Three-legged Model Research Articles

Comparative study of charge order in undoped infinite-layer nickelate superconductors

To understand the microscopic mechanism of the charge order observed in the parent compound of the infinite-layer nickelate superconductors, we consider a minimal three-legged model consisting of a two-legged Hubbard ladder for the Ni $3d_{x^2-y^2}$ electrons and a free conduction electron chain from the rare-earths. With highly accurate density matrix renormalization group calculations, when the chemical potential difference is adjusted to make the Hubbard ladder with $1/3$ hole doping, we find a long-range charge order with period $3$ in the ground state of the model, while the spin excitation has a small energy gap. Moreover, the electron pair-pair correlation has a quasi-long-range behavior, indicating an instability of superconductivity even at half-filling. As a comparison, the same method is applied to a pure two-legged Hubbard model with $1/3$ hole doping in which the period-3 charge order is a quasi-long range one. The difference between them demonstrates that the free electron chain of the three-legged ladder plays the role of a charge reservoir and enhances the charge order in the undoped infinite-layer nickelates.

A “three-legged model”: (De)constructing school autonomy, accountability, and innovation in the Italian National Evaluation System

The widespread adoption of school autonomy with accountability reforms in education has generated debate regarding the relationship between autonomy, innovation and accountability. While at the policy design level, these three elements are highly related, several authors highlight the contradictions among them. By analyzing key documents and interviews, this paper aims to identify the program ontology behind the current Italian National Evaluation System (SNV), with a focus on the way in which autonomy, accountability and innovation have been conceptualized and linked together. The paper also aims to explore whether pitfalls and/or tensions exist that might hamper the achievement of the SNV goals. The findings highlight the peculiarities of the Italian autonomy with accountability system, which has resulted from the involvement of different stakeholders in the design and implementation of the reforms. The findings also reveal contradictions regarding some of its premises. Various rationales (improvement, efficiency, equity and transparency) emerge that seem to have acted as drivers of the reforms, however, the influence of globalizing discourses on international competition and the benefits of datafication also appears significant. A number of contextual aspects are finally considered which hamper the expected change mechanisms, highlighting the discontinuous ground in which such policy dispositifs operate.

Experimental study on hydrodynamic characteristics of three truss-type legs of jack-up offshore platform

The research came from the project for 400-foot jack-up offshore platform's adaptation to environment. The empirical value of load coefficient is often selected by each Classification Society and the influence of other complex factors on hydrodynamic characteristics is ignored, which may make the result too conservative. Therefore, this paper took the jack-up offshore platform truss pile leg as research object, and the model experiments of single pile leg and three pile legs were conducted respectively, and the hydrodynamic coefficient under experimental conditions were studied. Based on the single-legged, the effects of three-legged model under different working conditions were quantitatively analyzed by defining the force component coefficient and the total force coefficients of group piles. This research may provide data support for structural optimization design and strength check analysis of jack-up offshore platform truss legs, which plays an important role in the fatigue detection and life evaluation of offshore platform.

A three-leg model producing tetrapod and tripod coordination patterns of ipsilateral legs in the stick insect.

Insect locomotion requires the precise coordination of the movement of all six legs. Detailed investigations have revealed that the movement of the legs is controlled by local dedicated neuronal networks, which interact to produce walking of the animal. The stick insect is well suited to experimental investigations aimed at understanding the mechanisms of insect locomotion. Beside the experimental approach, models have also been constructed to elucidate those mechanisms. Here, we describe a model that replicates both the tetrapod and tripod coordination pattern of three ipsilateral legs. The model is based on an earlier insect leg model, which includes the three main leg joints, three antagonistic muscle pairs, and their local neuronal control networks. These networks are coupled via angular signals to establish intraleg coordination of the three neuromuscular systems during locomotion. In the present three-leg model, we coupled three such leg models, representing front, middle, and hind leg, in this way. The coupling was between the levator-depressor local control networks of the three legs. The model could successfully simulate tetrapod and tripod coordination patterns, as well as the transition between them. The simulations showed that for the interleg coordination during tripod, the position signals of the levator-depressor neuromuscular systems sent between the legs were sufficient, while in tetrapod, additional information on the angular velocities in the same system was necessary, and together with the position information also sufficient. We therefore suggest that, during stepping, the connections between the levator-depressor neuromuscular systems of the different legs are of primary importance.

