How a flexible tail improves the power extraction efficiency of a semi-activated flapping foil system: A numerical study

The improvement of power extraction of a semi-activated flapping foil system via the use of a flexible tail is numerically investigated in this work. A NACA0015 airfoil arranged in a two-dimensional laminar flow synchronously executes a forced pitching motion and an induced plunging motion. A flat plate attached to the trailing edge of the foil is utilized to model a tail, and thereby they are considered as a unit for the purpose of power extraction. The tail is either rigid or deformable. At a Reynolds number of 1100 and the position of the pitching axis at third chord, the effects of the mass and flexibility of the tail as well as the frequency of pitching motion on the net power extraction are systematically examined. It is found that compared to the foil with a rigid tail, the efficiency of net power extraction for the foil with a deformable tail can be improved. Based on the numerical analysis, it is indicated that the enhanced power extraction, which is caused by the increased lift force, directly contributes to the net efficiency improvement. Moreover, owing to high enhancement of power extraction, a flexible tail with high flexibility is recommended in the semi-activated flapping foil based power extraction system.