A new microwave technique has been developed for the experimental study of small amplitude surface waves on an electrically conducting fluid. The fluid forms one of the walls of a resonating, microwave cavity. Surface waves with amplitudes as small as 10⁻³ cm. can be measured by observing the resulting change in the resonant frequency of the cavity. This technique has been successfully used to measure the viscous and magnetic damping coefficient of a small amplitude, standing, surface wave in liquid mercury. The magnetic damping, coefficient (for a vertical, magnetic field) was found to be in good agreement with a calculation that was made, for low magnetic Reynolds numbers. When the viscous damping coefficient was compared with the standard theory, which allows horizontal motion of the. surface, a disagreement of up to a factor of four was found. It, however, showed excellent agreement with a modified theory which assumes that there, is no horizontal motion of the surface. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Composite structures for aerospace and wind turbine applications are subjected to high acoustic and vibrational loading and exhibit very high amplitude displacements and thus premature failure. Materials with high damping or absorbing properties are crucially important to extend the life of structures. Traditional damping treatments are based on the combinations of viscoelastic, elastomeric, magnetic, and piezoelectric materials. In this work, the use of carbon nanofibers (CNFs) in the form of interconnected self-supportive paper as reinforcement can significantly improve damping performance. The interfacial friction is the primary source of energy dissipation in CNF paper based nanocomposites. The approach entailed making CNF paper by filtration of well-dispersed nanofibers under controlled processing conditions. The CNF paper was integrated into composite laminates using modified liquid composite molding processes including Resin Transfer Molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM). The rheological and curing behaviors of the CNF-modified polymer resin were characterized with Viscometry and Differential Scanning Calorimetry (DSC). The process analysis in mold filling and pressure distribution was conducted using Control Volume Finite Element Method (CVFEM) in an attempt to optimize the quality of multifunctional nanocomposites. The mold filling simulation was validated with flow visualization in a transparent mold. Several tests were performed to study the damping properties of the fabricated composites including Dynamic Mechanical Analysis (DMA) and piezoceramic patch based vibration tests. It was found that the damping performance was significantly enhanced with the incorporation of carbon nanofibers into the composite structures.

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Conventional electronics rely on the transport of electrons through a circuit to carry information. This comes with ever-present Joule heating as a result of the resistive scattering of electrons. Recent works in the field of spintronics have focused on using magnetic excitations (e.g., spin waves) instead of electrons as a means of information transport without Joule heating. However, realizing long distance information transport using conventional spin waves has proven difficult owing to their diffusive nature and the exponential decay of spin current. Theoretical studies have proposed a new form of magnetization dynamics, referred to as superfluid-like spin transport, as a way to overcome this shortfall. Instead of decaying exponentially with distance, the spin current associated with superfluid-like spin transport decays linearly with distance, potentially allowing for information transport beyond the micron-scale. In this dissertation, I discuss the work that I have done towards realizing this novel phenomenon in a metallic, ferromagnetic system. Results on a reduced damping and reduced magnetic moment Fe-based alloy, micromagnetic simulations that use established domain wall physics to explain superfluid-like spin transport, and an investigation of spin torques found in a current-in-plane spin valve structure with broken in-plane symmetry for excitation of superfluid-like spin transport dynamics are discussed. I conclude by discussing what steps remain before superfluid-like spin transport can be measured in an experimental system as well as the impacts this work could have on the wider spintronics field. This work was supported in part by National Science Foundation, Grant No. DMR-2003914. / Doctor of Philosophy / All of the electronics devices we use every day depend on tiny, charged particles called electrons moving through a wire. These particles bounce off of and collide with defects within that wire and cause the wire to heat up, dissipating their energy to the surrounding environment. If this could be avoided, the overall power needed to operate our devices could be lowered. To alleviate this problem, scientists take advantage of another property of the electron, its spin (which gives rise to magnetism), to send signals. Since the electron spin can interact with the spin of nearby electrons, information can be transported this way without actually moving the electrons themselves. These magnetic signals can be thought of as the electron spin wiggling a small amount about its axis, somewhat akin to a precessing top. The downside to these magnetic signals is that they decay away very quickly, typically much quicker than electron currents. In this dissertation, I focus on using a different form of magnetic signals, one that can be thought of like a fluid flowing through a pipe, to send information much further than before without significant energy losses. This phenomenon, which I call ``superfluid-like spin transport,'' has the potential to dramatically alter the future development path of next-generation devices. In the first experiment, I discuss the work done to choose a suitable material platform that can host superfluid-like spin transport. Starting with the common magnet iron, we show that by mixing it with the correct non-magnetic material, it is possible to improve the magnetic properties in a way that is beneficial to superfluid-like spin transport. In the next experiment, computer simulations were used to understand how superfluid-like spin transport might behave in a future device. We find that the fluid-like behavior found in this phenomenon can actually be understood by imagining a train of rigid particle-like entities being packed closely together. In the final experiment, I investigate whether a new and potentially simpler device geometry can be used to start the flow of superfluid-like spin transport. It turns out that the mechanism needed to start the flow is surprisingly weak in the material system studied. While this work does not achieve superfluid-like spin transport, it has taken essential steps towards understanding how one might do so in the future using common materials in an easy-to-make manner. I conclude by offering my thoughts of what the next steps would be as well as impacts this work might have on future next-generation energy-efficient devices.

