Flux Oscillations Research Articles

Mixed convection–radiation in lid-driven cavities with nanofluids and time-dependent heat-generating body

The cooling process of electronic devices having heat-generating elements is a major challenge allowing to develop electronics industry. Therefore, a creation of novel cooling techniques is an important task that can be solved numerically taking into account the multiparametric character of this problem. The mixed convection heat transfer combined with thermal radiation in a lid-driven cavity filled with an alumina–water nanofluid under the effect of sinusoidal time-dependent heat-generating solid element is studied numerically. The partial differential equations formulated in stream function–vorticity variables are solved by the finite difference method. Effects of the Rayleigh number, Reynolds number, thermal radiation parameter, heater location, volumetric heat flux oscillation frequency and nanoparticles volume fraction on liquid flow and heat transfer are analyzed. It has been found that an addition of nanoparticles leads to reduction of the heater temperature, while convective flow rate decreases also.

BeiDa Imaging Electron Spectrometer observation of multi-period electron flux modulation caused by localized ultra-low-frequency waves

Abstract. We present multi-period modulation of energetic electron flux observed by the BeiDa Imaging Electron Spectrometer (BD-IES) on board a Chinese navigation satellite on 13 October 2015. Electron flux oscillations were observed at a dominant period of ∼190 s in consecutive energy channels from ∼50 to ∼200 keV. Interestingly, flux modulations at a secondary period of ∼400 s were also unambiguously observed. The oscillating signals at different energy channels were observed in sequence, with a time delay of up to ∼900 s. This time delay far exceeds the oscillating periods, by which we speculate that the modulations were caused by localized ultra-low-frequency (ULF) waves. To verify the wave–particle interaction scenario, we revisit the classic drift-resonance theory. We adopt the calculation method therein to derive the electron energy change in a multi-period ULF wave field. Then, based on the modeled energy change, we construct the flux variations to be observed by a virtual spacecraft. The predicted particle signatures well agree with the BD-IES observations. We demonstrate that the particle energy change might be underestimated in the conventional theories, as the Betatron acceleration induced by the curl of the wave electric field was often omitted. In addition, we show that azimuthally localized waves would notably extend the energy width of the resonance peak, whereas the drift-resonance interaction is only efficient for particles at the resonant energy in the original theory.

Simulations of Electron Flux Oscillations as Observed by MagEIS in Response to Broadband ULF Waves

AbstractCoherent electron flux oscillations of hundreds of keV are often observed by the Van Allen Probes in the magnetosphere during quiet times in association with ultralow frequency (ULF) waves. They are observed in the form of periodic flux fluctuations, with a drift frequency that is energy dependent, but are not associated with drift echoes following storm‐ or substorm‐related energetic particle injections. Instead, they are associated with the resonant interaction of electrons with ULF waves and are an indication of ongoing electron radial diffusion. To investigate details of such flux oscillations, particle‐tracing simulations are conducted under the effect of realistic, broadband ULF electric and consistent magnetic fluctuations. Virtual detectors are simulated along spacecraft orbits and the results are compared to measurements. Through a parametric study, it is found that the width of electron energy channels is a critical parameter affecting the observed amplitude of flux oscillations, with narrower energy channel widths enabling the observation of higher‐amplitude flux oscillations; this potentially explains why such features were not observed regularly before the Van Allen Probes era, as previous spacecraft generally had lower energy resolution, which only enabled the observation of large‐amplitude drift echoes following a storm or substorm. Results are confirmed using the Magnetic Electron Ion Spectrometer (MagEIS) ultrahigh energy resolution data. Energy width effects are quantified through a parametric simulation study that matches flux oscillation observations during a period that is characterized by extremely quiet conditions, where the Van Allen Probes observed flux oscillations over multiple days.

