Main Oscillation Research Articles

Experimental and numerical investigation on the cavitation erosion effect and resonance effect of submerged water jets

ABSTRACT To analyse the cavitation erosion effect and resonance effect of submerged water jets generated by organ-pipe nozzles, experimental and numerical research is conducted in the nozzles with five different diffuser angles. The cavitation erosion intensity and the flow characteristics of submerged jets are obtained by the erosion test, high-speed photography and Large Eddy Simulation. The experimental and numerical results illustrate that the nozzle with a 60° diffuser angle is capable of producing a strong cavitation erosion effect. Compared to the fully developed submerged jet, the axial pressure oscillation of axially confined submerged jets is stronger, indicating that it has a strong resonance effect. The stress wave generated by cavitation modulates the jets after reflection from the impact wall. This modulation results in the excitation of the original main frequency oscillation of organ-pipe nozzle jets, which then gives rise to multiple frequency oscillations, thereby forming the strong resonance effect.

SPECIFIC FEATURES OF TIDAL EXCHANGE IN THE KURIL STRAITS ACCORDING TO MODELING DATA

Specific features of tidal fluctuations of sea level and tidal currents in the straits of the Kuril Islands are studied using numerical modeling. Two-time intervals, i. e. 02.06.1977-03.06.1977 and 11.10.1977-12.10.1977, were taken to compare the parameters of tropical and equatorial tides. The simulation results were verified with the field data. It was found that the straits of the northeastern part of the Kuril Ridge have a mixed type of tides with a predominance of the diurnal course, and in the southwestern part a semi-diurnal course prevails. The highest tides (up to 2,5-3 m) in the study period were observed on 02.06.1977 near the Kamchatka Peninsula and the Hokkaido Island, as well as large islands of the Kuril Ridge. Four main oscillation periods of 12, 12,4, 24 and 25,8 hours were identified, which corresponds to the S2, M2, K1 and O1 harmonics. Analysis of the data obtained showed that the maximum velocities of the depthaveraged currents reach 4,5 m/s. The highest speeds were recorded in the Kruzenshtern, Frieze and Sredny Straits. Estimates of the tidal flow have shown that semi-daily fluctuations are more pronounced in the straits with lower average depths during a tropical tide. In deeper straits, semi-daily fluctuations at tropical tides are very weak or absent at all, an example is the Boussol Strait. It is calculated that when using a two-dimensional model, the maximum values of the tidal flow are observed in the Boussol Strait, reaching ~40 Sverdrups (Sv).

Possible anticorrelations between pulsation amplitudes and the disc growth of Be stars in giant-outbursting Be X-ray binaries

ABSTRACT The mechanism of X-ray outbursts in Be X-ray binaries remains a mystery, and understanding their circumstellar discs is crucial for a solution of the mass-transfer problem. In particular, it is important to identify the Be star activities (e.g. pulsations) that cause mass ejection and, hence, disc formation. Therefore, we investigated the relationship between optical flux oscillations and the infrared (IR) excess in a sample of five Be X-ray binaries. Applying the Lomb–Scargle technique to high-cadence optical light curves from the Transiting Exoplanet Survey Satellite (${\it TESS}$), we detected several significant oscillation modes in the 3–24 h period range for each source. We also measured the IR excess (a proxy for disc growth) of those five sources, using J-band light curves from Palomar Gattini-IR. In four of the five sources, we found anticorrelations between the IR excess and the amplitude of the main flux oscillation modes. This result is inconsistent with the conventional idea that non-radial pulsations drive mass ejections. We propose an alternative scenario where internal temperature variations in the Be star cause transitions between pulsation-active and mass-ejection-active states.

Study on long term troposphere lower stratosphere temperature (TLST) trend in tropical and subtropical northern hemisphere using ground based and COSMIC satellite data

