Abrupt Breakdown Research Articles

Aerodynamic flow field analysis of vortex interactions on multi-swept wing configurations

The underlying flow physics of vortex breakdown and vortex-vortex interactions at moderate to high angles of attack is not well understood. This lack of understanding is a challenge to the development of optimized wing designs for fighter aircraft. The goal of this study is to gain insight into the breakdown of vortices on multi-swept wing configurations. Two generic models were considered, one with a gradual vortex breakdown and one with a rapid (abrupt) breakdown. Aerodynamic forces were calculated from the US Air Force Academy (USAFA) subsonic wind tunnel to validate the experimental setup using computational fluid dynamics (CFD) and previous work. Stereoscopic particle imaging velocimetry (SPIV) was used to visualize vortex-vortex interactions, measure vortex strengths, and identify vortex breakdown locations. The measured lift and drag forces showed flow physics that agreed with previous experiments and CFD, supporting the validity of the new experimental setup. Analysis of SPIV and CFD data improved our understanding of the different types of vortex interactions observed for both models. The only difference between the models was the location of the secondary sweep angle on the chord. The C4L80 model, with a secondary sweep angle at 40% of the root chord, had a large shear layer along the leading edge, which led to vortex merging and gradual vortex breakdown at lower angles of attack than its counterpart. The C6L80 model, with a secondary sweep angle at 60% of the root chord, had no such shear layer and instead developed strong, isolated vortices that were characterized by vortex braiding and abrupt breakdown at high angles of attack. These vortices were entrained into the main vortex at the leading edge of the strake (inboard vortex), creating a balanced vortex system that enhanced lift and delayed the onset of gradual vortex breakdown seen in the C4L80 model.

Education-to-Work Transitions in Former Communist Countries after 30-Plus Years of Transformation

This paper reviews how young people’s education-to-work transitions have changed since 1989 in former communist countries that have subsequently become full members of the European Union (EU). The sudden collapse of the command economies led to an equally abrupt breakdown in earlier routes into working life. Subsequently, the new independent states have reconstructed their education and training, and their market economies have developed. They now exhibit similar variations in rates of youth unemployment, progression through higher education, and mixtures of academic and vocational secondary education as older EU member states. However, there are features that continue to set all ex-communist countries apart, irrespective of whether they have become full EU members. These are low local rates of pay and westward migration. Its new member states have joined the Southern countries in a European periphery. Yet, there may be sufficient winners in the European core and periphery to keep Europe united.

Corona: An effective polarization strategy of polymer composites with high-k filler for piezoelectric nanogenerators

Piezoelectric nanogenerators (PENGs) are mushrooming owing to demand of swift transformation of power supply from fossil fuel to green energy. Piezoelectric ceramics are enthusiastically chased because glorious PENGs demand for high piezoelectricity. Generally, brittleness of ceramics can be compensated by polymer, while the dielectric mismatch between low permittivity of polymer and high permittivity of ceramics plaguing researchers so long. Traditional contact poling is always interrupted by abrupt dielectric breakdown due to electric field concentration at interface between polymer and fillers. Here, we propose a strategy of using corona polarization instead of traditional contact polarization to insistently polarize large-area polymer composite films with high-k filler. The corona polarization technique can ignore the negative effect of local short circuit on polarization failure owing to prominent lateral insulation between deposited charges on surface of composites film, consequently obtaining well-performed piezoelectricity. Henceforth, polarization failure did not occur using corona even under poling voltage of 26 kV. The corona-poled PVDF/Y-ZnO PENG with only 15 wt% content of fillers generated output power density of ∼1064 μW/cm3. Our work will pilot the development of polymer composites with high-k filler and piezoelectric nanogenerators.

