Flow curvature effects on dynamic behaviour of a novel vertical axis tidal current turbine: numerical and experimental analysis

The paper deals with performances analysis of vertical axis turbine to exploit tidal marine currents. Flow curvature effects on performences of a novel vertical axis turbine have been investuigated. It has been shown that the flow curvature effect allows to design properly an accurate airfoil shape to increase turbine performances

Acoustic and entropic transfer functions of a generalised subsonic nozzle

The knowledge of the acoustic and entropic transfer functions at the boundaries of combustors is crucial to understand the fate of flame-generated pressure perturbations and to predict and prevent the emergence of combustion instabilities. Traditional models often rely on the isentropic assumption for nozzle guide vanes. In real systems, however, pressure losses and local flow recirculations may occur, as evidenced by drops in the static pressure. In this work we relax the isentropic assumption and derive a parametric model to predict the acoustic and entropic transfer functions of generalised convergent-divergent nozzles with subsonic-to-sonic throat conditions in the low frequency domain. By tuning two parameters, this model can retrieve the impedance of three limit cases known from the literature: the isentropic nozzle, the orifice plate and the convergent nozzle duct termination. The generalised model also includes the conversion of entropy to sound through orifice plates and non-isentropic nozzles, yet to be considered in the literature. These analytical results are then compared with the experimental data acquired in the Cambridge Entropy Generator. The comparison highlights the need to correctly account for the losses in the system to properly explain the transfer functions of nozzles, as isentropic predictions differ substantially from the acquired experimental data.Qualcom

Measurements of the effect of boundary conditions on upstream and downstream noise arising from entropy spots

Pressure fluctuations in combustors arise either directly from the heat release rate perturbations of the flame (direct noise), or indirectly from the acceleration of entropy, vorticity or compositional perturbations through nozzles or turbine guide vanes (indirect noise). In this work, the generation of synthetic entropy spots via the Joule effect produces direct noise, and their acceleration through orifice plates and nozzles produces indirect noise. These acoustic waves reverberate, reflecting several times at the boundaries to add up to the measured pressure. Single travelling pulses are isolated by the introduction of a semiinfinite tube that acts as an anechoic termination for a limited time-window. It is shown how the shape of the converging nozzle does not affect the reflection of the direct noise wave, confirming the hypothesis of a compact nozzle. Further, it is demonstrated that the assumption of an isentropic nozzle does not hold, but that an alternative theory which takes into account the partial acoustic energy dissipation offers good agreement with the experiments. Finally, it is shown that the reflected indirect noise is underpredicted by isentropic theories. An extension of the present work is indicated for the measurement of the transmissivity of indirect noise.Francesca De Domenico is supported by the Honorary Vice-Chancellor’s Award and a Qualcomm/DTA Studentship (University of Cambridge). Erwan Rolland is supported by an EPSRC DTA studentship (University of Cambridge). Experiments were partly funded by EPSRC grant EP/K02924X/1

Low frequency generation, transmission and reflection of direct and indirect perturbations through nozzles

Pressure perturbations arise in combustors from direct noise related to the change in density in flames, as well as the indirect (entropic) noise associated with the acceleration of non-homogeneous regions of flow through nozzles. In this paper we review our recent work on quantifying the relative contributions of direct and indirect noise generated from perturbations in temperature and composition, and the resulting transmitted and reflected pressure perturbations. We show that (a) isentropic models are inadequate to capture the acoustic and entropic transfer functions across a nozzle; (b) corrections to non-isentropic behaviour are possible using existing models for orifices using a single parameter accounting for losses; (c) the behaviour of low frequency entropic noise generated in a chamber can be entirely accounted for when reverberation is taken into account; and (d) indirect noise due to compositional fluctuations can be as large as entropic noise arising from temperature fluctuations. The findings have implications for both the study of entropy noise in model systems, as well as for understanding how to separate the origins of noise in practical systems. In particular, the role of compositional noise in gas turbines (regarding for example the role of cooling in rich-quench-lean turbines) and the role of non-isentropic effects is not accounted for in current models, and should be revisited in the light of current findings.QUALCOM

