Abstract
This paper presents the overall pros and cons of the effect of surface roughness elements over a NACA 4412 tapered, swept back half wing with a sweep angle of 30º and a dihedral angle of 5º. The tests were conducted at a Reynolds number of 4 × 105 in the IIUM Low Speed wind tunnel. Different roughness sizes and roughness locations were tested for a range of angle of attack. Lift, drag and pitching moment coefficients were measured for the smooth wing and with roughness elements. Surface roughness delays the stall angle and decreases the lift. The wing with the roughness elements located at 75% to 95% of mean chord from leading edge shows minimum drag and maximum lift compared to other locations. Significant increase in the pitching moment coefficient was found for flexible roughness elements. In case of rigid surface roughness, the effect on pitching moment is small.
Highlights
The wing surface of birds does not follow a single smooth curve surface
Chen et al (2016) conducted a CFD and experimental study on diverse types of equivalent particle roughness elements over NACA 4418 airfoil at 0.4 × 106 Reynolds number. They concluded that there was aerodynamics performance decrement as roughness size increased at 10% chord length
The maximum lift coefficient (CLmax) of the smooth wing before stalling occurs at α = 14° with a CLmax of 0.9668, which indicates the beginning of the stall region
Summary
The wing surface of birds does not follow a single smooth curve surface. The construction of the body surface of mammals which fly or swim in the water reduces the drag by change in boundary layer dynamics over the surface area of their body (Bechert et al 2000). Small birds, e.g. swallows and swifts, can have thicker bugging out rachis (which is stronger and stiffer), without affecting aerodynamic properties. This aerodynamic role at low Reynolds numbers might have provided small birds a window to produce laminar flow and glide well, regardless of their basic wing surface construction. It can be used in the design of swift-sized micro air vehicles (MAVs) in which the efficiency does not depend on the smooth surface and the high-Reynoldsnumber (Laursen 2008)
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