Operating Regimes Research Articles

Nonlinear second order electromagnetic gyrokinetic theory for a tokamak plasma

The steep plasma pressure gradient that forms at the edge of the high confinement, H-mode regime of tokamak operation provides free energy to drive electromagnetic micro-instabilities that are widely believed to influence the transport processes in this so-called pedestal region. This high pressure gradient also provides a high current density (bootstrap current), known to influence ballooning mode stability and to be important for driving kink modes in the ideal magneto-hydrodynamic plasma model (so-called peeling-ballooning modes). Furthermore, efficient, steady state future tokamak power plants must operate with a large bootstrap current in the core and especially concerning spherical tokamaks, confinement will be influenced by electromagnetic turbulence. To accommodate these important situations, conventional electromagnetic gyrokinetic theory is extended to incorporate neoclassical effects in the equilibrium drives, allowing (B 0 is the confining magnetic field, and B ϑ is its poloidal component). This provides a global gyrokinetic model that self-consistently captures the consequences of large bootstrap current fractions on the equilibrium distribution functions.

Numerical investigation of h-Darrieus wind turbine aerodynamics at different tip speed ratios

PurposeThe purpose of the paper is to predict the aerodynamic performance of a complete scale model H-Darrieus vertical axis wind turbine (VAWT) with end plates at different operating conditions. This paper aims at understanding the flow physics around a model VAWT for three different tip speed ratios corresponding to three different flow regimes.Design/methodology/approachThis study achieves a first three-dimensional hybrid lattice Boltzmann method/very large eddy simulation (LBM-VLES) model for a complete scaled model VAWT with end plates and mast using the solver PowerFLOW. The power curve predicted from the numerical simulations is compared with the experimental data collected at Erlangen University. This study highlights the complexity of the turbulent flow features that are seen at three different operational regimes of the turbine using instantaneous flow structures, mean velocity, pressure iso-contours, blade loading and skin friction plots.FindingsThe power curve predicted using the LBM-VLES approach and setup provides a good overall match with the experimental power curve, with the peak and drop after the operational point being captured. Variable turbulent flow structures are seen over the azimuthal revolution that depends on the tip speed ratio (TSR). Significant dynamic stall structures are seen in the upwind phase and at the end of the downwind phase of rotation in the deep stall regime. Strong blade wake interactions and turbulent flow structures are seen inside the rotor at higher TSRs.Research limitations/implicationsThe computational cost and time for such high-fidelity simulations using the LBM-VLES remains expensive. Each simulation requires around a week using supercomputing facilities. Further studies need to be performed to improve analytical VAWT models using inputs/calibration from high fidelity simulation databases. As a future work, the impact of turbulent and nonuniform inflow conditions that are more representative of a typical urban environment also needs to be investigated.Practical implicationsThe LBM methodology is shown to be a reliable approach for VAWT power prediction. Dynamic stall and blade wake interactions reduce the aerodynamic performance of a VAWT. An ideal operation close to the peak of the power curve should be favored based on the local wind resource, as this point exhibits a smoother variation of forces improving operational performance. The 3D flow features also exhibit a significant wake asymmetry that could impact the optimal layout of VAWT clusters to increase their power density. The present work also highlights the importance of 3D simulations of the complete model including the support structures such as end plates and mast.Social implicationsAccurate predictions of power performance for Darrieus VAWTs could help in better siting of wind turbines thus improving return of investment and reducing levelized cost of energy. It could promote the development of onsite electricity generation, especially for industrial sites/urban areas and renew interest for VAWT wind farms.Originality/valueA first high-fidelity simulation of a complete VAWT with end plates and supporting structures has been performed using the LBM approach and compared with experimental data. The 3D flow physics has been analyzed at different operating regimes of the turbine. These physical insights and prediction capabilities of this approach could be useful for commercial VAWT manufacturers.

Approaches to the Realization of A Virtual Electric Machine Laboratory on Matlab&Simulink

Abstract This virtual lab integrates the Matlab&Simulink experimental model, which gives students the opportunity to verify their theoretical knowledge. The paper proposes the development of the Simulink model of the DC motor, aiming to achieve a more effective speed control. We describe the implementation of a virtual lab based on mathematical models. Our virtual lab in Matlab&Simulink in correspondence with the traditional one implies particular advantages: the time allocated to the development of a virtual laboratory is shorter than that required for a classic one, the scalability is wider and the cost is lower. The paper presents with the three main blocks: the excitation, the inductor and the mechanical block. The parameters defining both the transient regime of the excitation circuit (application duration and excitation voltage) and the stabilized operating regime at idle are established. The new transient regime of operation under load is studied, followed by the stabilized regime of operation under load.

