Statics and Dynamics of Skyrmions Interacting with Pinning: A Review

Magnetic skyrmions are topologically stable nanoscale particle-like objects that were discovered in 2009. Since that time, intense research interest has led to the identification of numerous compounds that support skyrmions over a range of conditions spanning cryogenic to room temperatures. Skyrmions can be set into motion under various types of driving, and the combination of their size, stability, and dynamics makes them ideal candidates for numerous applications. Skyrmions represent a new class of system in which the energy scales of the skyrmion-skyrmion interactions, sample disorder, temperature, and drive can compete. A growing body of work indicates that the static and dynamic states of skyrmions can be influenced strongly by pinning or disorder in the sample; thus, an understanding of such effects is essential for the eventual use of skyrmions in applications. In this article we review the current state of knowledge regarding individual skyrmions and skyrmion assemblies interacting with quenched disorder or pinning. We outline the microscopic mechanisms for skyrmion pinning, including the repulsive and attractive interactions that can arise from impurities, grain boundaries, or nanostructures. This is followed by descriptions of depinning phenomena, sliding states over disorder, the effect of pinning on the skyrmion Hall angle, the competition between thermal and pinning effects, the control of skyrmion motion using ordered potential landscapes such as one- or two-dimensional periodic asymmetric substrates, the creation of skyrmion diodes, and skyrmion ratchet effects. We highlight the distinctions arising from internal modes and the strong gyroscopic or Magnus forces that cause the dynamical states of skyrmions to differ from those of other systems with pinning. We also discuss future directions and open questions related to the pinning and dynamics in skyrmion systems.Comment: 66 pages, 71 figure

Enhanced Surface Melting of the Fennoscandian Ice Sheet during Stadials

Unexpected melting of Northern Hemisphere ice sheets during periods of regional cooling characterizes the climate of the last glacial period. While the Heinrich Events are the most well-studied example of this phenomenon, Samuel Toucanne and colleagues recently documented evidence of Fennoscandian Ice Sheet melting during Heinrich Stadials, the cold periods during which Heinrich Events occur. In this dissertation, I use the geographic provenance of sediments in the Bay of Biscay, a proxy for Fennoscandian Ice Sheet melting, along with other paleoclimate records to: (1) demonstrate the persistence of abrupt Fennoscandian Ice Sheet melting as a feature of the Pleistocene climate system, (2) develop a self-consistent explanation for the synchronous melting of ice sheets in the North Atlantic region, and (3) elucidate the timing of abrupt climate change in the Northern Hemisphere. I begin by introducing a framework for inferring the subglacial transport distance of fine sediments from coupled provenance and grain size analyses. This chapter untangles the relationships between the source, size, transport history, and geochemical signature of glacigenic sediments in northern Europe, clarifying the geographical significance of sediment provenance in the Bay of Biscay. I then develop a new method for the spectral analysis of unevenly sampled time series. In the following chapter, I apply the new spectral method to time series of Fennoscandian Ice Sheet melting, Laurentide Ice Sheet melting, and solar activity changes during the last glacial period. Doing so reveals a coherence between ice sheet melting and solar activity and helps explain the quasi-periodic melting of ice sheets on millennial timescales. I then extend the neodymium isotope provenance record of Fennoscandian Ice Sheet melting through Marine Isotope Stage 6, demonstrating that enhanced summertime melting of the FIS during Heinrich Stadials is a recurring feature of glacial periods. In the final chapter, I document a relationship between the occurrence of abrupt ice sheet melting in the Northern Hemisphere and the precession of Earth’s spin axis to reveal an astronomical forcing of millennial-scale climate change

NASA Thesaurus. Volume 2: Access vocabulary

The NASA Thesaurus -- Volume 2, Access Vocabulary -- contains an alphabetical listing of all Thesaurus terms (postable and nonpostable) and permutations of all multiword and pseudo-multiword terms. Also included are Other Words (non-Thesaurus terms) consisting of abbreviations, chemical symbols, etc. The permutations and Other Words provide 'access' to the appropriate postable entries in the Thesaurus

