published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Approved for public release, distribution is unlimited / When a burst of air is produced in water, the result can be a toroidal bubble. This thesis is concerned with experimental investigations of three acoustical properties of toroidal bubbles: (i) propagation through high-intensity noise, (ii) emission, and (iii) scattering. In (i), an attempt to observe a recent prediction of the acoustic drag on a bubble is described, which is analogous to the Einstein-Hopf effect for an oscillating electric dipole in a fluctuating electromagnetic field. No effect was observed, which may be due to insufficient amplitude of the noise. In (ii), observations of acoustic emissions of volume oscillations of toroidal bubbles are reported. Surprisingly, the emission occurs primarily during the formation of a bubble, and is weak in the case of very smooth toroidal bubbles. In (iii), we describe an experiment to observe the effect of a toroidal bubble on an incident sound field. In addition to the acoustical investigations, we describe the construction of a large hallway apparatus for further investigations and for hands-on use by the public. The tank has cross section 2 feet by 2 feet and height 6 feet, and the parameters of reservoir pressure and time between air bursts are adjustable by the observer. / Lieutenant, United States Navy

Super-tall buildings located in high velocity wind regions are highly vulnerable to large lateral loads. Designing for these structures must be done with great engineering judgment by structural professionals. Present methods of evaluating these loads are typically by the use of American Society of Civil Engineers 7-10 standard, field measurements or scaled wind tunnel models. With the rise of high performance computing nodes, an emerging method based on the numerical approach of Computational Fluid Dynamics has created an additional layer of analysis and loading prediction alternative to conventional methods. The present document uses turbulence modeling and numerical algorithms by means of Reynolds-averaged Navier-Stokes and Large Eddy Simulation equations applied to a square prismatic prototype structure in which its dynamic properties have also been investigated. With proper modeling of the atmospheric boundary layer flow, these numerical techniques reveal important aerodynamic properties and enhance flow visualization to structural engineers in a virtual environment. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2015. / FAU Electronic Theses and Dissertations Collection

This work uses computational fluid dynamics to study the flowfield around a hypersonic missile with two lateral jets to provide control in place of control surfaces. The jets exhaust an H2-O2 mixture at Mach number of 2.9 with a jet pressure ratio of roughly 10,500. The jets are staggered axially and circumferentially in such a way to produce pitch and yaw. The flowfield of such a jet configuration is characterized at several angles of attack and the corresponding force coefficients and amplification factors are provided. The freestream air and H2-O2 plume is treated as inert for the majority of the calculations. Special cases are treated with finite rate chemical kinetics and compared to the inert flowfield to ascertain the effects that chemical reactions have on the force coefficients. It was found that the flowfield was only slightly altered from the familiar one jet flowfield when the second jet is active. The flow topology and vortex structures tend to shift towards the second jet but the overall structure remains the same. The normal force amplification factors are close to unity over the range of angle of attack due to the thrust being so high with the two jet configuration having a lower amplification factor compared to firing a single jet. Treating the flowfield as chemically reacting did not affect the force values much: the difference being 0.3% for an angle of attack of 0°. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

To gain understanding of the physical and structural events in the turbulent bursting process, an effort to generate artificial bursts in a turbulent boundary layer was made. Turbulent bursts, which are both random in time and in space, and cause a large portion of drag of a turbulent boundary layer. Control of the bursts could yield a decrease in skin friction and hence drag. Data were taken in a turbulent boundary layer developed over a flat plate in a low-speed wind tunnel with an array of eight hot-wire probes. The turbulent burst like events were created by pitching a rectangular shaped piezoelectric bimorph actuator out into the flow. The actuator effect is proposed which models the production of counter-rotating vortices in a rectangular vortex filament configuration. The results are compared with naturally occurring bursts and data from previous studies. Knowing more about the turbulent bursting process gives us more opportunities to control the turbulent bursts and therefore reduce drag over airfoils.