Multi-Valley Superconductivity In Ion-Gated MoS2 Layers

Layers of transition metal dichalcogenides (TMDs) combine the enhanced effects of correlations associated with the two-dimensional limit with electrostatic control over their phase transitions by means of an electric field. Several semiconducting TMDs, such as MoS$_2$, develop superconductivity (SC) at their surface when doped with an electrostatic field, but the mechanism is still debated. It is often assumed that Cooper pairs reside only in the two electron pockets at the K/K' points of the Brillouin Zone. However, experimental and theoretical results suggest that a multi-valley Fermi surface (FS) is associated with the SC state, involving 6 electron pockets at the Q/Q' points. Here, we perform low-temperature transport measurements in ion-gated MoS$_2$ flakes. We show that a fully multi-valley FS is associated with the SC onset. The Q/Q' valleys fill for doping$\gtrsim2\cdot10^{13}$cm$^{-2}$, and the SC transition does not appear until the Fermi level crosses both spin-orbit split sub-bands Q$_1$ and Q$_2$. The SC state is associated with the FS connectivity and promoted by a Lifshitz transition due to the simultaneous population of multiple electron pockets. This FS topology will serve as a guideline in the quest for new superconductors.Comment: 12 pages, 7 figure

Fano collective resonance as complex mode in a two dimensional planar metasurface of plasmonic nanoparticles

Fano resonances are features in transmissivity/reflectivity/absorption that owe their origin to the interaction between a bright resonance and a dark (i.e., sub-radiant) narrower resonance, and may emerge in the optical properties of planar two-dimensional (2D) periodic arrays (metasurfaces) of plasmonic nanoparticles. In this Letter, we provide a thorough assessment of their nature for the general case of normal and oblique plane wave incidence, highlighting when a Fano resonance is affected by the mutual coupling in an array and its capability to support free modal solutions. We analyze the representative case of a metasurface of plasmonic nanoshells at ultraviolet frequencies and compute its absorption under TE- and TM-polarized, oblique plane-wave incidence. In particular, we find that plasmonic metasurfaces display two distinct types of resonances observable as absorption peaks: one is related to the Mie, dipolar resonance of each nanoparticle; the other is due to the forced excitation of free modes with small attenuation constant, usually found at oblique incidence. The latter is thus an array-induced collective Fano resonance. This realization opens up to manifold flexible designs at optical frequencies mixing individual and collective resonances. We explain the physical origin of such Fano resonances using the modal analysis, which allows to calculate the free modes with complex wavenumber supported by the metasurface. We define equivalent array dipolar polarizabilities that are directly related to the absorption physics at oblique incidence and show a direct dependence between array modal phase and attenuation constant and Fano resonances. We thus provide a more complete picture of Fano resonances that may lead to the design of filters, energy-harvesting devices, photodetectors, and sensors at ultraviolet frequencies.Comment: 6 pages, 5 figure

The physics of spreading processes in multilayer networks

The study of networks plays a crucial role in investigating the structure, dynamics, and function of a wide variety of complex systems in myriad disciplines. Despite the success of traditional network analysis, standard networks provide a limited representation of complex systems, which often include different types of relationships (i.e., "multiplexity") among their constituent components and/or multiple interacting subsystems. Such structural complexity has a significant effect on both dynamics and function. Throwing away or aggregating available structural information can generate misleading results and be a major obstacle towards attempts to understand complex systems. The recent "multilayer" approach for modeling networked systems explicitly allows the incorporation of multiplexity and other features of realistic systems. On one hand, it allows one to couple different structural relationships by encoding them in a convenient mathematical object. On the other hand, it also allows one to couple different dynamical processes on top of such interconnected structures. The resulting framework plays a crucial role in helping achieve a thorough, accurate understanding of complex systems. The study of multilayer networks has also revealed new physical phenomena that remain hidden when using ordinary graphs, the traditional network representation. Here we survey progress towards attaining a deeper understanding of spreading processes on multilayer networks, and we highlight some of the physical phenomena related to spreading processes that emerge from multilayer structure.Comment: 25 pages, 4 figure

Hidden geometric correlations in real multiplex networks

Real networks often form interacting parts of larger and more complex systems. Examples can be found in different domains, ranging from the Internet to structural and functional brain networks. Here, we show that these multiplex systems are not random combinations of single network layers. Instead, they are organized in specific ways dictated by hidden geometric correlations between the individual layers. We find that these correlations are strong in different real multiplexes, and form a key framework for answering many important questions. Specifically, we show that these geometric correlations facilitate: (i) the definition and detection of multidimensional communities, which are sets of nodes that are simultaneously similar in multiple layers; (ii) accurate trans-layer link prediction, where connections in one layer can be predicted by observing the hidden geometric space of another layer; and (iii) efficient targeted navigation in the multilayer system using only local knowledge, which outperforms navigation in the single layers only if the geometric correlations are sufficiently strong. Our findings uncover fundamental organizing principles behind real multiplexes and can have important applications in diverse domains.Comment: Supplementary Materials available at http://www.nature.com/nphys/journal/v12/n11/extref/nphys3812-s1.pd