Large-amplitude Fluctuations Research Articles

Evolution of the meridional shift of the subtropical and subpolar westerly jet over the Southern Hemisphere during the past 21,000 years

Deciphering the variations in the subtropical westerly jet (STJ) and the subpolar westerly jet (SPJ) over the Southern Hemisphere in Earth’s history will help us to better understand how the westerlies may respond to various external forcings in the future. Using Simulation of Transient Climate Evolution over the past 21,000 years simulations, we demonstrate that the STJ migrated equatorward in the Last Glacial Maximum relative to the present, which followed by abrupt poleward, equatorward, and poleward shifts during the Heinrich Stadial I, Antarctic Cold Reversal, and Younger Dryas, respectively. These large-amplitude fluctuations are controlled by changes in meltwater fluxes and polar ice-sheet topography. During the Holocene, the STJ shows no obvious long-term trend, due to the offsetting effect between increased orbital insolation in austral summer and decreased Antarctic ice-sheet topography. The meridional shift of the SPJ shows an in-phase change with the STJ during the past 21 kyr. In terms of the dynamic mechanisms, the meridional shift of the STJ and SPJ is induced by changes in the meridional temperature gradient and the maximum eddy growth rate, respectively; and is also linked to changes in the mean meridional circulations (that is, the Hadley and Ferrel cells). The modeled meridional shift of the southern westerlies is supported by the results from the Paleoclimate Modelling Intercomparison Project Phase Ⅲ and is consistent with several reconstructions, but it is difficult to constrain robustly by geological evidence as the reconstructed westerlies vary across sites and proxies. Nevertheless, our study provides a possible scenario for the evolution of the southern westerlies during the past 21 kyr and identifies the dominant forcing and the associated mechanisms, which may advance our knowledge on the future behavior of the southern westerlies.

Experimental investigation of combustion instabilities of a mesoscale multinozzle array in a lean-premixed combustor

Self-excited combustion instabilities in a mesoscale multinozzle array, also referred to as a micromixer-type injector, have been experimentally investigated in a lean-premixed tunable combustor operating with preheated methane and air. The injector assembly consists of sixty identical swirl injectors of 6.5 mm inner diameter, which are evenly distributed across the combustor dump plane. Their flow paths are divided into two groups – inner and outer stages – to form radially stratified reactant stoichiometry for the control of self-excited instabilities. OH PLIF measurements of stable flames reveal that the presence of radial staging has a remarkable influence on stabilization mechanisms, reactant jet penetration/merging, and interactions between adjacent flame fronts. In an inner enrichment case, two outer (leaner) streams merge into a single jet structure, whereas the inner (richer) reactant jets penetrate far downstream without noticeable interactions between neighboring flames. The constructed stability map in the 〈ϕi, ϕo〉 domain indicates that strong self-excited instabilities occur under even split and outer enrichment conditions at relatively high global equivalence ratios. This is attributed to large-scale flame surface deformation in the streamwise direction, as manifested by vigorous detachment/attachment movements. The use of the inner fuel staging method was found, however, to limit the growth of large-amplitude heat release rate fluctuations, because the center flames are securely anchored during the whole period of oscillation, giving rise to a moderate lateral motion. We demonstrate that the collective motion of sixty flames – rather than the individual local flame dynamics – play a central role in the development of limit cycle oscillations. This suggests that the distribution pattern of the injector array, in combination with the radial fuel staging scheme, is the key to the control of the instabilities.

Intermittent fluctuations due to Lorentzian pulses in turbulent thermal convection

Turbulent motions due to flux-driven thermal convection are investigated by numerical simulations and stochastic modeling. Tilting of convection cells leads to the formation of sheared flows and quasi-periodic relaxation oscillations for the energy integrals far from the threshold for linear instability. The probability density function for the temperature and radial velocity fluctuations in the fluid layer changes from a normal distribution at the onset of turbulence to a distribution with an exponential tail for large fluctuation amplitudes for strongly driven systems. The frequency power spectral density has an exponential shape, which is a signature of deterministic chaos. By use of a novel deconvolution method, this is shown to result from the presence of Lorentzian pulses in the underlying time series, demonstrating that exponential frequency spectra can also persist in turbulent flow regimes.

