Flux Fluctuations Research Articles

The interplay between environmental and magmatic conditions in eruption style transitions within a fissure-aligned monogenetic volcanic system of Auckland, New Zealand

Recent conceptual geological frameworks of continental monogenetic volcanism highlight that the small magma volume eruptions, resulting volcanic geology and edifice architecture in such settings are sensitive to variations in external or environmental conditions. These conditions, along with fluctuations in magma flux, can change rapidly over short time frames and cause dramatic changes in eruption style. Understanding the drivers of transitions in explosive to effusive behaviour within the short timescales of eruptions at individual volcanic centres is essential to accurately assess volcanic hazards in continental monogenetic settings. Wiri Mountain Volcanic Complex is one of the largest and most complex volcanic centres in the active Auckland Volcanic Field. Despite the significant removal of much of the original volcanic deposits, present-day exposures and historical images provide a unique opportunity to examine the growth and evolution of the volcanic complex. Wiri Mountain deposited an initial basal tuff ring (covering an area of approximately 0.67 km2) by predominantly pyroclastic density currents, followed by at least two smaller tuff rings erupted through the outer flanks of the first, in a transition from phreatomagmatic to Strombolian eruptive style. A 90 m high central scoria cone was then produced within the initial tuff ring, partially capped by lava spatter, clastogenic lava flows and lava flows that mostly covered all tuff rings, the scoria cone, and the surrounding area. A high-resolution stratigraphic study of the well-exposed tuff ring to capping magmatic succession was conducted to determine the changes in eruptive style and their driving forces. The deposit architecture of Wiri Mountain can be described using three volcanic stratigraphic units: a basal unit comprised of tuff, lapilli tuff and tuff breccia deposits, a middle unit comprised of juvenile-rich transitional tuff deposits of black scoria ash and lapilli, red scoria and spatter, and a capping unit comprised of agglutinate and lava flow successions. Most volcanic materials were either fragmented and ejected at near-optimal scaled depth, or the transition between near-optimal and shallower/deeper depths. Wiri Mountain provides a striking example of a fissure eruption likely controlled by a pre-existing tectonic fault that was fed by relatively stable melt sources over a sustained period. We infer that despite eruption within a water-saturated coastal plain, the initial phreatomagmatic phase was overridden by subsequent explosive and effusive magmatic phases through the formation of an increasingly established conduit, thus allowing sustained magma flux and melt supply. The transitions, both gradual and rapid, from an initial phreatomagmatic to subsequent magmatic explosive and effusive phases fit well to the general understanding of eruption style changes over time from other larger and more complex volcanoes of Auckland, and elsewhere worldwide. Wiri Mountain showcases the fine balance between the external and internal conditions that control eruption style variations and govern the formation of complex monogenetic volcanoes.

Efficient feedback linearization control for an IPMSM of EVs based on improved firefly algorithm

The conventional direct torque control (DTC) has high torque and stator flux fluctuation that causes the stator current distortion. This paper presents an efficient control method based on the feedback-linearization direct torque control (FL-DTC) method for an interior permanent magnet synchronous motor (IPMSM) drive by using an improved firefly algorithm. The proposed approach can greatly restrain the poor performance of torque and stator flux. Thus, it is suitable for IPMSM drives in electric vehicles. First, a decoupled linear model is derived to implement the proposed efficient feedback linearization control for the IPMSM. Two phase voltages in d-q axes and two additional control inputs take shape into an isomorphism mapping with the concept of orthogonal transformation. The torque generation is related to the additional control. Second, the Hamiltonian efficient control theory combined with an improved firefly algorithm is applied to obtain an analytical solution. An efficient linearization controller is designed with a cost function considering the maximum voltage of the inverter. Finally, simulation and experiment are carried out to compare the performance of the proposed efficient FL-DTC with the improved firefly algorithm and the conventional direct torque control. The results show that the proposed control method can reduce the torque and flux ripples at a steady state and maintains a good dynamic response with the variations of speed and torque.

