Abstract
The loudest source of high-speed jet noise, such as found on naval tactical fighters, appears to be unsteady wavepackets that are acoustically efficient but relatively weak compared to the main jet turbulence. These wavepackets can be usefully described by linear dynamics and connected to transient growth mechanisms. Through a component-wise structural sensitivity analysis of the turbulent jet baseflow, using both the equilibrium and time-average fields, estimates are given as to what location and kind of actuators and sensors are most effective, in a linear feedback context, to control the wavepackets to reduce their noise. Low and high frequency approaches are examined where the controlling mechanisms differ: the low-frequency control indirectly targets the slow variation of the mean on which the wavepackets propagate while the high-frequency control targets the wavepackets themselves. The predicted control strategy is evaluated using direct numerical simulations on a series of Mach 1.5 turbulent jets.