Experimental study on liquid-vapor two-phase pressure drop of pulsating flow in an evaporator

Pulsating liquid-vapor, two-phase, flows, characterized primarily by periodic oscillations in the mass flux, can occur naturally due to thermal-hydraulic conditions imposed on a flow, and they can be created by design in pursuit of improved thermal performance in heat pumping, air-conditioning, and refrigeration systems. Such flows are not fully understood, especially if the flow oscillations give rise to significant changes in liquid-vapor interface morphology (changes in two-phase flow regime). In this work, the pressure drop of R134a liquid-vapor, two-phase pulsating flows is measured under varying pulsation periods (2s to 24s), mass fluxes (100 to 200 kg•m−2•s−1), and inlet/outlet vapor qualities (0.1-0.4/0.6-0.8). The flow regimes and liquid-vapor interface evolution are also observed. Experimental results show the pressure drop increases sharply at the start of flow and drops quickly as the flow ceases in a pulsation cycle. The peak of pressure drop as the pulsation starts is larger at a higher inlet vapor quality, but the valley as flow ceases is lower. With an increasing mass flux, the rate of pressure drop increase is closely related to the pulsation period and vapor quality. A higher rate of increasing pressure drop is observed in flows with a shorter pulsation period and higher inlet vapor quality. Fluctuations in the pressure drop are also observed, and are more pronounces in low mass flux and low inlet vapor quality flows, a result attributed to fluctuations in the liquid-vapor interface and changes in two-phase flow regimes.

Drift echoes and flux oscillations: A signature of prompt and diffusive changes in the radiation belts

Prompt creation of new multi-MeV electron and proton radiation belts in the normally benign slot region between inner and outer Van Allen zones was observed by the short-lived Combined Release and Radiation Effects Satellite (CRRES) mission almost 30 years ago. Flux oscillations at lower energies coincident with ULF wave measurements and consistent with drift resonance were also a feature of the CRRES observations. The next mission designed to study the radiation belts in similar detail from a near equatorial plane orbit was the Van Allen Probes launched in August 2012. These twin spacecraft in similar orbits observed many milder events which produce drift echoes indicative of injection over a limited range of longitude on less than an electron drift time. Related substorm injection drift echoes were also observed by Van Allen Probes with much higher energy resolution than previously. Along with other flux oscillations associated with Ultra Low Frequency (ULF) waves, these events form a class which contributes to the overall radial transport and energization of radiation belt electrons as well as protons. This paper focuses on the greatly improved measurements from the Van Allen Probes relative to the CRRES era. These measurements illuminate our understanding of the importance of flux oscillations as a signature of radial transport relative to other energization and loss processes affecting the radiation belts.

Rapid Polarized Emission Variability of Blazar S5 0716+714 in Optical Range

The results of polarimetric observations of blazar S5 0716+714, carried out with the 6-meter BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences, are presented. The polarized emission microvariability of blazar S5 0716+714 was observed in the R band with an initial time resolution of 3–5 seconds. Exposures were integrated at 1-minute intervals with a total observation time of several hours over two nights. Microvariability of the polarization degree with amplitude of 3–4% was detected during 7–11 minutes and flux oscillations within 9–10% during 25 minutes. A partial synchronism of the flux oscillations and the polarization degree of this blazar was found. Possible interpretations of the detected microvariability are briefly described.

Experimental study on bubble characteristics of time periodic subcooled flow boiling in annular ducts due to wall heat flux oscillation

The significant effects of imposed wall heat flux oscillation on bubble characteristics of subcooled flow boiling with R-134a in horizontal annular ducts are experimentally performed in details. Specific emphasis has been applied on the bubble dynamics of the time periodic subcooled flow boiling characteristics influenced by the mean levels, amplitudes, and periods of the wall heat flux oscillations. The obtained experimental results have disclosed that the bubble parameters like the departure diameter, frequency and active nucleation site density, do oscillate periodically in time and are at the same frequency as the imposed wall heat flux oscillation. Nevertheless, these parameters also increase at long periods, high amplitudes of imposed heat flux oscillation or high values of mean imposed heat flux. Furthermore, the bubble size reduced after the time lag when the imposed heat flux decreases with time.

Neutronic Simulation of Fuel Assembly Vibrations in a Nuclear Reactor

The mechanical vibrations of core internals such as fuel assemblies (FAs) cause oscillations in the neutron flux that require in some circumstances nuclear power plants to operate at a reduced power level. This work simulates and analyzes the changes of the neutron flux throughout a nuclear core due to the oscillation of a single FA without considering thermal-hydraulic feedback. The amplitude of the FA vibration is bounded to a few millimeters, and this implies the use of fine meshes and accurate numerical solvers due to the different scales of the problem. The results of the simulations show a main oscillation of the neutron flux with the same frequency as the FA vibration along with other harmonics at multiples of the vibration frequency much smaller in amplitude. Also, this work compares time domain analysis and frequency domain analysis of the mechanical vibrations. Numerical results show a close match between these two approaches for the fundamental frequency.