Separated and vortical flow in aircraft aerodynamics: a CFD perspective

AbstractIn the early era of aviation, Frederick Lanchester was both an inventor and a theoretician driven by the need for a theory of flight that would reduce the guesswork in designing new aircraft. His book Aerodynamics in 1907 laid down the early foundations of such a theory. The theory with contributions from others, notably Ludwig Prandtl, was refined to become the basis for the sleek designs of WWII aircraft brought about with little guesswork. New technology changed aircraft design radically with the increased speed of jet propulsion reaching into the transonic range with nonlinear aerodynamics. In the late 1940s and early 1950s substantial guesswork returned to aircraft design. The legacy of Lanchester et al., however, lived on with the development of computational fluid dynamics (CFD) that could guide designers through nonlinear transonic effects. This article presents a historical sketch of how CFD developed, illustrated with examples explaining some of the difficulties overcome in the design of the first-generation swept-wing transonic fighters. The historical study is forensic CFD in search for the likely explanation of the designer’s choice for the wing shape that went into production a long time ago. The capability of current CFD applied to the aerodynamics of aircraft with slender wings is surveyed. The cases discussed involve flow patterns with coherent vortices over hybrid wings and wings of moderate sweep. Vortex-flow aerodynamics pertains to understanding the interaction of concentrated vortices with aircraft components. Modern Reynolds-Averaged Navier-Stokes (RANS) technology is useful to predict attached flow. But vortex interaction with other vortices and breakdown lead to unsteady, largely separated flow which has been found out of scope for RANS. Direct simulation of the Navier-Stokes equations is out of computational reach in the foreseeable future, and the need for better physical modeling is evident. Both cruise performance and stalling characteristics are influenced by strong interactions. Two important aspects of wing-flow physics are discussed: separation from a smooth surface that creates a vortex, and vortex bursting, the abrupt breakdown of a vortex with a subsequent loss of lift. Vortex aerodynamics of not-so-slender wings encounter particularly challenging problems, and it is shown how the design of early-generation operational aircraft surmounted these difficulties. Through use of forensic CFD, the article concludes with two case studies of aerodynamic design: how the Saab J29A wing maintains control authority near stall, and how the Saab J32 mitigates pitch-up instability at high incidence.

Dissolved Gas Analysis and Application of Artificial Intelligence Technique for Fault Diagnosis in Power Transformers: A South African Case Study

In South Africa, the growing power demand, challenges of having idle infrastructure, and power delivery issues have become crucial problems. Reliability enhancement necessitates a life-cycle performance analysis of the electrical power transformers. To attain reliable operation and continuous electric power supply, methodical condition monitoring of the electrical power transformer is compulsory. Abrupt breakdown of the power transformer instigates grievous economic detriment in the context of the cost of the transformer and disturbance in the electrical energy supply. On the condition that the state of the transformer is appraised in advance, it can be superseded to reduced loading conditions as an alternative to unexpected failure. Dissolved gas analysis (DGA) nowadays has become a customary method for diagnosing transformer faults. DGA provides the concentration level of various gases dissolved, and consequently, the nature of faults can be predicted subject to the concentration level of the gases. The prediction of fault class from DGA output has so far proven to be not holistically reliable when using conventional methods on account of the volatility of the DGA data in line with the rating and working conditions of the transformer. Several faults are unpredictable using the IEC gas ratio (IECGR) method, and an artificial neural network (ANN) has the hindrance of overfitting. Nonetheless, considering that transformer fault prediction is a classification problem, in this work, a unique classification algorithm is proposed. This applies a binary classification support vector machine (BCSVM). The classification precision is not reliant on the number of features of the input gases dataset. The results indicate that the proposed BCSVM furnishes improved results concerning IECGR and ANN methods traceable to its enhanced generalization capability and constructional risk-abatement principle.

Current-induced breakdown of the quantum anomalous Hall effect

The quantum anomalous Hall effect (QAHE) realizes dissipationless longitudinal resistivity and quantized Hall resistance without the need of an external magnetic field. However, when reducing the device dimensions or increasing the current density, an abrupt breakdown of the dissipationless state occurs with a relatively small critical current, limiting the applications of the QAHE. We investigate the mechanism of this breakdown by studying multi-terminal devices and identified that the electric field created between opposing chiral edge states lies at the origin. We propose that electric-field-driven percolation of two-dimensional charge puddles in the gapped surface states of compensated topological-insulator films is the most likely cause of the breakdown.

High Voltage and Low Leakage GaN-on-SiC MISHEMTs on a “Buffer-Free” Heterostructure

The performance of a novel ‘buffer-free’ AlGaN/GaN-on-SiC MISHEMTs for power switching applications is demonstrated in this letter. High voltage operation with exceptionally low gate and drain leakage currents is shown. A specific on-resistance of 3.61 m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\Omega } \cdot $ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and an abrupt breakdown voltage of 1622 V at a drain current of 22 nA/mm is achieved. Using two-terminal breakdown measurements, nitrogen-implanted GaN display breakdown fields of 0.96 MV/cm. The semi-insulating SiC substrate is capable of suppressing vertical leakage currents, ensuring that off-state operation is limited by lateral breakdown. The impact of electron trapping effects on dynamic on-resistance is small up to a drain quiescent voltage of at least 240 V. Drain current transient characteristics display a 14% increase in dynamic on-resistance with respect to quiescent drain bias, and a negligible change in resistance up to 100 ms. These types of ‘buffer-free’ heterostructures are of interest for power electronic applications above 1000 V and with potential for co-integration of power and RF-electronics.

