Magnetocaloric materials with first-order magnetic (FOM) phase transitions are of interest as low-cost working materials in magnetic cycles. Hysteresis is a property associated with first order transitions, and is undesirable as it can reduce performance. Devices using FOMs in active magnetic refrigeration have shown performance comparable to more expensive second-order materials, so some degree of hysteresis appears to be acceptable; however, the amount of hysteresis that may be tolerated is still an unanswered question. Among the FOM, the family of MnP-based is one of the promising materials for magnetic heat pump applications near room temperature. The present study describes the experimental investigation of a single-layer MnFeP1-xSix active magnetic regenerator (AMR), under different test conditions and following a protocol of heating and cooling processes. The results for the FOM are compared with a Gd AMR that is experimentally tested following the same protocol, with the objective to study the irreversibilities associated with FOM. The experimental tests are performed in a PM I test apparatus at a fixed displaced volume of 5.09 cm3 and a fixed operating frequency of 1 Hz. The results indicated a significant impact of the hysteresis on the heating and cooling temperature span for FOM regenerator. For certain operating conditions, multiple points of equilibrium (MPE) exist for a fixed hot rejection temperature. It is shown that the existence of MPEs can affect the performance of an AMR significantly for certain operating conditions. The present work advances our understanding since the combined hysteresis and MPE are two significant features which can impact layered AMR performance using MnFeP1-xAsx FOM by systematic experimental testing. With this objective, three multilayer MnFeP1-xAsx FOM regenerator beds are experimentally characterized under a range of applied loads and rejection temperatures. Thermal performance and the impacts of MPE are evaluated via heating and cooling experiments where the rejection (hot side) temperature is varied in a range from 283 K to 300 K. With fixed operating conditions, we find multiple points of equilibrium for steady-state spans as a function of warm rejection temperature. The results indicate a significant impact of MPE on the heating and cooling temperature span for multilayer MnFeP1-xAsx FOM regenerator. Unlike single material FOM tests where MPEs tend to disappear as load is increased (or span reduced), with the layered AMRs, MPEs can be significantly even with small temperature span conditions. A third experimental study examines the performance of MnFeP1-xAsx multilayer active magnetic regenerators. Five different matrices are tested: (i) one with three layers; (ii) one with six layers; and (iii) three, eight layer regenerators where the layer thickness is varied. The tests are performed using a dual regenerator bespoke test apparatus based on nested Halbach permanent magnets (PM II test apparatus). Operating variables include displaced volume (3.8 - 12.65 cm3), operating frequency (0.5 - 0.8 Hz) and hot-side rejection temperature (293-313 K).The results are mainly reported in terms of zero net load temperature span as a function of rejection temperature; a few tests with non-zero applied load are also presented. A maximum temperature span of 32 K is found for an 8-layer regenerator, which is similar to a previous work performed with gadolinium in the same experimental apparatus. A 1D active magnetic regenerator model accounting for thermal and magnetic hysteresis is developed and compared to experimental data for both a Gd-based and MnFeP1-xSix based AMR. Magnetic and thermal hysteresis are quantified using measured data for magnetization and specific heat under isothermal and isofield warming and cooling processes. Hysteresis effects are then incorporated in the model as irreversible work and reduced adiabatic temperature change. Model results are compared to measured temperature spans for regenerators operating with different thermal loads. Simulated results for temperature span as a function of cooling power and rejection temperature show good agreement with experimental data. The irreversible work due to hysteresis is found to have a small impact on predicted spans, indicating that useful cooling power is well predicted using cyclic measurements of adiabatic temperature change. / Graduate

