Pylon-Based Jet Noise Suppressors

This experimental investigation examined the potential of pylon-based deployable flaps to reduce jet noise of separate-flow turbofan engines with a bypass ratio of eight. The main function of the flap deflectors is to thicken the low-speed region surrounding the core jet in the downward and sideline directions. The study encompassed acoustic measurements, noise-source imaging, mean-velocity surveys, and aerodynamic estimates. Three types of deflectors were tested: solid flaps, porous flaps made of coarse perforation, and porous flaps made of fine perforation. It is shown that all the deflectors reduce noise sources near the end of the primary potential core. However, the solid flaps create excess noise in the vicinity of their location that can overwhelm this noise benefit, particularly at large polar angles. Porous flaps significantly reduce velocity gradients that cause excess noise. Noise generation from the perforations themselves can be shifted to very high frequency (rapidly attenuated by atmospheric absorption) by reducing the size of the perforation. Accordingly, the fine-perforation flaps provided superior acoustic results yielding effective perceived noise level benefits of 2.1 dB in the downward direction and 1.0 dB in the sideline direction. The static-thrust loss of these flaps is estimated at 0.7%.