Temporal changes in food web structure in semiarid waterholes

ABSTRACT High seasonality of semiarid aquatic ecosystems may be due to large-amplitude water level fluctuations. The resulting habitat changes impact aquatic communities and food web structures as a result of species replacement or loss. To study the impact of water level fluctuations on planktonic food web structure, we compared food web descriptors in contrasting seasons. We examined the planktonic food webs associated with semiarid natural and artificial waterholes sampled in Hwange National Pak, Zimbabwe, during the hot dry (HDS), wet (WS), and cold dry seasons (CDS). Natural waterholes are filled by rainwater, while underground water is pumped into artificial waterholes to provide drinking water to wildlife during the dry season. We computed the descriptors of 66 food webs. WS food webs tended to be more complex than those of HDS and CDS; the percentage of basal species was lower, whereas the food web connectance, percentage of intermediate species, omnivory index, mean trophic level, and maximum trophic height of species were higher. Pumping water to maintain pans in the dry season had no effect on food web properties. The drying up of waterholes in semiarid environments simplifies planktonic food webs. The dominance of small rotifers may explain the observed low omnivory index and food web connectance in HDS compared to those observed in other studies.

Route to Chaos in an Electrostatic Ion Cyclotron with Higher-Order Source Term

Many conventional experimental analyses have helped us to understand most of the properties of plasma wave interactions and its behavior; nonlinear dynamics has helped us to enhance our analysis. In this paper we have tried to analyze a plasma jerk system for an ion cyclotron plasma oscillation in a magnetic field by including a higher-order source term caused by large-amplitude fluctuations. The multistability and antimonotonicity properties of the plasma system are discussed which were not explored earlier. We have also shown that the large-amplitude limit cycle in the original jerk system model of plasma is now reduced to a much smaller chaotic cycle for the new proposed system with higher-order source term.

Dynamic behavior of intermittent combustion oscillations in a model rocket engine combustor

We experimentally study the dynamic behavior of intermittent combustion oscillations by time series analysis in terms of nonlinear forecasting, symbolic dynamics, and statistical complexity, including the detection of the change in dynamical state based on symbolic dynamics and graph networks. We observe sudden switching back and forth between irregular small-amplitude and regular large-amplitude pressure fluctuations. The nonlinear local prediction method, permutation spectrum test, and the Rényi complexity–entropy curve clearly identify the possible presence of chaotic dynamics in small-amplitude pressure fluctuations during intermittent combustion oscillations. The network entropy in ordinal partition transition networks allows us to capture a significant change in dynamical state switching between chaotic oscillations and noisy limit cycle oscillations.

High-Performance Beamforming Network Based on Si-Photonics Phase Shifters for Wideband Communications and Radar Applications

The evolution of microwave and mm-wave systems for communication and remote sensing is heading towards a massive deployment of phased-array antennas with beamforming capabilities. 5G wireless access networks are expected to employ beamforming to optimize resources allocation, whereas radars will benefit from precise and fast-scanning antennas. Moreover, in the next future, those systems are expected to widely employ photonic technologies for signal generation, acquisition, and distribution. In this scenario, photonics can allow for the development of high-performance beamforming networks, which can easily be integrated in photonics-assisted RF systems, combining broad operational bandwidth, low signal distortion, and high pointing precision with unprecedented flexibility. In this paper, a phase shift-based beamforming network for microwave applications is presented and its performance analyzed. The Si-photonics integrated phase shifters exhibit large phase shift (>360°) and limited amplitude fluctuations (∼2.5 dB). The packaged beamforming network elements show extremely low signal distortion, low steering error (<3°), and fast switching time (<5 ns), demonstrated by the transmission with beam-steering of a 2-Gb/s communication signal and of a 1-GHz chirped radar waveform. A comparison with commercial RF devices shows that the photonics-assisted beamforming network is well-suited for high bit-rate 5G communications and for wideband radar applications.