"This thesis documents the experimental proof of concept study for an ultrasonic method of wake vortex detection. A new acoustic technique is utilized to measure the circulation produced in the wake of lift-generating aircraft. Ultrasonic signals are transmitted in a path around the wake vortex, and are used to determine the average in-line velocity component along the acoustic path. It is shown herein that this velocity component is directly proportional to the net circulation value within the acoustic path. This is the first study to take this methodology and implement it in a realistic airport setting. This project included constructing a prototype and conducting field tests to prove the validity of this technology in a realistic environment setting. During field tests an acoustic path enclosed the vorticity shed behind one wing of a Piper PA-28 aircraft. Fourteen initial test flights were conducted in calm atmospheric conditions, and results show circulation values measured are comparable in magnitude and direction to expected circulations generated by the Piper PA-28 aircraft. Additional testing in various atmospheric conditions revealed the scope of practice for such a measurement technology. This study demonstrates the validity of the acoustic method in detecting aircraft wake vortices. Future investigations and applications utilizing this technique are discussed within."

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERO / Includes bibliographical references. / by Wen Liu. / M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Francesca De Siervi. / M.S.

Abstract not available

An experimental investigation of axial and near - axial flow over long slender cylinders, which involved both flow visualisation and hot - wire anemometry, is detailed. The investigation of this type of flow was instigated by the current interest in towed underwater sonar arrays. The need to discriminate between background noise of mechanical origin and the flow - induced noise generated on a moving underwater soundrecording device has produced a requirement for a greater understanding of the larger scale, lower frequency, turbulent flow processes in the wake and the boundary layer of a cylinder in both axial and near - axial flow. Of particular interest are any regular periodic fluid - dynamic processes. Thick axisymmetric boundary layers with the ratio of outer - layer length scale ( the boundary - layer thickness δ ) to cylinder radius a in the range 31 [approximately equal to or less than] δ / a [approximately equal to or less than] 38 and the corresponding ratio of cylinder radius to the inner - layer length scale ( the viscous length v / U [subscript τ] ) in the range 22 [approximately equal to or less than] aU [subscript τ] / v = a [superscript +] [approximately equal to or less than] 41 have been investigated. In accord with previous experimental results their mean - flow and turbulence properties are found to be strongly influenced by transverse curvature and to diverge significantly from those of flat - plate boundary layers. A characteristic feature of such thick axisymmetric layers is the occurrence of " spots " of low - speed fluid which are attributed to displacement of inner - layer fluid by large - scale turbulent cross - flows. A front of low - speed fluid which propagates radially across the boundary layer is identified as the primary large - scale, low - frequency, coherent structure within the boundary layer turbulence. A flow mechanism that describes the process by which these fronts are formulated on the basis of the experimental evidence formed from low - speed spots is obtained. The stripping of low - speed fluid from the cylinder surface by large - scale crossflows within the turbulent boundary layer is seen as an additional vorticity - and turbulence - generating mechanism, which cannot occur in a flat - plate layer. When the cylinder is yawed to the free - stream, an attached boundary layer persists over a small range of yaw angle, before flow separation occurs. In this range the boundary layer becomes extremely asymmetric, even at yaw angles less than 1 °. The asymmetry and mean - flow properties of such layers have been investigated for yaw angles of 0.25 ° and 0.5 ° at several Reynolds numbers in the range 300 [approximately equal to or less than] Re [subscript a] [approximately equal to or less than] 600. At somewhat larger yaw angles, a new regime of regular vortex - shedding in near - axial flow has been identified. From the experimental results, an empirical relation for the vortex - shedding frequency ( in terms of yaw angle, vortex - shedding angle, and a Reynolds number based on the component of free - stream velocity normal to the vortex axes ) has been derived as an extension of the Roshko formula for the frequency of vortex shedding from cylinders with their axes normal to the flow. The results presented advance the current understanding of the fundamental fluid mechanics of cylinders in axial and near - axial flow, and thereby have the potential to contribute to the advancement of the signal - processing techniques applied to towed underwater sonar arrays. / Thesis (Ph.D.)--School of Mechanical Engineering, 2005.