Active Control of Panel Oscillation Induced by Accelerating Boundary Layer and Sound

Measurements are made of the response and acoustic radiation of a panel structure forced by a constantly accelerating subsonic turbulent boundary layer with superimposed pure-tone sound in a wind tunnel. Both panel loading and response exhibit convective instability and high-dimensional complex behaviors because of nonlinear coupling between the convective short-scale instability wave in the boundary layer and the localized intense long scales of the pure-tone sound and its harmonics. In each run, the panel response exhibits unique dynamics, indicating a dependency on the initial conditions characteristic of a temporal chaotic system. Pure-tone and harmonic response over the broadband are stabilized and suppressed by a local external fixed temporal oscillator system. The controller almost suppresses the nonstationary multiharmonics in the panel. The response reduces to that of an accelerated turbulent boundary layer loading in the absence of sound. The experiments are motivated by considerations of aircraft interior noise and structural response during initial acceleration. T is known that a pure-tone acoustic excitation superimposed on a turbulent boundary layer along a flexible structure alters the boundary layer and the structure response. The incident wave acts as a trigger mechanism that transfers energy from the broadband response to the pure tone and then the harmonics. This transfer of energy was reported for the boundary layer and panel interaction for a flow at constant speed.1 In this coupled system, the convective instability of the turbulent boundary layer triggers convective waves in the flexible structure, and when the superimposed pure-tone amplitude exceeds a certain threshold value, the flexible structure exhibits chaotic and periodic response behaviors. The experiments present several challenges, one of which is characterization of the observed dynamics.2"4 One such characterization is synchronization of the system by linking some of the dynamic variables and using them in feedback control of the system variables during the acceleration state of the motion. Synchronization does not work when the initial state is not uniquely specified.5"8 Synchronism was necessary when a large number of driving oscillators were distributed over the surface of a structure whose forcing scale was smaller than the response scale. The control strategy then becomes dependent on the existence of a stable direction at each trajectory point.9"14 In the present investigation, the structure is forced by an accelerated boundary layer with superimposed pure-tone sound, representing a typical loading over an aircraft fuselage panel near an engine during the takeoff stage. Now in the absence of a stable direction, a new strategy for active control is adopted. In this system, control is achieved over the acceleration stage but the active control process varies with time. Pure-tone acoustic forcing of a jet shear layer has become widely used for investigating the stability of a shear layer.15'16 When the acoustic forcing is applied on the jet shear layer, the broadband farfield pressure decreases while the pure tone and harmonics exceed the broadband level of the undisturbed jet shear layer. We shall show in this paper that acoustic forcing of a turbulent boundary layer over a flexible structure also produces a decrease in broadband level at the same time that the amplitudes of the pure tone and harmonics exceed the level by several orders of magnitude. This mechanism of energy transfer is due to nonlinear coupling between the turbulent boundary layer and pure-tone sound.