages de Jacques Proust.

Pour tous les spécialistes de Diderot, pour tous les dix-huitiémistes mais aussi pour tous les universitaires, la figure de Jacques Proust (1926- 2005) conserve une présence singulière. En témoigne ce beau livre préparé par Marie Leca-Tsiomis qui réunit un ensemble d’évocations et d’études variées suscitées par le souvenir du grand professeur et du grand critique. Les cinq années écoulées depuis sa disparition permettent de donner à cet hommage quelque chose qui va plus loin que la piété et l..

Plankton dynamics controlled by hydrodynamic processes near a submarine canyon off NW corsican coast: A numerical modelling study

A three-dimensional (3D) non-linear high-resolution hydrodynamic model coupled to a coastal plankton ecosystem model is used to estimate the impact of hydrodynamic processes on the evolution of the spring phytoplankton bloom in the vicinity of a submarine canyon. Model results for the plankton distribution showed a clear 3D character around and in the canyon, with large horizontal and vertical gradients, induced by the hydrodynamic constraints. Phytoplankton concentrations were significantly larger all along the slope domain with maximum values obtained over the canyon. Upwelling of deep water rich in nitrate takes place both upstream (with respect to the current direction normal to the central axis of the canyon) and downstream of the canyon enhancing primary production. As phytoplankton-rich water enters into the western part of the canyon it is downwelled and trapped by the cyclonic circulation leading to accumulation of phytoplankton biomass there. The effect of wind events was to induce an upward nitrate flux into the upper layer through vertical turbulent diffusion, allowing the start of a short-live phytoplankton bloom. Maximum surface nitrate concentrations were found along the slope and particularly upstream and downstream of the canyon just after the wind stopped. Enhanced turbulent diffusion combined with upwelling motion in these areas resulted in larger upward nitrate transports, further enhancing primary production. © 2006 Elsevier Ltd. All rights reserved

A reduced-gravity model of the Catalan Sea

9 pages, 14 figuresA reduced-gravity model is used to study the effects of the wind on the upper layer circulation in the Catalan Sea. The model parameters were set by observed features of the circulation in the basin. It is shown that the results are particularly sensitive to the open sea boundary conditions. Simulations were done using the following boundary fluxes: (i) mean values estimated by Bethoux (1980) and (ii) more recent geostrophic transports computed from hydrographic data by Font et al. (1988). The latter seem to lead to more realistic circulation patterns. The influence of seasonal winds (climatological data) on the dynamics is clear, especially during the winter. © 199

Effects of extreme meteorological conditions on coastal dynamics near a submarine canyon

A 3-D hydrodynamic model is applied to assess shelf/slope exchanges in the Calvi Canyon region (Corsica, NW Mediterranean) during the violent storm that affected the Western Europe in December 1999. Simulations are carried out using high-frequency sampling meteorological data to take into account the short-term variability of the atmospheric conditions. It is shown that the combined effects of canyon topography and of the wind forcing during the storm are responsible for a large increase of both cross-shore and vertical transports in the area. Strong downwelling motion is simulated all along the continental slope with vertical velocities up to 2cms-1 within the canyon. High turbulent diffusion levels are obtained leading to the complete mixing of the water column within the canyon. Results suggest that increased turbulent diffusion and downwelling circulation in the canyon during the storm should result in a large transport of coastal water towards the abyssal plain. © 2004 Elsevier Ltd. All rights reserved