Intermittent Structures Research Articles

Effect of CaF2 addition on macro/microstructures and mechanical properties of wire and arc additive manufactured Ti-6Al-4V components

A titanium/CaF2 component was manufactured through the combination of wire and arc additive manufacturing systems as well as activating flux. The effects of CaF2 on macro/microstructures evolution and mechanical properties were investigated. The results showed that the addition of CaF2 to Ti-6Al-4V reduced the width of component and the aspect ratio of α-lath, increased β volume fraction, whereas the intermittent grain boundary-α structure was obtained. In addition, tensile strength of the components at room temperature was significantly improved. Consequently, these findings indicated that activating fluxes offered a possibility to produce the desired microstructures and improve mechanical properties of titanium alloys during additive manufacturing.

Structure of Plasma Heating in Gyrokinetic Alfvénic Turbulence.

We analyze plasma heating in weakly collisional kinetic Alfvén wave turbulence using high resolution gyrokinetic simulations spanning the range of scales between the ion and the electron gyroradii. Real space structures that have a higher than average heating rate are shown not to be confined to current sheets. This novel result is at odds with previous studies, which use the electromagnetic work in the local electron fluid frame, i.e., J·(E+v_{e}×B), as a proxy for turbulent dissipation to argue that heating follows the intermittent spatial structure of the electric current. Furthermore, we show that electrons are dominated by parallel heating while the ions prefer the perpendicular heating route. We comment on the implications of the results presented here.

Characterization of Flow Intermittency and Coherent Structures in a Gas–Solid Circulating Fluidized Bed through Electrostatic Sensing

Flow intermittency and coherent structures are important hydrodynamic phenomena in a gas–solid circulating fluidized bed (CFB). In this work, an electrostatic measurement system based on arc-shaped sensing electrodes is designed and implemented on a CFB test rig. Cross correlation, statistical analysis, wavelet transform, and probability density function (PDF) are applied to the electrostatic signal processing, providing a comprehensive description of the solids velocity, solids holdup, flow intermittency, and coherent structure behaviors. A conditional sampling method is used to extract the coherent structure signals from the electrostatic signals. By comparing the extended self-similarity (ESS) scaling law curves before and after the extraction, the effects of coherent structures on the flow intermittency are further confirmed. Experimental results have demonstrated that the electrostatic signals contain important information about the intermittent hydrodynamic behaviors in a CFB, and the analysis of el...

Formation of swift heavy ion tracks on a rutile TiO2 (001) surface.

Nanostructuring of surfaces and two-dimensional materials using swift heavy ions offers some unique possibilities owing to the deposition of a large amount of energy localized within a nanoscale volume surrounding the ion trajectory. To fully exploit this feature, the morphology of nanostructures formed after ion impact has to be known in detail. In the present work the response of a rutile TiO2 (001) surface to grazing-incidence swift heavy ion irradiation is investigated. Surface ion tracks with the well known intermittent inner structure were successfully produced using 23 MeV I ions. Samples irradiated with different ion fluences were investigated using atomic force microscopy and grazing-incidence small-angle X-ray scattering. With these two complementary approaches, a detailed description of the swift heavy ion impact sites, i.e. the ion tracks on the surface, can be obtained even for the case of multiple ion track overlap. In addition to the structural investigation of surface ion tracks, the change in stoichiometry of the rutile TiO2 (001) surface during swift heavy ion irradiation was monitored using in situ time-of-flight elastic recoil detection analysis, and a preferential loss of oxygen was found.

Non-extensive statistical analysis of magnetic field during the March 2012 ICME event using a multi-spacecraft approach

