Aircraft turbofan noise

Turbofan noise generation and suppression in aircraft engines are reviewed. The chain of physical processes which connect unsteady flow interactions with fan blades to far field noise is addressed. Mechanism identification and description, duct propagation, radiation and acoustic suppression are discussed. The experimental technique of fan inflow static tests are discussed. Rotor blade surface pressure and wake velocity measurements aid in the determination of the types and strengths of the generation mechanisms. Approaches to predicting or measuring acoustic mode content, optimizing treatment impedance to maximize attenuation, translating impedance into porous wall structure and interpreting far field directivity patterns are illustrated by comparisons of analytical and experimental results. The interdependence of source and acoustic treatment design to minimize far field noise is emphasized. Area requiring further research are discussed and the relevance of aircraft turbofan results to quieting other turbomachinery installations is addressed

Turbomachinery noise studies of the AiResearch QCGAT engine with inflow control

The AiResearch Quiet Clean General Aviation Turbofan engine was tested on an outdoor test stand to compare the acoustic performance of two inflow control devices (ICD's) of similar design, and three inlet lips of different external shape. Only small performance differences were found. Far-field directivity patterns calculated by applicable existing analyses were compared with the measured tone and broadband patterns. For some of these comparisons, tests were made with an ICD to reduce rotor/inflow disturbance interaction noise, or with the acoustic suppression panels in the inlet or bypass duct covered with aluminum tape to determine hard wall acoustic performance. The comparisons showed that the analytical expressions used predict many directivity pattern features and trends, but can deviate in shape from the measured patterns under certain engine operating conditions. Some patterns showed lobes from modes attributable to rotor/engine strut interaction sources

Importance of Symbol Equity in Coded Modulation for Power Line Communications

The use of multiple frequency shift keying modulation with permutation codes addresses the problem of permanent narrowband noise disturbance in a power line communications system. In this paper, we extend this coded modulation scheme based on permutation codes to general codes and introduce an additional new parameter that more precisely captures a code's performance against permanent narrowband noise. As a result, we define a new class of codes, namely, equitable symbol weight codes, which are optimal with respect to this measure

Experimental clean combustor program; noise measurement addendum, Phase 2

Combustor noise measurements were performed using wave guide probes. Test results from two full scale annular combustor configurations in a combustor test rig are presented. A CF6-50 combustor represented a current design, and a double annular combustor represented the advanced clean combustor configuration. The overall acoustic power levels were found to correlate with the steady state heat release rate and inlet temperature. A theoretical analysis for the attenuation of combustor noise propagating through a turbine was extended from a subsonic relative flow condition to include the case of supersonic flow at the discharge side. The predicted attenuation from this analysis was compared to both engine data and extrapolated component combustor data. The attenuation of combustor noise through the CF6-50 turbine was found to be greater than 14 dB by both the analysis and the data

Landscape-Scale Manipulation of the Acoustic Environment Alters the Distribution of Breeding Birds and Arthropods

Oil and gas development has rapidly increased across the world over the last several decades. Anthropogenic noise, an invisible pollutant that alters animal distribution and behavior, could be responsible for documented wildlife population declines near loud compressor stations in energy extraction fields. We experimentally played back compressor noise, creating a “phantom natural gas field” in a large-scale experiment, and tested the effects of noise on songbird distributions during the breeding season and on arthropod distributions. Further, to begin to understand the influence of noise produced by different types of extraction infrastructure, we examined the effects of sound intensity and bandwidth, or the amount of frequencies emanating from a noise source, on bird and insect abundance. Breeding songbird distributions were negatively affected by broadband, high sound level noise exposure. We observed a 25.9% decrease in abundance of the songbird community and three individual species showed declines in noise. Our results further show that higher intensity and bandwidth are positively associated with the arthropod abundance of most groups, where for instance sap-feeders, omnivores, and grazers increased over 30% with increased sound levels. In contrast, lower intensity and bandwidth playback was negatively associated with arthropod abundance, where omnivores and grazers decreased over 19% with increased sound levels. Noise could impact trophic relationships in the sage steppe ecosystem. Any increase in herbivore arthropod species, could intensify herbivory, resulting in changes in plant chemistry. We demonstrate the importance of understanding the potential landscape-scale costs of noise exposure and the acoustic structure of noise on wildlife

Advances In Internal Model Principle Control Theory

In this thesis, two advanced implementations of the internal model principle (IMP) are presented. The first is the identification of exponentially damped sinusoidal (EDS) signals with unknown parameters which are widely used to model audio signals. This application is developed in discrete time as a signal processing problem. An IMP based adaptive algorithm is developed for estimating two EDS parameters, the damping factor and frequency. The stability and convergence of this adaptive algorithm is analyzed based on a discrete time two time scale averaging theory. Simulation results demonstrate the identification performance of the proposed algorithm and verify its stability. The second advanced implementation of the IMP control theory is the rejection of disturbances consisting of both predictable and unpredictable components. An IMP controller is used for rejecting predictable disturbances. But the phase lag introduced by the IMP controller limits the rejection capability of the wideband disturbance controller, which is used for attenuating unpredictable disturbance, such as white noise. A combination of open and closed-loop control strategy is presented. In the closed-loop mode, both controllers are active. Once the tracking error is insignificant, the input to the IMP controller is disconnected while its output control action is maintained. In the open loop mode, the wideband disturbance controller is made more aggressive for attenuating white noise. Depending on the level of the tracking error, the input to the IMP controller is connected intermittently. Thus the system switches between open and closed-loop modes. A state feedback controller is designed as the wideband disturbance controller in this application. Two types of predictable disturbances are considered, constant and periodic. For a constant disturbance, an integral controller, the simplest IMP controller, is used. For a periodic disturbance with unknown frequencies, adaptive IMP controllers are used to estimate the frequencies before cancelling the disturbances. An extended multiple Lyapunov functions (MLF) theorem is developed for the stability analysis of this intermittent control strategy. Simulation results justify the optimal rejection performance of this switched control by comparing with two other traditional controllers

On linear H∞ equalization of communication channels

As an alternative to existing techniques and algorithms, we investigate the merit of the H∞ approach to the linear equalization of communication channels. We first give the formulation of all causal H∞ equalizers using the results of and then look at the finite delay case. We compare the risk-sensitive H∞ equalizer with the MMSE equalizer with respect to both the average and the worst-case BER performances and illustrate the improvement due to the use of the H∞ equalizer

Impact of Channel Disturbances on Current Narrowband Power Line Communications and Lessons to Be Learnt for the Future Technologies

[EN] The electricity network is a complex communication medium with properties that depend on both the topology of the grid and the usage pattern of the connected devices. These devices generate channel disturbances during normal operation, which need to be overcome by power line communications (PLC) transmission technologies for ensuring communication. This paper analyzes the influence of the channel disturbances on the performance of the physical layer of the main narrowband PLC technologies approved by international communication organisms and currently deployed in Europe: PoweRline Intelligent Metering Evolution (PRIME) 1.3.6, PRIME 1.4 and G3-PLC. The methodology of this paper applies a standardized test method, metrics and a set of representative channel disturbances defined by the European Telecommunications Standards Institute (ETSI). Moreover, noise recordings from field measurements in an environment equipped with distributed energy resources (DER) complete the subset of the types of noise used in the study. This paper develops a replicable, fully automated, and cost optimized test scenario, based on an innovative Virtual PLC Laboratory, which provides a replicable and automated test process, where a wide range of channel disturbances can be accurately replicated, and the performance of the PLC technologies can be compared under the same conditions. The results of this paper provide important conclusions to be applied in the development of future PLC technologie