Kanthasamy Chelliah

Aeroacoustics of Twin Supersonic Jets: A Review

Interaction of twin supersonic jets can dramatically alter the dynamic pressures in the near-field and cause fatigue of aircraft structures. This review paper focuses on the twin jet coupling situation that is rich in fluid flow physics, requires sophisticated analysis tools and is of relevance to aircraft with closely spaced engines and other industrial situations involving multiple jet flows. The review consists of four sections: (i) a historical survey of twin jet exhaust coupling, (ii) rectangular twin jets with both spanwise uniform and non-uniform geometry, (iii) a description of multi-modes and mode jumps using higher order spectral analysis, and (iv) the suppression of twin jet coupling. The parameters that affect twin jet coupling include nozzle geometry, internozzle spacing and nozzle operating conditions. Certain modes of resonant coupling increase the dynamic pressures drastically in the inter-nozzle region, whereas other coupling modes can suppress unsteady pressure levels. Since coupled resonances can cause fatigue failure of components, mitigation approaches need to be considered. Successful techniques include nozzle inserts, beveled nozzles, water injection, and miniature pins and cavities.

Enhanced nearfield acoustic holography for larger distances of reconstructions using fixed parameter Tikhonov regularization.

This paper evaluates the performance of various regularization parameter choice methods applied to different approaches of nearfield acoustic holography when a very nearfield measurement is not possible. For a fixed grid resolution, the larger the hologram distance, the larger the error in the naive nearfield acoustic holography reconstructions. These errors can be smoothed out by using an appropriate order of regularization. This study shows that by using a fixed/manual choice of regularization parameter, instead of automated parameter choice methods, reasonably accurate reconstructions can be obtained even when the hologram distance is 16 times larger than the grid resolution.

Demonstration of the Possibilities of Using a Phased Microphone Array as the Next-Generation Airport Noise Monitoring System

Aircraft noise is a form of environmental noise pollution that is a cause for resident complaints, especially near larger airports. Noise monitoring is usually performed by placing single microphones at various locations in neighborhoods that are near airports. Single omnidirectional microphones, however, record every sound wave that is incident on the sensor. The sound amplitudes estimated by these single microphones include the contributions from environmental sources other than the aircraft, such as traffic noise, sirens, powered landscaping equipment, and barking dogs. Use of a phased array of microphones in combination with advanced beam-forming algorithms makes possible the selective monitoring of aircrafts’ localized sound amplitudes and thereby more accurately identifies aircraft-specific sound levels while minimizing the contributions from other sound sources. This paper demonstrates the noise source localization abilities of the phased-array system for the application of aircraft environmental noise monitoring.

Leakage Detection Techniques Using Nearfield Acoustic Holography

This paper attempts to explore the possibilities of using various methods of nearfield acoustic holography (NAH) to detect the air/sound leakages from a container by locating the noise sources. The detection capabilities of three different NAH methods are compared. An acoustic source is placed inside the container and the hologram measurement is performed near the surface of the container. The NAH methods are applied on the hologram data to detect and quantify the leakage locations. Reconstructed acoustic pressure and particle velocity results are presented. NAH algorithms are found to work better for the higher input frequency cases. Tikhonov regularization is used to alleviate the singularity in the transformation matrices. However, generalized cross validation over-predicts the Tikhonov parameter for the present geometry. The resolution limitations of the NAH methods with respect to the hologram distance are addressed.Copyright

Building air-infiltration quantification based on sound transmission loss calculated using nearfield acoustic holography

This talk will demonstrate the abilities of nearfield acoustic holography (NAH) to detect and quantify leakages in building envelopes. A tonal sound source was placed inside a building model which has known leakages and microphone array measurements were obtained from the outside. Equivalent sources model based NAH was applied on the measured data to reconstruct the sound pressure field on the wall of the building model. Present results show that the NAH method was able to successfully locate the major leakages. A single microphone was used to measure the sound pressure level inside the building model which was used as a reference for quantification calculations. The difference between the inner and outer sound pressure levels was related to the area of the leakage. Various sizes of pinholes and rectangular cracks were investigated, and the detection limits of the current method were explored.