Energy Saving through Automation of the Lightweight Floor Heating System

The overview of the literature sources showed that over the last decade, the attention to the study of heat transfer processes with the automation and periodic use of underfloor heating systems has increased significantly. The aim of this study was to investigate the heat flux density from the surface of the lightweight floor heating system dependence on different parameters. These parameters are material of the finishing coating, average temperature difference between indoor air and heat carrier, heat carrier temperature and its flow rate, thickness of the thermal insulation layout under the lightweight floor heating system. The article shows results of both experimental and computational modelling studies of the lightweight floor heating system which was operated in different regimes and with different (ceramic tile and laminate) finishing coatings. A new experimental stand has been created in the thermophysical laboratory of the Institute of Engineering Thermophysics of the National Academy of Sciences of Ukraine for the purposes of experimental studies. For the purposes of comparison of different operational regimes as well as the determination of the energy saving through automation of the operation regimes of the lightweight floor heating system, the CFD modelling was conducted. The results showed that such systems have significant potential for automation and heat supply purposes of different premises since they have low thermal inertia due to the absence of the concrete screed monolith. On the other hand, the research showed that use of quantitative regulation of the lightweight floor heating system, by reducing the flow of the heat carrier is more effective in this case than qualitative with change of heat carrier temperature.

Experimental observation of anomalous supralinear response of single-photon detectors

The linearity of single-photon detectors allows accurate optical measurements at low light levels and using non-classical light in spectroscopy, biomedical imaging, optical communication, and sensing. However, in practice, the response of single-photon detectors can exhibit intriguing nonlinear effects that may influence the performed measurements. Here, we demonstrate a direct single-source measurement of the absolute nonlinearity of single-photon detectors with unprecedented accuracy. We discover a surprising supralinear behavior of single-photon avalanche diodes and show that it cannot be explained using known theoretical models. We also fully characterize sub- and supra-linear operation regimes of superconducting nanowire single-photon detectors and uncover the supralinearity under faint continuous illumination. The results identify new detector anomalies that supersede existing knowledge of nonlinear effects at the single-photon level.

Performance analysis of SPAD-based optical wireless communication with OFDM

In recent years, there has been a growing interest in the use of a single-photon avalanche diode (SPAD) in optical wireless communication (OWC). The SPAD operates in the Geiger mode and can act as a photon counting receiver obviating the need for a transimpedance amplifier. Although a SPAD receiver can provide higher sensitivity compared to traditional linear photodetectors, it suffers from dead-time-induced nonlinearity. To improve the data rates of SPAD-based OWC systems, optical orthogonal frequency division multiplexing (OFDM) can be employed. This paper provides a comprehensive theoretical analysis of the SPAD-based OWC systems using direct-current-biased optical OFDM signaling considering the effects of signal clipping, SPAD nonlinearity, and signal-dependent shot noise. An equivalent additive Gaussian noise channel model is proposed to describe the performance of the SPAD-based OFDM system. The statistics of the proposed channel model and the analytical expressions of the signal-to-noise ratio and bit error rate are derived in closed forms. By means of extensive numerical results, the impact of the unique receiver nonlinearity on the system performance is investigated. The results demonstrate new insights into different optical power regimes of reliable operation for SPAD-based OFDM systems even well beyond the SPAD saturation level.

‘We work for the Devil’: Oil extraction, kinship and the fantasy of time on the offshore frontier

In the offshore oil industry of Takoradi, Ghana, white expatriate workers describe oil extraction as both ‘the work of the Devil’ and a ‘labour of love’. While companies strive to produce the offshore as a timeless and spaceless fantasy of ‘frictionless profit’, workers emphasize oil work as a sacrificial economy where risk, loss and distance are traded in the pursuit of an ideal of family life. In this article, I argue that the operational structures and labour regime of the offshore (characterized by a rotation pattern, continuous production, distant locations, a segregated workforce, and mobile installations) create not only a model of capital accumulation, but a mode of being and making kin. I describe oil workers’ aspirations to a ‘good family life’ and parental care, pitting time against distance, and the interpersonal ruins that remain when they fray. In probing how oil workers make petro-capitalism affectively workable, by exploring the entangled processes of extractive and reproductive labour, this article contributes to recent scholarship on the role of kinship in sustaining global capitalism.

Large-mode-area soliton fiber oscillator mode-locked using NPE in an all-PM self-stabilized interferometer.

