Statistical Analysis of Nonlinearly Propagating Acoustic Noise in a Tube

Acoustic fields radiated from intense, turbulent sound sources such as military jets and rockets are not well understood. In addition to the inherent random nature of the field, the amplitudes of the acoustic vibration are great enough that nonlinear considerations are necessary for modeling. In order to better understand these complex fields, high-amplitude noise in a tube is measured and analyzed. The basics of nonlinear acoustics will be covered briefly in this talk. Additionally, some statistical tools that are useful in analyzing random systems, such as probability density functions and skewness, will be explained. The measured evolution of the skewness of the first time derivative of high-amplitude noise in a tube will be presented

Application of Beamforming Methods to Full-Scale Military Jet Noise

Over the past decade, beamforming in aeroacoustics applications have undergone significant advances. Cross beamforming methods improve upon traditional beamforming in that they relax the assumption of multiple-source incoherence. This paper compares the abilities of three cross beamforming methods to reproduce source and field characteristics for an extended, partially correlated source that mimics supersonic jet noise radiation. Standard cross beamforming and two related methods that involve regularization—the hybrid method and improved generalized inverse beamforming—are applied to a numerically generated dataset along a near-field line. Estimated levels and coherence lengths are compared with benchmarks at the source as well as near and far-field locations. All three methods are successful in reproducing the field and source properties in high-amplitude regions. Although regularization generally helps to improve both source and field reconstructions, results are sensitive to regularization parameters, particularly for the generalized inverse method. The successful application of the three methods demonstrate the utility of cross-beamforming in formulating equivalent source models for accurate field prediction of complex sources, including jet noise

Energy-Based Tetrahedron Sensor for High-Temperature, High-Pressure Environments

An acoustic energy-based probe has been developed that incorporates multiple acoustic sensing elements in order to obtain the acoustic pressure and three-dimensional acoustic particle velocity. With these quantities, the user can obtain various energy-based quantities, including acoustic energy density, acoustic intensity, and acoustic impedance. In this specific development, the probe has been designed to operate in an environment characterized by high temperatures and high pressures as is found in the close vicinity of rocket plumes. Given these capabilities, the probe is designed to be used to investigate the acoustic conditions within the plume of a rocket engine or jet engine to facilitate greater understanding of the noise generation mechanisms in those plumes. The probe features sensors mounted inside a solid sphere. The associated electronics for the probe are contained within the sphere and the associated handle for the probe. More importantly, the design of the probe has desirable properties that reduce the bias errors associated with determining the acoustic pressure and velocity using finite sum and difference techniques. The diameter of the probe dictates the lower and upper operating frequencies for the probe, where accurate measurements can be acquired. The current probe design implements a sphere diameter of 1 in. (2.5 cm), which limits the upper operating frequency to about 4.5 kHz. The sensors are operational up to much higher frequencies, and could be used to acquire pressure data at higher frequencies, but the energy-based measurements are limited to that upper frequency. Larger or smaller spherical probes could be designed to go to lower or higher frequency rang

Summary of Supersonic Jet and Rocket Noise

This paper summarizes a two-part special session, “Supersonic Jet and Rocket Noise,” which was held during the 174th Meeting of the Acoustical Society of America in New Orleans, Louisiana. The sessions were cosponsored by the Noise and Physical Acoustics Technical Committees and consisted of talks by government, academic, and industry researchers from institutions in the United States, Japan, France, and India. The sessions described analytical, computational, and experimental approaches to both fundamental and applied problems on model and full-scale jets and rocket exhaust plumes

Characterization of High-Power Rocket and Jet Noise Using Near-Field Acoustical Holography

Structural fatigue, hearing damage, and community disturbances are all consequences of rocket and jet noise, especially as they become more powerful. Noise-reduction schemes require accurate characterization of the noise sources within rocket plumes and jets. Nearfield acoustical holography (NAH) measurements were made to visualize the sound field in the jet exhaust region of an F-22 Raptor. This is one of the largest-scale applications of NAH since its development in the 1980s. A scan-based holographic measurement was made using a 90-microphone array with 15 cm regular grid spacing, for four engine power settings. The array was scanned through 93 measurement positions, along three different planes in a region near 7 m from the jet centerline and 23 m downstream. In addition, 50 fixed reference microphones were placed along the ground 11.6 m from the jet centerline, spanning 30.8 m. The reference microphones have been used to perform virtual coherence on the measurement planes. Statistically-optimized NAH (SONAH) has been used to backpropagate the sound field to the source region for low frequencies, and to identify jet noise characteristics. Ground reflection interference and other non-ideal measurement conditions must be dealt with. Details relating to jet coherence lengths and their relation to reference microphone requirements will be discussed. Preliminary results of this ongoing work will be presented. [Work supported by Air Force SBIR.

Prediction of Nonlinear Propagation of Noise from a Solid Rocket Motor

Nonlinear acoustic wave propagation predictions (Generalized Burgers equation-based) of noise propagation are compared with measurements from a static, horizontally-fired solid rocket motor over a range of 76-1220 m during an 80 s burn time. The modeling suggests the nature of the geometric spreading between 76 and 305 m varies from cylindrical at low-frequencies to spherical at high frequencies. The predicted waveforms and high-frequency spectral slopes associated with significant shock content are in agreement with properties of the measured noise. At 1220 m, the relatively simple nonlinear model again approximates the measured spectrum despite the complexities of the measurement environment and atmospheric propagation

Characterization of Rocket and Jet Noise using Near-Field Acoustic Holography Methods

As rockets and jets on military aircraft become more powerful, the noise they produce can lead to structural fatigue, hearing damage, and community disturbances. Noise-reduction technologies and sound radiation prediction require accurate characterization of the noise sources within rocket plumes and jets. Near-field acoustical holography techniques were used to visualize the sound field in the region of the jet exhaust on a high-performance military jet. Holography requires a coherent measurement of the sound field, but the size of the jet made a dense measurement over the entire source region impractical. Thus, a scan-based measurement was performed, after which a partial field decomposition (PFD) procedure was used to tie together incoherent scans. Then, the effective aperture of the measurement was extended utilizing the rigid ground reflection and a processing technique called analytic continuation. Finally, the three-dimensional sound field was reconstructed using statistically-optimized near-field acoustical holography (SONAH). This is the first time such a map has been obtained for a full-scale military aircraft. [Work supported by Air Force SBIR.

LESSONS FROM THE MOTORIZED MIGRATIONS

Ten experiments have been conducted to determine if cranes can be led on migration and if those so trained will repeat migrations on their own. Results have been mixed as we have experienced the mishaps common to pilot studies. Nevertheless, we have learned many valuable lessons. Chief among these are that cranes can be led long distances behind motorized craft (air and ground), and those led over most or the entire route will return north come spring and south in fall to and from the general area of training. However, they will follow their own route. Groups transported south and flown at intervals along the route will migrate but often miss target termini. If certain protocol restrictions are followed, it is possible to make the trained cranes wild, however, the most practical way of so doing is to introduce them into a flock of wild cranes. We project that it is possible to create or restore wild migratory flocks of cranes by first leading small groups from chosen northern to southern termini

Rocket Motor Microphone Investigation

At ATK's facility in Utah, large full-scale solid rocket motors are tested. The largest is a five-segment version of the reusable solid rocket motor, which is for use on the Ares I launch vehicle. As a continuous improvement project, ATK and BYU investigated the use of microphones on these static tests, the vibration and temperature to which the instruments are subjected, and in particular the use of vent tubes and the effects these vents have at low frequencies