Measurements, Models, Systems and Design

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A gravel-sand bifurcation:a simple model and the stability of the equilibrium states

A river bifurcation, can be found in, for instance, a river delta, in braided or anabranching reaches, and in manmade side channels in restored river reaches. Depending on the partitioning of water and sediment over the bifurcating branches, the bifurcation develops toward (a) a stable state with two downstream branches or (b) a state in which the water discharge in one of the branches continues to increase at the expense of the other branch (Wang et al., 1995). This may lead to excessive deposition in the latter branch that eventually silts up. For navigation, flood safety, and river restoration purposes, it is important to assess and develop tools to predict such long-term behavior of the bifurcation. A first and highly schematized one-dimensional model describing (the development towards) the equilibrium states of two bifurcating branches was developed by Wang et al (1995). The use of a one-dimensional model implies the need for a nodal point relation that describes the partitioning of sediment over the bifurcating branches. Wang et al (1995) introduce a nodal point relation as a function of the partitioning of the water discharge. They simplify their nodal point relation to the following form: s*=q*k , where s* denotes the ratio of the sediment discharges per unit width in the bifurcating branches, q* denotes the ratio of the water discharges per unit width in the bifurcating branches, and k is a constant. The Wang et al. (1995) model is limited to conditions with unisize sediment and application of the Engelund & Hansen (1967) sediment transport relation. They assume the same constant base level for the two bifurcating branches, and constant water and sediment discharges in the upstream channel. A mathematical stability analysis is conducted to predict the stability of the equilibrium states. Depending on the exponent k they find a stable equilibrium state with two downstream branches or a stable state with one branch only (i.e. the other branch has silted up). Here we extend the Wang et al. (1995) model to conditions with gravel and sand and study the stability of the equilibrium states

Design, Analysis, Optimization and Control of Rotor Tip Flows

Developments in turbomachinery focus on efficiency and reliability enhancements, while reducing the production costs. In spite of the many noteworthy experimental and numerical investigations over the past decades, the turbine tip design presents numerous challenges to the engine manufacturers, and remains the primary factor defining the machine durability for the periodic removal of the turbine components during overhaul. Due to the hot gases coming from the upstream combustion chamber, the turbine blades are subjected to temperatures far above the metal creep temperature, combined with severe thermal stresses induced within the blade material. Inadequate designs cause early tip burnouts leading to considerable performance degradations, or even a catastrophic turbine failure. Moreover, the leakage spillage, nowadays often exceeding the transonic regime, generates large aerodynamic penalties which are responsible for about one third of the turbine losses. In this view, the current doctoral research exploits the potential through the modification and optimization of the blade tip shape as a means to control the tip leakage flow aerodynamics and manage the heat load distribution over the blade profile to improve the turbine efficiency and durability

The chronology of prehistoric high-energy wave events (tropical cyclones, tsunamis) in the southern Caribbean and their impact on coastal geo-ecosystems – a case study from Bonaire (Leeward Antilles)