Glacial drainage development controls temporal and spatial fluctuations of sediment flux in Mont Blanc’s North face

Glacial drainage development controls temporal and spatial fluctuations of sediment flux in Mont Blanc’s North face

Impact of coherent mode on divertor particle and heat flux in a type I ELMy H mode plasma on EAST tokamak

The paper investigates the effect of pedestal turbulence, i.e. coherent mode (CM), on the divertor total particle flux, the particle flux distribution and heat flux distribution, and their decay lengths in the scrape-off layer (SOL) region on EAST tokamak. The experimental evidence indicates the coherence between the pedestal CM and the divertor particle flux fluctuation, which implies that the pedestal CM can affect the divertor plasma behavior. The increase of the particle flux decay length correlates with the density radial decay length (λ n,SOL) in the SOL region where the λ n,SOL is largely affected by the change of pedestal CM amplitude. A quantitative statistical analysis shows that the divertor total particle flux and in the SOL region increase with increasing amplitude of the electron density fluctuation and the electron temperature fluctuation induced by the pedestal CM. The temporal decay process of the total particle flux is interrupted by the appearance of CM at the pedestal. Meanwhile, the spatial distributions of the divertor particle flux and heat flux are enhanced when the pedestal CM appears, which are observed in both the upper single null and lower single null configurations.

Metal Lines Associated with the Lyα Forest from eBOSS Data

We investigate the metal species associated with the Lyα forest in eBOSS quasar spectra. Metal absorption lines are revealed in stacked spectra from cross-correlating the selected Lyα absorbers in the forest and the flux fluctuation field. Up to 13 metal species are identified as being associated with relatively strong Lyα absorbers (those with flux fluctuations − 1.0 < δ Lyα < − 0.6 and with a neutral hydrogen column density of ∼ 1015−16 cm−2) over the absorber redshift range of 2 < z abs < 4. The column densities of these species decrease toward higher redshift and for weaker Lyα absorbers. From modeling the column densities of various species, we find that the column density pattern suggests contributions from multiple gas components, both in the circumgalactic medium (CGM) and the intergalactic medium (IGM). While the low-ionization species (e.g., C ii, Si ii, and Mg ii) can be explained by high-density, cool gas (T ∼ 104 K) from the CGM, the high-ionization species may reside in low-density or high-temperature gas in the IGM. The measurements provide inputs for modeling the metal contamination in the Lyα forest baryon acoustic oscillation measurements. Comparisons with metal absorptions in high-resolution quasar spectra and hydrodynamic galaxy formation simulations can further elucidate the physical conditions of these Lyα absorbers.

Wall heat transfer in high-enthalpy hypersonic turbulent boundary layers

In this paper, we investigate the differences in wall heat transfer between the low- and high-enthalpy turbulent boundary layers by exploiting direct numerical simulation databases of hypersonic turbulent boundary layers at the free-stream Mach number of 4.5 and the friction Reynolds number of 800. For that purpose, we refine the integral formula of decomposing the wall heat flux proposed by Sun et al. [“A decomposition formula for the wall heat flux of a compressible boundary layer,” Adv. Aerodyn. 4, 1–13 (2022)], enabling us to scrutinize the contribution of different physical processes. Statistical results show that the mean wall heat transfer is primarily contributed by the heat conduction, the turbulent heat transfer, viscous dissipation of mean kinetic energy, and turbulent kinetic energy production. Among these processes, the contribution of the turbulent heat flux in the high-enthalpy case is 10% higher than that in the low-enthalpy case. Such discrepancy is caused by the turbulent–chemistry interaction consisting of velocity and species mass fraction fluctuations. Coherent structures in the conditionally averaged fields related to this process reveal that the sweep in the viscous sublayer and ejection in the logarithmic layer bringing the hot fluid downward and upward, respectively, significantly alter the distribution of the species mass fraction. The wall heat flux fluctuations are slightly enhanced in the high-enthalpy flows, which is ascribed to be the intensification of traveling wave packets.

