Experimental study of the propulsive performance and wake interactions of tandem flapping foils

Tandem flapping foils can achieve high propulsive efficiency through beneficial wake interactions. Present study investigates the propulsive performance of tandem foils through experiment; the combined effects of kinematic parameters (Strouhal number St and effective angle of attack α0) and spatial conFiguration parameters (inter-foil spacing L and flap phase φ) are studied extensively. The kinematics and spatial configurations have coupled influences on the hind foil. The wake advection velocity U‾v of the upstream foil is a function of St and α0; it can be 1.2 times of the uniform flow velocity U∞ and stretches the wavelength of upstream wake. A global wake phase Ψ is adopted to unify the influence of kinematic and spatial configuration parameters. A mapping of Ψ, the wake interaction pattern and the efficiency can be found. The peak propulsive efficiency of the hind foil consistently appears when St∈[0.4,0.5] and Ψ/π∈[0.4,0.6], although the effective angle of attack and tandem configurations are different. The findings can be used to identify and optimize the wake interaction patterns of tandem flapping foils.