Many researchers have devoted their work to the development of modal analysis extraction techniques in order to obtain more reliable identification of the modal parameters. Also, as a consequence of all this work, there are some other works devoted to the evaluation and comparison of these methods in order to find which one is the most reliable method with respect to certain characteristics. In this thesis the Rational Fraction Polynomial (RFP) Method, the Prony or Complex Exponential Method (CEM), the Ibrahim Time Domain (ITD) Method, and Hilbert Envelope Method are used to evaluate how the accuracy of the damping ratio is affected with respect to various parameters and conditions. The investigation focuses in the estimation of damping ratio because among the modal parameters, it is the most difficult to model. Each method is evaluated individually in order to understand how the damping ratio estimation is affected with respect to each method when the characteristics of the FRF are changed. Also, they are compared to show that, in general, the Rational Fraction Polynomial Method is a more reliable method than the other methods. To investigate this, a simulated analytical data and an experimental data are processed to estimate the modal parameters, but focusing in the damping ratio. For the simulated analytical data the damping ratio's percent of error were calculated. The highest damping ratio's percent of error of the RFP was 0.0073501%. In the other hand, for the CEM, ITD, and Hilbert Envelope Method their highest damping ratio's percent of error were 83.02%, 99.82%, and 4.077%, respectively. / Master of Science

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The compromise between ride comfort and handling for a passive suspension system is a well-known and often researched problem. Semi-active suspension systems offer significant improvements to this compromise. One example of a semi-active system, that can change both spring and damper characteristics between two discrete values is the 4-state semi-active hydro-pneumatic suspension system. This system can switch between a ”ride comfort mode” (soft spring and low damping) and a ”handling mode” (stiff spring and high damping) within 100ms, improving both ride comfort and handling. The discrete 4S4 could be improved upon further by adding continuous variable damping. Work on this topic showed great promise but also posed its challenges in achieving this in a safe and controllable manner. In order to make continuous variable damping a reality a new configuration for the 4S4 is proposed. This new configuration incorporates a blow-off damper in parallel with a proportional flow control valve. The system ensures that, in the highly non-linear closing region of the proportional flow control valve, adequate damping for handling is maintained and uncontrollable peak pressure differences are avoided. Experimental work conducted showed that the system was capable of achieving the required spring and variable damping characteristics in a safe and controllable manner. The experimental data was used for parametrizing and validating a physics based mathematical model of the suspension system. The mathematical model incorporates the: pressure drop vs: flow characteristics for both the blow-off and proportional valves, response time for the on-off valves as well as the gas pressure vs: flow characteristic incorporating the compressibility of the oil and thermal properties of the gas. This model can be used to make informed decisions on further prototype development or in full vehicle simulations. The system makes continuous variable damping possible ranging from the optimal damping characteristic for handling to the low damping characteristic required for ride comfort. The system also shows a significant reduction in friction. / Dissertation (MEng)--University of Pretoria, 2020. / VDG / University of Pretoria / Mechanical and Aeronautical Engineering / MEng / Unrestricted

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy