Analysis of modulation of wake–wave coupling by pycnocline structure in stratified flow

Nonlinear pycnoclines are frequently encountered by underwater vehicles operating in the ocean. This study investigates the impact of pycnocline strength on the dynamics of the cylinder wake. Using large eddy simulation, the research investigates how varying pycnocline strength within hyperbolic-tangent density stratification influences wake evolution and internal wave characteristics. The study reveals the combined effects of pycnocline strength and incoming flow velocity on the wake, as well as the key role of internal waves in energy dissipation under two-dimensional constraints. When buoyancy predominates in wake evolution, the internal waves are primarily Lee waves. Enhanced stratification suppresses internal wave development, accelerates wake energy decay, and inhibits vortex shedding and recirculation. Conversely, when both buoyancy and inertial forces are influential, internal waves consist of Lee waves and coherent waves. Weak stratification promotes internal wave radiation, with a more pronounced leeward wave range, whereas strong stratification suppresses this effect. The rate of energy dissipation in the wake exhibits a nonlinear relationship with stratification strength. Weak stratification may induce wake-wave resonance, leading to faster decay, while strong stratification results in a distinct envelope structure. When inertial forces dominate, substantial stratification delays wake energy decay by inhibiting internal wave radiation. Coherent structures, induced by vortex shedding and other instabilities, significantly enhance the modulation of wake energy by internal waves.