Flow visualization and aerodynamic-force measurement of a dragonfly-type model

Abstract

An unsteady flow visualization and force measurement were carried out in order to investigate the effects of the reduced frequency of a dragonfly-type model. The flow visualization of the wing wake region was conducted by using a smoke wire technique. An electronic device was mounted below the test section in order to find the exact position angle of the wing for the visualization. A load-cell was employed in measuring aerodynamic forces generated by a plunging motion of the experimental model. To find the period of the flapping motion in real time, trigger signals were also collected by passing laser beam signals through the gear hole. Experimental conditions were as follows: the incidence angles of the fore-and hind wing were 0° and 10°, respectively, and the reduced frequencies were 0.150 and 0.225. The free-stream velocities of the flow visualization and force measurement were 1.0 and 1.6 m/sec, respectively, which correspond to Reynolds numbers of 3.4 × 10$^3$ and 2.9 × 10$^3$. The variations of the flow patterns and phase-averaged lift and the thrust coefficients during one cycle of the wing motion were presented. Results showed that the reduced frequency was closely related to the flow pattern that determined flight efficiency, and the maximum lift coefficient and lift coefficient per unit of time increased with reduced frequency.