The troposphere, the lowest and closest layer of the atmosphere, is where all meteorological events take place. The tropospheric — lower stratospheric (TLS) temperature trend is determined using linear regression and is essential to comprehending the consequences of climate change in the future. In this article, we explored the long-term temperature variabilities and trends of TLS (1–25 km) temperature and its responses by natural drivers such as El Nino southern oscillation (ENSO), solar flux (SF), quasi-biannual oscillation (QBO), Indian ocean dipole (IOD), and aerosol indexes (AI) using monthly averaged zonal mean COSMIC satellite and ground — based Radiosonde (RS) observations for the period of 2006 – 2020 over tropical station Addis (90 N, 38.80 E) and subtropical station Cairo (30.030 N, 31.230 E). The tropopause is located at the tropical station Addis at 17 km with a temperature of 190–194 K, and for the subtropical station Cairo, it is located at 15 km with a temperature of 201 K, which supports the decrement of tropopause height from the tropics to the subtropics with a slight increase in temperature. The two main oscillations in the TLS region can be seen by using the wavelet analysis technique: the semiannual oscillation (SAO) and the annual oscillation (AO), with the AO being especially strong in the lower troposphere. Furthermore, Morlet wavelet analysis on cold-point tropopause temperature CPTt displays AO and cold point tropopause height CPTh reveals a QBO-like signal. The TLS region has positive peaks at heights of 7, 21, 22, 13, and 4 km for the Addis station, and at 15, 19, 25, 15, and 16 km for the Cairo station in response to natural drivers such as ENSO, SF, QBO, IOD, and aerosol. Lag analyses demonstrate a one-month delay for all natural forcings, except for oceanic indices and SSF, up to three months below the tropopause (below 15 km). There is a noticeable 3 to 4 months lag in every oscillation above the tropopause. A warming trend in the tropospheric region and a cooling trend in the UTLS regions are revealed by MLR trend analysis. In contrast to the subtropical Cairo station, which has the highest warming rate of 0.38 K/decade at 2 km and the maximum cooling rate of −0.2 K/decade at 10 km, the tropical Addis station has the highest cooling rate of −0.38 K/decade at 12 km and the highest warming rate of 0.28 K/decade at 3 km. Our trend findings are consistent with previous research.

Sweeping jet impingement heat transfer on concave surface: Influence of trapped vortex ring revealed by generalized k-ω (GEKO) model

Jet impingement-based heat transfer enhancement is widely employed due to the significant mass, momentum, and energy it accompanied. While the steady jet with high energy flux is commonly favored, it still exhibit few defects, including heat transfer deterioration away from the stagnation point and mutual flow interference in the jet arrays, which are particularly critical in demanding applications. Recently, the sweeping jet has been proposed to resolve the above challenges owing to its self-excited and self-sustaining unsteady oscillation. However, current research predominately focuses on impinging on the flat surface, neglecting impinging on the concave surface, such as internal cooling at the gas turbine blade leading-edge as well as the hot gas de-icing at the aircraft wing leading-edge. Understanding the heat transfer characteristics in these scenarios is crucial as they notably differ from those on flat surfaces. Therefore, this paper investigates the heat transfer characteristics and the related essential flow dynamics of a sweeping jet with a Reynolds number of 2577 impinging on a concave surface, and those of a steady jet under the same conditions are compared. The jet-to-wall distance and the curvature of the impingement surface are 4 and 10 times the jet's hydraulic diameter, respectively. To correctly capture the three-dimensional characteristics of the sweeping jet, this research employs the Reynolds stress model for turbulence modeling and the Generalized k-ω (GEKO) model for better calibration of the jet's spreading and dissipation rates, as well as the characteristics of trapped vortex generated due to the confined concave flow channel. The study initially focuses on the average Nusselt number on the impingement surface, revealing a 15.2% increase in heat transfer intensity of the sweeping jet compared to the steady jet, and the sweeping jet covers a larger area with enhanced uniformity. Subsequent analysis on streamlines in two perpendicular planes and Q criterion iso-surfaces in the spatial domain shows the formation of a trapped vortex ring with a constant location around the jet, restricting the effective cooling range to its inner side and thus resulting in a steep temperature rise on the two lateral sides of the vortex ring. The stability of the trapped vortex is enhanced by the curved impingement wall in the oscillation plane, which on the one hand forces the high-temperature fluid that has exchanged heat with the wall to repeatedly wash the same location, and on the other hand significantly reduces the jet oscillation angle compared to a free sweeping jet. Consequently, the heat transfer intensity of the sweeping jet is lower in the main oscillation plane than in the non-oscillation plane. Analysis of the near-wall flow parameters indicates that the sweeping jet generates higher velocity and turbulence kinetic energy in the wall jet zone far away from the stagnation point compared to the steady jet, which results in a higher local heat transfer intensity. The unsteady transverse oscillation of the sweeping jet disturbs the wall jet zone and disrupts the formation of the wall boundary layer, causing a higher velocity gradient at the wall and hence the lower thicknesses of the velocity boundary layer and the thermal boundary layer than those of the steady jet. In conclusion, this study reveals the flow and heat transfer features of the sweeping jet impingement on confined concave surface, with the influence of the trapped vortex ring mattering significantly. The findings provide a reference for designing a more effective jet arrangement. Future work will focus on investigating the effects of jet Reynolds number and impingement distance variations on the trapped vortex ring, and how to mitigate the adverse effects of the trapped vortex ring on impingement heat transfer.

