How orcas capture seals resting on ice floes

Flow around a submerged cylinder near a free surface reveals that adjusting the Froude number and gap ratio influences the underwater jet pattern, vortex shedding frequency and free-surface deformation. The jet typically separates near the trough, leading to vorticity concentration and breaking waves that dissipate wave energy. Antarctic orcas collaborate to generate deep depression waves, breaking ice and washing seals from floes. Orcas raise their heads and tap their tails downward when approaching ice, which may benefit strong wave generation. We investigate the wave-generating hydrodynamics using a towing tank and particle image velocimetry. A scaled model with an elliptical body and wedge-shaped tail was tested under Froude number similarity. Experiments covered towing speeds of $0.3- 0.7\,\textrm{ms}^{-1}$ , combining different body ( $10^\circ$ / $0^\circ$ / $-10^\circ$ ) and tail angles ( $30^\circ$ / $0^\circ$ / $-30^\circ$ ), at chord-based Reynolds numbers of $17\,030- 40\,506$ . Four wake regimes are identified: small-scale vortex emergence triggered by capillary waves; extensive wave breaking due to flow separation at the trough; smooth depression wave caused by jet reattachment and downward advection of wake vortices; and large-scale vortex impingement generated by wake vortex perturbations. Under the pitched posture, the jet attaches successively to the solid surface and the trough via the Coandâ effect, suppressing flow separation, creating the most pronounced wave. The strong jet maintained a low-potential-energy state of the wave and led to large ice floes flipping and fracturing through the bending effect, while smaller ice floes were overwashed. This study suggests a novel flow-control strategy for objects near the free surface through jet attachment.