A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Magnetorheological fluid dynamics for high speed energy absorbers(2017) Sherman, Stephen Gilman; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids with a controllable yield stress allow rapid variations in viscous force in response to an externally applied field. These fluids are used in adaptive energy dissipating devices, which have a controllable force response, reducing shock and vibration loads on occupants and structures. This thesis investigates the physics of these fluids at high speeds and shear rates, through particle modeling and fluid dynamics. The focus is on the experimentally observed reduction in controllable force at high speeds seen in magnetorheological (MR) fluid, a suspension of magnetizable particles that develop a yield stress when a magnetic field is applied. After ruling out particle dynamic effects, this dissertation takes the first rigorous look at the fluid dynamics of a controllable yield stress fluid entering an active region. A simplified model of the flow is developed and, using computational fluid dynamics to inform a control volume analysis, we show that the reduction in high speed controllable force is caused by fluid dynamics. The control volume analysis provides a rigorous criteria for the onset of high speed force effects, based purely on nondimensional fluid quantities. Fits for pressure loss in the simplified flow are constructed, allowing yield force prediction in arbitrary flow mode geometries. The fits are experimentally validated by accurately predicting yield force in all of the known high speed devices. These results should enable the design of a next generation of high performance adaptive energy absorbers.Item Semi-Active Magnetorheological Seat Suspensions for Enhanced Crashworthiness and Vibration Isolation of Rotorcraft Seats(2007-09-27) Hiemenz, Gregory J; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research focuses on the use of magnetorheological (MR) dampers for enhanced occupant protection during harsh vertical landings as well as isolation of the occupant from cockpit vibrations. The capabilities of the current state-of-the-art in helicopter crew seat energy absorption systems are highly limited because they cannot be optimally adapted to each individual crash scenario (i.e. variations in both occupant weight and crash load level). They also present an unnecessarily high risk of injury by not minimizing the load transmitted to the occupant during a crash. Additionally, current rotorcraft seats provide no means of isolating the occupant from harmful cockpit vibrations. The objective of this research was to investigate and demonstrate the feasibility and benefits of an MR-based suspension for rotorcraft seats. As such, this research began with an in-depth investigation into design feasibility. Three MR seat suspension design cases are investigated: 1) for only vibration isolation, 2) for adaptive occupant protection, and 3) for combined adaptive occupant protection and vibration isolation. It is shown that MR-based suspensions are feasible for each of these cases and the performance benefits and tradeoffs are discussed for each case. Next, to further illustrate the occupant protection benefits gained with an MR-based suspension, three control strategies were developed and performance metrics were compared. It was shown that MR dampers can be controlled such that they will automatically adapt to the crash load level as well as occupant weight. By using feedback of sensor signals, MR dampers were adjusted to utilize the full stroke capability of the seat suspension regardless crash level and occupant weight. The peak load transmitted to the occupant and the risk of spinal injury, therefore, was always minimized. Because this control significantly reduced or eliminated injury risk during less severe landings, it is a significant advance over the current state-of-the-art rotorcraft seat suspensions which can provide no better than 20% risk of occupant injury. Finally, an MR-based seat suspension designed solely for the purposes of vibration isolation was designed, analyzed, and experimentally demonstrated. MR dampers were integrated into the current crashworthy SH-60 crew seat with minimal weight impact such that the original crashworthy capabilities were maintained. Then, utilizing semi-active control, experimental vibration testing demonstrated that the system reduced vertical cockpit vibrations transmitted to the occupant by 76%. This is a significant advance over current state-of-the-art rotorcraft seats which provide no attenuation of cockpit vibrations.