An Investigation of Active, Suction, Shock and Boundary Layer Control Techniques

Abstract

This report describes experimental and theoretical studies at high subsonic speeds on a two-dimensional airfoil of novel drag reduction systems. The control systems are all ‘active’ in the sense that they utilise suction to remove some of the mass flow within the boundary layer. The systems considered were: 1) Active Suction:‘Passive’ control by means of a porous region on the surface in the region of the shock wave is very effective at reducing wave drag by controlling the development of the shock wave, but this is at the expense of an increase in viscous drag mainly due to the flow through the surface. In the ‘active’ variant of this system a small amount of air is removed from the plenum with the aim of reducing the streamwise growth of the boundary layer thickness in the control region, thus arriving at a nett drag reduction. 2) Discrete Suction: The effect of applying suction locally, for example by a narrow porous strip or slot, will be to reduce the boundary-layer momentum thickness across the strip. If the strip is placed on the airfoil or wing upper surface, where the local velocity is higher than that of the free-stream, the effect will be ‘magnified’, so that the drag reduction will be greater than that of the equivalent flat plate at the same free-stream speed. 3) Hybrid Suction: This system combines the benefits of ‘Passive’ control and the potential of Discrete Suction, by reducing wave drag in the passive control region and eliminating the viscous drag increase by a narrow suction strip downstream. In assessing the potential benefits of these ‘active’ suction systems it is necessary to allow for the internal or ‘pump’ drag to arrive at a nett drag for the system.