Dynamic stress prediction for a Pump-Turbine in Low-Load Conditions: Experimental validation and phenomenological analysis

To meet the increasing demand for higher flexibility in hydropower, an operating range extension in an existing hydroelectric powerplant is considered. To this end, the dynamic flow and structural behavior of the low-head pump-turbine is investigated in turbine mode in partial load and deep partial load conditions, to evaluate the feasibility of this flexibilization due to the increased high cycle fatigue damage. In a first step, different numerical simulation approaches that combine fluid dynamics and structural dynamics are applied and compared to in-situ dynamic strain and pressure measurements on the impeller. As a result, the modelling requirements for a numerical workflow that leads to unmatched accuracy in predicting the broad-band dynamic stresses of deep partial load are identified. In a second step, the operating-point-dependent fatigue crack initiation spots are identified based on dynamic stresses. Herein, the emergence of an additional dominant critical spot near the blade leading edge is observed, which is unexpected for low-head pump turbines. Thanks to the detailed numerical flow and structural analyses, the root cause for the dynamic stress concentration is studied and the emergence of the new critical spot is understood. It turns out in deep partial load, that the main source of pressure oscillations on the impeller blade surfaces is the vortex-dominated flow detachment zone around the blade leading edge, which, in combination with the geometry of the investigated impeller, translates into the main stress oscillations. The results of this work were later used to extend the operating range of the hydroelectric powerplant from initially 25% to 100% power output to now 0% to 100%.

Neuropsychopharmacological Induction of (Lucid) Dreams: A Narrative Review.

Lucid dreaming (LD) is a physiological state of consciousness that occurs when dreamers become aware that they are dreaming, and may also control the oneiric content. In the general population, LD is spontaneously rare; thus, there is great interest in its induction. Here, we aim to review the literature on neuropsychopharmacological induction of LD. First, we describe the circadian and homeostatic processes of sleep regulation and the mechanisms that control REM sleep with a focus on neurotransmission systems. We then discuss the neurophysiology and phenomenology of LD to understand the main cortical oscillations and brain areas involved in the emergence of lucidity during REM sleep. Finally, we review possible exogenous substances-including natural plants and artificial drugs-that increase metacognition, REM sleep, and/or dream recall, thus with the potential to induce LD. We found that the main candidates are substances that increase cholinergic and/or dopaminergic transmission, such as galantamine. However, the main limitation of this technique is the complexity of these neurotransmitter systems, which challenges interpreting results in a simple way. We conclude that, despite these promising substances, more research is necessary to find a reliable way to pharmacologically induce LD.

Feature statistics and analysis of transient overvoltage waveform measured from 220 kV substation

The electromagnetic transient overvoltage in the substation is an important factor that affects the life of insulation equipment. For evaluate the economy and reliability of the insulation design of substation equipment, it is necessary to study and analyze the actual measured overvoltage waveforms in the substation. In this paper, firstly, the 3018 pieces of overvoltage data monitored in the 220 kV substation from 2018 to 2019 are preprocessed to obtain the required overvoltage waveform. Next, by analyzing and classifying the overvoltage data, four typical overvoltage waveforms suffered by the insulation equipment in the substation are extracted. Combined with the IEC60060 standard, the time parameters of the overvoltage waveform are selected and defined: wave-front time (Tf), half-value time (Thalf), main oscillation frequency (f), and attenuation constant (α). Further, statistical analysis is performed on the parameters of each waveform. The distribution law, distribution characteristics and distribution range of the parameters are obtained. Finally, the parameters of four typical overvoltage waveforms are summarized and compared, the cumulative distribution functions are obtained. This results can guide and optimize the insulation design of the insulation equipment in the substation, reduce design errors. It provide data support for the economy and reliability of insulation design.

