Hydrodynamic independence and passive control application of twist and flapwise deformations of tidal turbine blades

The load-induced deformations experienced by axial-flow rotor blades can result in significant hydrodynamic impacts on rotor operation. These changes in hydrodynamics are dominated by the flapwise and twist deformation components, affecting blade loading and performance. This work uses blade-resolved computational fluid dynamics simulations to explore the hydrodynamic interactions of coupled flapwise and twist deformations, and their potential for use in passive control strategies. The rotor blades were simulated under parametrically prescribed flapwise-only, twist-only and coupled flapwise–twist deformations. The results show that the hydrodynamic effects are adequately described by blade-element theory for twist deformations regardless of the presence of flapwise deformations, whereas flapwise deformations induce changes in the local lift and drag coefficients that are independent of twist. For moderate blade deflections, the hydrodynamic changes generated by the two deformation components can be approximated to be independent from each other. The observed hydrodynamic independence between the two deformation components is used to explore passive deformation strategies for a tidal rotor. By extrapolating an existing dataset containing CFD simulations of twist-only and flapwise-only deformation cases at different tip-speed ratios, control paths are designed within a tip-speed ratio, flapwise and twist deformation parameter space. These control paths demonstrate passive control strategies as a potential alternative to active pitch control on tidal turbines, showing similar performance and maximum loading, compared with an active pitch strategy, over a full tidal cycle. In particular, it is shown that flapwise deformations have an important role in power capping above rated flow speed

A numerical study on the hydrodynamics of a floating tidal rotor under the combined effects of currents and waves

This work examines the hydrodynamics of a 20 m diameter axial-flow tidal rotor supported by a catamaran-style floating platform. Using a time-domain seakeeping model of the float, coupled with a dynamic model of the rotor based on blade-element momentum theory, the floating tidal turbine was analysed under the combined effects of following waves and currents. The rotor loads were analysed in scenarios with and without platform motions, starting from equivalent initial conditions. While the results show that mean power and thrust are not significantly affected, thrust and power fluctuations are substantial for the rotor under waves with and without platform motions. When platform motions were considered, amplification and reduction of load fluctuations were observed at different wave periods. These effects are associated with the phase interactions between waves and platform motion response. The reductions in thrust and power fluctuations at certain ranges of wave periods suggest that platform motions do not necessarily have an adverse impact on the operation of floating tidal rotors and could potentially be exploited to reduce fatigue damage and improve the quality of power delivery. The amplification of transient loads, on the contrary, suggests that consideration is required when designing floating systems to avoid potentially damaging effects

Dynamic Mode Decomposition of merging wind turbine wakes

The design and operation of wind farms is significantly affected by the impact that upstream turbine wakes have on the power production and fatigue loading of subsequent turbines; often called the wake effect. In this work, two types of flows are considered: the wake of a single turbine with a laminar inflow and the combined wake of two turbines operating in-line where the upstream wake results in an unsteady inflow for the downstream turbine. Those two scenarios are simulated using large eddy simulation (LES) and the actuator line method (ALM). The spatio-temporal velocity fields are analyzed by means of high order dynamic mode decomposition (HODMD), a well established variant of the DMD. The results show that most of the higher frequencies characterizing the laminar case are instead dominated by the lower frequency modes in the combined wake. This suggests that structures emerging from the blade rotations in a wind turbine wake may be less significant for describing the wake dynamics when the rotor is operating in the unsteady wake of an upstream rotor

Observation of squeezed light from one atom excited with two photons

Single quantum emitters like atoms are well-known as non-classical light sources which can produce photons one by one at given times, with reduced intensity noise. However, the light field emitted by a single atom can exhibit much richer dynamics. A prominent example is the predicted ability for a single atom to produce quadrature-squeezed light, with sub-shot-noise amplitude or phase fluctuations. It has long been foreseen, though, that such squeezing would be "at least an order of magnitude more difficult" to observe than the emission of single photons. Squeezed beams have been generated using macroscopic and mesoscopic media down to a few tens of atoms, but despite experimental efforts, single-atom squeezing has so far escaped observation. Here we generate squeezed light with a single atom in a high-finesse optical resonator. The strong coupling of the atom to the cavity field induces a genuine quantum mechanical nonlinearity, several orders of magnitude larger than for usual macroscopic media. This produces observable quadrature squeezing with an excitation beam containing on average only two photons per system lifetime. In sharp contrast to the emission of single photons, the squeezed light stems from the quantum coherence of photon pairs emitted from the system. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emittersComment: Main paper (4 pages, 3 figures) + Supplementary information (5 pages, 2 figures). Revised versio

Latent atrophy factors related to phenotypical variants of posterior cortical atrophy

OBJECTIVE: To determine whether atrophy relates to phenotypical variants of posterior cortical atrophy (PCA) recently proposed in clinical criteria; dorsal, ventral, dominant-parietal and caudal, we assessed associations between latent atrophy factors and cognition. METHODS: We employed a data-driven Bayesian modelling framework based on latent Dirichlet allocation to identify latent atrophy factors in a multi-center cohort of 119 individuals with PCA (age:64±7, 38% male, MMSE:21±5, 71% amyloid-β-positive, 29% amyloid-β status unknown). The model uses standardized gray matter density images as input (adjusted for age, sex, intracranial volume, field-strength and whole-brain gray matter volume) and provides voxelwise probabilistic maps for a predetermined number of atrophy factors, allowing every individual to express each factor to a degree without a-priori classification. Individual factor expressions were correlated to four PCA-specific cognitive domains (object-perception, space-perception, non-visual/parietal functions and primary visual processing) using general linear models. RESULTS: The model revealed four distinct yet partially overlapping atrophy factors; right-dorsal, right-ventral, left-ventral, and limbic. We found that object-perception and primary visual processing were associated with atrophy that predominantly reflects the right-ventral factor. Furthermore, space-perception was associated with atrophy that predominantly represents the right-dorsal and right-ventral factors. However, individual participant profiles revealed that the vast majority expressed multiple atrophy factors and had mixed clinical profiles with impairments across multiple domains, rather than displaying a discrete clinical-radiological phenotype. CONCLUSION: Our results indicate that particular brain-behavior networks are vulnerable in PCA, but most individuals display a constellation of affected brain-regions and symptoms, indicating that classification into four mutually exclusive variants is unlikely to be clinically useful

Contribution to the understanding of tribological properties of graphite intercalation compounds with metal chloride

Intrinsic tribological properties of lamellar compounds are usually attributed to the presence of van der Waals gaps in their structure through which interlayer interactions are weak. The controlled variation of the distances and interactions between graphene layers by intercalation of electrophilic species in graphite is used in order to explore more deeply the friction reduction properties of low-dimensional compounds. Three graphite intercalation compounds with antimony pentachloride, iron trichloride and aluminium trichloride are studied. Their tribological properties are correlated to their structural parameters, and the interlayer interactions are deduced from ab initio bands structure calculations