In this work, we investigate an approach to scale up the output pulse energy in an all-polarization-maintaining 17.3 MHz Yb-doped fiber oscillator via implementation of a 25 µm core-diameter large-mode-area fiber. The artificial saturable absorber is based on a Kerr-type linear self-stabilized fiber interferometer, enabling non-linear polarization rotation in polarization-maintaining fibers. Highly stable mode-locked steady states in the soliton-like operation regime are demonstrated with 170 mW average output power and a total output pulse energy of ∼10n J distributed between two output ports. An experimental parameter comparison with a reference oscillator constructed with 5.5 µm core-sized standard fiber components reveals an increase of pulse energy by a factor of 36 with simultaneously reduced intensity noise in the high-frequency range >100k H z.

A Reconfigurable Setup for the On-Wafer Characterization of the Dynamic RON of 600 V GaN Switches at Variable Operating Regimes

Charge-trapping mechanisms observed in high-voltage GaN switches are responsible for the degradation of power converter efficiency due to modulation of the effective dynamic ON-resistance (RON) with respect to its static value. Dynamic RON degradation is typically dependent on the blocking voltage and the commutation frequency and is particularly significant in new technologies under development. The possibility to characterize this phenomenon on GaN switch samples directly on-wafer, under controlled operating conditions that resemble real operations of the DUT in a switching mode power converter is extremely valuable in the development phase of new technologies or for quality verification of production wafers. In this paper, we describe a setup that allows this characterization: dynamic RON degradation of on-wafer 600 V GaN switches is characterized as a function of the VDS blocking voltage, the VGS driving voltage, and at different temperatures. The dependency on the switching frequency is identified by measuring the current recovery of the switch after the application of blocking voltages of different durations.

High-power, high-repetition-rate tunable longwave mid-IR sources based on DFG in the OPA regime.

We demonstrate high-power longwave mid-IR ultrafast sources based on a high-power Er-fiber laser system at 1.55 µm with a 32-MHz repetition rate. Compared with previous 1.03-µm-driven difference frequency generation (DFG), our current configuration allows tighter focusing in the GaSe crystal thanks to an increased damage threshold at 1.55 µm. Consequently, the 1.55-µm-driven DFG can operate in the regime of optical parametric amplification (OPA), in which the mid-IR power grows exponentially with respect to the square root of the pumping power. We experimentally demonstrate this operation regime and achieve broadband mid-IR pulses that are tunable in the 7.7-17.3 µm range with a maximum average power of 58.3 mW, which is also confirmed by our numerical simulation.

Selection of Relative DEM Time Step for Modelling Fast Fluidized Bed of A-Type FCC Particles

In chemical production processes, the most suitable operation regime for A-Type powders such as typical FCC particles is high-speed fast fluidization owing to their uniquely advantageous properties. Discrete element method (DEM) for modelling fast fluidization with A-Type powders has rarely been reported. How to appropriately select the DEM time step and the stiffness coefficient is one of the most critical problems for stable and accurate calculation. This article mainly discusses the effect of the stiffness coefficient and DEM time step on simulations of A-type FCC particles. In order to describe the effect of both parameters and their complex interaction, a dimensionless relative DEM time step is introduced. In total, nine cases with different numbers of relative time steps are adopted for modelling a microfluidized bed of A-Type FCC particles, the regime of which is proved to be fast fluidization. Results show that three bifurcations occur in all the simulations. Only the moderate relative time step possesses the capability of modelling the process of particle collision and thus predicts the right flow regime with asymmetric and heterogeneous typical fast fluidized structures. When the relative time step increases to large rank, simulations predict untrue fluidization regimes with symmetric and homogeneous structures. Moreover, both over-large and over-low relative time steps cause excessive particle overlap and thus a divergence of simulation. The further optimization of moderate relative DEM time step in relation to real particle property is unidentifiable and is thus an outstanding issue. That the range of the moderate relative time step is limited indicates that the common soft-sphere model is poor at modelling fast fluidization of A-Type particles. It is suggested that possible future work should be focused on improving the simulation frame and employing the molecular dynamics model to more appropriately deal with particle contact.

Micro Turbojet Engine Nozzle Ejector Impact on the Acoustic Emission, Thrust Force and Fuel Consumption Analysis

This paper explores the implementation of an ejector to a micro turbojet engine and analysis of the advantages in terms of acoustic and thrust/fuel consumption. Starting with the analytical equations and a series of numerical simulations, the optimal ejector geometry for maximum thrust was obtained. The ejector was manufactured and integrated with the Jet Cat P80 micro turbo engine for testing. The purpose of this article is to report on an improved geometry that results in no significant increase in the frontal area of the turbo engine, which could increase drag. The tests were completed using various functioning regimes, namely idle, cruise and maximum. For each of them, a comparative analysis between engine parameters with and without an ejector was performed. During the experiments, it was observed that, when the ejector was used, the thrust increased for each regime, and the specific consumption decreased for all regimes. The stability of the engine was tested in transient regimes by performing a sudden acceleration sequence, and one carried out the operating line and the modification of temperature values in front of the turbine for both configurations. For each regime, the acoustic noise was monitored at a few points that were different distances from the nozzle, and a decrease was identified when the ejector was used. The advantages of using the ejector on the Jet Cat P80 turbo jet engine are an increased thrust, a lower specific consumption and a reduced noise level, and at the same time, the integrity of the engine in stable operational states and transient operating regimes is not affected.