Spatial structure of resonance cavities in sunspots

ABSTRACT We present a study of wave processes in sunspots from active regions NOAA 11131 on 2010 December 10 and NOAA 12565 on 2016 July 14 observed by SDO/AIA in the 1600, 304, and 171 Å temperature channels. To study the spatial structure of the resonance cavities previously found by Jess et al., we applied spectral data processing techniques such as pixelized wavelet filtering and mode decomposition. For the first time, we found stable regions as waveguides of the oscillations in the sunspot umbra, occupying specific frequency ranges without spatial overlap. The sizes of these regions depend on the frequency oscillations, and the maximum frequency coincides with the values of the harmonics of the main oscillation mode. Frequency drifts were observed in the band occupied by these regions, with different spectral slopes depending on the location of the sources in the sunspot umbra. We suggest that the observed distribution of wave sources in the umbra is a set of resonant cavities where successive amplification of oscillations at selected multiple harmonics is observed. The distribution of sources at low frequencies indicates the influence of the atmospheric cut-off due to the inclinations of the magnetic field lines.

The Global Asteroseismology Project Proof of Concept: Asteroseismology of Massive Stars with Continuous Ground-based Observations

Massive (≳8M ⊙) stars are the progenitors of many astrophysical systems, yet key aspects of their structure and evolution are poorly understood. Asteroseismology has the potential to solve these open puzzles; however, sampling both the short period pulsations and long period beat patterns of massive stars poses many observational challenges. Ground-based single-site observations require years or decades to discern the main oscillation modes. Multisite campaigns were able to shorten this time span, but have not been able to scale up to population studies on sample of objects. Space-based observations can achieve both continuous sampling and observe large numbers of objects; however, most lack the multiband data that is often necessary for mode identification and removing model degeneracies. Here, we develop and test a new ground-based observational strategy for discerning and identifying the main oscillation modes of a massive star in a few months, in a way that can be scaled to large samples. We do so using the Las Cumbres Observatory—a unique facility consisting of robotic, homogeneous telescopes operating as a global network, overcoming most of the challenges of previous multisite efforts, but presenting new challenges which we tailor our strategy to address. This work serves as the proof of concept for the Global Asteroseismology Project, which aims to move massive star asteroseismology from single-objects to bulk studies, unleashing its full potential in constraining stellar structure and evolution models. This work also demonstrates the ability of the Las Cumbres Observatory to perform multisite continuous observations for various science goals.

Multi-scale free oscillations and resonances over the continental shelf of the East China Sea from the 2011 Tohoku-Oki tsunami

A number of coastal tide gauges and offshore Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys across the Pacific Basin recorded persistent oscillations excited by the 2011 Tohoku-Oki tsunami. The hazardous persistent oscillations prompted authorities to maintain a tsunami warning for dozens of hours. Tsunami waves reached the eastern China coast about five hours after the earthquake. The duration of detected prolonged oscillations was more than 80 h. In this study, we accurately reproduced tsunami generation and its evolution over the shallow and wide continental shelf of the East China Sea (ECS). For resonance analysis, we used an advanced numerical model that can consider simultaneously the effects of kinematic rupture process, horizontal displacement on tsunami generation, and Earth elasticity, seawater compressibility, and wave dispersion on tsunami propagation. The consistency of the simulated results was validated with both coastal and offshore tsunami records. In addition, we performed a combined spectral analysis of the observed tsunami records, background noise, and synthetic tsunami wave fields to identify tsunami oscillation modes and estimate their spatial distribution, including the spectral amplitude and phase angles. Eight main oscillation modes of the 2011 Tohoku tsunami were determined with periods of 24, 28, 37, 47, 57, 93, 136, and 157 min. They were all constrained within the 200-m isobaths for dozens of hours. The 57 min oscillations mode had the highest energy amplification across most of eastern China's coast. The spatial patterns of the resonance oscillations were used to propose hazard-prone areas. Based on the comprehensive spectral analysis results, the local and shelf topography properties played a key role in the dominant resonance modes of 136, 57, and 37 min. Identification of the shelf resonance oscillations coupled with the nearshore basin oscillations can provide valuable information for coastal tsunami hazard mitigation.