Asymmetrical LVRT of DFIG incorporating feed‐forward transient current control and controllable resistive‐type fault current limiter

AbstractDuring the low‐voltage ride‐through (LVRT) of the doubly fed induction generator (DFIG), the change rate of the stator flux is introduced as the feed‐forward compensation (FFC) to prevent the stator flux oscillation from affecting the rotor current control. The dual‐sequence FFC is proposed for the asymmetrical LVRT of the DFIG. To reduce the rotor voltage increase due to the FFC, the controllable resistive‐type fault current limiter (CRTFCL) is implemented at the stator side to improve damping to the stator flux oscillation. The resistance of the CRTFCL is controlled based on the ramp function to relieve oscillations resulted from the switch‐out of the CRTFCL. With the dual‐sequence FFC and CRTFCL applied to the asymmetrical LVRT, analytical expressions of the stator current, rotor current, and rotor voltage are derived. The accuracy of the analytical expressions and the improved LVRT performance with the dual‐sequence FFC and the CRTFCL are verified based on the time‐domain simulations. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Torque ripples improvement of direct torque controlled five-phase induction motor drive using backstepping control

The paper proposes Direct Torque Control (DTC) of a five-phase induction motor drive with reduced torque ripple. The method presented here is the DTC Backstepping based on the classic DTC working with a constant switching frequency of the inverter. Another remarkable aspect is the complexity of the method proposed, both in the control unit of the inverter and in the number of correctors necessary for the control of the torque. The selection table and hysteresis have been eliminated. This method significantly improves the torque and flux oscillations and improves the dynamics of the drive by making it less sensitive to load torque disturbances. The proposed method is developed and designed using Matlab/SIMULINK to show the eectiveness and performances of the DTC-Backstepping.

Proposed controller and stability analysis for DFIG to suppress stator flux oscillations during autonomous operation

Doubly fed induction generator (DFIG) driven wind turbines experience poorly damped stator flux oscillations at frequencies around the fundamental, limited low voltage ride through capability and output power fluctuation. In this study, a proposed controller to mitigate stator flux oscillations of DFIG is proposed during autonomous operation mode. In addition, super-capacitors based energy storage is integrated to manipulate wind speed vagaries and contribute to load demand. A 53rd-order small-signal analytical model and eigenvalues study is carried out to optimally determine control system parameters and pinpoint latent system dynamics and stability margins under various operation conditions. Also, a detailed non-linear system is modelled and simulated in Matlab/Simulink environment to assure small-signal model results. The results obtained confirm the accuracy of the analytical model and explore the significance of the proposed damping controller to mitigate stator flux oscillations even during contingencies.

Pitch Angle Structures of Ring Current Ions Induced by Evolving Poloidal Ultra‐Low Frequency Waves

AbstractUltra‐low frequency (ULF) waves can effectively accelerate ring current ions through drift‐bounce resonance. Conventional theories have been developed to understand this resonance since the dawn of the space age, which usually assume ULF waves are quasi‐steady in time. However, ULF waves would experience growth and damping stages in the real magnetosphere, which requires an adjustment of the conventional theories. Here, we attempt to reveal how ions respond to evolving ULF waves. By analyzing an event observed by Van Allen Probe A and reproducing the observations via simulation, we find the phase shift of ion flux oscillation across resonant pitch angles varies with time, when wave growth and damping cannot be neglected. As a result, the inclination angle of stripes in pitch angle spectrogram varies with time, causing “fishbone‐like” pitch angle structures. One should take into account such ion behaviors when considering the role of drift‐bounce resonance in the real magnetosphere.