Experimental performance of gas-solid countercurrent fluidized bed particle solar receiver with high-density suspension

• A particle solar receiver based on high-density countercurrent fluidized bed proposed. • The lab-scale single tube prototype for high-temperature testing established. • Particles downward stably under the fluidized state of countercurrent flow with bubbling. • Stable operation, high solids holdup and particle temperature increase achieved simultaneously. The particle-based concentrated solar power plant has attracted more attention since its higher working temperature. A particle solar receiver based on the gas-solid countercurrent fluidized bed was proposed to realize the high-density suspension and high outlet temperature of particles. To demonstrate the feasibility of this concept, a lab-scale single tube prototype was established and an electromagnetic heating furnace with an electric power of 10–40 kW el was selected to provide heating power. Based on a series of experimental testing, the results revealed that the countercurrent fluidized bed receiver could operate stably in the fluidized state as gas-solid countercurrent flow with bubbling and the average solids holdup of ∼40% was achieved, resulting in a particle temperature increase ranging between 101 and 312 °C. The global wall-to-bed heat transfer coefficient with a range of 311 to 1481 W/(m 2 ·K) was obtained by the experimental data. The effects of the input heat flux fluctuation on the countercurrent fluidized bed particle solar receiver were studied to reveal its operating characteristics. These results demonstrated that the receiver may combine the merits of the up bubbling fluidized bed and the Downer, which are long residence time, high solids holdup, lower solids back mixing, avoid slugs and excellent heat transfer. The suspension operating in the fluidized state of countercurrent flow with bubbling has the potential to open a new domain of gas-solids two-phase flow. The novel concept appears to provide a potential technique to applicate in concentrated solar power generation.

Flux roughening in spin ice with mixed ±J interactions

Spin ice presents a typical example of a classical spin liquid, where conserved magnetic fluxes emerge from microscopic spin degrees of freedom. In this paper, we investigate the effect of perturbation by magnetic charge disorder in two-dimensional spin ice. To this aim, we develop a cluster update algorithm which enables fast relaxation of magnetic charges. The efficient Monte Carlo calculation reveals a drastic change of the spin structure factor as doping magnetic charges: the pinch point, characterizing the spin ice, is gradually replaced by a diffusive peak. We derive an analytical relation connecting the flux fluctuation and the spin structure factor, and we explain the evolution of the diffusive peak in terms of the roughening of magnetic fluxes.

Energetic Electron Microinjections Observed by MMS in the Dusk Plasma Sheet and Drift Resonance Interpretation

AbstractMicroinjection phenomena, characterized by dispersive oscillations of electron fluxes at the Pc5 period and bi‐directional pitch angle anisotropy, are frequently observed by Magnetospheric Multiscale in the dusk to midnight plasma sheet. In this paper, two such events are analyzed and the features of toroidal mode drift resonance measured meanwhile are shown in detail. The prominent observation is that the fluctuations of the electron flux and the electric field have either −90° or +90° phase difference at the resonant energy, and the phase difference rises as the energy increases. We extend the present theory for drift resonance of toroidal mode wave with only the equatorial moving electrons in a dipole field to include bouncing electrons. The predicted phase differences based on the new theory are consistent well with the observations in the microinjection events. It is thus suggested that drift resonance may act as the forming mechanism for the observed microinjections.

Mechanical model for the motion of RPV internals affecting neutron flux noise

In the decade after 2000, the amplitude of the neutron flux fluctuations in German 1300 MWe class PWR built by Kraftwerk Union (KWU) increased and later decreased significantly. To further investigate the hypothesis that changes in the mechanical properties of the fuel assemblies (FAs) and subsequent stronger mechanical oscillations of fuel assemblies and other RPV internals are responsible for the observations and to find possible excitation mechanisms, a simple mechanical model is developed. It describes the dynamic motion response of the mechanically coupled system of Reactor Pressure Vessel (RPV), core barrel and a row of fuel assemblies and takes reactive fluidic forces into account. To determine the components to be considered in this model, RPV internals with an effect on neutron flux are identified in a prior step.The dynamic answers of the model to generic excitation scenarios and parametric studies reveal distinct properties of the system. They indicate 1) that fluidic near-field coupling can equalize the fuel assemblies’ reaction amplitudes within a certain region regardless of their individual stiffnesses, 2) that the fluid must be part of the oscillation, 3) that a non-periodicity might stem from a superposition and interaction of several oscillators, 4) that a local excitation source does not spread or synchronize over the whole core, 5) that a neglection or separate consideration of the RPV and core barrel may be justified in some cases. The findings altogether affirm that core-wide oscillations of the fluid flow leading to a simultaneous oscillation of individual fuel assembly groups, possibly including bidirectional effects between fluid and structure, in combination with the changes of the fuel assemblies’ mechanical properties, might be responsible for the temporary increase of neutron flux fluctuations observed in PWR built by KWU.A variant of the mechanical model was studied in a full core investigation with coupled neutron kinetics simulations. These simulations qualitatively reproduce important features of measurement data, while being overall too small in amplitude to explain the observations quantitatively, which confirmed the necessity of considering further effects.