Patterns of runoff fluctuations of the Dniester River (Ukraine) under conditions of climate change at the beginning of the 21st century

The relevance of the chosen topic is associated with the impact of climate change on fluctuations of water content in Ukrainian rivers. The consequences of warming depend on physical and geographical conditions and vary significantly across different natural zones. The behaviour of catchment areas affected by climate change varies depending on landscape (mountainous or flat terrain) and latitude. This results in instability of boundaries of catchment areas with annual runoff synchronous and out-of-phase fluctuations. The study aims at establishing the patterns of annual runoff fluctuations in the Dniester basin with relation to fluctuations in the maximum runoff of rainfalls and spring floods. The spring floods are the ones that determine a significant part of annual runoff and depend on the impact of warming during the winter season. The study's main method is the method of constructing difference integral curves of annual and maximum rainfalls and maximum spring runoff and their analysis. There are 7 groups (zones) within the Dniester River catchment area identified by the type of difference integral curves of annual runoff. Fluctuations in annual, maximum rain and spring runoff were compared using the curves averaged within each group. The degree of synchronicity of fluctuations was estimated via correlation coefficients between the ordinates of difference integral curves. It was found that the main river oscillations retain their cyclicity along its entire length, even below the Dniester HPP. The last transition to a low-water phase occurred in 2010-2011. It was discovered that the main river's cyclical nature mostly results from fluctuations in the runoff of mountainous Carpathian tributaries that form the Dniester's runoff formation zone. This part of the catchment area is characterized by a high amount of snow and rainfalls feeding the river with the role of rainfalls having a predominant character. In the upper reaches of Carpathian rivers, where the impact of warming is not yet sufficiently pronounced at high altitudes, spring floods play a significant role in runoff formation. In the upper reaches of Carpathian rivers there is an additional zone where fluctuations in annual runoff are caused by both rainfalls and spring floods. The boundaries of this zone (2nd district) may change as the effects of warming increase high in the mountains. On the left-bank tributaries, the runoff fluctuations in Verkhnepodilskyi and Serednepodilskyi districts are similar to those of the main river, however, there is a shift in the dates of the last transition to a low-water phase. The study indicates the likelihood of karst and artificial reservoirs influence. It also indicates a significant difference in the nature of runoff fluctuations of the upper left-bank tributaries (Strvyazh, Vereshchytsia, Shcherek) and lower left-bank tributaries. They are affected by other atmospheric processes and climatic factors associated with such processes.

Generation mechanism of high-frequency oscillation in proportional valve with a high-speed on/off valves bridge as the pilot stage

A digital proportional valve has a main spool that operates with a high-speed on/off valves bridge as the pilot stage. However, this can cause the spool to undergo high-frequency oscillation during movement, leading to a slower valve response, lower control accuracy, and even control failure. Therefore, it is important to study the oscillation mechanism of the valve. To address this issue, we establish a detailed dynamic model of the digital proportional valve, and analyze the factors affecting the response performance of the proportional valve spool. A method of half-bridge control based on duty cycle is proposed to eliminate the oscillation. The influence regularity of each factor on the response performance of the valve is explored. It is proved that the control method of the high-speed on/off valves bridge and the Pulse-width modulation (PWM) signal-switching frequency of the high-speed on/off valves are the main reasons for the oscillation of the main spool. The proposed half-bridge control method based on duty cycle can eliminate the oscillation in a lower frequency range. In addition, the results show that the pilot oil pressure is an important contributor to the oscillation; the higher the pilot oil pressure, the easier the main spool oscillation occurs. However, the spool mass, oil temperature, and coil temperature of the high-speed on/off valve do not affect the stability of the digital proportional valve.

OPRM1 and BDNF polymorphisms associated with a compensatory neurophysiologic signature in knee osteoarthritis patients