Malgré une apparente concurrence entre les monnaies, le dollar américain est la monnaie adoptée par la majorité des participants aux échanges internationaux, exportateurs et importateurs. C’est ce que montre l’état des lieux statistique proposé dans cette thèse. Partant d’un tel constat, l’objectif de cette thèse est d’expliquer les choix de monnaies de facturation du commerce international. Différents déterminants ont été établis par la littérature académique pour expliquer les choix de monnaies de facturation. Trois approches, aux méthodologies sensiblement distinctes, y contribuent, et peuvent dès lors être confrontées : l’approche macroéconomique standard, l’approche institutionnaliste, et l’approche de l’économie politique internationale. Le principal résultat de cette thèse est de montrer que la compréhension du choix du dollar américain ne peut se contenter des déterminants mis en évidence par les approches instrumentales de la monnaie qui dominent la macroéconomie internationale, et dans lesquelles la monnaie est avant tout vue comme un outil de maximisation d’utilités individuelles : leurs apports sont utiles, mais insuffisants. En appliquant la lecture institutionnaliste aux échanges internationaux, la thèse introduit l’importance des rapports de confiance et d’adhésion des acteurs à l’égard de la monnaie et de l’ordre défendu par ses institutions régulatrices. Cette importance se vérifie dans l’histoire du XXe siècle, à la fois dans l’échec des monnaies concurrentes du dollar, et dans la persistance de la monnaie américaine elle-même. / Despite the appearance of competition between currencies, the US dollar remains the invoicing currency among the actors of international trade. Such observation is derived from ample statistical evidence contained in this thesis. This observation represents the starting-point of a broader reflection on the rationale underlying the choice of an invoicing currency in international trade, which is the central question which this thesis aims to address. Various determinants have been closely examined by the academic literature to that end. Three approaches dealt with the subject, each with its distinct methodology, and can therefore be analyzed comparatively: the standard macroeconomic approach, the institutionalist approach, and the international political economy approach. The main result of this thesis is that preference for the US dollar cannot be explained exclusively by the instrumental approaches of money which usually prevail in international macroeconomics: their contributions are useful but inadequate as they envision money primarily as a tool for maximizing individual utilities. By applying the institutionalist reading to international exchanges, the thesis introduces the necessity of trust and ethical appeal that money inspires, and the centrality of the social order that its regulatory institutions pursue. This necessity was reasserted throughout the twentieth century, as competing currencies failed to challenge the supremacy of the dollar. It is also evidenced by the resilience of the American currency itself.

O trabalho trata da formulação, implementação e aplicação de um modelo para análise não-linear de estruturas reticuladas planas em concreto armado, baseado no conceito simplificado de localização dos processos físicos irreversíveis (dano, plasticidade e histerese) em zonas previamente definidas denominadas rótulas que se localizam nas extremidades de um elemento finito de barra. O modelo proposto tem por objetivo reproduzir os deslocamentos residuais relacionados tanto ao comportamento plástico do concreto danificado quanto ao escoamento do aço, além de possibilitar a reprodução de laços de histerese observados experimentalmente em diagramas momento-rotação, ou equivalentemente força-deslocamento, quando a história de carregamento incluir ciclos de descarregamento e recarregamento. A formulação termodinâmica é desenvolvida fazendo-se referência as variáveis de tensão e deformação generalizadas em razão de sua aplicação no âmbito da teoria clássica de flexão de barras. Neste modelo a não-linearidade do elemento estrutural é totalmente decorrente da dissipação de energia localizada nas rótulas e é representada por três variáveis escalares a elas associadas: a variável adimensional de dano, a rotação plástica e a rotação por escorregamento, que representa o fenômeno físico do escorregamento entre as faces das microfissuras no concreto (encaixe e desencaixe dos agregados). Na fase de aplicações as respostas numéricas obtidas com a utilização do modelo proposto foram avaliadas mediante o confronto com diversos resultados experimentais disponíveis na literatura para estruturas de concreto armado sob solicitações estáticas e dinâmicas. Os resultados obtidos evidenciam o bom funcionamento do modelo no caso das solicitações estáticas, tendo sido obtidos resultados limitadamente satisfatórios no caso das solicitações dinâmicas. / This work deals with the formulation, implementation and application of a model for nonlinear analysis of reinforced concrete planed frame structures, based on the simplified concept of localization of the irreversible physical processes (damage, plasticity and hysteresis) in zones previously defined called hinges which are located in the extremities of a bar finite element. The proposed model has as objective to reproduce the residual displacements related to the plastic behavior of the damaged concrete as well as to the steel yielding, moreover making possible the reproduction of hysteresis loops experimentally observed in moment-rotation diagrams, or equivalently force-displacement, when the loading history includes unloading and reloading cycles. The thermodynamic formulation is developed referring to generalized stress and deformation variables due to their application in the scope of the classic theory of bars bending. In this model the nonlinearity of the structural element is totally due to the energy dissipation in the hinges and is represented by three scalar variables associated to them: the non-dimensional damage variable, the plastic rotation and the sliding rotation, which represents the physical phenomenon of sliding between the microcracks faces in the concrete (aggregate interlocking). In the applications phase, the numerical solutions obtained with the use of the proposed model had been evaluated by confrontation with various experimental results available in the literature of reinforced concrete structures under static and dynamic solicitations. The obtained results evidence the good performance of the model in the case of static solicitations, having been obtained limited satisfactory results in the case of dynamic solicitations.

