VIDELER, J.J.; STAMHUIS, E.J.; VAN DUREN, L.A.; M�LLER, U.K.: Structure and scaling of aquatic animal wakes

Water movements in the wakes of plankton and fish were visualised and quantified using two-dimensional particle image velocimetry (PIV). The resulting vector flow fields allowed the study of the spatial distribution of velocity, acceleration, vorticity, strain and shear in the fluid near the animals. If present, the location and size of vortices could be detected and measured. Calculations of divergence were used to determine the magnitude of the flow in the third dimension. Flow induced by foraging and escape responses of different larval stages of a marine copepod offers insight in vorticity in viscosity dominated flow regimes. The Re number values vary between 0.2 during slow foraging of 0.3 mm long nauplius larvae and 75 during the fastest escape responses when 0.8 mm long copepodites can swim at speeds up to 108 mm/s. Foraging movements are continuous; escape responses intermittent. Larvae and small adult fish (zebra danio) use a burst-and-coast-swimming mode at Re numbers between 100 and 5000, shedding two vortices at the tail during the burst phase. The flow patterns differ between larvae and adults due to the changes in importance of viscosity. Adult fish swimming in a continuous mode can generate different wake patterns. Mullet and eel swim at Re numbers between 10000 and 20000 in inertia dominated regimes. The generation of two fundamentally different wake vorticity patterns will be discussed.