ObjectiveThe present study investigated the relationship between three genetic polymorphisms of OPRM1 (rs1799971 - A118G and rs1799972 - C17T) and BDNF (rs6265 - C196T) and EEG-measured brain oscillations in Knee Osteoarthritis (KOA) patients. Materials and MethodsWe performed a cross-sectional analysis of a cohort study (DEFINE cohort), KOA arm, with 66 patients, considering demographic (age, sex, and education), clinical (pain intensity and duration), OPRM1 (rs1799971 - A118G and rs1799972 - C17T) and BDNF (rs6265 - C196T) genotypes, and electrophysiological measures. Brain oscillations relative power from Delta, Theta, Alpha, Low Alpha, High Alpha, Beta, Low Beta and High Beta oscillations were measured during resting state EEG. Multivariate regression models were used to explore the main brain oscillation predictors of the three genetic polymorphisms. ResultsOur findings demonstrate that Theta and Low Beta oscillations are associated with the variant allele of OPRM1-rs1799971 (A118G) on left frontal and left central regions, respectively, while Alpha brain oscillation is associated with variant genotypes (CT/TT) of BDNF-rs6265 on frontal (decrease of oscillation power) and left central (increase of oscillation power) regions. No significant model was found for OPRM1-rs1799972 (C17T) in addition to the inclusion of pain intensity as a significant predictor of this last model. ConclusionOne potential interpretation for these findings is that polymorphisms of OPRM1 – that is involved with endogenous pain control - lead to increased compensatory oscillatory mechanisms, characterized by increased theta oscillations. Along the same line, polymorphisms of the BDNF lead to decreased alpha oscillations in the frontal area, likely also reflecting the disruption of resting states to also compensate for the increased injury associated with knee OA. It is possible that these polymorphisms require additional brain adaption to the knee OA related injury.

Large eddy simulation of film cooling for a forward expansion hole under main flow oscillation

This study conducted a large eddy simulation (LES) to predict the flow structure and effectiveness of film cooling in a gas turbine. The study focused on a forward expansion hole on a flat plate with an inclined injection angle of 35° and spanwise injection angles (orientation angles) of 0° and 30° under main flow oscillation. In gas turbines, the mainstream flow can exhibit unsteadiness owing to stator–rotor interactions or freestream turbulence. Here, sinusoidal functions are used to approximate the flow oscillations, and their effects are evaluated at frequencies of 0, 8, and 32 Hz, corresponding to Strouhal numbers of 0, 0.603, and 2.413, respectively. The study aims to characterize the film cooling at two time-averaged blowing ratios: 0.5 and 1.0. The range of Strouhal numbers considered in this study is relevant to actual transonic gas turbines. The computational results obtained through LES are validated using experimental data and compared with the Reynolds-averaged Navier–Stokes simulation results. The findings demonstrate the efficiency of LES to predict accurately the distribution of spanwise film cooling effectiveness and spanwise-averaged effectiveness for forward expansion holes on turbine blades under flow oscillations. Comprehensive analyses of various aspects such as the instantaneous turbulent structures of the film cooling flow fields, time-averaged temperature, and root-mean-squared values of the coolant temperature and a proper orthogonal decomposition analysis for velocity vectors are performed. The results show that when the oscillation frequency of the main flow varied from 0 Hz to 32 Hz for a time-averaged blowing ratio of 0.5, the film cooling effectiveness for the forward expansion hole reduced significantly. However, when the frequency increased under a time-averaged blowing ratio of 1.0, the film cooling effectiveness for the hole was only slightly affected by the oscillations. Moreover, an orientation angle of 30° enhanced the film-cooling performance even under flow oscillations. Thus, this paper reports the development of highly effective film-cooling strategies that can guide researchers aiming to mitigate the adverse effects of flow oscillations, thereby enhancing the reliability and efficiency of gas turbine engines.

Along-wind and cross-wind coupled nonlinear oscillations of wind turbine towers close to 1:1 internal resonance

An approximate analytical solution is obtained to detect the along-wind and cross-wind coupled oscillations of wind turbine towers, in the neighbourhood of a 1:1 internal resonance. The obtained formulas are general, and they are applied to the NREL 5-MW reference wind turbine. It is shown that the coupled solution is branched (by a pitchfork bifurcation) from the main uncoupled along-wind oscillation. The coupled motion is initially stable, but for increasing excitation amplitudes it becomes unstable in favour of quasi-periodic solutions, a fact that is particularly dangerous from a practical point of view and so must be avoided. The critical threshold is determined for varying excitation amplitude, and the analytical solution is checked by means of numerical simulations.