Abstract In this study we present some new and significant results concerning the dynamics of interplanetary coronal mass ejections (ICMEs) observed in the near Earth at L1 solar wind environment, as well as its effect in Earth’s magnetosphere. The results are referred to Tsallis non-extensive statistics and in particular to the estimation of Tsallis q -triplet, ( q s t a t , q s e n , q r e l ) of magnetic field time series of the ICME observed at the Earth resulting from the solar eruptive activity on March 7, 2012 at the Sun. For this, we used a multi-spacecraft approach based on data experiments from ACE, CLUSTER 4, THEMIS-E and THEMIS-C spacecraft. For the data analysis different time periods were considered, sorted as “quiet”, “shock” and “aftershock”, while different space domains such as the Interplanetary space (near Earth at L1 and upstream of the Earth’s bowshock), the Earth’s magnetosheath and magnetotail, were also taken into account. Our results reveal significant differences in statistical and dynamical features, indicating important variations of the magnetic field dynamics both in time and space domains during the shock event, in terms of rate of entropy production, relaxation dynamics and non-equilibrium meta-stable stationary states. So far, Tsallis non-extensive statistical theory and Tsallis extension of the Boltzmann–Gibbs entropy principle to the q -entropy principle (Tsallis, 1988, 2009) reveal strong universality character concerning non-equilibrium dynamics (Pavlos et al. 2012a,b, 2014a,b; Karakatsanis et al. 2013). Tsallis q -entropy principle can explain the emergence of a series of new and significant physical characteristics in distributed systems as well as in space plasmas. Such characteristics are: non-Gaussian statistics and anomalous diffusion processes, strange and fractional dynamics, multifractal, percolating and intermittent turbulence structures, multiscale and long spatio-temporal correlations, fractional acceleration and Non-Equilibrium Stationary States (NESS) or non-equilibrium self-organization process and non-equilibrium phase transition and topological phase transition processes according to Zelenyi and Milovanov (2004). In this direction, our results reveal clearly strong self-organization and development of macroscopic ordering of plasma system related to strengthen of non-extensivity, multifractality and intermittency everywhere in the space plasmas region during the CME event.

PERSISTENCE MAPPING USING EUV SOLAR IMAGER DATA

ABSTRACT We describe a simple image processing technique that is useful for the visualization and depiction of gradually evolving or intermittent structures in solar physics extreme-ultraviolet imagery. The technique is an application of image segmentation, which we call “Persistence Mapping,” to isolate extreme values in a data set, and is particularly useful for the problem of capturing phenomena that are evolving in both space and time. While integration or “time-lapse” imaging uses the full sample (of size N ), Persistence Mapping rejects (N − 1)/N of the data set and identifies the most relevant 1/N values using the following rule: if a pixel reaches an extreme value, it retains that value until that value is exceeded. The simplest examples isolate minima and maxima, but any quantile or statistic can be used. This paper demonstrates how the technique has been used to extract the dynamics in long-term evolution of comet tails, erupting material, and EUV dimming regions.

Influence of intermittent flow sub-patterns on erosion-corrosion in horizontal pipe

Thinning of pipe wall due to erosion and corrosion has resulted in many catastrophic failures and huge economic losses in petroleum refineries (Wood et al., 2013) and nuclear power stations (Ahmed, 2012). The intermittent structure of two-phase flow through pipe (intermittent flow regime) is an important factor responsible for such erosion and corrosion. The present research is an effort to develop a regime map showing possible erosion and corrosion phenomena due to intermittent flows in pipe. In this direction, flow visualization experiments are carried out for three distinct sub-regimes of intermittent flow namely, plug flow with or without bubble rim, less aerated slug flow and highly aerated slug flow. Images captured at a rate of 1600 frames per second at 288 diameter downstream of inlet are utilised for analysing the dynamics of intermittent flow structure in terms of expansion, contraction, breaking, coalescence, collapse, and collision of bubbles. Quantitative measurements of intermittent flow characteristics (including plug/slug frequency, plug/slug velocity and length of liquid plug/slug) and their influence on erosion-corrosion phenomena in the pipe are reported in detail. Based on these qualitative and quantitative analysis, a regime map for prediction of erosion-corrosion phenomena in pipes is established as a function of inlet flow conditions for both the phases. Transition boundaries for four distinct erosion-corrosion phenomena: shear stress induced erosion, cavitation erosion, liquid impact induced erosion and flow accelerated corrosion are represented in the map and eight distinct regimes of erosion-corrosion are illustrated. The regime map is represented in terms of non-dimensional superficial Reynolds numbers of both the phases to account for pipe diameter, flow rate and fluid viscosity. This erosion-corrosion regime map developed in this research will immensely aid to the effective design of piping systems and optimization of operating conditions for safer operation of petroleum refineries and nuclear power stations.

