Characterizing the Role of Hind Flippers in Hydrodynamics of A Harbor Seal

In this paper, we investigate the hydrodynamic characteristics of harbor seal locomotion, focusing on the role of hind flippers in thrust generation and wake dynamics. Through three-dimensional numerical simulations using an immersed boundary method at Reynolds number of 3000, we analyze the impact of varying Strouhal number (St = 0.2-0.35) and propulsive wavelength ($λ^\ast = 1.0-1.2$) on swimming performance. Our findings reveal two distinct wake patterns: a single-row structure at lower Strouhal numbers ($St \leq 0.25$) and a double-row configuration at higher St ($St \geq 0.3$). Increasing wavelength generally enhances thrust production by reducing both pressure and friction of drag components. Additionally, we identify critical vortex interactions between the front and hind flippers, with destructive interference occurring at lower St and constructive patterns emerging at higher St. Circulation analysis confirms stronger vortex formation at higher St and $λ^\ast$}, particularly during the left stroke phase. These results provide novel insights into the hydrodynamic mechanisms underlying seal locomotion and contribute to our understanding of efficient aquatic propulsion systems.