The force and impulse of a flapping plate performing advancing and returning strokes in a quiescent fluid

Force measurements are conducted on a low-aspect-ratio ([InlineMediaObject not available: see fulltext.]) trapezoidal flat plate that rotates at 90 angle of attack and performs an advancing stroke from rest followed by a returning stroke. The parameters varied are the rotational amplitude of the motion and the rest time between the advancing and returning strokes. The unsteady normal forces track with the angular acceleration of the plate, with the added mass force peak in the returning stroke being larger than that in the advancing stroke. However, as the rest time is increased, the normal forces generated in each stroke become dynamically similar. The maximum total impulse is calculated from the force measurements and rapidly decays from its largest value at zero rest time and asymptotes to a constant with increased rest time. The direction of this impulse is also calculated and quickly approaches the direction about which the plate motion is symmetric. The largest additional impulse contribution obtained from executing a returning stroke within a finite time is approximately 18 %. Increases in rotational amplitude initially increase the maximum total impulse, but it then plateaus at an amplitude of around 90. For nonzero rest times, any maxima of the impulse in a fixed direction are weak and necessarily reduced from the maximum possible impulse. For a nearly 100 range of directions, the impulse is largest for rotational amplitudes between 75 and 90. The results are also applied to three types of propulsive configurations.