Engine Cycle and Exhaust Configuration for Quiet Supersonic Propulsion

The thermodynamics and acoustics of a fixed-cycle, bypass ratio 3 supersonic engine with an innovative noise suppression scheme is explored. The silencing method entails installation of variable turning vanes in the by-pass exhaust of a separate-flow turbofan engine. During noise-sensitive segments of flight, the vanes impart a slight downward tilt to the bypass plume relative to the core plume, thus thickening the bypass stream on the underside of the jet. This results in a reduction of the convective Mach number of instability waves that produce intense downward sound radiation. Subscale experiments show that, relative to the mixed-flow exhaust, the coaxial separate-flow exhaust with vanes reduces the peak overall sound pressure level by 8 dB and the effective perceived noise level by 7 dB. The noise-equivalent specific thrust on takeoff is reduced from 490 to 390 m/s. Compared to a current-generation low-bypass turbofan engine, the bypass ratio 3 engine is estimated to be 13 dB quieter with the mixed-flow exhaust and 20 dB quieter with the aforementioned suppression scheme. The vane configuration of this study is estimated to cause a thrust loss of 1 % at takeoff and 0.25 % at supersonic cruise. Nomenclature D = diameter D = drag Fx = axial force of vanes Fy = transverse force of vanes f = frequency M = Mach number m ̇ = mass flow rate r = distance from jet exit S = planform area of deflector vanes T = thrust t = time from liftoff U = velocity W = weight x = horizontal distance from brake release y = altitude α = geometric angle of attack γ = climb angle θ = polar angle relative to jet centerline φ = azimuth angle relative to vertical plane Subscripts com = compressor fan = fan tip LO = liftoff p = primary (core) exhaust s = secondary (bypass) exhaust TOM = takeoff monitor tot = total v = vanes ∞ = flight conditions