A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
Browse
3 results
Search Results
Item Nonlinear Interactions in Planar Jet Flow with High Frequency Excitation(2016) Kreutzfeldt, Timothy; Chopra, Inderjit; Glaz, Bryan; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental active flow control study was conducted involving excitation of a tabletop planar turbulent jet with a high frequency piezoelectric actuator. The excitation frequencies considered corresponded to the dissipative subrange of turbulent kinetic energy and were orders of magnitude greater than classical shear layer instability modes. Single-wire and dual-wire hot wire probes were used to determine how excitation induces alterations to bulk flow quantities as well as nonlinear interactions. Differences in flow receptivity to high frequency excitation were investigated by varying the development length of the turbulent jet at a Reynolds number of 8,700 and Strouhal number of 21.3. Excitation of developed turbulent flow yielded larger increases in the energy dissipation rate and higher magnitude velocity power spectrum peaks at the forcing frequency than undeveloped turbulent flow. Further tests with excitation of reduced mean velocity flow at a Reynolds number of 6,600 and a Strouhal number of 27.8 demonstrated that high frequency forcing resulted in transfer of energy from large to small scales in the turbulent kinetic energy spectrum. This phenomenon appeared to support past literature that indicated that the mechanics of high frequency forcing are fundamentally different from conventional instability-based forcing. Theoretical arguments are presented to support these experimental observations where it is shown that coupling between the applied forcing and background turbulent fluctuations is enhanced. An eddy viscosity model first proposed under the assumption of instability-based forcing was shown to be an effective approximation for the experimental measurements presented here in which the flow was forced directly at turbulence scales. Dimensional analysis of the coupling between the induced oscillations and the turbulent fluctuations supported experimental findings that receptivity to excitation was increased for forced flow with higher turbulent kinetic energy, higher excitation amplitude, and lower energy dissipation rate. This study is the first to present such results which validate a model that offers theoretical insight into flow control mechanics when directly forcing small scale turbulent fluctuations.Item Miniaturized Power Electronic Interfaces for Ultra-compact Electromechanical Systems(2015) Tang, Yichao; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advanced and ultra-compact electromechanical (EM) systems, such as kinetic energy harvesting and microrobotic systems are deemed as enabling solutions to provide efficient energy conversion. One of the most critical challenges in such systems is to develop tiny power electronic interfaces (PEIs) capable of addressing power conditioning between EM devices and energy storage units. This dissertation presents technologies and topological solutions toward fabricating miniaturized PEIs to efficiently regulate erratic power/voltage for kinetic energy harvesting and drive high-voltage actuators for microrobotic systems. High-frequency resonant-switching topologies are introduced as power stages of PEIs that allow small footprint of the circuit without suffering from switching losses. Two types of bridgeless resonant ac-dc converters are first introduced and developed to efficiently convert arbitrary input voltages into a regulated dc output voltage. The proposed topologies provide direct ac-dc power conversion with less number of components, in comparison to other resonant topologies. A 5-mm×6-mm, 100-mg, 2-MHz and 650-mW prototype is fabricated for validation of capability of converting very-low ac voltages into a relatively higher voltage. A resonant gate drive circuit is designed and utilized to further reduce gating losses under high-frequency switching and light-load condition. The closed-loop efficiency reaches higher than 70% across wide range of input voltages and output powers. In a multi-channel energy harvesting system, a multi-input bridgeless resonant ac-dc converter is developed to achieve ac-dc conversion, step up voltage and match optimal impedance. Alternating voltage of each energy harvesting channel is stepped up through the switching LC network and then rectified by a freewheeling diode. The optimal electrical impedance can be adjusted through resonance impedance matching and pulse-frequency-modulation (PFM) control. In addition, a six-input standalone prototype is fabricated to address power conditioning for a six-channel wind panel. Furthermore, the concepts of miniaturization are incorporated in the context of microrobots. In a mobile microrobotic system, conventional bulky power supplies and electronics used to drive electroactive polymer (EAP) actuators are not practical as on-board energy sources for microrobots. A bidirectional single-stage resonant dc-dc step-up converter is introduced and developed to efficiently drive high-voltage EAP actuators. The converter utilizes resonant capacitors and a coupled-inductor as a soft-switched LC network to step up low input voltages. The circuit is capable of generating explicit high-voltage actuation signals, with capability of recovering unused energy from EAP actuators. A 4-mm × 8-mm, 100-mg and 600-mW prototype has been designed and fabricated to drive an in-plane gap-closing electrostatic inchworm motor. Experimental validations have been carried out to verify the circuit’s ability to step up voltage from 2 V to 100 V and generate two 1-kHz, 100-V driving voltages at 2-nF capacitive loads.Item Dynamics of Slender, Flexible Structures(2014) Vlajic, Nicholas A.; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dynamics of slender beam-like structures subjected to rotational motions is studied experimentally, numerically, and analytically within this dissertation. As the aspect ratio of beam-like structures is increased (i.e., as the structures become slender), the structure can undergo large elastic deformations, and in addition, the torsional and lateral motions can be strongly coupled. Two different paradigms of rotor systems are constructed and used to investigate coupled torsional-lateral motions in slender rotating structures. The first rotor model is a modified version of the classical Jeffcott rotor, which accounts for torsional vibrations and stator contact. Analysis and simulations indicate that torsional vibrations are unlikely to exist during forward synchronous whirling, and reveal the presence of phenomena with high-frequency content, such as centrifugal stiffening and smoothening, during backward whirling. The second rotor model is a nonlinear distributed-parameter system that has been derived with the intent of capturing dynamics observed in an experimental apparatus with slender, rotating structures. Nonlinear oscillations observed in the experiments contain response components at frequencies other than the drive speed, a feature that is also captured by predictions obtained from the distributed-parameter model. Further analysis of the governing partial-differential equations yields insights into the origins (e.g., nonlinear gyroscopic coupling and frictional forces) of the nonlinear response components observed in the spectrum of the torsion response. Slender structures are often subject to large deformations with pre-stress and curvature, which can drastically alter the natural frequencies and mode shapes when in operation. Here, a geometrically exact beam formulation based on the Cosserat theory of rods is outlined in order to predict the static configuration, natural frequencies, and mode shapes of slender structures with large pre-stress and curvature. The modeling and analysis are validated with experiments as well as comparisons with a nonlinear finite element formulation. The predictions for the first eight natural frequencies are found to be in excellent agreement with the corresponding experimentally determined values. The findings of this dissertation work have a broad range of applications across different length scales, including drill strings, space tethers, deployable structures, cable supported structures (e.g., bridges and mooring cables), DNA strands, and sutures for non-invasive surgery to name a few.