Perturbations In Stratified Flows Research Articles

Investigation on the thermohaline structure of the stratified wake generated by a propagating submarine

The in situ thermohaline distribution of the wake contains a critical source of information for submersible detection. In this work, a stratified thermohaline wake model, in which major fluid properties vary with temperature and salinity, is presented to address heat and mass transfer characteristics in the stratified wake signatures induced by a propagating submarine via Large-Eddy Simulation (LES) approach. Of particular interests are the range and intensity of the wake signatures. The instantaneous fields reveal the evolutions of the temperature and salinity fields, showing the regional features of wake signatures. As the key diagnostic variables, the perturbation variations in temperature and salinity are quantified at the wake region defined by wake amplitude. Subsequently, the range and intensity of the wake signatures generated by submarines with different velocities are analyzed to illustrate the effects of velocity. The magnitudes of the perturbations in stratified flows are at significantly detectable levels, indicating that the submersible detection based on the temperature and salinity variations is a feasible and promising approach. The scientific findings provide useful insight for the deep understanding of the evolution of stratified flow, and support further research on optical and lidar tracking of the submersible vessels.

Stability of stratified flow with inhomogeneous shear

The temporal evolution of perturbations in stratified flow with inhomogeneous shear is examined analytically by an extension of the nonmodal approach to flows with inhomogeneous shear. The solutions of the equations that govern the linear evolution and the weak nonlinear evolution of perturbations of the stream function for stratified flow with monotonic inhomogeneous shear are obtained. It is shown that stabilization of perturbations arises from nonmodal effects due to flow shear. Conditions at which these nonmodal effects may be strong enough to stabilize the Rayleigh-Taylor instability are presented. These analytical results are also compared to numerical simulations of the governing equations performed by Benilov, Naulin, and Rasmussen.

Transient Development of Perturbations in Stratified Shear Flow

Transient development of perturbations in inviscid stratified shear flow is investigated. Use is made of closed form analytic solutions that allow concise identification of optimally growing plane-wave solutions for the case of an unbounded flow with constant shear and stratification. For the case of channel flow, variational techniques are employed to determine the optimally growing disturbances. The maximum energy growth attained over a specific time interval decreases continuously with increasing stratification, and no special significance attaches to Ri = 0.25. Indeed, transient growth can be substantial even for Ri = O(1). A general lower bound on the energy growth attained by an optimal perturbation in a stratified flow over a given time interval is the square root of the growth attained by the corresponding perturbation in unstratified flow. Enhanced perturbation persistence is found for mean-flow stratification lying in the range Ri between 0.1 and 0.3. Small but finite perturbations in mean flow with Ri less than 0.4 produce regions with locally negative total density gradient, which are expected to overturn. Although the perturbations are of wave form, buoyancy fluxes mediate transfer between perturbation kinetic and potential energy during transient development, thus implying that buoyancy flux is not a determinative diagnostic for distinguishing between waves and turbulence in stratified flows.