Wave drag reduction by means of aerospikes on transonic wings

Abstract

Wave drag and shock induced boundary layer separation are important issues of transonic wings. The negative effect of the transonic flow regime can be mitigated by controlling the shock terminating the supersonic region above the wing. In the past many different concepts based, for example, on passive ventilation, active suction, contour bumps or on adaptive walls have been pursued (see references). These approaches have in common that measures for controlling the shock are applied directly at the surface of the wing. However, a control of the shock wave is also possible by external devices placed above the surface of the wing in the supersonic flow regime. Experiments related to the latter concept that is related to the one of aerospikes on blunt bodies, will be presented in the present contribution. In a test series the effectiveness of a variety of different spike-shaped bodies placed above a transonic wing was tested in the transonic wind tunnel DNW-TWG, Göttingen. In addition to pressure measurements a colour schlieren system was set up for providing information about the influence of spikes on the flow field. The drag reduction mechanism of spike-shaped bodies that are placed in the supersonic flow above a transonic wing is based on the generation of wake flows and oblique shock waves interfering with the normal shock terminating the supersonic region. In this manner the pressure increases more gradually thus limiting losses. Since the spike is located above the surface of the wing the boundary layer on the wing is not directly disturbed. In the streamwise direction the exact position of the spike is of less importance than in the case of measures applied at the surface of the wing. The height at which the spike is arranged above the surface is chosen so that the shock is especially weakened in its lower part where the shock strength is greatest. Typical dimensions depend on the chord length and the Mach angle. Similarly, in order to weaken the shock over the whole span width several spikes are placed next to each other in spanwise direction. Bodies of various geometries that are acting as wake and shock inducing spikes, have been studied. Results are reported that were obtained with a cylindrical body having a needle-like tip, a punctured pipe that was open at its leading edge and a cone. A 400mm-chord model of the transonic airfoil VC-opt was mounted in the 1m x 1m adaptive walls test section of the TWG. Initially, a single spike was placed on the suction side of the VC-Opt model, the tip of the spike being located about in the middle of the chord. A colour schlieren system was set up for observing the flow field. A comparison of colour schlieren pictures of the flow about a clean airfoil and the flow about an airfoil with a conical spike shows only little differences. This is due to the span width (1m) being much greater than the size of the spike (diameter about 12 mm). However, a shock wave and Mach lines originating from the tip and the surface of the spike, respectively, can well be seen. Lift and drag were determined by pressure measurements. On the wing the static pressure was obtained via pressure taps that were arranged in a slightly diagonal manner thus avoiding interferences between the taps. The drag was calculated from total pressure data obtained by wake-rake measurements one and a half chord lengths behind the trailing edge. Initially, the rake was laterally displaced with respect to the spike by ten percent of the chord length. Tests were performed at a Reynolds number of Re ≈ 5⋅106 and at two Mach numbers, M = 0.775 and M = 0.795, respectively. Lift and drag polars were obtained for different configurations such as a clean airfoil and for an airfoil with shock inducing bodies. At certain angles of attack a gain in lift is observed and the drag is clearly reduced. This effect is most pronounced for a conical spike, i.e., for a body which produces a notable displacement in the flow. Hence, the concept of using aerospikes on transonic wings clearly shows a potential for reducing wave drag.