Investigating the interaction of two oscillating foils in tandem arrangement, using 3D unsteady boundary element method

Abstract

Interaction of an oscillating foil in uniform flow would produce a forwarding thrust. The efficiency of this phenomenon is very dependent on the heave and pitch frequencies, and especially different phase angles. It is also well known that in the case of two foils in tandem arrangement, the frequency of heave and pitch motion of forward foil affects the lift/drag force of aft ward foil and thus the shape of emanating vortex sheet. In this paper, the thrust coefficient and efficiency of two oscillating foils in tandem arrangement are investigated. The interference of airfoils frequencies is investigated. A three-dimensional unsteady code is developed, based on boundary element method, and applied to simulate all the targeted cases. The trailing edge vortex sheet is also modeled using vortex filament and time stepping method. This approach is adopted due to its powerful ability for estimating the hydrodynamic forces on the lifting bodies. In order to model the heave/pitch motion, it is imperative to solve the 2-DOF equations for each foil in conjunction with BEM solver. For validation purpose, the developed program is applied to a single stationary and flapping foil and the thrust coefficient is compared against existing experimental data. Subsequently, the developed code is used to analyze tandem flapping foils. The induced wake shape behind foils is compared with the single flapping foil. Based on the obtained results, the effects of forward flapping foil are clearly observed on the aft ward foil wake shape. It is also demonstrated that for better efficiency, the optimum angle of attack for the aft ward foil is zero.