Relationship Between Intermittent Separation and Vortex Structure in a Three-Dimensional Shock/Boundary-Layer Interaction

The relationship between the three-dimensional vortex structures and flow-separation zones generated by a shock wave/boundary-layer interaction within a low-aspect-ratio duct was studied using stereoscopic particle imaging velocimetry measurements. In this configuration, the interaction of the incident shock with all walls was important in controlling the flowfield; the three interactions coupled to produce a strongly distorted flowfield. Conditional sampling was used to construct the local probability of reverse flow maps, and thus quantify the distribution of regions of intermittent separation on both bottom-walls and side-walls. The latter regions were found to be significantly larger and more likely to separate than the former. Thus, it was concluded that the sidewall and corner flow interactions dominate in this configuration. A triple decomposition of motion was used to construct a three-dimensional representation of the vortex features generated by the interaction. The results indicated that the flowfield was dominated by three vortex systems: 1) the vortex associated with the sidewall swept-shock interaction; 2) a complex, possibly branched, vortex pair induced on the bottom wall; and 3) a vortex pair induced by the flow at the corner, which coupled the two interactions. The role of the three vortex systems on the onset of flow separation was also explored and discussed.

Transitional structures in annular Poiseuille flow depending on radius ratio

The transitional regime of incompressible pressure-driven flows inside an annular pipe is investigated using accurate direct numerical simulation in long computational domains. At marginally low friction Reynolds number $Re_{{\it\tau}}$, turbulence occurs in the form of intermittent localised structures. Different types of localisation are identified as the aspect ratio is varied from ${\it\eta}=0.8$ to $0.1$. These coherent structures vary from helical turbulence at ${\it\eta}=0.8$ to streamwise-localised puffs at ${\it\eta}=0.1$. They are respectively analogous to the stripe patterns and puffs formerly identified in plane channel flow and cylindrical pipe flow. Helical turbulence has been tracked down to ${\it\eta}=0.3$, accompanied by a monotonic reduction of the pitch angle. For ${\it\eta}=0.3$ and marginally low $Re_{{\it\tau}}$, these turbulence structures localise in the streamwise direction, giving rise to a new regime of helical puffs with chirality. The present results suggest that helical puffs mediate the transition from globally axisymmetric puffs to helical stripe patterns.

Scalings of intermittent structures in magnetohydrodynamic turbulence

Turbulence is ubiquitous in plasmas, leading to rich dynamics characterized by irregularity, irreversibility, energy fluctuations across many scales, and energy transfer across many scales. Another fundamental and generic feature of turbulence, although sometimes overlooked, is the inhomogeneous dissipation of energy in space and in time. This is a consequence of intermittency, the scale-dependent inhomogeneity of dynamics caused by fluctuations in the turbulent cascade. Intermittency causes turbulent plasmas to self-organize into coherent dissipative structures, which may govern heating, diffusion, particle acceleration, and radiation emissions. In this paper, we present recent progress on understanding intermittency in incompressible magnetohydrodynamic turbulence with a strong guide field. We focus on the statistical analysis of intermittent dissipative structures, which occupy a small fraction of the volume but arguably account for the majority of energy dissipation. We show that, in our numerical simulations, intermittent structures in the current density, vorticity, and Elsässer vorticities all have nearly identical statistical properties. We propose phenomenological explanations for the scalings based on general considerations of Elsässer vorticity structures. Finally, we examine the broader implications of intermittency for astrophysical systems.

Scale properties of turbulent transport and coherent structure in stably stratified flows

The empirical mode decomposition (EMD) is used to study the scale properties of turbulent transport and coherent structures based on velocity and temperature time series in stably stratified turbulence. The analysis is focused on the scale properties of intermittency and coherent structures in different modes and the contributions of energy-contained coherent structures to turbulent scalar counter-gradient transport (CGT). It is inferred that the velocity intermittency is scattered to more modes with the development of the stratified flow, and the intermittency is enhanced by the vertical stratification, especially in small scales. The anisotropy of the field is presented due to different time scales of coherent structures of streamwise and vertical velocities. There is global counter-gradient heat transport close to the turbulence-generated grid, and there is local counter-gradient heat transport at certain modes in different positions. Coherent structures play a principal role in the turbulent vertical transport of temperature.

