Danielle J. Moreau

Broadband trailing edge noise from a sharp-edged strut

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 10(5) to 5.0 × 10(5)). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.

A Review of Virtual Sensing Algorithms for Active Noise Control

Traditional local active noise control systems minimise the measured acoustic pres- sure to generate a zone of quiet at the physical error sensor location. The resulting zone of quiet is generally limited in size and this requires the physical error sensor be placed at the desired location of attenuation, which is often inconvenient. To overcome this, a number of virtual sensing algorithms have been developed for active noise control. Using the physical error signal, the control signal and knowledge of the system, these virtual sensing algorithms estimate the error signal at a location that is remote from the physical error sensor, referred to as the virtual location. Instead of minimising the physical error signal, the estimated error sig- nal is minimised with the active noise control system to generate a zone of quiet at the virtual location. This paper will review a number of virtual sensing algorithms developed for active noise control. Additionally, the performance of these virtual sensing algorithms in numerical simulations and in experiments is discussed and compared.

Active noise control in a pure tone diffuse sound field using virtual sensing

Local active noise control systems generate a zone of quiet at the physical error sensor using one or more secondary sources to cancel acoustic pressure and its spatial derivatives at the sensor location. The resulting zone of quiet is generally limited in size and as such, placement of the error sensor at the location of desired attenuation is required, which is often inconvenient. Virtual acoustic sensors overcome this by projecting the zone of quiet away from the physical sensor to a remote location. The work described here investigates the effectiveness of using virtual sensors in a pure tone diffuse sound field. Stochastically optimal virtual microphones and virtual energy density sensors are developed for use in diffuse sound fields. Analytical expressions for the controlled sound field generated with a number of control strategies are presented. These expressions allow the optimal control performance to be predicted. Results of numerical simulations and experimental measurements made in a reverberation chamber are also presented and compared.

Flow-Induced Sound of Wall-Mounted Finite Length Cylinders

This paper presents the results of an experimental investigation of the sound produced by flow interaction with a wall-mounted finite-length cylinder of circular or square cross section. Acoustic measurements have been taken in an anechoic wind tunnel at a range of flow speeds and for a wide variety of aspect ratios (cylinder length-to-diameter ratio). Unsteady velocity data have also been measured in the cylinder wake using hot-wire anemometry, and these data are related to far-field noise measurements to determine the flow mechanisms responsible for noise generation. The cylinder aspect ratio was found to be an important parameter that controls vortex shedding behavior and hence tonal noise generation. Multiple peaks in the noise spectra can be attributed to different vortex cells in the near wake, the number and strength of which are controlled by aspect ratio influencing flow over the tip or cylinder–wall junction.

On the aeroacoustic tonal noise generation mechanism of a sharp-edged plate

This letter presents an experimental study on the tonal noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number. Flow and far-field noise data reveal that, in this particular case, the tonal noise appears to be governed by vortex shedding processes. Also related to the existence of the tonal noise is a region of separated flow slightly upstream of the trailing edge. Hydrodynamic fluctuations at selected vortex shedding frequencies are strongly amplified by the inflectional mean velocity profile in the separated shear layer. The amplified hydrodynamic fluctuations are diffracted by the trailing edge, producing strong tonal noise.

On the noise reduction mechanism of a flat plate serrated trailing edge at low-to-moderate Reynolds number

<4:2 10 ). Acoustic and aerodynamic measurementshave been taken using a at plate with both sharp and serrated trailing edges in theanechoic wind tunnel at the University of Adelaide. Trailing edge serrations are foundto achieve up to 13 dB of attenuation in the narrowband noise levels without modifyingthe directivity of the radiated noise. The noise reduction achieved with trailing edgeserrations is found to be dependent on their geometrical wavelength and Strouhal number,St

A nonlinear active noise control algorithm for virtual microphones controlling chaotic noise.

In active noise control (ANC) systems, virtual microphones provide a means of projecting the zone of quiet away from the physical microphone to a remote location. To date, linear ANC algorithms, such as the filtered-x least mean square (FXLMS) algorithm, have been used with virtual sensing techniques. In this paper, a nonlinear ANC algorithm is developed for a virtual microphone by integrating the remote microphone technique with the filtered-s least mean square (FSLMS) algorithm. The proposed algorithm is evaluated experimentally in the cancellation of chaotic noise in a one-dimensional duct. The secondary paths evaluated experimentally exhibit non-minimum phase response and hence poor performance is obtained with the conventional FXLMS algorithm compared to the proposed FSLMS based algorithm. This is because the latter is capable of predicting the chaotic signal found in many physical processes responsible for noise. In addition, the proposed algorithm is shown to outperform the FXLMS based remote microphone technique under the causality constraint (when the propagation delay of the secondary path is greater than the primary path). A number of experimental results are presented in this paper to compare the performance of the FSLMS algorithm based virtual ANC algorithm with the FXLMS based virtual ANC algorithm.

Active control of transformer noise by using power line signal as reference

The hum noise generated from the distribution transformers is annoying and is significant when the transformer is installed near the residential area. This hum noise consists of harmonics of 100 Hz, when the power line frequency is 50 Hz. A new type of active noise control method has been proposed in this paper to combat such low-frequency harmonically related noise. The method is based on generation of required harmonics from the power line signal consisting of 50 Hz frequency using two adaptive filters. Through simulation study it has been shown that the proposed technique is able to accurately generate the mix of noise frequency components with the frequency change. The method also tracks the change in power line frequency and generates the corresponding anti-noise to nullify the noise.

An experimental application of aeroacoustic time-reversal to the Aeolian tone

This paper presents an experimental application of the aeroacoustic time-reversal (TR) source localization technique for studying flow-induced noise problems and compares the TR results with those obtained using conventional beamforming (CB). Experiments were conducted in an anechoic wind tunnel for the benchmark test-case of a full-span circular cylinder located in subsonic cross-flow wherein the far-field acoustic pressure was sampled using two line arrays (LAs) of microphones located above and below the cylinder. The source map obtained using the signals recorded at the two LAs without modeling the reflective surfaces of the contraction-outlet and cylinder during TR simulations revealed the lift-dipole nature of aeroacoustic source generated at the Aeolian tone; however, it indicates an error of 3/20 of Aeolian tone wavelength in the predicted location. Modeling the reflective contraction-outlet during TR was shown to improve the focal-resolution of the source and reduce side-lobe levels, especially in the low-frequency range. The experimental TR results were shown to be comparable to (a) the simulation results of an idealized dipole at the cylinder location in wind-tunnel flow and (b) that obtained by monopole and dipole CB, thereby demonstrating the suitability of TR method as a diagnostic tool to analyze flow-induced noise generation mechanism.

Active noise control at a moving location in a modally dense three‐dimensional sound field using virtual sensing

Local active noise control systems generate a zone of quiet at the physical error sensor using secondary sources to cancel the acoustic pressure at the sensor location. The resulting zone of quiet is generally limited in size and as such, placement of the physical error sensor at the location of desired attenuation is required, which is often inconvenient. Virtual acoustic sensors overcome this by projecting the zone of quiet away from the physical error sensor to a remote location. While virtual acoustic sensors have shown potential to improve the performance of local active noise control systems, it is, however, likely that the desired location of maximum attenuation is not spatially fixed. The work described here presents a virtual sensing method capable of tracking a desired location in a modally dense three‐dimensional sound field. The developed algorithm has been experimentally verified in a three‐dimensional enclosure and the experimental results are presented.