Selective Acceleration of O+ by Drift‐Bounce Resonance in the Earth's Magnetosphere: MMS Observations

AbstractWe studied O+ drift‐bounce resonance using Magnetospheric Multiscale (MMS) data. A case study of an event on 17 February 2016 shows that O+ flux oscillations at ~10–30 keV occurred at MLT ~ 5 hr and L ~ 8–9 during a storm recovery phase. These flux oscillations were accompanied by a toroidal Pc5 wave and a high‐speed solar wind (~550 km/s). The azimuthal wave number (m‐number) of this Pc5 wave was found to be approximately −2. The O+/H+ flux ratio was enhanced at ~10–30 keV corresponding to the O+ flux oscillations without any clear variations of H+ fluxes, indicating the selective acceleration of O+ ions by the drift‐bounce resonance. A search for similar events in the time period from September 2015 to March 2017 yielded 12 events. These events were mainly observed in the dawn to the afternoon region at L ~ 7–12 when the solar wind speed is high, and all of them were simultaneously identified on the ground, indicating low m‐number. Correlation analysis revealed that the O+/H+ energy density ratio has the highest correlation coefficient with peak power of the electric field in the azimuthal component (Ea). This statistical result supports the selective acceleration of O+ due to the N = 2 drift‐bounce resonance.

Tidal Variations in the Ionosphere and Mesosphere Over Eastern China During 2014

AbstractThe sources for the variabilities of the ionospheric tidal oscillations are complex and not totally understood yet. In this paper, both the seasonal and short‐term variations of the ionospheric diurnal and semidiurnal oscillations over a meteor radar chain (Wuhan [114.4°E, 30.6°N], Beijing [116.5°E, 39.9°N], and Mohe [121.1°E, 50.1°N]) are investigated with critical frequency of the F2 layer (foF2) observations during 2014. The collocated mesospheric horizontal winds, the daily 10.7‐cm solar flux, and the daily Ap index are also examined to determine the sources of the ionospheric tidal variabilities from the lower atmosphere, solar and geomagnetic activities, respectively. We found that the seasonal variations of the ionospheric tidal oscillations at the three sites are consistent, which are not in‐phase with the seasonal variations of mesospheric tides at the same location. This indicates that mesospheric tides are unlikely the main cause of the seasonal variations of the ionospheric tides. Besides, the ionospheric tidal oscillations show clear periodical signals, which could correspond well with those in the local mesospheric wind or Ap index. Nevertheless, the periodical solar flux oscillation does not induce similar short‐term ionospheric tidal variabilities. We also found that the influence of geomagnetic activities on the ionospheric tidal variabilities is with either enhancement or depression patterns during geomagnetic storms.

The Physics of Changing Tectonic Regimes: Implications for the Temporal Evolution of Mantle Convection and the Thermal History of Venus

AbstractGiven similar sizes and compositions, Venus invites comparisons with Earth. While Earth convects in the plate tectonic regime, Venus' current and past tectonic states remain uncertain. Venus' impact crater distribution has led to competing models for its tectonic state, steady (e.g., stagnant lid) or catastrophic (e.g., episodic lid). Terrestrial geochemical evidence and geodynamic models suggest that planets may transition between tectonic states over time. Transitions can be triggered by increasing yield stress due to loss of water or by increasing surface temperature. Here, we revisit the classic endmember models of Venusian tectonics by modeling the transition from stable mobile lid convection into a stable stagnant lid in 3D spherical geometry. Transitions between states are governed by both regional and global scale instabilities, resulting in oscillations in heat flux, velocity, and magma production over Gyr time scales. The thermal and tectonic evolution of a convectively transitioning planet is neither catastrophic nor steady. Volcanism and tectonic yielding may be non‐global in nature. At any given time, portions of the planetary surface may reflect apparently different styles of convection, with some regions being highly active and other regions being sluggish and inactive. For Venus, the implications are that global melt production and resurfacing are a natural consequence of lid‐state evolution, without needing ad hoc resurfacing mechanisms which rely on a single governing lid‐state. Geologic evidence suggests that the transition from mobile lid to stagnant lid is still on‐going. Venus may have lost liquid surface water in the last third of its history.

Analytical LVRT analysis of doubly fed induction generator with MPC‐basedDSCC/DRCC

Under the low-voltage ride-through (LVRT) of the doubly fed induction generator, extra efforts are required for the model predictive control (MPC)-based direct stator/rotor current controls (DSCC/DRCC) against the stator flux oscillation. Constant stator/rotor currents are maintained at the cost of rotor/stator current oscillations, respectively, due to stator flux oscillation. Necessity to incorporate the stator flux transient in the MPC-based DSCC/DRCC under the LVRT is analysed based on tracking effects of the current references. To quantify LVRT effect with the MPC-based DSCC/DRCC, analytical expressions of the stator current, rotor current and rotor voltage are proposed, whose accuracy is validated based on comparison with the time-domain simulations. On the basis of analytical study, advantages and disadvantages of the DSCC/DRCC are analysed considering impacts of the current oscillations on the security of the rotor-side converter. With the DRCC, rotor voltage increase is suppressed with damping to the stator flux oscillation and rotor current constraint is satisfied without decreasing stator output power; thus, the LVRT effect is desirable compared with the DSCC. The MPC-based DSCC/DRCC is extended to LVRT under the asymmetrical fault to realise effective control to both the positive and negative sequence stator/rotor currents.

