Noise reduction by interaction of flexible filaments with subsonic jets

Experiments were conducted at the Boeing Interior Noise Test Facility (INTF) to evaluate the reduction of jet noise by a small diameter cord in a cold subsonic jet plume Different cord diameters, lengths and styles were used to determine the optimum configuration for noise reduction The optimized configuration of the cord was multiple stranded Kevlar fibers, 6 3 nozzle diameters long and having a thickness of 2% of the primary nozzle diameter A small knot at the downstream tip of the cord stabilized it along the jet centerline The tests were performed using a single jet and coaxial cold jet The jet velocities ranged from 7 to 9 Mach for the primary jet Acoustic and flow visualization data showed the effect of the cord on the jet noise and on the flow structure The filament changed both magnitude and directivity of the sound emission The sound field became nearly uniform, with predominant perpendicular orientation With a 6 3 nozzle diameter long multiple stranded cord in place noise reductions occurred in the lower frequencies up to 4OkHz with a maximum reduction of 2dB in the far field.