AIAA 95-1775, Applied Aerodynamics Conference, June 1995 FOREBODY VORTEX CONTROL AT HIGH INCIDENCE USING A MOVEABLE NOSE STAGNATION POINT Darden, L.A.* and Komerath, N.M. 1 Georgia Institute of Technology Atlanta, Georgia 30332-0150 ABSTRACT A direct causal relationship is shown between lateral displacement of the nose stagnation point and lateral asymmetry of the vortex system over a wing-body configuration at angle of attack. Steady asymmetry is created and eliminated by moving the nose tip for angles of attack up to 40 deg. Rapid movement of the nose tip is then used to induce and eliminate dynamic vortex asymmetry. Video images of smoke in laser sheets at two cross-flow planes, and of the shadow of the nose, are analyzed to construct time series representations of the vortex asymmetry and compared with the simultaneous nose position. The intersection point of the zero-vorticity contour (ZVC) with the body surface serves as a metric of vortex asymmetry. The dynamics of the vortex asymmetry are analyzed in the frequency domain using these series. Correspondence between nose movement and variations in asymmetry shows that forebody asymmetry is a deterministic phenomenon directly related to the location of the nose stagnation point. The assumption of linear response is valid upstream of the wing vortices, but deteriorates downstream. The frequency-domain transfer function between nose movement and asymmetry is used to extract the steady-state sensitivity of the vortex in response to nose deflection, the time lag, and the rate dependence of sensitivity. The lag in the vortex response is an order of magnitude larger than the freestream convection time, and increases with downstream distance. The moving nose tip is shown to be a simple method for creating, studying, modifying, and eliminating forebody vortex flow asymmetry, statically and at high rates. * Graduate Fellow, School of Aerospace Engineering. Student Member, AIAA. t Professor, School of Aerospace Engineering. Associate Fellow, AIAA. Copyright O 1995 by Leigh Ann Darden and Narayanan Komerath. Published by the American Institute of Aeronautics and Astronautics with permission r: ZVC: a: $1 8: NOMENCLATURE body radius at the local cross-section. Zero Vorticity Contour; line of demarcation between the vortex systems on either side of the aircraft model. angle of attack. azimuth of the point at which ZVC intersects the body surface, in degrees from the plane of lateral symmetry. Nose tip deflection from the plane of lateral symmetry, degrees. INTRODUCTION Lateral instability due to asymmetry of the vortices over the forebody is a well-known problem with aircraft and missiles at high angles of attack1. In this paper, we explore the control of such asymmetries using a moving nose tip. The concept is then used as a tool to study the dynamics of the forebody vortex system. An experimental result illustrates the concept. The model shown in Fig. 1, from Ref. 2, consists of a pointed body of revolution with double-swept beveled-edge flat plate cropped delta wings. It was used in the Georgia Tech 2.7m x 2.5m Low Speed Wind Tunnel to measure vortex flow characteristics over the wings at high steady incidence. Flow visualization, performed to verify lateral symmetry, showed a large stable lateral asymmetry of the vortex system at a = 25 deg. An intensive study of this phenomenon showed it to be (a) stable and (b) repeatable. The asymmetry was insensitive to opening and closing test section side windows, moving a large probe traverse nearby, and even to small yaw errors. The asymmetry was traced to the very tip of the nose, where a minor dent was found, whose net effect was to shift the stagnation point laterally from the vertical plane of symmetry. The aymmetry was eliminated by gradually correcting the geometric imperfection, and the resulting symmetric flowfield is shown in Fig. l(b). This suggests that by varying the location of the stagnation point at the nose, desired degrees of asymmetry can be induced or suppressed in the entire vortex flow downstream. The shear layer