Effect on drag–thrust transition for flapping airfoil with chordwise flexibility

The objective of the current study is to leverage hereditary flexibility to enhance an airfoil's aerodynamic performance. The aim is to understand the effect of drag reduction by imparting chordwise flexibility and thereby increasing thrust generation. The study is done on NACA0015 airfoil to examine the effects of chordwise foil flexibility on the dynamical features of flapping-based propulsion. The experimental work considers pitching oscillations of the rigid and flexible airfoil at three pitching amplitudes and three frequencies in a subsonic wind tunnel for the Reynolds number (Re) in the range 1.2–5.1 × 103. Flow visualization, a hotwire anemometer, and particle image velocimetry are used to study the flow field. The findings demonstrate that the foil flexibility alters force parameters and efficiently increases thrust generation. For a higher value of Strouhal number (StD), the effect is more pronounced. The minimum drag coefficient ratio (Cdr) for the flexible foil is −3.17 compared to −1.55 for the case of a rigid foil. The result suggests that flexibility could impart twice the propulsion in the flapping range studied. The drag–thrust transition occurs in the 2P wake regime for the low oscillation frequency. The results show that the undulating flexible foil enhances the flow around the trailing edge, which causes local acceleration in the fluid. The vortex is shed closer to the leading edge for the flexible case because of the large flap deformation. A region of a wider and stronger envelope of velocity jet is formed behind the airfoil for the flexible case. The excess momentum enables the flexible foil to generate higher thrust. The result suggests that jet deflection and meandering are suppressed in the flexible case compared to the rigid case. The experimental results would assist in designing flexible wings with higher thrust-generation capacity for improved propulsion.