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 foreand hind-wing were 0° and 10°, respectively, and the reduced frequencies were 0.150 and 0.225. The freestream velocities of the flow visualization and force measurement were 1.0 and 1.6m/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.

A modified blade element theory for estimation of forces generated by a beetle-mimicking flapping wing system

“…Many studies on unsteady aerodynamic phenomena at low Reynolds numbers have been conducted in attempts to explain the exceptional force generation by flapping motions of insects [1][2][3][4][5][6]. For example, smoke visualization of the flow around flapping insect wings has been studied [7][8][9]. In [2], a dynamically scaled robotic fly was used to identify force generation mechanisms in a low Reynolds number environment.…”

Delta-wing vortex visualization using micro-sized water droplets generated by an ultrasonic humidifier

“…Smoke generated by combusting organic material or vaporizing hydrocarbon oils are used most frequently because they are small-sized particles and easily visible; but all smoke is toxic to some degree. In addition to this problem of toxicity, the use of smoke in a re-circulating wind tunnel also creates the problem that after a certain time of operation, the tunnel is completely filled with smoke (Kim et al, 2008). For this reason, there has been a continuous effort to use a steam as the visualization tracer.…”