Prograde and retrograde precession of a fluid-filled cylinder

We numerically study precession driven flows in a cylindrical container whose nutation angle varies between 60 and 90 degrees for prograde and retrograde precession. For prograde precession we observe sharp transitions between a laminar and a turbulent flow state with low and high geostrophic axisymmetric flow components related with a centrifugal instability, while for retrograde precession a rather smooth transition between a low state and a high state occurs. At the same time prograde and perpendicular precession shows an abrupt breakdown of the flow directly excited by the forcing mechanism, which is not the case for retrograde motion. We characterize the corresponding flow states in terms of the directly driven, non-axisymmetric Kelvin mode, the axisymmetric geostrophic mode, and an axisymmetric poloidal flow which is promising for precession-driven dynamo action. The latter issue is discussed with particular view on an optimal parameter choice for the DRESDYN dynamo project.

Structural performance of exterior beam-column joint using biochar impregnated pond ash concrete

RC Structures when exposed to seismic waves Beam-Column joints are an important part of the construction and play a significant role in the overall behavior. The most significant explanation behind the failure and abrupt breakdown during the seismic tremors is the insufficient provision of shear reinforcement in joints RC Structures. In reinforced concrete frames beam-column joint is the critical zone that should be designed to provide sufficient deformation capacity and ultimate strength. More vulnerable failures of joints around the exterior border of the structure occur than interior joints. In the static load analysis, the priority is strength and ductility So, in ordinary structures joints are designed by considering strength to resist wind and gravity loads. Experimental research is the most significant way to know the parameters which influence both joint performance and those that should be improved to upgrade joint performance. In the present study, two Specimens of Beam-Column joints were prepared one is with conventional concrete and other with concrete using 5% binder replaced with Biochar and 20% fine aggregate replaced with pond ash and the test results showed that the energy absorption and ductility were increased but stiffness was reduced in pond ash concrete when compared to conventional concrete and there is no much difference in deflections within yield stage but deflections were reduced in Ultimate stage in pond ash concrete compared to traditional.

Bearing performance degradation assessment based on topological representation and hidden Markov model

Bearing performance degradation assessment is essential to avoid abrupt machinery breakdown. However, background noise, outliers, and other interferences in the monitoring data may restrict the accuracy and stability of bearing performance degradation assessment in practical applications. In this study, a bearing performance degradation assessment method based on the topological representation and hidden Markov model is proposed. To construct a robust and representative feature space, the topological representations, specifically, topological meshes of the original features are obtained by self-organizing map, which can represent the general structure of the original feature space and eliminate outliers and other interferences. Then, the weight vectors of topological meshes are used as degradation features. Finally, the hidden Markov model is adopted as the assessment model to evaluate the bearing performance degradation tendency and detect the initial degradation effectively. To validate the effectiveness and superiority of the proposed method, two experimental datasets are analyzed. Compared with peer methods, the performance indicator curve of the proposed method presents a more smooth and accurate degradation tendency than comparative methods. Moreover, initial degradation can be identified accurately.

Sulfadiazine dissipation in acidic tropical soils.

Sulfadiazine (SDZ) residues have been detected in manured soils as well as their adjacent water resources, but its behavior is still poorly understood in acidic tropical soils. This research aimed to evaluate sorption, leaching, and biodegradation of 14C-SDZ in four acidic soils from Brazil, using OECD guidelines. Except for the sand soil (Kd = 2.6Lkg-1), SDZ sorption tended to be higher (Kd > 8.4Lkg-1) and more hysteretic (ΔH >> 1) in acidic soils. When freshly applied, SDZ leaching was low (< 0.11% of applied radioactivity (AR)) and could not always be predicted by Kd values; but leaching was restricted when SDZ was aged for 62days. SDZ mineralization was low (< 3%) but its dissipation was fast (DT50 < 2.3days and DT90 < 6.3days) due to fast initial degradation (an unknown metabolite was immediately formed, likely 4-hydroxysulfadiazine) and mainly to fast formation of non-extractable residues (NER) (> 78% of AR up to 7days). For certain acidic soils, the abrupt breakdown of the SDZ suggests that degradation should be initially chemical and then followed by enzymatically driven reactions. The fast formation of NERs was attributed mostly to chemical bounding to soil humic substances (Type II-NER), but SDZ sequestration cannot be ruled out (Type I-NER). NERs represent a long-term environmental reservoir of SDZ that may cause deleterious effects on non-target organisms as well as promote antibiotic resistance to soil microbes.

