Vortex evolution of the flow around an underwater vehicle in density-stratified flow

Density stratification is a natural existing phenomenon observed in marine environ-ments. When underwater vehicles navigate near density stratification interface, the wake flow disrupts the initial stratification interface, resulting in a stratified wake. As one of the non-acoustic target features, stratified wake becomes a typical signal for underwater vehicle detection. To establish theoretic basis for wake flow detection in stratified environments, it is imperative to understand the characteristics of vortex generation and evolution around underwater vehicles in the density-stratified flows. This work focuses on the fine flow field around a SUBOFF scale model with full-appendage, by experiment and Large Eddy Simulation (LES) method, to figure out the characteristics of vortex evolution around the SUBOFF scale model in density-stratified flow. It is found that the mixing of saline water and fresh water in the wake displays multi-scale features, due to the strong unsteadiness of the separated vortices in the wake flow. The isolated and intensified vortex tends to evolve into a relatively regular configuration in the process of stratified fluid mixing, and causes this part of the fluid to detach from the primary interface, leading to a stratified interface with discrete characteristics in a given lateral range in the wake flow. Therefore, wake detection based on density gradients of the wake flow of an underwater target might be feasible.