Study on the Simulation of the EMU Body Operating Main Breaker Overvoltage and High-Voltage Side Suppression Method Based on the 3D Structure

Current circuit simulation research on EMU (Electric Multiple Unit) overvoltage has focused on overvoltage amplitude distribution and the oscillation frequency of the operating main circuit breaker overvoltage. However, when studying the suppression method, the overvoltage amplitude and frequency characteristics must be considered simultaneously, and few studies have reported on the overvoltage frequency distribution. In this paper, a new simulation model of train-operational main breaker overvoltage based on the 3D structure of the EMU body was proposed, which provided guidance for the study of overvoltage suppression in operational main circuit breakers. This model simulated the transient overvoltage characteristics and distribution of the vehicle body when the train operated the main circuit breaker. The simulated results showed that the first vehicle had the highest operating overvoltage amplitude, and the fourth vehicle had the lowest. The overvoltage amplitudes of vehicles 1–4 were all above 1 kV, and the main overvoltage oscillation frequencies of each vehicle were at 0.70 MHz and 1.30 MHz. The accuracy of the distribution characteristics and spectral characteristics of the operating main breaker overvoltage was verified by static tests on trains, and the simulation was considered to be in agreement with the test. Aiming at the problem that there was no universal suppression method for the operating main breaker overvoltage, a general suppression method for the overvoltage on the high-voltage side of the EMU was studied, and an overvoltage suppression device based on a grounding inductor was proposed. The inductance parameter requirements of the suppressor were theoretically analyzed, and the validity of the design of the inductance suppressor was experimentally verified.

Generation of Hypersonic Oscillations in the Scheme of a Magnetostriction Transducer with Positive Feedback

In this paper, we consider the problem of generation of hypersonic oscillations in the circuit of a magnetostrictive converter covered by a positive feedback circuit. A “coupling parameter” that reflects the amplification of the signal in the feedback loop was introduced. The development of magnetic and elastic oscillations in time is considered. As the coupling parameter increases, three main oscillation modes are revealed: initial relaxation, stationary self-oscillations, and chaotic self-oscillations. The effect of the coupling parameter and the damping parameter of the magnetization on the time of development and the amplitude of oscillations was studied. The characteristic values of both parameters corresponding to transitions between modes are revealed. Practical recommendations for choosing system parameters for the most efficient excitation of hypersonic oscillations are given.

Comprehensive review of power system oscillations in large-scale power electronic-based renewable energy power plants

Recently, the large-scale integration of power electronic-based renewable energy power plants has changed the operation and response mechanism of the power system, resulting in several emerging oscillation issues that have seriously been threatening the system's stability. It helps us to recognize the similarities and differences among the triggering causes and formation mechanisms of oscillation scenarios. Following several typical oscillation events in the real world and the timescale decomposition method, this paper comprehensively reviews the wide-bandwidth oscillation study from the aspects of the analysis methods, possible cause, mechanism, and mitigation solution. The paper provides a perspective to classify the oscillations in the modern power systems on the basis of the oscillation frequency and the main oscillation module. This classification framework involves not only emerging oscillations in the power system with large-scale renewable energy sources integration but also includes typical oscillations in traditional power systems. It also systematically presents the relative relationship, development process, and inner influence between emerging oscillations and typical oscillations. Based on this review, the future research is suggested to focus on the relationship between different analytical methods or oscillation mechanisms, as well as the stability risk assessment of hybrid alternating current and direct current power systems.

The origin of Ti 1s XANES main edge shifts and EXAFS oscillations in the energy storage materials Ti2CT x and Ti3C2T x MXenes

A potential application of two-dimensional (2D) MXenes, such as Ti2CT x and Ti3C2T x , is energy storage devices, such as supercapacitors, batteries, and hydride electrochemical cells, where intercalation of ions between the 2D layers is considered as a charge carrier. Electrochemical cycling investigations in combination with Ti 1s x-ray absorption spectroscopy have therefore been performed with the objective to study oxidation state changes during potential variations. In some of these studies Ti3C2T x has shown main edge shifts in the Ti 1s x-ray absorption near-edge structure. Here we show that these main edge shifts originate from the Ti 4p orbital involvement in the bonding between the surface Ti and the termination species at the fcc-sites. The study further shows that the t 2g –eg crystal field splitting (10Dq) observed in the pre-edge absorption region indicate weaker Ti–C bonds in Ti2CT x and Ti3C2T x compared to TiC and the corresponding MAX phases. The results from this study provide information necessary for improved electronic modeling and subsequently a better description of the materials properties of the MXenes. In general, potential applications, where surface interactions with intercalation elements are important processes, will benefit from the new knowledge presented.