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A weakly nonlinear analysis of the precessing vortex core oscillation in a variable swirl turbulent round jet
No full textWe study the emergence of precessing vortex core (PVC) oscillations in a swirling jet experiment. We vary the swirl intensity while keeping the net mass flow rate fixed using a radial-entry swirler with movable blades upstream of the jet exit. The swirl intensity is quantified in terms of a swirl number S. Time-resolved velocity measurements in a radial-axial plane anchored at the jet exit for various S values are obtained using stereoscopic particle image velocimetry. Spectral proper orthogonal decomposition and spatial cross-spectral analysis reveal the simultaneous emergence of a bubble-type vortex breakdown and a strong helical limit-cycle oscillation in the flow for S > S-c where S-c = 0.61. The oscillation frequency, f(PVC), and the square of the flow oscillation amplitudes vary linearly with S - S-c. A solution for the coherent unsteady field accurate up to O(epsilon(3)) (epsilon similar to O((S - S-c)(1/2)) is determined from the nonlinear Navier-Stokes equations, using the method of multiple scales. We show that onset of bubble type vortex breakdown at Sc, results in a marginally stable, helical linear global hydrodynamic mode. This results in the stable limit-cycle precession of the breakdown bubble. The variation of f(LC) with S - S-c is determined from the Stuart-Landau equation associated with the PVC. Reasonable agreement with the corresponding experimental result is observed, despite the highly turbulent nature of the flow in the present experiment. Further, amplitude saturation results from the time-averaged distortion imposed on the flow by the PVC, suggesting that linear stability analysis may predict PVC characteristics for S > S-c
Broadly tunable and low power penalty radio frequency phase shifter using a coupled silicon microcavity
No full textWe present a continuously tunable silicon photonics assisted radio frequency (RF) phase shifter using a coupled microring resonator. Using the coupled cavity, we demonstrate a sub-1 dB power penalty for a RF bandwidth of 34.5 GHz (9�43.5 GHz) and a phase shift of � over the reported frequency range. Rigorous optimization of the cavity design using the coupled-mode theory is carried out to realize ultranarrow resonance peaks with a low-extinction ratio and large phase shift. Thermal tuning of the cavity is used to tune the phase while all-optical tuning is exploited to achieve broadband operation. We present a detailed simulation and experimental study of the proposed configuration. The proposed device configuration exhibits a configurable resonance linewidth and extinction ratio that allows for a broad bandwidth and an extremely low power penalty microwave phase shifter. We believe the demonstration would allow better integration of the on-chip functional elements of integrated microwave photonics
Microscopic Study of Proton Kinetic Energy Anomaly for Nanoconfined Water
No full textThe reported anomalies of the proton mean kinetic energy, Ke(H), in nanoconfined water, as measured by deep inelastic neutron scattering (DINS), constitute a longstanding problem related to proton dynamics in hydrogen-bonded systems. A considerable number of theoretical attempts to explain these anomalies have failed. The mean vibrational density of states (VDOS) of protons in water nanoconfined inside single wall carbon nanotubes (SWCNTs) is calculated as a function of temperature and SWCNT diameter, DCNT, by classical molecular dynamics (MD) simulation using the TIP4P-2005f water model. The calculated VDOS are utilized for deducing the mean kinetic energy of the water protons, Ke(H), by treating each phonon state as a harmonic oscillator. The calculation depicts a strong confinement effect as reflected in the drop of the value of Ke(H) at 5 K for DCNT < â12 à , while absent for larger diameters. The results also reveal very significant blue and red shifts of the stretching and bending modes, respectively, compared to those in bulk ice, in agreement with experiment
A Single-Stage Soft-Switched Isolated Three-Phase DC-AC Converter with Three-Phase Unfolder
No full textThis article proposes a unidirectional, single-stage, three-phase, and high-frequency-link dc-ac converter. The converter uses three high-frequency transformers to provide galvanic isolation. In the primary, the converter has three half-bridge legs. In the secondary, three diode-bridge rectifiers are used. Each rectifier is followed by a half-bridge (unfolder) leg. The modulation strategy proposed in this article ensures zero-voltage switching of all six primary side switches over the entire line cycle without additional snubber circuit. The secondary half-bridge legs are switched at line frequency, incurring negligible switching loss. The proposed converter does not require any interstage bulky dc-link filter capacitor. Thus, the overall filtering requirement is reduced, and the converter reliability is improved. The high-frequency galvanic isolation improves the converter power density. In this article, the modulation strategy and converter operation are described in detail. The operation is validated in a 3.7-kW hardware prototype. Key experimental results are presented in this article
New protection scheme for maintaining coordination time interval among relay pairs in micro-grid by employing centralised master controller
No full textThe penetration of distributed generators (DGs) of large capacities in micro-grid introduces a considerable difference of current seen by the relay, during faults in its primary and backup region. This leads to several issues with the protection system such as relay miscoordination, delay in relay operation or failure in the relay operation (blinding). This study discusses a methodology to solve such issues by identifying the current status of micro-grid configuration. The proposed method performs well by maintaining the coordination time interval (CTI) between the relays, irrespective of the source strength (weak or strong), the mode of operation of the micro-grid (islanded or grid connected) or the micro-grid configuration (radial or looped). The method proposes three curves with different settings in a single relay. A suitable algorithm has been developed to calculate the relay settings for all the three curves of each relay, well in advance and this program is made to run in the central master controller (MC). Thereafter, MC sends the settings of all relays through a reliable communication channel. The proposed relaying scheme provides desirable CTI margin as compared to conventional over current relay. The simulation results of the novel protection scheme are validated through hardware emulation
Thermoelectric properties of Al substituted tetrahedrite
