Analytical investigation of propeller-wing interaction noise

This paper investigates the noise generated by propeller-wing configuration at take-off condition with the propeller mounted upstream. This study makes use of various axisymmetric noise models developed for contra-rotating propellers to estimate the noise generated by propeller-wing configuration and later integrate them to estimate total noise. First, using well-published theory, rotor-alone (loading, thickness, and self-noise) and interaction noise sources (viscous-wake, potential field, tip-vortex) including tonal and broadband components are estimated. Later, a systematic parametric study is carried out by changing the blade number and tip Mach, while maintaining the propeller thrust and blade solidity. The noise generated is represented by Overall Acoustic Sound Power Level (OSWLs), which is an integrated value over the emission angles and frequency range, in a matrix form for the range of blade number and tip Mach. This matrix shows the regions dominated by rotor-alone and interaction noise and found that the noise characteristics of a rotor in uninstalled conditions (rotor-alone) are significantly altered due to the presence of a wing (installed condition). Further, it is found that the balance between these regions shifts with the variation in separation distance between the propeller and the wing. These results are further discussed with the individual interaction noise source mechanism and their dominance at various blade numbers, tip Mach, and separation distances. In addition, the non-axisymmetric viscous-wake interaction noise is investigated for even and odd numbers of blades and found that viscous-wake interaction noise has considerable directivity in the azimuthal direction. The results presented in the study are preliminary findings of propeller-wing noise, however, it gives give a quantitative picture of the behaviour of various noise sources and their balance with respect to geometric and operating parameters. This study will help to understand the dominant noise sources involved in propeller-wing configuration and will provide a quick guide for designing a low-noise configuration

Aeroacoustics source mechanisms of fixed-wing VTOL configuration

This paper is a predominantly experimental investigation into the noise generation mechanisms of Fixed-wing Vertical Take-Off and Landing (VTOL) propeller – wing configurations. These new aircraft combine the benefits of being able to land vertically in urban areas while being able to fly like conventional propeller-driven aircraft. However, their unique arrangement of wing and propeller that can rotate relative to the wing leads to aeroacoustic source mechanisms that have never, or are poorly, understood. During forward flight the aeroacoustic mechanisms of propeller-wing interaction have been studied theoretically and experimentally in recent work. However, during transition, and particularly during vertical takeoff, the flow from the propeller is incident upon the wing at high incidence angle, leading to new source mechanisms and balance of sources. This paper reports the results of acoustic and velocity measurements of a scale-model propeller adjacent to a flat plate or wing for the purpose of identifying and characterising the dominant noise generation mechanisms of propeller -wing interaction. In this paper we focus on the hover condition and investigate the variation in noise due to the vertical separation distance and horizontal offset distance. The paper will demonstrate that the overall noise is the summation of the noise due to the interaction of the tip vortex of the propeller with the wing leading edge, rotor-alone noise, potential field interactions at close separation distance, turbulent-wing interaction at larger separation distance, and the interference effects due to reflections from the wing. The balance of these various sources with vertical and horizontal separation distance is explored

On the single mono porous line trailing edge

This experimental works investigate simplistic porous arrangement at the trailing edge on the aerofoil self-noise radiation. The basis of this works is to utilise the trailing edge geometry to facilitate acoustic interference between two sources that are physically displaced in the longitudinal direction. The study investigated two trailing edge configurations: Baseline (B) and Selective Interference Mono Porous Line trailing Edge (SIMPLE). The experimental results found that SIMPLE trailing edge cases demonstrated the co-existence of destructive and constructive interference between the porous holes and the trailing edge for all free-stream velocities and longitudinal displacements. It was also established that flow communication is necessary to facilitate as a scattering location for acoustic interference. A simple analytical prediction model, based on phase cancellation, has produced good agreement with the experimental results