Stochastic Turbulent Acceleration in a Fractal Environment

Abstract We analyze the stochastic acceleration of particles inside a fully developed turbulent plasma. It is well known that large-amplitude magnetic fluctuations and coherent structures in such an environment obey a fractal scaling, and our specific aim is to study for the first time the effects of the fractality of these environments on stochastic acceleration. We have shown that an injected Maxwellian energy distribution is heated and forms a high-energy tail in a very short time. Using standard parameters for the low solar corona, the injected Maxwellian distribution of electrons gets heated from the initial 100 eV to 10 KeV, and the power-law index of the high-energy tail is about −2.3. The high-energy tail starts around 100 keV, and reaches 10 MeV. The index of the power-law tail depends on the system size, and it is in good agreement with observed values for realistic system sizes. The heating and acceleration process is very fast (∼2 s). The reason why the acceleration time is so short is that the particles are trapped within small-scale parts of the fractal environment, and their scattering mean free path reduces drastically. The presence of small-scale activity also easily pulls particles from the thermal pool, so there is no need for a seed population. The mean square displacement in space and energy is superdiffusive for the high-energy particles.

Nuclear and magnetic small-angle neutron scattering in self-organizing nanostructured Fe1−xCrx alloys

Small-angle neutron scattering (SANS) in self-organizing nanostructured Fe1-xCrx (x = 0.21, 0.43 and 0.53) alloys has been studied. The relation between the nuclear and magnetic contributions depends on the alloy composition and the extent of self-organizing due to the demixing of Fe and Cr. When the demixing is pronounced with a large concentration fluctuation amplitude, the two scattering contributions are very similar, whereas when the concentration fluctuation amplitude is very small, the magnetic contribution is negligible compared to the nuclear contribution. The relation between the scattering signals is critical when assessing the demixing in Fe1-xCrx alloys by SANS, which has been mostly ignored in prior works in the literature.

Fluid–structure interaction in Z-shaped pipe with different supports

Fluid–structure interaction (FSI) has a strong relation with layout of fluid delivery system. FSI is liable to cause local damage. Thus, FSI analysis is necessary in many cases, especially for flexible pipe systems. FSI modeling consists of eight governing equations and then completely solved via the finite volume method (FVM). Friction, Poisson and joint couplings were discussed in detail to reveal the influence of a Z-shaped pipe with different supports and elbows on FSI. After the feasibility of solving FSI by FVM was verified, the different effects of free, fixed and elastic supports on FSI in the commonly used and simplified Z-shaped pipe were further analyzed. Results indicated that different support stiffness lead to various FSI responses. If coupling occurs at the elbow and less support is considered, then the pipe has a relatively large amplitude and complex pressure fluctuation.

Radial Evolution of Magnetic Field Fluctuations in an Interplanetary Coronal Mass Ejection Sheath

Abstract The sheaths of compressed solar wind that precede interplanetary coronal mass ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations. As ICMEs propagate radially from the Sun, the properties of these fluctuations may evolve significantly. We have analyzed magnetic field fluctuations in an ICME sheath observed by MESSENGER at 0.47 au and subsequently by STEREO-B at 1.08 au while the spacecraft were close to radial alignment. Radial changes in fluctuation amplitude, compressibility, inertial-range spectral slope, permutation entropy, Jensen–Shannon complexity, and planar structuring are characterized. These changes are discussed in relation to the evolving turbulent properties of the upstream solar wind, the shock bounding the front of the sheath changing from a quasi-parallel to quasi-perpendicular geometry, and the development of complex structures in the sheath plasma.

Continuum electromagnetic gyrokinetic simulations of turbulence in the tokamak scrape-off layer and laboratory devices

We present algorithms and results from Gkeyll, a full-f continuum, electromagnetic gyrokinetic code, designed to study turbulence in the edge region of fusion devices. The edge is computationally very challenging, requiring robust algorithms that can handle large-amplitude fluctuations and stable interactions with plasma sheaths. We present an energy-conserving high-order discontinuous Galerkin scheme that solves gyrokinetic equations in Hamiltonian form. Efficiency is improved by a careful choice of basis functions and automatically generated computation kernels. Previous verification tests were performed in the straight-field-line large plasma device [Shi et al., J. Plasma Phys. 83, 905830304 (2017)] and the Texas Helimak, a simple magnetized torus [Bernard et al., Phys. Plasmas 26, 042301 (2019)], including the effect of end-plate biasing on turbulence. Results for the scrape-off layer for NSTX parameters with a model helical magnetic geometry with bad curvature have been obtained [Shi et al., Phys. Plasmas 26, 012307 (2019)]. In this paper, we present algorithms for the two formulations of electromagnetic gyrokinetics: the Hamiltonian and the symplectic. We describe each formulation and show results of benchmark tests. Although our scheme works for the Hamiltonian formulation, the presence of spurious numerical modes for high-β and large k⊥2ρs2 regimes shows that the symplectic formulation is more robust. We then review our recent algorithm for the symplectic formulation [Mandell et al., J. Plasma Phys. 86, 905860109 (2020)], along with example application of this new capability. Maintaining positivity of the distribution function can be challenging, and we describe a new and novel exponential recovery based algorithm to address this.

