Vortex–wave coupling and internal wave radiation in stratified sphere wakes

Large-eddy simulations are conducted to investigate the wake collapse dynamics and internal wave mechanisms in stratified flow past a sphere at a Reynolds number of 3700 and Froude numbers Fr=U∞/ND=1,5,∞, where U∞ is the free-stream velocity, N is the background buoyancy frequency, and D is the sphere diameter. The wake evolution follows a classical regime transition from the near wake to the non-equilibrium regime and eventually to a quasi-two-dimensional (Q2D) regime, with stratification intensity modulating vortex dynamics, momentum transport, and energy pathways. The simulations reveal that strong stratification (Fr=1) suppresses vertical transport and accelerates the onset of Q2D regime, whereas weaker stratification (Fr=5) maintains turbulence and delays wake collapse. A three-mode classification of internal waves is proposed, comprising lee waves, coherent waves, and collapse waves. These wave types exhibit distinct spatial structures and sensitivities to stratification, with strong stratification producing organized, narrow wavefields and weaker stratification leading to broadband, fragmented radiation. Vorticity and energy transport analyses reveal a spatial decoupling between vortex dissipation and wave excitation, with horizontal and vertical vorticity components governing lateral energy radiation and core decay, respectively. These findings offer new insights into vortex–wave coupling in stratified wakes and provide theoretical guidance for modeling internal wave processes in geophysical and engineering applications.