Aerodynamic Control Using Distributed Bleed

Abstract

Aero-effected flight control using distributed active bleed driven by pressure differences on lifting surfaces and regulated by low-power louver actuators is investigated in wind tunnel experiments. The interaction between unsteady bleed and the cross flows alters the apparent aerodynamic shape of the lifting surface and consequently the distributions of aerodynamic forces and moments. The experiments are conducted using a 2-D dynamically pitching VR-7 model over a wide range of angles of attack from pre- to post-stall. Induced changes in surface vorticity concentrations are measured using PIV with emphasis on the effects of leading edge bleed at high angles of attack on the evolution of the dynamic stall vorticity concentrations. Leading edge bleed leads to large variations in lift and pitching moment and extends the static stall margin, enabling over 50% variation in the baseline lift without external control surfaces. At high reduced frequencies (k > 0.1), the bleed alters the dynamics of vorticity production and advection and interacts with the dynamic stall vortex to increase the stability of the pitch motion and minimize negative damping with low lift penalty. I. Overview Aerodynamic bleed, by exploiting pressure differences over moving lifting surfaces or bluff bodies, is attractive for flow control since the actuation is derived from regulation of the bleed flow through surface openings and therefore requires relatively little power. However, while in common with zero net mass flux (synthetic jet) actuation, active bleed utilizes fluid from the embedding flow, unlike synthetic jets, and the operating bandwidth is limited by the characteristics of the fluidic circuit and the magnitude of the available pressure drop. The use of passive porosity for aerodynamic flow control by exploiting inherent pressure differences over surfaces of moving bodies is not new and has been explored in a number of earlier investigations. In a typical application, fluid is bled from high- to low-pressure domains through porous surface segments and an internal passage or a plenum. The ensuing interaction of the ejection or suction with the cross flow can result in a local modification of the pressure distribution (and pressure gradients), lead to alteration of the flow over the surface, and in some cases result in global changes in aerodynamic forces. Several examples include reduction of the drag of bluff bodies by base bleed (Tanner, 1975), laminar flow control (Carpenter and Porter, 2001), tip vortex control (Han and Leishman, 2004), aerodynamic maneuvering forces (Hunter, Viken, Wood, and Bauer, 2001), mitigation of flow separation (Lopera, Ng, and Patel, 2004), and reduction of shock-induced separation on transonic airfoils (Savu and Trifu, 1984). It is noteworthy that studies of multiple slots that act as conduits for bleed flow between the pressure and suction side of various airfoils were investigated as early as the 1920s (Lachmann, 1924, and Weick and Shortal, 1932).