By using sediment archives along the coast of Bonaire (Leeward Antilles; political status: special municipality of the Netherlands), such as bokas (enclosed lagoons), narrow floodplains and mangrove swamps, and by investigating supralittoral blocks and boulders, the goal of this thesis is (i) to identify prehistoric high-energy wave events, (ii) to differentiate between extraordinary storms and tsunamis, and (iii) to date these events. Furthermore, the spatio-temporal impact of high-energy wave events on coastal geo-ecosystems is evaluated. A total of 45 vibracores (max. depth 11 m) and push cores (max. depth 1.62 m) was taken from all around the island (Lagun, Boka Washikemba, Lac Baai, Saliña Tam, Klein Bonaire, Boka Funchi, Wayaka, Boka Bartol, Playa Grandi). Cores from the most promising locations were investigated in terms of sedimentology, geochemistry, mineralogy and faunal remains. The coasts of the Caribbean Sea are prone to tsunami hazard, proven by numerous observed events of the last 500 years. Bonaire was chosen as the site of interest for this study due to the antagonism of a total lack of historical accounts on tsunami occurrence and its massive record of coarse-clast coastal landforms and single blocks and boulders, which are attributed to deposition by extreme wave events (tsunamis, hurricanes). The largest coastal blocks were studied as evidence for the occurrence of more powerful waves than those of strong tropical cyclones (locally known as hurricanes). By comparing calculated minimum storm wave heights required to quarry and move the largest blocks of Spelonk and Boka Olivia on northeastern Bonaire, with maximum wave heights observed during recent high-category hurricanes and the buoy record, little doubt remains that one or more major tsunamis occurred in Mid- to Late Holocene times. This confirmation of the previously established (and in the meantime challenged) tsunami hypothesis provides a very important framework condition for the interpretation of the subsurface overwash deposits in the coastal stratigraphical archives. A tentative chronology of tsunami occurrence was established, based on a comparison of subsurface overwash deposits identified in the sediment cores from the stratigraphical geoarchives with characteristic signature types of modern tsunamis and calibrated 14C data. Uncertainties in sediment interpretation, associated with the fact that most of these signature types may also occur in storm deposits, were reduced according to (i) the results from the numerical boulder study, (ii) the high tsunami potential of the southern Caribbean (abundant potential tsunami trigger mechanisms), (iii) the identification of sediment source areas which are out of reach for storm waves, and (iv) the lack of littoral and marine sand/mud input during recent high-category hurricanes. Several sites reflect long-lasting environmental change subsequent to palaeotsunami impact. At the open embayment of Lagun, this is confirmed by palynological investigations. Pollen spectra prior to the wave impact are dominated by mangrove types (Rhizophora), whereas significantly lower percentage of Rhizophora pollen was detected in the post-tsunami samples. Mangroves seem to have only gradually recovered from the impact, probably on centennial scales. In contrast to Lagun, the bokas and saliñas of Bonaire are cut off from the sea by a quasi-impermeable barrier of beachrock and coral rubble. The candidate tsunami deposit of Boka Bartol is vertically confined by mangrove peat (below) and evaporite-rich mud (above). This pattern is interpreted as the transformation of an open mangrove-fringed embayment (pre-tsunami) into a poly- to hypersaline lagoon (post-tsunami) due to the establishment or closure of the barrier of coral material during the event. The high amount of gypsum and halite crystals and thin layers of inorganic carbonate are the result of limited water input and high evapotranspiration rates during the dry season. The absence of marine faunal remains also indicates very limited exchange with the ocean. The chronology of potential tsunamis based on data from the sediment cores starts with extreme wave event I (EWE I), around 3600 BP, represented by a carbonate sand deposit bracketedby man-grove peat, which was found on Klein Bonaire. A well-preserved candidate tsunami deposit from Boka Bartol with a maximum age of 3300 BP (EWE II) has counterparts on the leeward coast (Klein Bonaire, Saliña Tam, possibly between Saliña Tam and Punt’i Wekua) and the windward coast (Playa Grandi, possibly Boka Washikemba). EWE II can be identified as the best-documented candidate palaeotsunami on Bonaire. EWE XII, with a maximum age of 2000 BP, left massive shell-dominated deposits at Lagun (windward side) and carbonate sands at Saliña Tam, both of which were interpreted as tsunamigenic. Deposits of another younger, unspecified EWE XVI (pre-500 BP) were found between Saliña Tam and Punt’i Wekua, and Boka Washikemba. Based on these results, a preliminary estimation of the recurrence interval of high-energy wave events on Bonaire, which reveal magnitudes significantly exceeding those of recent high-category hurricanes and which are therefore likely to be major tsunamis, is in the order of roughly 1000 years or less. This study corroborates conclusions of predecessor studies that the hazard of tsunami does exist on Bonaire, although historical accounts on tsunami ocurrence are lacking. It is strongly suggested to the authorities of Bonaire, to conduct a local analysis of tsunami risk, similar to the one recently established for the neighbour island of Curaçao. A study of modelling tsunami inundation based on the spectrum of tsunami triggering mechanisms and past inundation inferred from the overwash deposits identified in this research may be useful to determine high-risk areas, which are prone to potential future tsunami damage