Time scales of forebody vortex response

Abstract

When the vortex system of a pointed forebody at high angle of attack is perturbed, the response involves phenomena with various time scales. Basic knowledge of these is sought to help generalize a roll-yaw control system. The forebody vortex system is controlled using lateral displacement of a nosetip stagnation point actuator (SPA) both statically and dynamically. In previous work, piecewise linear transfer functions were shown to represent the dynamic response of the vortex asymmetry, as well as the lateral pressure differences and the rolling moment. Wing rock oscillations were induced and suppressed using the SPA. In the current work, the time scales in the problem are studied. The stagnation point is moved in square wave excitation at 0.1 to 3.5 Hz for freestream speeds from 6.7 to 25 m/s, and sting angles of 35 to 45 deg. Surface pressure difference across the forebody, asymmetry of the vortex patterns, and wing rolling moment are correlated with hot-film anemometer signals to gauge propagation lag and relaxation time. The vortex pattern asymmetry propagates at convection speed. The velocity field near the surface starts responding with a lag one order of magnitude larger than convection time. A relaxation and oscillatory response appear in the hot-film signal over the wings, similar to those seen in the rolling moment response: this time scale is two orders of magnitude longer than convection. A two-timescale system simulates the roll response. In addition, a very long-scale switching of the vortex system is observed, attributable to viscous processes. On a wingless body, the time lags are shorter by a factor of 2 to 4, but the amplitude of asymmetry is greatly increased, so that isolated forebody tests are likely to overestimate the yawing moment and its speed of response. NOMENCLATURE