A Finite-Time Differentiator with Application to Nuclear Reactor Inverse Period Measurement

The measurement of the growth rate, or the so-called inverse period, of a nuclear reactor is crucial for safety monitoring and control purposes. Due to the inevitable statistical fluctuation of neutron flux at low power-levels, it is difficult to precisely estimate the inverse period from the pulse counting data in the source range. Motivated by the equivalence of the measurement of inverse period and the differentiation of the logarithm of pulse count, a new differentiator is proposed, which is finite-time convergent with a bounded steady estimation error. The feasibility of this newly-built finite-time differentiator is verified by numerical simulation. Then, based on the pulse count data recorded during the startup of a test reactor, the differentiator is used to estimate the inverse period and its derivative, as well as the period and the reactivity of the reactor. The results show that the differentiator is capable of providing a satisfactory estimation of signal derivatives under strong noise.

Numerical and experimental study of the transient conjugate heat transfer and fluid flow of passive residual heat removal heat exchanger

The transient heat transfer performance of passive residual heat removal heat exchanger (PRHR HX) with C-tube bundle directly influences nuclear reactor safety under accident condition. This paper presents an investigation of transient conjugate heat transfer and fluid flow phenomena and characteristics of scaled-down single C-tube submerged in water tank under atmospheric pressure at the initial period before saturation pool boiling occurs. Empirical calculation model, experimental setup and numerical model were established separately, and both numerical and experimental studies were carried out. The distributions of tube outlet temperature and water temperatures at horizontal and vertical monitoring points in tank were obtained. Detailed comparisons of the results show good agreements. Also, the change tendencies of heat flux, heat transfer coefficients and heat load were analyzed. The fluctuation of heat flux along normalized length from tube inlet firstly increases but finally tends to keep stable with the change of temperature difference and transformation of heat transfer mechanism. The heat transfer coefficient inside tube shows a slight downtrend along normalized length from tube inlet, but the heat transfer coefficient outside tube shows obvious change in the transient process. The total heat load continuously decreases with time. Furthermore, the volume fraction of vapor is relatively low during the subcooled boiling period. The two-phase flow enhances the agitation in water tank and the fluid velocity near tube outside wall is much larger.

Multistable dynamics in a Hopfield neural network under electromagnetic radiation and dual bias currents

This paper investigates a Hopfield neural network under the simulation of external electromagnetic radiation and dual bias currents, in which the fluctuation of magnetic flux across the neuron membrane is used to emulate the influence of electromagnetic radiation. Utilizing conventional analytical methods, the basic properties of the proposed Hopfield neural network are discussed. Due to the addition of electromagnetic radiation and dual bias currents, the Hopfield neural network shows high sensitivity to system parameters and initial conditions. The proposed Hopfield neural network possesses multistability with periodic attractor, quasi-periodic attractor, chaotic attractor and transient chaotic attractor, and all of the attractors are hidden attractors because there is no equilibrium point in the system. In particular, when the neuron membrane magnetic flux is different, the system can present transient chaos with different chaotic times. More interestingly, with the change of system parameters, the proposed Hopfield neural network can exhibit parallel bifurcation behaviors. Finally, the Multisim simulation and hardware experiment results based on discrete electronic components are conducted to support the numerical ones. These results could give useful information to the study of nonlinear dynamic characteristics of the Hopfield neural network.

Wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence, part I: One-point statistics

This two-part study investigates the effects of Mach number and wall temperature on the statistics of wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence. In the first part, we focus on their one-point statistics, including the root mean square (r.m.s.), skewness factor (third-order moment), flatness factor (fourth-order moment), and their correlations. By exploiting the direct numerical simulation databases, we found that the r.m.s. of the streamwise wall shear stress and pressure, the skewness factor of all the flow quantities considered, and the flatness factor of streamwise wall shear stress monotonically vary with the friction Mach number (Mτ), while for the rest, the wall heat flux and global temperature parameters should be taken into account as well for a monotonic trend of variation. The correlation coefficients between wall shear stress, pressure, and heat flux fluctuations increase with the Mach number Mτ, suggesting the underlying interactions between dynamic and thermodynamic processes. The distributions of spectra and probability density functions indicate that the increased correlation is induced by the highly intermittent traveling wave packets among the streaky structures, as reflected by the “double-peak” feature of the spectra that gradually emerges with the increasing compressibility effects. The probability density distribution also manifests the alteration of the occurrence of extreme events caused by these structures. By accordingly decomposing the fluctuations with cutoff filtering, it is found that the root mean squares of streamwise wall shear stress and heat flux fluctuations related to the streaky structures are Mach number-independent, while those related to the traveling wave packets monotonically increase with the friction Mach number.

Wall shear stress, pressure and heat flux fluctuations in compressible wall-bounded turbulence. II. Spectra, correlation and nonlinear interactions

Wall shear stress, pressure, and heat flux are of significant importance in engineering applications. In this two-part study, we investigate the compressibility effects on wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence by exploiting direct numerical simulation databases. In Paper I, we primarily deal with the one-point statistics, whereas in this second part, we report the effects of compressibility on the frequency spectra, wavenumber-frequency spectra of these flow quantities, and the two-point cross-correlations between them. It is found that the scaling laws of the spectra at low and high frequencies are retained as those of incompressible flows, whereas the spectra intensities at mid frequencies increase with the enhancement of compressibility effects, which is identified to be related to the ever-predominating traveling wave packets. These wave packets are convected downstream at the same velocity of 0.87Ub as that of pressure fluctuations, higher than that of the streaky structures 0.65Ub (Ub the bulk velocity), and enhance the space and time cross correlation between wall shear stress, pressure, and heat flux fluctuations. By extracting the envelopes of the traveling wave packets and inspecting the time and space correlations between the envelopes and the streaky structures, we found that the emergence of traveling wave packets comes later than the streaky structures, both in time and space. Based on these observations, we provide a depiction of the physical processes regarding the formation and evolution of the traveling wave packets.

Hybrid fiber laser integrating fast and slow active media for accurate synthesis of high-energy arbitrary optical waveforms by cavity dumping

We demonstrate the possibility of the accurate direct laser synthesis of high-energy arbitrary optical waveforms by the programmable driving of partial cavity dumping in a specific continuous-wave fiber laser. To this effect we have developed an original hybrid laser configuration which integrates two different active media. The first medium, a semiconductor optical amplifier (SOA), acts as a saturated lumped preamplifier. It features a relatively fast (sub-nanosecond) gain recovery, and thus effectively suppresses the intracavity power fluctuations induced by cavity dumping. The second active medium, an erbium-doped fiber amplifier (EDFA), acts mainly as a booster amplifier. This distributed inertial amplifying medium effectively accumulates pump energy, thereby providing an enhancement of output energy upon cavity dumping. Our simple proof-of-concept laser setup has allowed the synthesis of nanosecond arbitrary optical waveforms with an energy up to 40 nJ and arbitrarily tunable repetition rate. The proposed combination of a slow (EDFA) and fast (SOA) amplifying stages prevents the laser from strong relaxation oscillations and power flux fluctuations which essentially restrict cavity dumping in conventional rare-earth-doped fiber lasers. The applied two-stage intracavity spectral filtering ensures spectral purity of a rather narrowband (⩽0.1 nm) laser output. For the purpose considered, the integrated SOA-EDFA laser configuration is preferable to a conventional architecture ‘master oscillator—power amplifier’ whose nonlinear gain can obstruct the accurate synthesis of high-energy optical waveforms.

