Aerodynamic performance of semi-wing with multiple winglets operating at low- and medium-range Reynolds numbers

Birds have traits that can induce better aerodynamic efficiency along with high manoeuvring capability during its flight, which could be shared with unmanned aerial vehicles for improving their aerodynamic performances. One such feature of the wing tip, i.e. the primary feathers of the birds could be an effective geometrical feature to reduce the wing tip vortices. This paper presents the bio-inspired wing tip devices, i.e. three-and four-tipped multiple winglets in reducing the strength of vortices emanating from the wing tip of the wing operating in the Reynolds number (Re) of $0.9794\times 10^5$ and $0.9794\times 10^6$. Different combinations of both three- and four-tipped multiple winglets have been designed by varying the cant angle of each tip. Numerical simulations were carried out using Ansys-Fluent by solving three-dimensional Reynolds averaged Navier–Stokes formulations coupled with k-$ϵ$ turbulence model to resolve the features of tip vortices. The simulation clearly indicates that there is a strong correlation between the size of the vortices and the aerodynamic performance parameters such as $C_L/C_D$, ${(C_L)}_{max,}$ $C_L{}^{0.5}/C_D$, $C_L{}^{1.5}/{\mathrm{\text{C}}}_D$. The three- and four-tipped multiple winglets are effective in reducing vortex drag by disintegrating large strength vortex which occurs in the tip of straight wing, into few numbers of small strength vortices. When compared to straight wing, three-tipped multiple winglet with the cant angle combination of 50, 30, 10 improves the aerodynamic efficiency by 22% to 23% and the four-tipped winglet with the cant angle combination of 60, 50, 40, 30 enhances the same by 21% to 22% in the Re of $0.9794\times 10^5$. Even the longitudinal static stability has seen considerable improvement for four-tipped multiple winglets than three-tipped multiple winglets and straight wing.