Turbofan Jet Noise Reduction via Deflection of the Bypass Stream

We describe a parametric experimental study of noise reduction via de ection of the secondary stream in dual-stream jets. The jets simulated the exhaust of turbofan engines with bypass ratios 4.5 and 6.0. The secondary (bypass) stream was de ected downward by means of vanes installed inside the bypass duct. Variables included the number of vanes, vane angle of attack, azimuthal placement, and axial position. The shape of the bypass duct was also varied. Noise measurements were converted to estimates of yover and sideline e ective perceived noise level (EPNL). The quietest con gurations used a rapidly convergent bypass duct with vanes installed in the proximity of the duct exit. Two pairs of vanes, installed at azimuth angles 70 and 110 deg with respect to the vertical, constitutes one of the most promising con gurations. Substantial bene t in EPNL occurs when the vanes have dissimilar angles of attack, with the upper pair at smaller incidence than the lower pair. The best arrangement o ered a 5.3-dB reduction in the sum of yover and sideline EPNL.