Active Magneto-Optical Control of Near-Field Radiative Heat Transfer between Graphene Sheets

Graphene provides an ideal platform for active modulation of near-field radiative heat transfer. While much research here focuses on tuning the chemical potential by doping or gating, this work proposes using a magnetic field to control near-field heat flux between graphene sheets. The authors predict giant thermal magnetoresistance and negative thermal magnetoresistance of the near-field radiative heat flux for different Fermi energies, and Shubnikov-de-Haas-like oscillations in the spectral heat flux. In a static magnetic field, coupling of excited magnetoplasmon-polariton modes of graphene boosts radiative heat transfer beyond the blackbody limit by several orders of magnitude.

An Improved Nonlinear Flux Observer Based Sensorless FOC IM Drive With Adaptive Predictive Current Control

This paper presents a novel adaptive predictive current controller (PCC) with improved flux estimator for a sensorless direct field oriented control (DFOC) induction motor (IM) drive. Classical PCC performance depends on the stator resistance of the motor which varies with temperature. In the proposed method, a discrete stator resistance estimator is developed to increase the robustness of the system toward stator resistance variations. An improved nonlinear discrete stator flux estimator is also developed to replace the pure integrator which reduces the initial value and dc offset problems. Sensorless operation of IM drive has advantages of reduced overall cost and increased reliability of motor operation. Dynamic and steady-state performances of the IM under different operating conditions are analyzed in MATLAB/Simulink as well as experimentally using dSPACE1104 controller and have been compared with conventional PCC. The simulation results of the classical and proposed methods are also validated experimentally. Smooth starting, reduced oscillations in speed and flux at low speed operation, fast speed and torque dynamics, robust operation of drive against load and variation in stator resistance, fast and smooth speed reversal are the main advantages of the proposed method over classical PCC-based DFOC IM drive.

Diurnal Rhythms Spatially and Temporally Organize Autophagy

SUMMARYCircadian rhythms are a hallmark of physiology, but how such daily rhythms organize cellular catabolism is poorly understood. Here, we used proteomics to map daily oscillations in autophagic flux in mouse liver and related these rhythms to proteasome activity. We also explored how systemic inflammation affects the temporal structure of autophagy. Our data identified a globally harmonized rhythm for basal macroautophagy, chaperone-mediated autophagy, and proteasomal activity, which concentrates liver proteolysis during the daytime. Basal autophagy rhythms could be resolved into two antiphase clusters that were distinguished by the subcellular location of targeted proteins. Inflammation induced by lipopolysaccharide reprogrammed autophagic flux away from a temporal pattern that favors cytosolic targets and toward the turnover of mitochondrial targets. Our data detail how daily biological rhythms connect the temporal, spatial, and metabolic aspects of protein catabolism.

Meshless local Petrov–Galerkin solution of the neutron transport equation with streamline-upwind Petrov–Galerkin stabilization

The meshless local Petrov–Galerkin (MLPG) method is applied to the steady-state and k-eigenvalue neutron transport equations, which are discretized in energy using the multigroup approximation and in angle using the discrete ordinates approximation. To prevent oscillations in the neutron flux, the MLPG transport equation is stabilized by the streamline upwind Petrov–Galerkin (SUPG) method. Global neutron conservation is enforced by using moving least squares basis and weight functions and appropriate SUPG parameters. The cross sections in the transport equation are approximated in accordance with global particle balance and without constraint on their spatial dependence or the location of the basis and weight functions. The equations for the strong-form meshless collocation approach are derived for comparison to the MLPG equations. The method of manufactured solutions is used to verify the resulting MLPG method in one, two and three dimensions. Results for realistic problems, including two-dimensional pincells, a reflected ellipsoid and a three-dimensional problem with voids, are verified by comparison to Monte Carlo simulations.