Variability in the noise-induced modes of climate dynamics

New features of noise-induced climate variability are revealed on the basis of the three-dimensional model derived by Saltzman and Maasch. It is shown that the climate system can be highly noise excitable and it possesses the large-amplitude fluctuations even in those regions where its akin deterministic model does not contain any self-sustained oscillations. Intermittency in small- and large amplitude climate fluctuations between different basins of attraction of a limit cycle and stable equilibria substantially influencing the climate state (from warm to cold and vice versa) are found at various noise intensities. Suddenly occurring jumps between the basins of attraction of two stable equilibria corresponding to the warm and cold climate states are statistically confirmed under a certain diapason of noise intensities. The climate system undergoes transitions between its equilibria in the presence of noise in its prognostic variables. In addition, such transitions become more likely with increasing the noise intensity.

Large Amplitude Fluctuations in the Alfvénic Solar Wind

Large amplitude fluctuations, often with characteristics reminiscent of large amplitude Alfven waves propagating away from the Sun, are ubiquitous in the solar wind. Such features are most frequently found within fast solar wind streams and most often at solar minimum. The fluctuations found in slow solar wind streams usually have a smaller relative amplitude, are less Alfvenic in character and present more variability. However, intervals of slow wind displaying Alfvenic correlations have been recently identified in different solar cycle phases. In the present paper we report Alfvenic slow solar wind streams seen during the maximum of solar activity that are characterized not only by a very high correlation between velocity and magnetic field fluctuations (as required by outwardly propagating Alfven modes) – comparable to that seen in fast wind streams – but also by higher amplitude relative fluctuations comparable to those seen in fast wind. Our results suggest that the Alfvenic slow wind has a different origin from the slow wind found near the boundary of coronal holes, where the amplitude of the Alfvenic fluctuations decreases together with decreasing the wind speed.

The Enhancement of Proton Stochastic Heating in the Near-Sun Solar Wind

Abstract Stochastic heating (SH) is a nonlinear heating mechanism driven by the violation of magnetic moment invariance due to large-amplitude turbulent fluctuations producing diffusion of ions toward higher kinetic energies in the direction perpendicular to the magnetic field. It is frequently invoked as a mechanism responsible for the heating of ions in the solar wind. Here, we quantify for the first time the proton SH rate Q ⊥ at radial distances from the Sun as close as 0.16 au, using measurements from the first two Parker Solar Probe encounters. Our results for both the amplitude and radial trend of the heating rate, Q ⊥ ∝ r −2.5, agree with previous results based on the Helios data set at heliocentric distances from 0.3 to 0.9 au. Also in agreement with previous results, Q ⊥ is significantly larger in the fast solar wind than in the slow solar wind. We identify the tendency in fast solar wind for cuts of the core proton velocity distribution transverse to the magnetic field to exhibit a flattop shape. The observed distribution agrees with previous theoretical predictions for fast solar wind where SH is the dominant heating mechanism.

Gyrokinetic full-f particle-in-cell simulations on open field lines with PICLS

While in recent years gyrokinetic simulations have become the workhorse for theoretical turbulence and transport studies in the plasma core, their application to the edge and scrape-off layer (SOL) region presents significant challenges. In particular, steep density and temperature gradients as well as large fluctuation amplitudes call for a “full-f” treatment. To specifically study problems in the SOL region, the gyrokinetic particle-in-cell (PIC) code PICLS has been developed. The code is based on an electrostatic full-f model with linearized field equations and uses kinetic electrons. Here, the well-studied parallel transport problem during an edge-localized mode in the SOL shall be investigated for one spatial dimension. The results are compared to previous gyrokinetic continuum and fully kinetic PIC simulations and show good agreement.