Compact Modeling of Two-Dimensional Field-Effect Biosensors.

A compact model able to predict the electrical read-out of field-effect biosensors based on two-dimensional (2D) semiconductors is introduced. It comprises the analytical description of the electrostatics including the charge density in the 2D semiconductor, the site-binding modeling of the barrier oxide surface charge, and the Stern layer plus an ion-permeable membrane, all coupled with the carrier transport inside the biosensor and solved by making use of the Donnan potential inside the ion-permeable membrane formed by charged macromolecules. This electrostatics and transport description account for the main surface-related physical and chemical processes that impact the biosensor electrical performance, including the transport along the low-dimensional channel in the diffusive regime, electrolyte screening, and the impact of biological charges. The model is implemented in Verilog-A and can be employed on standard circuit design tools. The theoretical predictions obtained with the model are validated against measurements of a MoS2 field-effect biosensor for streptavidin detection showing excellent agreement in all operation regimes and leading the way for the circuit-level simulation of biosensors based on 2D semiconductors.

Parametric study of pulsed nanosecond laser interaction with carbon-nanotube composite bipolar plate for PEMFCs

A laser processing technique is proposed for the processing of a 2.5 mm thick carbon nanotube (CNT) composite bipolar plate for proton exchange membrane fuel cells (PEMFCs). This study aims to understand laser interaction with the CNT composite plate experimentally using a pulsed nanosecond laser. Penetration depth, top width, spatter width, and overall physical morphologies are studied. Scanning electron microscope (SEM) and 3D Scanning Confocal Microscope were used for observation and measurements. Based on that, a parametric investigation is conducted and reported systematically. Most importantly, the pulse repetition rate presents a unique nature of interaction that resulted in a critical repetition rate distinguishing three operational regimes. The physical and chemical properties of the regimes are further analyzed by Vickers microhardness testing and energy dispersive X-ray (EDX) analyses performed on the surface and cross-section of each specimen. The results reveal that the pulse repetition rate introduces changes in mechanical properties and chemical compositions in the vicinity of the processed region. In conclusion, lower pulse repetition should be favored for less impact on mechanical properties, chemical composition, and morphological aspects.

Distribution of strong input energy in base-isolated structures with complex nonlinearity: a parametric assessment

PurposeOver the past few decades, several base isolation systems have been developed to enhance the performance of structures under extreme earthquake shaking intensities. Recently, to achieve high energy dissipation capabilities, a new generation of multi-stage friction pendulum (FP) bearings known as the “Quintuple Friction Pendulum (QFP)” was introduced in the literature. With the help of its five effective pendula and nine operational regimes, this bearing's major benefits stem from its ability to accomplish complicated multi-stage adaptive behavior with smoothed loading and unloading when subjected to lateral forces.Design/methodology/approachWithin the assessment context, five finite element models of reinforced concrete frames supported on QFP isolators with different properties will be developed in OpenSees. Thereafter, a set of 60 earthquakes will be analyzed using the nonlinear time history analysis approach, and the impact of each ground motion record's properties will be evaluated.FindingsOverall, the study's findings have demonstrated that the characteristics of the isolator, combined with the type of earthquake being applied, have a substantial impact on the isolator's behavior.Originality/valueCurrently, no studies have examined the energy distribution of structural systems equipped with this type of isolation system while considering the influence of earthquake characteristics. Thus, this study is intended to extend the findings available in the literature by discussing and illustrating the distribution of strong ground motions input energy into highly nonlinear base-isolated systems that account for the bearing and superstructural materials' nonlinearity, geometric nonlinearity and leakage-prevented viscous damping nonlinearity. Besides, it investigates the influence of various earthquake characteristics on the energy dissipation of such buildings.

