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The physical properties and subjective characteristics of sound and several special methods of measuring sound levels are discussed in order to provide a basic understanding of sound in general and noise in particular. The Federal Aviation Administration's regulation, FAR36, which stipulates the allowable perceived noise levels produced by commercial jet aircraft, is examined in detail. The principle of jet propulsion, the basic components of turbojet and turbofan engines, the theory of aerodynamic sound and the origin of the perceived noise decibel (which is the basic unit for measuring aircraft noise) are presented to provide an understanding of the fundamentals of jet aircraft noise. The origin of the jet noise problem is traced to the introduction of commercial jet aircraft in 1958. The sources of jet engine noise, their generating mechanisms and the applications of acoustic design technology incorporated to reduce the various components of jet engine noise are identified for each generation of jet engines powering subsonic commercial jet aircraft: turbojet, low bypass ratio turbofan and high bypass ratio turbofan engines. The technique used to identify a source of jet engine noise, specifically compressor noise, is demonstrated by presenting the spectral analysis (obtained by utilizing Fast Fourier Transform Software) of noise produced by a single stage axial flow fan rig. A review of airport noise, due to jet aircraft approaches and takeoffs, throughout the history of commercial jet aircraft, demonstrates the progress the aircraft industry has made in reducing the noise produced by jet engines powering commercial jet aircraft.

A long aspect ratio synthetic jet is produced through an axial slit along part of the length of a cylinder. The jet is excited acoustically by a pair of loudspeakers mounted at the cylinder terminations. The study compares between the performance of two different slits with aspect ratios of 273 and 773. The comparison is based on the spanwise distribution of the mean jet velocity and phase between the jet velocity fluctuations and the excitation signal. Three different frequencies and amplitudes are used to excite the speakers covering the range of frequencies used in the control application. For both cases studied the mean centerline velocity of the jet increases with increasing the amplitude of the exciting signal, but decreases with increasing its frequency. Moreover, velocity deficits of up to 30% are evident as the midspan of the cylinder is approached from either end. Similar trends are also observed for the centerline phase distributions of the velocity fluctuations, with deficits of up to 130°. However, it is observed that for the long slit case the deficits in both the velocity and phase distributions are much larger than those for the short one. The synthetic jet is then mounted in the upstream cylinder of a tandem cylinder arrangement to be used as a control actuator for controlling the vibrations of the downstream cylinder. A simple feedback control mechanism is used at a Reynolds number of about 6.3x104. This Reynolds number corresponds to the case' where the downstream cylinder’s response is dominated with two frequency components, one at the resonance frequency of the cylinder, which is excited by broadband turbulence in the flow, and the other at the vortex shedding frequency. Both slits studied for the characterization experiments are used to compare their performance as control actuators. Both jets produce comparable reductions in the vibration of the downstream cylinder. A reduction of about 20% in the total RMS amplitude of the vibrations signal is achieved. This amounts to a reduction of about 50% in the resonant peak and an average value of about 40% in the vortex shedding peak. The optimal values of gain and time lag of the controller are then used to investigate the effect of the jet on the flow. It is found that the short slit jet produced an effect that was traced up to 1.875 diameters downstream, while the effect of the long slit jet dropped dramatically very close to the upstream cylinder. / Thesis / Master of Applied Science (MASc)

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Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 108-110. / Ph.D.