Non-stationary flow and heat transfer in a synthetic confined jet impingement

Heat transfer in a synthetic impinging jet in the range of pulse frequencies is studied experimentally and numerically. A heat flux sensor is used to measure averaged and pulsating heat transfer at the point of flow stagnation. Mean and fluctuational structure and heat transfer in a synthetic impinging air jet are simulated using the axisymmetric non-steady-state Reynolds averaged Navier-Stokes equations and the Reynolds stress model. The measured results on the instantaneous and pulsation values of the heat flux at the stagnation point and along the radius of the target surface are obtained. The experimental spectra of heat flux fluctuations on the plate are given. The measurements and numerical predictions of the heat transfer are carried out with variation in the nozzle-to-target distance, and frequency of pulsations of the synthetic jets.

Frequency-domain models in the SP[formula omitted] approximation for neutron noise calculations

Simulations of the neutron flux fluctuations, known as neutron noise, can be performed by means of the spherical harmonics equations (SPN) approximation with higher accuracy than with the diffusion equation. In this sense, one can solve these equations in the time-domain or in the frequency-domain. This last approach permits solving the neutron noise without performing complete time-dependent simulations for monochromatic perturbations. This work presents two formulations of the SPN equations in the frequency domain, that are obtained by using different treatments of the time derivatives of the field moments. The methodology is verified with several neutron noise problems where the numerical results are compared with the time-domain computations of FEMFFUSION code. The C5G7 noise benchmark compares both SPN formulations, showing the applicability of the diffusive SPN approximation.

Effect of permeability heterogeneity on the dissolution process during carbon dioxide sequestration in saline aquifers: two-and three-dimensional structures

The convection–diffusion process of carbon dioxide (CO2) dissolution in a saline reservoir is investigated to shed light on the effects of the permeability heterogeneity. Using sequential Gaussian simulation method, random permeability fields in two and three-dimension (2D and 3D) structures are generated. Quantitative (average amount of the dissolved CO2 and dissolution flux) and qualitative (pattern of the dissolved CO2 and velocity streamlines) measurements are used to investigate the results. A 3D structure shows a slightly higher dissolution flux than a 2D structure in the homogeneous condition. Results in the random permeability fields in 2D indicates an increase in the standard deviation of the permeability nodes enhances the dissolution efficiency, fluctuations in CO2 dissolution flux, separation between the different realizations from the same input parameters, and tendency toward more jagged convective fingers’ shape. Furthermore, the distance between the permeability nodes increases the convective fingers’ dissolution efficiency and jagged structure. The degree of freedom in 3D structures results in a higher chance of escaping from the low permeability zones and reduces the interactions between convective fingers in 3D systems. With the same variance and correlation length between permeability nodes, connectivity between high permeable zones in 3D cases are less than that of 2D cases; therefore, 2D realizations overestimate the dissolution flux of real heterogeneous 3D structures, which should be considered carefully.Article HighlightsCO2 sequestration in two and three dimensional heterogeneous saline aquifers are investigated.3D structures in homogeneous conditions show higher dissolution than 2D structures.2D realizations overestimates the dissolution flux over real heterogeneous 3D reservoirs.

Structural design and analysis of a solar thermochemical reactor partially filled with phase change material based on shape optimization

Solar parabolic trough thermochemical reactor (SPTR) exhibits poor comprehensive performance due to the intermittent solar influence. In this work, a Bézier parametric curve-based shape optimization model is presented for designing the methanol steam reforming reaction-based SPTR filled with metal foam enhanced phase change material. Combined with the SPTR multi-field model, the catalytic tube shape in SPTR is well optimized by Globally Convergent Method of Moving Asymptotes, and finally an optimized rose-shaped catalytic tube with maximum methanol conversion efficiency is obtained. The SPTR performances of unoptimized original, customized and optimized tubes under constant and fluctuating solar flux are also analyzed and discussed in detail. The results show that the optimized tube and the small inlet cone-frustum tube have a better overall performance than the original tube, with about 67% reduction in catalyst usage. Especially for the optimized tube with optimal steady performance, methanol conversion efficiency, solar thermochemical efficiency and hydrogen yield are relatively improved by 4.3%, 8.2% and 2.4%, respectively, as compared to the original tube. The transient performance of the optimized tube also shows a relative increase of 8.6% and 8.2% in average conversion and hydrogen yield, and the reaction is more stable and safe in flux fluctuations.