Estômatos são poros responsáveis pela troca gasosa entre a folha e o meio externo. A partir da década de 80, experimentos revelaram um complexo padrão espaço temporal na abertura e fechamento dos estômatos. As experiências apontam para uma possível coordenação entre estômatos em algumas áreas da folha chamada de patches. Esse fenômeno é conhecido na literatura como patchy stomatal conductance. Frequentemente a coordenação dinâmica dos estômatos está associada à oscilações temporais na condutãncia estomática (média especial da abertura dos estômatos). Em 1997 Haefner, Buckley e Mott (HBM) publicaram uma análise numérica de um modelo dinâmico para explorar o comportamento complexo dos estômatos. O modelo é baseado em algumas características conhecidas dos estômatos e assume transporte hídrico em uma rede definida por uma geometria simples e bastante restritiva. De acordo com os autores, o modelo reproduz qualitativamente os dados experimentais. Recentemente, Ferraz e Prado mostraram que esse modelo não é capaz de reproduzir os resultados experimentais. Usando ingredientes do modelo sugerido por HBM, Ferraz e Prado sugeriram uma geometria realística de distribuição reservatórios hídricos. Embora essa configuração reproduza os patches, eles permanecem estáticos e nenhuma oscilação é observada. Sem explorar detalhes significativos, Ferraz e Prado afirmaram que a histerese na abertura estomatal poderia explicar vários aspectos dos resultados experimentais. No presente estudo comprovamos, através de uma abordagem computacional baseada em transdutores histeréticos, que a hipótese de histerese na abertura dos estômatos de fato reproduz qualitativamente os dados experimentais. Em nossa abordagem a histerese na abertura dos estômatos é emulada através de operadores chamados de histerons. A robustez da hipótese é testada usando diferentes tipos de histerons. Analisamos a correlação entre os estômatos na rede que simula a superfície da folha. Observamos que a correlação entre estômatos depende da geometria da veia. Uma análise detalhada dos parâmetros envolvidos revela uma dependência entre o período de oscilação na condutância estomática e o déficit de vapor d\'água entre a folha e o meio ambiente. Esta característica subjacente ao modelo pode inspirar novas experiências para testar a hipótese da histerese na abertura dos estômatos. / Stomata are pores on the surface of leaves responsible for controlling the exchange of gas between the plant and the environment. Experiments revealed a complex spatial-temporal pattern in the opening and closing mechanism of stomata. The main feature of the phenomenon is that stomata appear to be synchronized into clusters, known as patches. The dynamical coordination of stomata often involves oscillations in stomatal conductance. In 1997 Haefner, Buckley, and Mott (HBM) published a numerical analysis of a dynamic model to explore the complex behavior of stomata. The model is based on some known features of the stomata, and assumes that water diffuses within the leaves according to a simple geometric arrangement. According to the authors, the model reproduces qualitatively the experimental data. Recently, Ferraz and Prado showed that the computational approach of HBM is not able to reproduce the experimental results. Inspired by this model, Ferraz and Prado introduced a new geometric features that leads to static patches of stomata; however no oscillation was observed and the patches remained static. The authors suggested that hysteresis in stomatal aperture could explain several experimental aspects. We now report a further investigation of the changes suggested by Ferraz and Prado in the original model of HBM. The theoretical approach confirmed that hysteresis in the aperture mechanism of pores reproduces a variety of behaviors of stomatal conductance described in experiments. We explore the hysteresis feature through the formalism of hysteretic transducer. The robustness of the hysteretic assumption is tested by different kinds of hysteresis operators. We analyzed the correlation among stomata in the lattice. We observed that the correlation depends on the geometry of the veins. Finally, the analysis of the model reveals a dependence between the period of oscillation in the stomatal conductance time series and water vapor pressure deficits Δω - an external parameter. Further experiments might explore this underlying feature of the model.

