Hysteresis of a separated variable-velocity flow about a straight-wing model

Abstract

STRAIGHT-WING MODEL B. Yu. Zanin UDC 533.69.011 It is known that if the angle of attack of a wing increases and decreases sequentially, its aerodynamic characteristics, such as lift and drag [1], change, which leads to an ambiguous flow pattern within a certain range of the angles of attack [2, 3]. The author [4] and Lushin [5] have revealed that the hysteresis phenomena can be observed in the flow about a wing mounted at a constant angle of attack if the flow velocity first increases and then decreases. These papers dealt with a global separation (leading-edge stall) on a straight-wing model. The separation was shown in [4] to be eliminated by an increase in velocity, whereas it occurs again at a different (lower) velocity, i.e., the hysteresis is observed within a certain range of flow velocities. In this case, the angle of attack was not changed. It was also found [4] that the irreversible flow can reattach to the wing surface under the acoustic action on the flow in the hysteresis range of velocities, i.e., the separation is not restored when the sound is switched off. For flow velocities lower than the hysteresis ones, acoustic forcing leads to reversible reattachment -- the separation is restored after the sound effect is eliminated. With a global flow separation (stall) from the leading edge, a vast separation region is formed above the wing. This region extends over the entire wing surface from the separation line to the trailing edge. An important feature of global separation, which has been noted by many authors, is flow three-dimensionality in the separation region, which manifests itself in the emergence of a pair of large-scale vortices rotating in the wing plane (see, for example, [3, 5-8]). A typical flow pattern on the upper wing surface for a global stall is shown in Fig. 1. The contribution of the three-dimensional vortex structure of the separated flow to the appearance of hysteresis has not yet been studied. The goal of the present paper is to obtain flow patterns on the wing surface in the hysteresis range of flow velocities and to determine which changes in the three-dimensional structure of the separated flow are caused by an increase and a sequential decrease of the flow velocity and how these changes are related to the hysteresis. In addition, the effect of acoustic forcing on the flow pattern has been studied. The experiments were performed in a low-turbulence wind tunnel at the Institute of Theoretical and Applied Mechanics, Siberian Division, Russian Academy of Sciences. Use was made of the model with a rectangular wing in plan and with a symmetric profile whose relative thickness was 10%. The model span was 945 mm, and the chord length was 196 mm (aspect ratio 4.82). Top plates were installed at the model edges to avoid flow spillage. A 8 ~ angle of attack was kept constant during the experiments. The results were obtained using the oil-film visualization technique. With an increase in the flow velocity from 0 to 22 m/sec, stalling from the leading edge (global separation) occurred, which was confirmed by a hot-wire anemometer. Upon reaching a velocity of