Measurement extension limits of patch nearfield acoustic holography

Patch nearfield acoustic holography is widely used when the measurement aperture is smaller than the area of the sound source. This paper discusses the factors affecting the limit of aperture extension. One step patch nearfield acoustic holography based on equivalent sources model was considered in this study. Available literature provide constant estimations on the ratio of measurement area to the area of the source. However, the present study shows that this extension limit is a variable depending on a few parameters. Wave number, hologram distance and choice of regularization technique were found to affect the extended area of reconstruction significantly. By choosing the right combination of these parameters, accurate reconstructions were achieved even when the measurement area was only ten percent of the source surface in size. This paper provides a systematic comparison of reconstruction errors for cases with various parameter combinations. A model relating the parameters and the extension limit is ...

Aircraft noise monitoring using multiple passive data streams

Monitoring of aircraft in the skies surrounding an airport is important to ensure the safety of the aircraft as well as quality of life for nearby residents. The traditional method of tracking aircraft position involves the ubiquitous rotating active radar, and the traditional method of monitoring aircraft noise is the single omnidirectional microphone. Both of these systems have significant disadvantages. In the past several decades, microphone array acoustic monitoring systems have become widely known although they are typically practical only for research purposes. More recently, advanced beamforming algorithms have set the stage for much smaller compact microphone arrays. Meanwhile, the Automatic Dependent Surveillance-Broadcast position tracking system has quickly risen in popularity. In this study, an advanced compact microphone array is used along with Automatic Dependent Surveillance-Broadcast and more conventional methods to monitor and track aircraft that are approaching and departing Chicago’s Midway Airport. The systems are portable and compact, and the unified streams of data can more fully characterize an individual aircraft, offering distinct advantages over existing monitoring systems.

Building leakage detection and quantification using statistically optimized nearfield acoustic holography technique

This paper presents a building infiltration detection and quantification technique using statistically optimized nearfield acoustic holography (SONAH) technique. A model building with known cracks on its wall was investigated in this study. The model building housed a synthetic acoustic source. A hologram measurement was performed outside the model building and the sound pressure levels on the walls were reconstructed. The correlation between the reconstructed pressure levels and the area of the crack was obtained. It was found that the acoustic technique can successfully detect and quantify the leakages from the building model. Two different frequencies of reconstructions are compared and it was found that the lower frequency reconstructions were more accurate. Effects of various regularization methods, phase matching, and quantization error are discussed. Various methods to suppress the wrap-around error in the reconstructions will be addressed.

Detecting building leakages using nearfield acoustic holography technique: A numerical simulation

A crack on a building wall and sound generated inside the building using an artificial sound source were simulated using Matlab. Various noise types affecting the pure sound signal, such as background noise in the room, were simulated. The forward problem of sound propagation from the crack surface to a virtual microphone array was simulated using the free-space green’s function. Nearfield acoustic holography (NAH) was applied to perform the inverse problem to calculate the sound pressure at the crack surface and determine the crack location on the building wall. Effects of various errors and noise on the sound pressure reconstruction using NAH were studied. The reconstructed sound pressure levels on the wall surface were compared with the originally simulated sound pressure data at the crack surface. Various crack sizes and shapes were simulated to determine the correlation between the size of the crack and the reconstructed pressure field.

Phased Array Measurements of Tandem Cylinder Noise

Landing gear noise is a key component of airframe noise during aircraft approach. In the present study we experimentally investigate a simplified landing gear model consisting of two cylinders in a tandem configuration. The cylinders are located at the exit of a jet flow facility operating at low subsonic speeds. The goal of this study is to use a compact acoustic phased array with advanced beamforming algorithms to locate and identify noise sources of the cylinder pair at various spacing and flow speeds. Narrowband analysis is conducted using the CLEAN-SC algorithm for the tandem cylinder arrangement and the baseline case (the plain wall jet without the cylinders). The TIDY algorithm is used for broadband analysis. Sources corresponding to lower frequencies were located near the downstream cylinder. The medium frequency sources were found to appear in the regions around the upstream cylinder. The noise from the flow (jet stream) was found to dominate in the higher frequency range. However, the noise levels were relatively low in this higher range of frequency. The spectra of the two cylinders are separated by integrating the results of TIDY in the middle frequency range. Comparing the spectra, the downstream cylinder appears to dominate. Finally, when the distance between the two cylinders is increased from 3.7 to 8.64 cylinder diameters, the downstream cylinder no longer appears to dominate. The results obtained in this paper suggest that it is indeed possible to locate the noise source and separate the noise spectrum on a simplified model landing gear configuration. The effect of Mach number on the sound levels was also studied using the microphone array and flow measurements.