Selected Publications
Alan T. Wall, Kent L. Gee, Tracianne B. Neilsen, David W. Krueger, Scott D. Sommerfeldt, and Jonathan D. Blotter (et al.)
Array-based visualization techniques have been used for several decades to explore jet-noise properties such as source strength, size, and distribution. However, near-field acoustical holography (NAH) has only recently been utilized to study jet noise, and experiments are typically performed on laboratory jets in controlled environments. In this work, NAH was applied to measurements of a full-scale, installed jet engine. This paper outlines three different NAH methods applied to the data and addresses practical problems faced when NAH is performed on a full-scale jet. Holographic reconstruction of the jet-noise field at low frequencies shows phenomena consistent with radiation from large-scale turbulence structures. A comparison of reconstructed levels using both planar and cylindrical NAH methods suggests that cylindrical NAH outperforms planar NAH for jet noise sources.
Kent L. Gee, Tracianne B. Neilsen, and Alan T. Wall (et al.)
Nonlinear propagation of noise measured during static run-up measurements of a military jet aircraft is considered. Measurements made of the noise radiation from the F-35AA Joint Strike Fighter indicate levels commensurate with finite-amplitude sound propagation, both through a Gol’dberg number analysis and numerical modeling. Comparisons with a nonlinear propagation model based on the generalized Mendousse-Burgers equation and augmented for weak-shock theory agree favorably with the measurements.
Kent L. Gee and Micah R. Shepherd (et al.)
This paper describes the use of a spectrally-based “nonlinearity indicator” to complement ordinary spectral analysis of jet noise propagation data. The indicator, which involves the cross spectrum between the temporal acoustic pressure and the square of the acoustic pressure, stems directly from ensemble averaging the generalized Burgers equation. The indicator is applied to unheated model-scale jet noise from subsonic and supersonic nozzles. The results demonstrate how the indicator can be used to interpret the evolution of power spectra in the transition from the geometric near to far field. Geometric near-field and nonlinear effects can be distinguished from one another, thus lending additional physical insight into the propagation.
Alan T. Wall, Kent L. Gee, and Tracianne B. Neilsen (et al.)
The sources of jet noise have been investigated through the use of near-field array measurements. An understanding of spatial coherence properties in the jet near field leads to insight about source composition. A metric called coherence length is a useful tool for summarizing near-field coherence information. Near-field coherence lengths are presented in this work for a full-scale jet, which show that jet noise is composed of two distinct components: (1) highly spatially coherent, low-frequency, large-scale turbulent structures and (2) high-frequency fine-scale turbulence with very low spatial coherence. The spatial extent and coherence of jet noise sources support the use of near-field acoustical holography as a viable jet-noise-source imaging technique.
Jeffery M. Fisher, Jonathan D. Blotter, Scott D. Sommerfeldt, and Kent L. Gee
Active structural acoustic control has been an area of research and development for over two decades with an interest in searching for an "optimal" error quantity. Current error quantities typically require the use of either a large number of transducers distributed across the entire structure, or a distributed shaped sensor, such as polyvinylidene difluoride. The purpose of this paper is to investigate a control objective function for flat, simply-supported plates that is based on transverse and angular velocity components combined into a single composite structural velocity quantity, termed V-comp. Although multiple transducers are used, they are concentrated at a single location to eliminate the need for transducers spanning most or all of the structure. When used as the objective function in an active control situation, squared V-comp attenuates the acoustic radiation over a large range of frequencies. The control of squared V-comp is compared to other objective functions including squared velocity, volume velocity, and acoustic energy density. The analysis presented indicates that benefits of this objective function include control of radiation from numerous structural modes, control largely independent of sensor location, and need to measure V-comp at a single location and not distributed measurements across the entire structure. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.3699264]
Curtis P. Wiederhold, Kent L. Gee, Jonathan D. Blotter, and Scott D. Sommerfeldt
One design for three-dimensional multimicrophone probes is the four-microphone orthogonal design consisting of one microphone at an origin position with the other three microphones equally spaced along the three coordinate axes. Several distinct processing methods have been suggested for the estimation of active acoustic intensity with the orthogonal probe; however, the relative merits of each method have not been thoroughly studied. This comparative study is an investigation of the errors associated with each method. Considered are orthogonal probes consisting of matched point sensor microphones both freely suspended and embedded on the surface of a rigid sphere. Results are given for propagating plane-wave fields for all angles of incidence. It is shown that the lowest error for intensity magnitude results from having the microphones in a sphere and using just one microphone for the pressure estimate. For intensity direction, the lowest error results from having the microphones in a sphere and using Taylor approximations to estimate the particle velocity and pressure. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.3692242]