Model-Based Development of Leaping in a Hexapod Robot

We report on the model-based development of leaping behavior in a RHex-style hexapod robot. A three-legged model is proposed to analyze the dynamic behavior of leaping, and this serves as a guide for implementing the behavior on the empirical robot. The model has a rigid body and three massless and compliant legs, which have rolling contact with the ground for better modeling the leg behavior of the empirical robot. By investigating the model's behavior, a two-step leaping maneuver is developed. The first step is utilized for adjusting the body pitch, synchronizing the phases of all six legs, and speeding up the body's forward velocity. This provides adequate initial conditions for the second step leaping, which creates a long-distance flight and adequate landing for follow-up running. In addition, we also report on the strategy of stride length regulation. With implementation of the range sensor, the robot can regulate its stride in order to reach a specific and desired position for leaping. The gait transition and initiation of leaping is fully autonomous. The behavioral development is implemented in the RHex-style robot and is evaluated experimentally.

Modeling OCB for Hotels

Most research examining organizational citizenship behavior (OCB) has focused on employees’ efforts that benefit the organization or the individuals’ coworkers. A third dimension that is critical for the hospitality industry is behavior above and beyond the specific job description that is directed at customers. While most OCB studies have considered what behaviors are essential to corporate citizenship, but specific behaviors might be culturally bound. To avoid cultural issues, a more effective approach is to analyze the targets of citizenship behavior, that is, the organization, coworkers, and customers. A study of 240 hotel workers in China found support for a three-leg model of OCB, combining behavior aimed at these targets: at the organization itself, at coworkers, and at customers. Citizenship behavior aimed at the organization includes such activities as promoting a hotel’s products and making favorable comments about the property outside of work. Citizenship behavior in support of coworkers includes assisting them as needed and taking time to listen to coworkers’ concerns. Customer-focused OCB includes such activities as maintaining a positive attitude at work and performing duties carefully and accurately. Although this study did not expressly measure the results of such actions, previous work has shown increases in guest satisfaction and company revenue when OCB activities are high.

Assessing appropriate stiffness levels for spudcan foundations on dense sand

Before a jack-up can operate at a given location, a site-specific assessment of its ability to withstand a design storm during operation must be performed. During this assessment, the complex state of stress and strain under a spudcan is usually simplified to a value of foundation stiffness that is integrated as a boundary condition into the structural analysis. Soil stiffness is a critical parameter affecting the foundation and structural load distribution and displacements, and the jack-up natural period and dynamic response. The level of spudcan stiffness is an area of intense interest and debate. This paper assesses appropriate stiffness levels for numerical simulation. Utilising results from a detailed “pushover” experiment of a three-legged model jack-up on dense sand, the paper compares the experimental pushover loads and displacements on the hull and spudcans to numerical simulations using different assumptions of spudcan stiffness. These include pinned and encastré footings, linear springs and a force-resultant model based on displacement-hardening plasticity theory. Constant stiffness levels are shown to be inadequate in simulating the experimental pushover test. The non-linear degradation of stiffness associated with the latter force-resultant model is critical.

Physical modelling of the push-over capacity of a jack-up structure on sand in a geotechnical centrifuge

Offshore jack-up drilling rigs are subjected to loading from wind, waves, and current in addition to their self-weight. This applies combined loading in all six degrees-of-freedom in space on the footings. Although the foundation–soil interaction is crucial to the overall response of a jack-up structure, current state-of-the-art models to predict jack-up footing behaviour, developed using data from single footing experiments, have not been validated for such multi-footing systems under general combined loading. This paper introduces the experimental development of a three-legged model jack-up and loading apparatus designed to investigate the rig’s response — in particular the footing load paths — under combined loading in three dimensions. Push-over experiments were performed in a geotechnical beam centrifuge on silica sand. Experimental results of two tests on dense sand are discussed, highlighting differences in response and mode of failure depending on the loading direction of the jack-up. The importance of three-dimensional modelling is also stressed by experimentally demonstrating that the symmetric load case is not necessarily conservative.

Experimental investigation of the system behaviour of a model three-legged jack-up on clay

Results from combined loading experiments dedicated to evaluating the capacity and stiffness of a single footing have significantly improved the understanding of the response of circular shallow foundations. However, due to experimental practicalities, little corroborative evidence is available to confirm predicted behaviour when multiple footings act within a structural system. This paper addresses this concern by presenting the results of a series of experimental tests on a three-legged model jack-up unit founded on soft, heavily overconsolidated clay. The model adopted geometric features representative of a large prototype rig by appropriately scaling leg height, leg separation and flexural stiffness, as well as the ‘spudcan’ footing diameter and profile. Pushover events were considered by subjecting the model jack-up to monotonic horizontal loading at hull level. By measuring loads and displacements at the hull and at each spudcan, the system behaviour of the jack-up could be analysed. Nine separate tests were performed, revealing how load orientation, leg length and preload ratio changed the system capacity and affected the load–displacement paths of the jack-up and spudcan system. The test results are compared with those derived from single spudcan experiments, and are interpreted within the combined load yield surface approach gaining acceptance within the offshore industry.