Computational study on the effect of initial pitch angle on dragonfly hovering performance

A numerical analysis has been conducted to study the role of hindfoil initial pitch angle on aerodynamic performance of dragonfly hovering flight. The inclined oscillation of two elliptic airfoils with tandem arrangment at \(\:Re=157\) is analysed using 2D numerical simulation. The pitch amplitude (\(\:{\alpha\:}_{m}\)) is kept constant for both foils and hindfoil initial pitch angle (\(\:{\alpha\:}_{{o}_{h}}\)) is varied from \(\:{15}^{o}\) to \(\:{75}^{o}\) for three different phase oscillations: \(\:\varphi\:=\:{0}^{o}\), \(\:{90}^{o}\) and \(\:{180}^{o}\). The results indicate, for \(\:{\alpha\:}_{{o}_{h}}\) < \(\:{45}^{o}\), the lower \(\:{\alpha\:}_{{o}_{h}}\) reduces total lift for all phase differences. It occurs due to the detrimental wake capture and downward dipole jet encountered by hindfoil during downstroke, resulting in less hindfoil \(\:\stackrel{-}{{C}_{V}}\). However, for \(\:{\alpha\:}_{{o}_{h}}\) > \(\:{45}^{o}\), lift enhancement of up to \(\:46\:\%\) is observed with increase in \(\:{\alpha\:}_{{o}_{h}}\) during \(\:\varphi\:=\:{180}^{o}\). Also, the higher thrust is obtained during lower \(\:{\alpha\:}_{{o}_{h}}\) and it reduces with increase in \(\:{\alpha\:}_{{o}_{h}}\).