Johansson L Christoffer, Norberg R Ake
Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138, USA.
Nature. 2003 Jul 3;424(6944):65-8. doi: 10.1038/nature01695.
Most foot-propelled swimming birds sweep their webbed feet backwards in a curved path that lies in a plane aligned with the swimming direction. When the foot passes the most outward position, near the beginning of the power stroke, a tangent to the foot trajectory is parallel with the line of swimming and the foot web is perpendicular to it. But later in the stroke the foot takes an increasingly transverse direction, swinging towards the longitudinal axis of the body. Here we show that, early in the power stroke, propulsion is achieved mostly by hydrodynamic drag on the foot, whereas there is a gradual transition into lift-based propulsion later in the stroke. At the shift to lift mode, the attached vortices of the drag-based phase turn into a starting vortex, shed at the trailing edge, and into spiralling leading-edge vortices along the sides of the foot. Because of their delta shape, webbed feet can generate propulsive forces continuously through two successive modes, from drag at the beginning of the stroke, all the way through the transition to predominantly lift later in the stroke.
大多数用脚划水的鸟类会沿着与游泳方向一致的平面，以弯曲路径向后摆动它们有蹼的脚。当脚经过最向外的位置时，即在动力冲程开始附近，脚轨迹的切线与游泳方向线平行，且脚蹼与之垂直。但在冲程后期，脚的方向越来越横向，朝着身体的纵轴摆动。我们在此表明，在动力冲程早期，推进主要通过脚受到的水动力阻力实现，而在冲程后期会逐渐过渡到基于升力的推进方式。在转换到升力模式时，基于阻力阶段的附着涡旋会变成一个起始涡旋，从后缘脱落，并沿着脚的两侧形成螺旋状的前缘涡旋。由于其三角形形状，有蹼的脚可以通过两种连续模式持续产生推进力，从冲程开始时的阻力，一直到冲程后期主要转换为升力。