Democratization as institutional change: Hong Kong 1992–2015

The limited democratization in Hong Kong has been on-going for more than three decades. Unlike typical democratizations as the abrupt breakdown of authoritarian regimes, the case of Hong Kong is characterized by gradual and incremental changes under a constitutional framework. The process, involving actors such as the Chinese government, colonial government, and local democratic forces, provides a wide variety of forms of institutional changes. By utilizing concepts from the literature on institutional change, namely path dependence, layering, and conversion, important junctures of democratization in Hong Kong are analyzed. It is found that the case can be suitably explained by the framework of institutional change. This research carries implications for the study of central-regional interactions, democratization, and Hong Kong politics.

Design and Development of InP DHBTs With High Breakdown Voltage for Ka-Band PA Applications

Type-I (straddled) InP DHBT devices have been fabricated to study the impact of different base-collector junction transition layer designs on the collector to emitter breakdown behavior, breakdown voltage, BVceo and the device output performance such as knee voltage, Vknee at high collector current densities, Jc. An abrupt breakdown behavior with BVceo of 11V and Vknee less than 1V at Jc>100kA/cm2 are achieved using an optimized base collector junction transition layer design. Using 2D numerical simulations, components of leakages dominating Iceo at low and high collector to emitter bias voltages are explained. It is further shown that the optimized device exhibits good RF and power performance achieving f $_{T}$ of 115GHz and f $_{max}$ of 150GHz at emitter width >1um. An MMIC power amplifier (PA) is also designed and fabricated using the optimized Type-I InP DHBT device. At 28GHz, good power performance is achieved with PAE of 54% and Psat of 26dBm. Thus, it is shown that InP DHBTs developed in this work are a good candidate for the next phase of 5G millimeter wave Ka-band power amplifier applications.

Graphene thermal break-down induced by anharmonic bending mode

A novel approach is proposed allowing us to prove, self-consistently, that free-standing graphene reaching a certain temperature loses its mechanical stability resulting in abrupt breakdown, which can be interpreted as melting. Our study is based on the idea of the crucial role of the anomalously soft bending ‘sound’ mode in the jump transition of graphene from the state with relatively small bending fluctuations to a state with fluctuations close in amplitude to the graphene lattice constant. The acme of the developed theory is in establishing a quantitative relationship connecting the graphene elastic moduli of second, third (negative!), and fourth orders at the melting temperature Tm that permits us to calculate Tm. The results obtained lay a theoretical foundation for an analog of Lindemann criterion for graphene.

Spatio-Temporal Signal Recovery Based on Low Rank and Differential Smoothness

The analysis of spatio-temporal signals plays an important role in various fields including sociology, climatology, and environmental studies, etc. Due to the abrupt breakdown of the sensors in the sensor network, there always are missing entries in the observed spatio-temporal signals. In this paper, we study the problem of recovering spatio-temporal signals from partially known entries. Based on both the global and local correlated property of spatio-temporal signals, we propose a low rank and differential smoothness based recovery method (LRDS), which novelly introduces the differential smooth prior of time-varying graph signals to the field of spatio-temporal signal analysis. The performance of the proposed method is analyzed theoretically. Considering the case where a priori information about the signal's global pattern is available, we propose prior LRDS to further improve the reconstruction accuracy. Such improvement is also verified by synthetic experiments. Besides, experiments on several real-world datasets demonstrate the improvement on recovery accuracy of the proposed LRDS over the state-of-the-art spatio-temporal signal recovery methods.

Nano-composites for enhanced catastrophic failure temperature of organic light-emitting diodes

Thermal stability is one of the key technical challenges in developing high brightness organic light-emitting diodes (OLEDs). In this study, thermal catastrophic failure, a process involving temperature-induced abrupt device breakdown, is found to have a high degree of correlation to the glass transition process of the organic molecules. It is also found that C60-organic nano-composites can be used effectively to increase the glass transition temperature of the organic small molecular thin-films leading to the improvement of the thermal stability of OLEDs. In addition, a universal mathematical formula with only one variable, concentration, is discovered to quantify well the glass transition temperatures of all C60 nano-composites. The thermal catastrophic failure temperature of OLEDs with C60 nano-composites as a hole transport layer is found to be proportional to the glass transition temperature of C60 nano-composites.