Turbulent flow over a rough surface produces sound that radiates outside the near wall region. This noise source is often at a lower level than the noise created by edges and bluff body flows, but for applications with large surface area to perimeter ratios at low Mach number, this noise source can have considerable levels. In the first part of this dissertation, a detailed study is made of the ability of the Glegg & Devenport (2009) scattering theory to predict roughness noise. To this end, comparisons are made with measurements from cuboidal and hemispherical roughness with roughness Reynolds numbers, hu_Ï /ν, ranging from 24 to 197 and roughness height to boundary layer thickness ratios of 5 to 18. Their theory is shown to work very accurately to predict the noise from surfaces with large roughness Reynolds numbers, but for cases with highly inhomogeneous wall pressure fields, differences grow between estimation and measurement. For these surfaces, the absolute levels were underpredicted but the spectral shape of the measurement was correctly determined indicating that the relationship of the radiated noise with the wavenumber wall pressure spectrum and roughness geometry appears to remain relatively unchanged. In the second part of this dissertation, delay and sum beamforming and least-squares analyses were used to examine roughness noise recorded by a 36-sensor linear microphone array. These methods were employed to estimate the variation of source strengths through short fetches of large hemispherical and cuboidal element roughness. The analyses show that the lead rows of the fetches produced the greatest streamwise and spanwise noise radiation. The least-squares analysis confirmed the presence of streamwise and spanwise aligned dipoles emanating from each roughness element as suggested by the LES of Yang & Wang (2011). The least-squares calculated source strengths show that the streamwise aligned dipole is always stronger than that of the spanwise dipole, but the relative magnitude of the difference varies with frequency. / Ph. D.

Plasma railguns are electromagnetic accelerators used to produce controlled high velocity plasma jets. This thesis discusses the design and characterization of a small coaxial plasma railgun intended to accelerate argon-helium plasma jets. The railgun will be used for the study of plasma shocks in jet collisions. The railgun is mounted on a KF-40 vacuum port and operated using a 90 kA, 11 kV LC pulse forming network. Existing knowledge of coaxial railgun plasma instabilities and material interactions at vacuum and plasma interfaces are applied to the design. The design of individual gun components is detailed. Jet velocity and density are characterized by analyzing diagnostic data collected from a Rogowski coil, interferometer, and photodiode. Peak line-integrated electron number densities of approximately 8 × 10¹⁵ cm^-2 and jet velocities of tens of km/s are inferred from the data recorded from ten experimental pulses. / Master of Science / Plasma is a gaseous state of matter which is electrically conductive and interacts with electric and magnetic fields. Plasmas are used in many everyday objects such as fluorescent lights, but some of the physics of plasmas are still not entirely understood. One set of plasma interactions that have not been fully explored are those which occur during high-velocity collisions between plasmas. Experiments aimed to further the understanding of these interactions require the generation of plasmas with specified properties at very high velocities. A device known as a plasma railgun can be used to produce plasmas which meet these experimental demands. In a plasma railgun, a short pulse of current is passed through a plasma located between two parallel electrodes, or "rails". This current generates a magnetic field which propels the plasma forward. The plasma is accelerated until it leaves the muzzle of the railgun. In coaxial plasma railguns, the electrodes are concentric. This paper discusses the design and testing of a small, relatively low power coaxial plasma railgun. Specific elements of the design are examined and the inherent physical and material difficulties of a coaxial design are explored. The experiment which was performed to confirm the properties of the plasma jets produced by the coaxial plasma railgun is explained. The results of this experiment confirm that the design succeeds in producing plasmas which meet targets for plasma properties.

A liquid jet in a crossflow is often used to as a fuel injection method for combustion systems. Parameters such as penetration and core trajectory are used as characterization for the spray and specification of design criteria for combustor geometry. In addition to penetration and core trajectory, mapping the mass flux in space and time is an important part of modeling evaporation and global equivalence ratio throughout the combustor. Accurate prediction of these spray characteristics allows for a stable and robust combustor design. The break up of a liquid jet in a crossflow is an extremely complex phenomenon in both combination of mechanisms and variability of possible paths progressing from a liquid column to a distribution of individual droplets. In each region separate governing forces control the behavior of the liquid phase. Accordingly, different measurement techniques and different factors must be considered in each region. Presented are the results of measurements using Phase Doppler Analyzer, PDA, and a time resolved, digital, particle imaging velocimetry system, TRDPIV. The measurements include instantaneous and time-averaged liquid phase velocity fields, spray penetration and core location in the near field and far field of the spray resulting from the liquid jet breakup. With the TRDPIV system, the holistic properties of all three segments of a jet in crossflow were acquired with a single measurement. This allowed for comparison of system characteristics across not only individual pieces of one segment of the jet, for example PDA measurements of many droplets in one point of the far field spray, but characteristics across the entire system including the liquid column, near field spray, and far field spray simultaneously in a fashion that allowed for direct comparison between the different segments. / Master of Science