Thrust Enhancement For Parallel Pitching Foils

A numerical investigation is done to study the thrust enhancement for parallel pitching foils at fixed Reynolds number(Re = 220).Effect on thrust is examined of certain parameters like pitching frequency(StD = 0.12,0.15,0.175,0.2,0.225,0.25,0.3),pitching amplitude(AD = 1.2,1.4,1.6,1.8 2.0)and pitching center which changes from leading edge point to mid-point of the foil.The average longitudinal velocity is shown in this paper.The map of power coefficient,thrust coefficient and propulsive efficiency of single pitching foil is obtained depending on the systematic numerical study.And the effect of pitching center is numerically investigated.Two cases,i.e.low and high propulsive efficiency of single pitching foil are investigated.The effect of gap ratio(S*= S/D= 2.5,3.1,4,5,6,8)and phase-lag(? = 0° and 180°)on the power coefficient,thrust coefficient and propulsive efficiency of two side-by-side foils are investigated systematically.The current study shows that,maximum propulsive efficiency is obtained at leading edge point.And when pitching center moves from leading edge point to trailing edge point,the thrust coefficient and power coefficient will decrease.For the parallel configuration,anti-phase pitching will amplify the propulsive efficiency regardless of the conditions of high and low propulsive efficiency for single pitching foil.But for the high propulsive efficiency of single pitching foil,the enhancement effect is smaller.In-phase oscillation has the opposite effect.Besides,the in-phase oscillation results in a drop of the thrust production and power consumption.At S* = 2.5,anti-phase condition can render the maximum propulsive efficiency enhancement.Besides,four flow patterns are obtained,and the corresponding characteristics are presented.