Abrupt breakdown of the predictability of early season typhoon frequency at the beginning of the twenty-first century

Maintenance of the anomalous anticyclone (cyclone) over the Philippine Sea following strong peak El Nino (La Nina) leads to a negative correlation between the pre-winter (October–December) El Nino-Southern Oscillation (ENSO) index and the tropical cyclone (TC) genesis frequency in the ensuing early TC season (January–June). This negative lead-lag correlation has established the predictability of the western North Pacific (WNP) TC genesis frequency in the early season. However, little is known about whether this relationship remains steady to date, especially in the context of evolving features of ENSO in recent decades. Here we show that the negative relationship experiences an abrupt breakdown around the early 2000s (1999–2006), which primarily occurs in the southeastern quadrant of the WNP. During the breakdown period, the anomalous anti-cyclone shifts westward to the South China Sea and an anomalous cyclonic circulation occupies the southeastern quadrant of the WNP, causing enhanced TC formation. We further reveal that the westward displacement of the WNP anticyclonic anomaly during 1999–2006 is induced by the westward shift of sea surface temperature (SST) and associated precipitation anomalies during the decaying phase of ENSO, which increases cyclonic circulation and ascending motion while reduces vertical wind shear, and thereby enhances TC formation over the southeastern WNP. The results suggest that the frequent occurrence of decaying ENSO events with SST anomaly center over the central Pacific (CP) accounts for the abrupt breakdown of predictability, and the impacts of ENSO on TC formation in the early TC season are sensitive to the location of the maximum SST anomaly. Distinguishing eastern Pacific (EP) and CP types of ENSO and/or strong and weak ENSO events may help to improve the early season TC genesis forecast over the WNP. The results also have important implications for understanding the future projection of TC changes under global warming scenarios.

The Control of Meiotic Recombination in the Human Genome.

Meiotic recombination plays a key role in reshuffling haplotypes in human populations and thus affects evolution profoundly. However, our understanding of recombination dynamics is largely limited to descriptions of variation in populations and families. Higher-resolution analysis (≤ 0.0001 cM) of de novo recombination events in human sperm DNA has revealed clustering into very narrow hotspots (1-2kb) that generally coincide with abrupt breakdown of linkage disequilibrium. Recent findings have highlighted an unexpected molecular control of the distribution of meiotic double-strand breaks (DSBs) in mammals by a rapidly evolving gene in trans, PR-domain-containing 9 (PRDM9), and specific DNA sequence motifs incis. In addition, the understanding of new regulators in DSB repair processes has allowed the delineation of recombination pathways that have two major outcomes, cross-overs and non-cross-overs, which have distinct mechanistic roles and consequences for genome evolution. Further molecular studies are needed to gain information about how hotspots originate, function, and evolve.

Current quantization and fractal hierarchy in a driven repulsive lattice gas.

Driven lattice gases are widely regarded as the paradigm of collective phenomena out of equilibrium. While such models are usually studied with nearest-neighbor interactions, many empirical driven systems are dominated by slowly decaying interactions such as dipole-dipole and Van der Waals forces. Motivated by this gap, we study the nonequilibrium stationary state of a driven lattice gas with slow-decayed repulsive interactions at zero temperature. By numerical and analytical calculations of the particle current as a function of the density and of the driving field, we identify (i) an abrupt breakdown transition between insulating and conducting states, (ii) current quantization into discrete phases where a finite current flows with infinite differential resistivity, and (iii) a fractal hierarchy of excitations, related to the Farey sequences of number theory. We argue that the origin of these effects is the competition between scales, which also causes the counterintuitive phenomenon that crystalline states can melt by increasing the density.

Geometric Correlations Mitigate the Extreme Vulnerability of Multiplex Networks against Targeted Attacks

We show that real multiplex networks are unexpectedly robust against targeted attacks on high-degree nodes and that hidden interlayer geometric correlations predict this robustness. Without geometric correlations, multiplexes exhibit an abrupt breakdown of mutual connectivity, even with interlayer degree correlations. With geometric correlations, we instead observe a multistep cascading process leading into a continuous transition, which apparently becomes fully continuous in the thermodynamic limit. Our results are important for the design of efficient protection strategies and of robust interacting networks in many domains.