Investigating the nature of the link between magnetic field orientation and proton temperature in the solar wind

Solar wind fluctuations are a mixture of propagating disturbances and advected structures that transfer into the interplanetary space the complicated magnetic topology present at the basis of the corona. The large-scale interplanetary magnetic field introduces a preferential direction in the solar wind, which is particularly relevant for both the propagation of the fluctuations and their anisotropy and for the topology of the structures advected by the wind. This paper focusses on a particular link observed between angular displacements of the local magnetic field orientation from the radial direction and values of the proton temperature. In particular, we find that observations byHeliosandWindshow a positive correlation between proton temperature and magnetic field orientation. This is especially true within Alfvénic wind characterized by large-amplitude fluctuations of the background field orientation. Moreover, in the case ofWind, we found a robust dependence of the perpendicular component of the proton temperature on the magnetic field angular displacement. We interpret this signature as possibly due to a physical mechanism related to the proton cyclotron resonance. Finally, by simulating the sampling procedure of the proton velocity distribution function (VDF) of an electrostatic analyzer, we show that the observed temperature anisotropy is not due to instrumental effects.

Numerical study on vapor–liquid phase change in an enclosed narrow space

Based on the multiphase lattice Boltzmann method, a transient model of vapor–liquid phase change in an enclosed narrow space during startup operation is developed and numerically analyzed to investigate the two-phase dynamics and heat transfer behaviors in the confined flat two-phase thermosiphon. The transient temperature distribution, vapor–liquid interface evolution and thermal performance are investigated over a wide range of heat load and liquid filling ratio. The results indicate that, as heat load increases, the intermittent nucleation boiling, fully developed nucleation boiling and film boiling sequentially takes place at the evaporator section of the confined flat two-phase thermosiphon. Correspondingly, the transient temperature evolution shows the periodic large-amplitude fluctuation, the random medium-amplitude fluctuation, and gentle small-amplitude fluctuation. The fully developed boiling is desirable for the heat transfer enhancement on the evaporator surface due to smaller temperature fluctuation and lower thermal resistance. The moderate liquid filling ratio ensures the desired interaction between the evaporator and condenser, resulting in superior thermal performance. To optimize the thermal performance, the optimal liquid filling ratio is about 40%.

A vortex interaction mechanism for generating energy and enstrophy fluctuations in high-symmetric turbulence

Turbulent vortex dynamics is investigated in triply periodic turbulent flow with Kida’s high symmetry (Kida, J. Phys. Soc. Japan, vol. 54, 1985, pp. 2132–2136) by means of unstable periodic motion representing both the statistical and dynamical properties of turbulence (van Veen et al., Fluid Dyn. Res., vol. 38, 2006, pp. 19–46). In the periodic motion, the large-scale columnar vortices, the smaller-scale vortices and the large-amplitude axial waves on the large-scale columnar vortices are detected. In terms of mutual dynamical interaction between the large-scale columnar vortices and smaller-scale vortices, we demonstrate a cyclic process of excitation of the axial waves, which leads to large-amplitude fluctuations of the total kinetic energy and enstrophy. This cyclic process is characterised by three distinct phases and is therefore reminiscent of the regeneration cycle of near-wall turbulence structures (Hamilton et al., J. Fluid Mech., vol. 287, 1995, pp. 317–348). Notably, such oscillatory behaviour is observed even in freely decaying turbulence as a consequence of the instantaneous energy transfer from smaller to larger scales.

Liquid-Liquid Phase Separation Is Driven by Large-Scale Conformational Unwinding and Fluctuations of Intrinsically Disordered Protein Molecules.

Liquid-liquid phase separation occurs via a multitude of transient, noncovalent, and intermolecular interactions resulting in phase transition of intrinsically disordered proteins/regions (IDPs/IDRs) and other biopolymers into mesoscopic, dynamic, nonstoichiometric, and supramolecular condensates. Here we present a unique case to demonstrate that unusual conformational expansion events coupled with solvation and fluctuations drive phase separation of tau, an IDP associated with Alzheimer's disease. Using intramolecular excimer emission as a powerful proximity readout, we show the unraveling of polypeptide chains within the protein-rich interior environment that can promote critical interchain contacts. Using highly sensitive picosecond time-resolved fluorescence depolarization measurements, we directly capture rapid large-amplitude torsional fluctuations in the extended chains that can control the relay of making-and-breaking of noncovalent intermolecular contacts maintaining the internal fluidity. The interplay of these key molecular parameters can be of prime importance in modulating the mesoscale material property of liquid-like condensates and their maturation into pathological gel-like and solid-like aggregates.