Effect of body angle on the aerodynamics of a rhinoceros beetle: Smoke-wire visualization in a wind tunnel

In the present study, flow structure around a live rhinoceros beetle in a tethered flight is investigated experimentally using a smoke-wire visualization technique in a wind tunnel with a free-stream velocity of 1.2 m/s, which is close to that of a typical flight speed. While varying the body angle (from 5 to 85 degrees), the flow structures generated by the hindwings, elytra, and body are visualized along the spanwise direction. During the flapping period, the complex flow structures comprised leading-edge, trailing-edge, and tip vortices generated on the hindwing, but the flow structure is quite simple on the elytra (attached flow) and body (separated flow). As the body angle increases, these vortices convect in the downward direction, which matches the observation that the body angle of a hovering flight is larger than that of a forward flight. When the body angle matches the condition of forward flight, it is also found that the Strouhal number of a flapping hindwing is tuned to 0.4, which is known as an optimal value for thrust efficiency. Further, the effect of free-stream velocity (i.e., advance ratio) on the formation and evolution of these coherent vortical structures are investigated.