Exotic wakes of flapping fins

We present, in 8 chapters, experiments on and numerical simulations of bodies flapping in a fluid. Focus is predominantly on a rigid foil, a model fish, that performs prescribed pitching oscillations where the foil rotates around its leading edge. In a flowing soap film is measured, with unprecedented accuracy, the vortex wake structure behind the flapping foil in the space spanned by dimensionless flapping frequency 0 <StD <0.3 and amplitude 0 <AD <2. We measure not only the ubiquitous von K´arm´an wake, but also wakes where up to sixteen vortices are shed each oscillation period. The wake measurements are supplemented with numerical simulations of the flow and fluid forces, in settings relevant for the experiments. It is shown that wake transitions and average fluid forces are described by a single parameter, the Strouhal number, which is a measure of both the dimensionless foil tip-speed and the strength ratio of the vortices formed at the foil’s leading and trailing edge. The simulated vortex particles and measured thickness variations in the soap film show similar behaviour which indicates that the soap film provides a good approximation the flow of a two-dimensional incompressible and Newtonian fluid. Also, measurements of the swimming speed of a pitching foil in a water tank are presented. Finally, an experimental study of the surprisingly strong fluid-mediated interaction of two tandem flappers is presented. It is shown that a passively flapping flag in general is affected by its downstream neighbour. When this neighbour is a second flag close by, they synchronise in frequency and the leader experiences a reduced drag compared to that on the lone flag. In case the follower is replaced by a flapping plate, upstream synchronisation and drag reduction is again found over a wide range of frequencies. Drag reductions up to a factor 3 are measured. Many results presented are obtained through flow visualisations. A great effort is made to produce visualisations of primarily high scientific quality, but often also with a certain aesthetic appeal.