Microstability of β ~ 1 tokamak equilibria

High-power-density tokamaks offer a potential solution to design cost-effective fusion devices. One way to achieve high power density is to operate at a high $\beta$ value (the ratio of thermal to magnetic pressure), i.e. $\beta \sim 1$ . However, a $\beta \sim 1$ state may be unstable to various pressure- and current-driven instabilities or have unfavourable microstability properties. To explore these possibilities, we generate $\beta \sim 1$ equilibria and investigate their stability. First, we demonstrate the generation of high- $\beta$ equilibria with the computer code VMEC. We then analyse these equilibria to determine their stability against the infinite- $n$ ideal-ballooning mode. We follow that by engaging in a detailed microstability study using the GS2 code, beginning with assessments of electrostatic ion-temperature-gradient and trapped election mode instabilities. We observe interesting behaviour for the high- $\beta$ equilibria – stabilization of these modes through two distinct mechanisms – large negative local shear and reversal of electron precession drift. Finally, we perform electromagnetic gyrokinetic simulations and observe enhanced stability in the outer core of high- $\beta$ equilibria and absence of kinetic ballooning modes in the negative-triangularity, high- $\beta$ equilibria. The enhanced outer-core stability of high- $\beta$ equilibria is different from their lower- $\beta$ counterparts and offers an alternative, potentially favourable regime of tokamak operation.

Tailoring Of Poly(N-Methyl Pyrrole) Thin Film Surface With Au-Nanoparticles For Selective Sensing Of H2S

We have demonstrated a highly selective H2S sensor fabricated out of Au-nanoparticles decorated Poly (N-Methyl Pyrrole) thin film. Electrochemical process parameters were initially optimized for uniform deposition of Poly (N-Methyl Pyrrole) on planar platinum substrate. Single step chronopotentiometric deposition resulted in a uniform and adhesive polymeric matrix which served as the basic sensing platform. To inculcate selectivity to the synthesized sensor backbone, the polymeric surface was tailored with Au nanoparticles via cyclic voltammetry. Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDAX) and FTIR spectroscopy were carried out to investigate morphological and structural aspects of the sensory material. The semiconductive regime of operation of the sensor was revealed by V-I characterization. Comparative investigations clearly indicated inclusion of Au nanoparticles resulted in better sensing behaviour than pristine polymeric matrix. Most interesting results were obtained on validating the sensor under independent atmospheres of NH3, NO2, SO2, and H2S where the sensor reflected fingerprinting of H2S. Successful detection of H2S with commendable response and recovery behaviour was possible down to concentration of 5 ppm. The overall sensing behaviour of the nanoparticles tailored polymeric matrix could open up opportunities for complex sensing platforms like e-noses.

Implementation and performance evaluation of controller signal embedded sensorless speed estimation for wind turbine doubly fed induction generators

This paper reports a novel sensorless speed estimation method for wind turbine doubly fed induction generators that is highly non-invasive and utilises the readily available generator drive controller signals. A generalised analytic analysis is first presented that allows the assessment, identification and study of the controller signal spectral content needed to allow the establishment of real time speed estimation. An overlapping window parabolic interpolation algorithm is then proposed for tracking the controller signal speed dependent spectral content, with a view to provision of an increased real-time estimation rate. The reported method is implemented on a vector controlled doubly fed induction generator laboratory system and tested in transient operating regimes representative of wind generator application. The test results show that high accuracy high estimation rate is achievable in field application characteristic transientdynamics.

Effect of the Discharging Flap on Particle Separation in a Cyclone

AbstractThe flow field, pressure drop, and separation efficiency of an industrial cyclone separator were computationally investigated using the Reynolds stress model simulations. The effects of the flapper valve on the static pressure drop, cut size, and slope of the grade efficiency curve were revealed. For that, the cyclone was simulated in three operation regimes, when the flapper valve is open, closed, and without the flapper valve. The results showed that a flapper valve increases the cyclone cut size by 3.4 % when it is in the position “closed” and by 11.4 % when it is in the position “open”. The slope of the grade efficiency curve increases when a valve is switched to the position “open” but the static pressure drop increases by 4.8 % when a flapper valve is in the position “closed”.

Intermodal excitation in time-varying plasmonic structures

Space and time-varying electromagnetic structures give access to regimes of operation and effects that ordinarily do not occur in their time-invariant counterparts due to modal orthogonality constraints. Here, we present the theory of intermodal energy transfer in time-varying plasmonic structures. After identifying a suitable physical mechanism of permittivity modulation, we introduce an appropriate time-domain formalism to study the evolution of the dielectric polarization density in the system. Using a perturbative approach, we obtain closed-form solutions describing the intermodal energy transfer between a directly excited dipolar mode and a higher order subradiant mode. We further show that the modal amplitudes reach a steady state and determine the optimal modulation conditions that maximize the amplitude of the high-order mode. Finally, we identify a coherent control strategy to enhance the conversion efficiency to higher order modes.