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A study has been conducted on the transmission loss of perforated plates at normal incidence. The investigation includes a theoretical analysis of the problem with validation through experimentation. The experiments comprised microphone measurements of transmission loss for 11 perforated plates with variable thickness, hole size, and porosity. The theoretical model is based on planar wave propagation through a single contraction/expansion chamber with modifications to account for hole interaction effects. The resulting formula for transmission loss yields superior predictions over past theories for the range of properties investigated. Deviations between experimental measurements and theoretical predictions of transmission loss are less than about 1.5 dB for dimensionless hole diameter d/λ < 0.5. The accuracy of the model does not show a strong dependence on plate thickness-to-diameter ratio or porosity.

The noise source distribution of coaxial jets with a diameter ratio of 1.6 and variable velocity ratio is investigated with a small-aperture microphone phased array. The array design enables differentiation of noise emitted by large-scale and fine-scale turbulence structures, which have different directivities. The acoustic data are complemented by pitot surveys of the mean flow, which yield measurements of the primary and secondary cores of the jet. For zero velocity ratio (single-stream jet), the region near the nozzle emits strong high-frequency noise. Increasing the secondary-to-primary velocity ratio suppresses the near-nozzle noise and extends the location of the peak noise downstream, which increases moderately. The axial location of peak noise is approximately situated at the end of the primary core. The suppression of high-frequency noise is explained by the creation and elongation of the secondary core as the velocity ratio increases. The acoustic trends with velocity ratio are similar for small and large array observation angles from which large-scale and fine-scale turbulence noise, respectively, have been shown to radiate. However, the increase in peak noise is more pronounced for the large-scale noise. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

Provides information on a study which measured the noise radiated from perfectly expanded supersonic coaxial jets in an anechoic chamber for operating conditions with the same total mass flow and thrust and with the same temperature ratio. Experimental method used; Numerical results and calculations; Conclusion.