ABSTRACT
The motion and turbulent wake characteristics of underwater vehicles in stratified fluid is of great importance and not yet fully understood. In this work, a methodology is proposed to predict density stratified flows and wakes past a sphere. The density difference is set based on the thermal stratification background, turbulence is implemented with a Shear Stress Transport (SST) based Delayed Detached Eddy Simulation (DDES) approach. Two kind of simulations are performed in both non-stratified and stratified conditions. Drag force, wakes as well as coherent vortex structures are studied to assess the influences of the stratification. Results show that internal waves are generated by the presence of the sphere in stratified fluid, which results in an increase in resistance. The vertical motions and vortices are suppressed due to the stratification in vertical direction compared to that of horizontal plane.
The research of wakes induced by the motion of three-dimensional blunt bodies through homogenous fluid has been of long-standing interest to the marine industry. As a canonical case, the sphere geometry draws obvious attention for validating numerical methods due to its wide applicability and simplicity. Plenty of researchers have predicted its hydrodynamic performance in homogenous flow, special focus has been devoted to flow separation and evolution (Son, Choi, Jeon and Choi, 2010), the Reynolds number effect on wakes (Ploumhans, Winckelmans, Salmon, Leonard and Warren, 2002), and validation for innovative numerical methods (Bassi, Botti, Colombo, Crivellini, Ghidoni and Massa, 2015).
It is well known that stratification exists extensively in underwater environment of oceans, where naval vessels are commonly operated. On the other hand, most of the fundamental researches on flow field characteristics of moving bodies have neglected the stratification effects. According to the early experiments by Lin and Pao (1979) of a grid, a two-dimensional body and an axisymmetric body in stratified fluids, density stratification would induce changes in the flow such as the generation of internal gravity waves, the alteration in the level and distribution of turbulent wakes and fluctuations, and the appearance of coherent vortex structures. Spedding (2014) showed that bluff-body wakes in stably environments have an unusual degree of coherence and organization, such as arrays of alternating-signed vortices have very long lifetimes, as measured in units of buoyancy timescales, or in the distance scaled by a body length.
