AIAA Paper 97-2061 4th Shear Flow Control Conference
"Time Scales of Forebody Vortex Response "
Darden, L.A., Villarreal, L., and Komerath, N.M.
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-time-scale
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.
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