We study the buckling of an oil-water interface populated by micron-sized latex particles using a Langmuir trough. We extend pre-existing results to the micron-range with different capillary length and compare the experimental data to the existing theoretical framework. An unexpected trend for the dominant wavelength of buckling is observed, suggesting that there is a transition between regimes in the micron-range. A mechanism for the new regime is proposed. Cascading is reported, as well as novel kinds of transition between wavelengths within the same particle raft. Lastly, the effect of compression on the macroscopic arrangement of particles is investigated, as well as its effect on the buckling wavelength.

The Inverse Temperature Crystallization (ITC) and seed-induced method were used to grow high-quality cubic samples within hours using a 1M solution of methylammonium lead bromide (MAPbBr3) samples. The current-voltage (I-V) hysteresis observed within the MAPbBr3 perovskite sample demonstrates anomalous dependence on scan rate and various preconditioning pulses. We investigate this dependence and the relationship of current-transient with slow migrating ions. The current transients fitted using a bi-exponential decay model produced two distinct time constants t1 = 38.4 s and t2 = 6.49 s associated with migrating ionic species. From the Arrhenius plot an activation energy of Ea = 0.410 associated with migrating Br ions was extracted. Future research is required towards the understanding of I-V hysteresis and the link to ion migration in MAPbBr3 perovskite. / Dissertation (MSc (Physics))--University of Pretoria, 2020. / NRF Funding / Physics / MSc (Physics) / Restricted

The word hysteresis is derived from a Greek word meaning "to lag 'behind". As specifically applied to fluid flow around bodies with transient angles of attack in and near the stall region, "aerodynamic hysteresis" is used to describe the effect of delay in boundary layer separation and reattachment upon the lift, drag and pitching moment. Experimental work done on airfoils by H. Studer showed that for increasing angles of attack flow "separation is delayed to an angle of attack appreciably greater than that for a stationary airfoil. On the return movement, re-establishment of a smooth flow is also delayed." [1]* The result is that under transient conditions "more than one value of flow coefficient (and thus lift, drag and pitching moment) can be obtained for a single angle of attack. . ., depending upon the direction in which the particular angle of attack is approached." [2]

>Magister Scientiae - MSc / Titanium dioxide (TiO2) has potential as an adsorbent within the 68Ge/68Ga generator context. To function effectively, three basic requirements are required: optimum 68Ga elution efficiency, stability and removal of the 68Ge breakthrough. After analysis of the 68Ga elution efficiency and the 68Ge breakthrough, two models are proposed, namely; surface modification and molecular imprinted polymer, to address factors influencing 68Ga elution efficiency and minimize 68Ge breakthrough. The surface modifiers that were investigated are 3-aminopolypropyltriethoxy silane (3-APTES), 3-mercaptopropyltrimethoxy silane (3-MPTMS), bis(3-aminopropyl)amine (BAPA) and 2-chloro-4,6-dimethoxy-1,3,5-triazine silane (CDMT). / 2023

