Jeudi 16 mars - J. Penney - Simulation of Double-Diffusion
de 11:00 à 12:00
|Où ?||Salle Lyot|
Jared Penney, post-doctorant Legos
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Title : Direct Numerical Simulation of Double-Diffusion in Gravity Currents and Rayleigh-Taylor Instabilities
When the density of a fluid is a function of the concentration of two different constituents, differences in the diffusivities of those constituents may result in a class of instability called double-diffusion. For example, when a layer of warm, salty water rests above a layer of cool, fresh water, the rapid diffusion of heat compared to the slow diffusion of salt results in localized parcels of high density fluid above the pycnocline. These parcels then sink out in the form of long, thin, convective cells called salt fingers. In the ocean, double-diffusion is thought to be an important mechanism in the transport of heat and salt. However, the scales of double-diffusive phenomena are considerably finer than many oceanographic processes (e.g., salt fingers typically range from a few millimetres to centimetres wide). The small scale of these structures can make their direct measurement in the environment difficult, and they are often considered sub-grid scale processes in many oceanographic models. In this talk, we discuss the results of high-resolution direct numerical simulations of fine-scale double-diffusive phenomena. The ability to compute quantities that cannot be measured directly through experiment, such as dissipation, stirring, and mixing, is a considerable advantage afforded by numerical simulation. We consider two different types of flow. The first set of simulations examines the development of gravity currents that are favourable to salt fingering, and explores how different vertical boundary conditions and current volumes affect the flow. The second set of simulations examines a three-layer system giving way to double-diffusive Rayleigh-Taylor (RT) instabilities, in an effort to characterize double-diffusive turbulence without imposed shear. The fluid in these simulations is assumed to be incompressible, and governed by the Navier-Stokes equations subject to the Boussinesq approximation. Density is defined by an approximation to the UNESCO equation of state that is third-order in temperature and first-order in salinity. High-quality three-dimensional visualization techniques are used to characterize the flow fields. Simulations of the salt fingering gravity currents revealed that no-slip boundaries cause the formation of standard lobe-and-cleft instabilities in the current head. Simulations with no-slip boundaries also encourage both an earlier onset and a greater degree of three-dimensionalization, as well as greater viscous dissipation, stirring, and mixing when compared to similar configurations using free-slip boundaries. In the three-layer simulation, RT instabilities were observed to dominate the length scales of kinetic energy, while double-diffusion dominated the length scales associated with the density field. This was confirmed through spectral analysis, in addition to displaying strong similarity between the dominant scales of salinity and density. Double-diffusion was also observed to be the cause of densities greater than the initial maximum value.