Alexander R. Quayle

Beamforming correction for dipole measurement using two-dimensional microphone arrays

In this paper, a beamforming correction for identifying dipole sources by means of phased microphone array measurements is presented and implemented numerically and experimentally. Conventional beamforming techniques, which are developed for monopole sources, can lead to significant errors when applied to reconstruct dipole sources. A previous correction technique to microphone signals is extended to account for both source location and source power for two-dimensional microphone arrays. The new dipole-beamforming algorithm is developed by modifying the basic source definition used for beamforming. This technique improves the previous signal correction method and yields a beamformer applicable to sources which are suspected to be dipole in nature. Numerical simulations are performed, which validate the capability of this beamformer to recover ideal dipole sources. The beamforming correction is applied to the identification of realistic aeolian-tone dipoles and shows an improvement of array performance on estimating dipole source powers.

Mechanisms for Model Scale Landing Gear Noise Generation

th scale to determine the main noise generating features on simplified, four-wheel bogies aligned with the flow. Previous experiments have indicated that changes to wheel shape and model layout can have substantial effects on model noise. In this paper we explain these changes in noise level and present surface oil visualization of the mean flow for different wheel shapes. Acoustic measurements describe the noise generated by major components of the landing gear model and identify sources at the wheel edges, rear axle and oleo/beam junction. The most important noise sources were the result of wake-body interactions and we present changes to the wheel shape, layout of the landing gear and local fairings as successful methods of noise reduction. Results suggest that modest changes in design could yield a much more stable flow over the landing gear, substantially reducing noise generated by all components.

Phased Array Measurements from Landing Gear Models

Landing gear is now well known to be a major source of aircraft noise. Several studies have also identified the importance of surface details in the noise generated by landing gears at high frequencies. However, the basic mechanisms of noise generation at lower frequencies are less well understood. In this study, we examine the effect of changes to the overall gear layout on the noise produced. 1/12th scale models containing only the wheels, axles and main struts have been studied using two nested, 48-microphone arrays in the closed-section Markham wind tunnel at Cambridge University. Local fairings have also been added to isolate and identify individual noise sources. Results indicate that shape and placement of the wheels can affect the overall noise level by at least 6dB on simplified four wheel models. Changes in geometry were also found to substantially affect high frequency sources close to the main oleo, suggesting that noise sources from conventional, fully dressed gears might be equally susceptible to small changes in the overall design.

Experimental study of surface roughness noise

A turbulent boundary-layer flow over a rough wall generates a dipole sound field as the near-field hydrodynamic disturbances in the turbulent boundary-layer scatter into radiated sound at small surface irregularities. In this paper, phased microphone arrays are applied to the experimental study of surface roughness noise. The radiated sound from two rough plates and one smooth plate in an open jet is measured at three streamwise locations, and the beamforming source maps demonstrate the dipole directivity. Higher source strengths can be observed in the rough plates than the smooth plate, and the rough plates also enhance the trailing-edge noise. A prediction scheme in previous theoretical work is used to describe the strength of a distribution of incoherent dipoles over the rigid plate and to simulate the sound detected by the microphone array. Source maps of measurement and simulation exhibit encouraging similarities in both source pattern and source strength, which confirms the dipole nature and the predicted magnitude of roughness noise. The simulations underestimate the streamwise gradient of the source strengths and overestimate the source strengths at the highest frequency.

Surface Roughness Noise Prediction for Silent Aircraft eXperimental Design SAX-40

Surface roughness noise is a potentially important contributor to airframe noise. In this paper, noise assessment due to surface roughness is performed for a conceptual Silent Aircraft design SAX-40 by means of a prediction model developed in previous theoretical work and validated experimentally. Estimates of three idealized test cases show that surface roughness could produce a significant noise level above that due to the trailing edge at high frequencies. Roughness height and roughness density are the two most significant parameters influencing surface roughness noise, with roughness height having the dominant effect. The ratio of roughness height to boundary-layer thickness is the relevant non-dimensional parameter and this decreases in the streamwise direction. The candidate surface roughness is selected for SAX-40 to meet an aggressive noise target and keep surface roughness noise at a negligible level. Copyright