Gravity driven trickle flow of a liquid over a fixed bed in the presence of a gaseous phase is widely encountered throughout the process industry. It is one of the most common ways of contacting multi-phase fluids for reaction or mass transfer purposes. The presence of three phases greatly complicates the mathematical modelling of trickle-bed reactors and makes a description from first principles difficult. Trickle flow performance is usually characterized in terms of hydrodynamic parameters. One such parameter is the liquid holdup. The value and morphology (shape or texture) of the holdup influences the catalyst contacting, wetting, mass transfer characteristics and ultimately the performance of the trickle flow unit. This study is limited to the air-water-glass spheres system with no gas flow. It is partitioned into three sections. An investigation into the nature of the residual liquid holdup in beds of spherical particles revealed that the general assumption that all residual liquid is held in the form of pendular rings at particle contact points proves to be untrue. Instead, indication is that 48 % of the residual holdup is present in the form of agglomerated liquid globules in interstices of low local porosity. Theoretical residual liquid holdup models and residual liquid holdup-based mass transfer models should include this phenomenon. In a subsequent section, the influence of the prewetting procedure on the operating holdup is investigated. Three distinct limiting cases are identified: Kan-wetted, Levec-wetted and non-wetted. A volumetric utilization coefficient that describes the extent to which the bed is irrigated is developed. It indicates that large fractions of the bed remain non-irrigated in the Levec- and non-wetted modes. A momentum balance-based model is adopted to predict the Kan-wetted mode holdup. This model was successfully extended to predicting the holdup in the Levec- and non-wetted modes by simple incorporation of the volumetric utilization coefficient. The predictive capability of this model is highly satisfactory, especially in light of it using only the classical Ergun constants and no fitted parameters (AARE = 9.6 %). The differences in the hysteresis behaviour of holdup and pressure drop in the different modes are attributed to differences in the morphology of the operating holdup. The existence of the three limiting prewetted modes is confirmed by residence time distribution (RTD) analysis of the stimulus-response behaviour of the system. This behaviour was quantified using a NaCl tracer and conductivity measurements at both the inlet and outlet of a bench scale bed. The analyses show that: · There are large fractions of the holdup that is inaccessible to the tracer in the Levec-wetted and non-wetted modes. · The mixedness in the three prewetted modes differ appreciably, with the Kan-wetted mode clearly less mixed than the Levec-wetted mode. The RTD analyses also confirm the existence of the three prewetting modes in a porous system (spherical a-alumina), with a large fraction of the holdup being inaccessible to the tracer in the Levec-wetted mode. This study emphasizes the role of the morphology of the various types of liquid holdup on the hydrodynamic performance of a trickle flow unit. It is apparent that aspects of the morphology depend strongly on phenomena like globule formation, hysteresis and flow and prewetting history that have not been adequately recognized to date. The visualization of the various modes of trickle flow is an intellectual platform from which future studies may be directed. / Dissertation (MEng)--University of Pretoria, 2004. / Chemical Engineering / Unrestricted

Indiana University-Purdue University Indianapolis (IUPUI) / Suspension has been the most important subsystem of the vehicle viewed as a system. The ride comfort and vehicle handling performance are affected by the suspension design. Automotive technology has been continuously incorporating developments over the past few decades to provide the end users with a better comfort of driving. Multi-objective optimization of MR damper with objective function of maximizing damping force generated by MR damper with the geometrical parametric constraint function is achieved in this research using pattern search optimization technique. Research focuses on design, modeling, and simulation of active suspension using non-linear theory of the Magneto-Rheological (MR) damper with consideration of the hysteresis behavior for a quarter car model. The research is based on the assumption that each wheel experiences same disturbance excitation. Hysteresis is analyzed using Bingham, Dahl’s, and Bouc-Wen models. Research includes simulation of passive, Bingham, Dahl, and Bouc-wen models. Modeled systems are analyzed for the six road profiles, including road type C according to international standards ISO/TC108/SC2N67. Furthermore, the comparative study of the models for the highest comfort with less overshoot and settling time of vehicle sprung mass are executed. The Bouc-Wen model is 36.91 percent more comfortable than passive suspension in terms of damping force requirements and has a 26.16 percent less overshoot, and 88.31 percent less settling time. The simulation of the Bouc-Wen model yields a damping force requirement of 2003 N which is 97.63 percent in agreement with analytically calculated damping force generated by MR damper. PID controller implementation has improved the overshoot response of Bouc-Wen model in the range of 17.89 percent-81.96 percent for the different road profiles considered in this research without compromising on the settling time of system. PID controller implementation further improves the passenger comfort and vehicle ride handling capabilities. The interdisciplinary approach of systems engineering principles for the suspension design provides unique edge to this research. Classical systems engineering tools and MBSE approach are applied in the design of the MR damper. Requirement traceability successfully validates the optimized MR damper.