Role of Valleys in the Aerodynamic Characteristics of Flat-Plate Corrugated Wings

It is known that corrugated airfoils with wavy surfaces exhibit high aerodynamic performance in the Reynolds number range of several tens of thousands; however, the detailed flow mechanisms around such wings remain unclear. In this study, we investigate a flat corrugated wing with two valleys based on a NACA symmetric profile. We conducted lift and drag measurements, flow visualization using the smoke-wire method, and Particle Image Velocimetry (PIV). The experiments were carried out in a closed-circuit wind tunnel. For the force measurements, the angle of attack was varied from -20° to +20° in 1° increments. The Reynolds number ranged from 3.0 × 10⁴ to 6.0 × 10⁴ in increments of 1.0 × 10⁴. PIV was performed at a frame rate of 10,000 fps with a resolution of 1024 × 672 pixels. The main findings are as follows: A notable feature of the flat corrugated wing is that it exhibits a positive lift coefficient even at zero angle of attack. Among the three flat corrugated wings tested, with thicknesses of 6%, 9%, and 12% relative to chord length, the one with intermediate thickness (9%) demonstrated the highest aerodynamic performance. With increasing angle of attack, the direction and magnitude of circulating flow in the two valleys changed significantly. The main reason why high lift is maintained even at high angles of attack is that the recirculating flow in the second valley acts to draw the vortex flow, which involves vertical velocity fluctuations, close to the downstream surface of the corrugated wing.