No full textIn this study, aluminum, a p-block element, is substituted at the Cu(1) site, and its effect on the structural and thermoelectric properties of tetrahedrite Cu12-xAlxSb4S13 (x = 0.1, 0.25, 0.5, and 0.75) was investigated. The samples were prepared via solid-state synthesis followed by induction hot pressing. The theoretical calculations, using density functional theory (DFT), showed that the Al substitution results in lowering the band degeneracy near the Fermi level (EF) with EF moving towards the bandgap, indicating effective compensation of holes. The projected density of states (PDOS) revealed almost negligible hybridization of Al states with Cu 3d and S 3p states near EF, thus resulting in relatively low DOS near EF. The electrical resistivity and Seebeck coefficient increased with increasing Al content due to the compensation of holes and reduction of the charge carrier concentration. However, the Seebeck coefficient values were relatively low due to a low DOS near EF, as indicated by the DFT calculations. Although the electronic thermal conductivity (κe) decreased with increasing Al concentration, the magnitudes of the total thermal conductivity (κT) could not be reduced significantly. As a result, a maximum zT of 0.6 at 673 K was obtained for Cu11.9Al0.1Sb4S13. Based on the current study and previously reported results, the paper demonstrates how the phase stability and transport properties of the tetrahedrite are affected significantly by the nature of the substituent at the Cu(1) tetrahedral site. © 2020 Author(s)
Exploiting Power Adaptation with Transmit Antenna Selection for Interference-Outage Constrained Underlay Spectrum Sharing
No full textIn underlay spectrum sharing, the interference constraint limits transmissions by the secondary transmitter, which concurrently accesses the spectrum, to protect the primary user from excessive interference. Transmit antenna selection enables a secondary user to overcome the limitations imposed by the interference constraint using low-complexity hardware. We develop an optimal and novel joint antenna selection and power adaptation rule that minimizes the average symbol error probability (SEP) of a secondary user that is subject to two practically well-motivated constraints. The first is the less-studied but general interference-outage constraint, which limits the probability that the interference power at the primary receiver exceeds a threshold. The second constraint limits the peak transmit power of the secondary transmitter. We show that the optimal rule for the interference-outage constraint has a novel structure that is markedly different from the rules considered in the literature. We then present an insightful geometric interpretation of its structure. Using this, we also propose a practically amenable and near-optimal variant of the optimal rule called the linear rule, and analyze its performance. Our numerical results show that the optimal rule reduces the average SEP by one to two orders of magnitude compared to the rules in the literature
Evaluation of developmental toxicity and genotoxicity of aqueous seed extract of Croton tiglium L. using zebrafish
No full textCroton tiglium L. has been used in Ayurvedic and Chinese herbal medicinal formulations from ancient times. Although its seeds are widely prescribed as traditional medicine, there is a dearth of information, regarding its toxic effects, and the mechanisms underlying its toxicity. This study aims to investigate the developmental toxicity and genotoxicity of the aqueous seed extract of C. tiglium L. (AECT) in zebrafish. We have examined the effects of AECT on the early embryonic development of zebrafish. Zebrafish embryos, treated with different concentrations of the AECT, suffered embryonic lethality and displayed various developmental defects. The 96 h-LC50 of AECT was found to be 162.78 µg/ml. Interestingly, the developmental abnormalities observed, such as pericardial edema (PE), yolk sac edema (YSE), spinal curvature (SC), and delayed hatching, varied in severity, in a dose-dependent manner. Zebrafish embryos, treated with different concentrations of AECT, exhibited exaggerated cell death in the anatomical regions of brain, heart, and trunk. Our data suggest that the phenomenon of apoptosis is probably responsible for both embryonic lethality and developmental toxicity in zebrafish embryos. Furthermore, the genotoxic potential of the AECT, in vivo, was evaluated using micronucleus assay and comet assay, on the peripheral blood of zebrafish. The results suggest that AECT has the potential to cause genotoxicity in the peripheral blood of zebrafish
Bioinspired Aluminum Composite Reinforced with Soft Polymers with Enhanced Strength and Plasticity
No full textComposites have played a key role in revolutionizing the automobile, marine, and aerospace industries. There is a constant attempt for the development of low-density composite materials with superior mechanical and corrosion-resistant properties for elevated temperature applications. Herein, an attempt is made to develop a nature-inspired unique aluminum-based composite with low-density polymer (polyethylene terephthalate, i.e., soft material) reinforcement, which shows an enhancement in strength and toughness. The composite is processed using the easily scalable and simple friction stir processing technique. Mechanical properties of the uniformly reinforced aluminum composite show double ultimate strength and fivefold improvement in plasticity. The ultimate strength of the composite increases at elevated temperatures. The experimental observations are further supported by theoretical calculations and molecular dynamics simulations
Lower bound limit analysis of unsupported vertical circular excavations in rocks using Hoek-Brown failure criterion
No full textThe stability of vertical unsupported circular excavations in rock media, obeying generalized Hoek-Brown yield criterion, has been investigated by using the lower bound finite elements limit analysis. An axisymmetric analysis, composed of a planar domain with a mesh of three-noded triangular elements, has been carried out. The optimization problem is dealt with by using the semidefinite programming technique avoiding the need of either smoothing the yield surface or making any assumption associated with the circumferential stress (�θ). A detailed parametric study has been executed, and the effects of different input material parameters, namely, geological strength index (GSI), yield parameter (mi), and the disturbance factor (D) on the results have been studied. For different height to radius ratios of the excavation, the computed results are presented in the form of nondimensional stability numbers. Failure mechanisms have also been investigated for a few typical cases. The results from the analysis have been compared with that evaluated separately with the application of the software OptumG2