Turbulent–laminar patterns in shear flows without walls

Turbulent–laminar intermittency, typically in the form of bands and spots, is a ubiquitous feature of the route to turbulence in wall-bounded shear flows. Here we study the idealised shear between stress-free boundaries driven by a sinusoidal body force and demonstrate quantitative agreement between turbulence in this flow and that found in the interior of plane Couette flow – the region excluding the boundary layers. Exploiting the absence of boundary layers, we construct a model flow that uses only four Fourier modes in the shear direction and yet robustly captures the range of spatiotemporal phenomena observed in transition, from spot growth to turbulent bands and uniform turbulence. The model substantially reduces the cost of simulating intermittent turbulent structures while maintaining the essential physics and a direct connection to the Navier–Stokes equations. We demonstrate the generic nature of this process by introducing stress-free equivalent flows for plane Poiseuille and pipe flows that again capture the turbulent–laminar structures seen in transition.

粗面クエット流において発生する様々な準秩序構造

Most wall-bounded shear flows undergo a subcritical transition scenario. The lowest limit of the transitional regime features intermittent coherent structures that are, for instance, turbulent-laminar patterns in plane channel flows. The transitional phenomena on smooth walls have been considered well; however, walls in industrial situations have hydro-dynamically roughened surface, since a perfectly smooth wall is ideal and uneconomic for production. The aim of this study is to elucidate the influence of roughness on the turbulent-laminar patterns and to evaluate the robustness of the turbulent-laminar patterns on the roughness wall. We study plane Couette flows with one/two rough wall(s) by means of direct numerical simulation and found several flow structures in roughness induced regime.

Effect of Oscillation Structures on Inertial-Range Intermittence and Topology in Turbulent Field

AbstractUsing the incompressible isotropic turbulent fields obtained from direct numerical simulation and large-eddy simulation, we studied the statistics of oscillation structures based on local zero-crossings and their relation with inertial-range intermittency for transverse velocity and passive scalar. Our results show that for both the velocity and passive scalar, the local oscillation structures are statistically scale-invariant at high Reynolds number, and the inertial-range intermittency of the overall flow region is determined by the most intermittent structures characterized by one local zero-crossing. Local flow patterns conditioned on the oscillation structures are characterized by the joint probability density function of the invariants of the filtered velocity gradient tensor at inertial range. We demonstrate that the most intermittent regions for longitudinal velocity tend to lay at the saddle area, while those for the transverse velocity tend to locate at the vortex-dominated area. The connection between the ramp-cliff structures in passive scalar field and the corresponding saddle regions in the velocity field is also verified by the approach of oscillation structure classification.

Non-diffusive transport of suprathermal ions by intermittent turbulent structures

We report on recent experimental, numerical and theoretical investigations of suprathermal ion transport and turbulent plasma dynamics in TORPEX. TORPEX is a toroidal device in which field-aligned blobs are intermittently generated and propagate across a confining magnetic field. Suprathermal ions are locally injected using a miniaturized source and detected using gridded energy analyzers. We show evidence for subdiffusive and superdiffusive transport of suprathermal ions, using an unprecedented combination of uniquely resolved three-dimensional measurements and first-principles numerical simulation. We present time-resolved measurements of suprathermal ion current fluctuations, which exhibit a clear causality relation with blobs, as revealed by a transfer entropy method for the superdiffusive case. This link is further investigated by using a conditional sampling technique, which allows resolving the cross-field dynamics of both blobs and the suprathermal ion beam, revealing that suprathermal ions experiencing superdiffusive transport are associated with bursty displacement events, resulting in highly intermittent time traces. This work links observations usually inaccessible in fusion devices and astrophysical plasmas, namely energy resolved three-dimensional time-averaged measurements, with Eulerian time-resolved measurements, which are often the only accessible measurements in such systems.

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

The suspension of sediments by oscillatory flows is a complex case of fluid–particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and event-driven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds’ stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly-evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect.

THE SPECTRAL FEATURES OF LOW-AMPLITUDE MAGNETIC FLUCTUATIONS IN THE SOLAR WIND AND THEIR COMPARISON WITH MODERATE-AMPLITUDE FLUCTUATIONS

The power-spectrum index of magnetic fluctuations is a crucial parameter for the characterization of nonlinear interactions affecting the solar wind turbulence. Most previous observations were made for large- and moderate-amplitude magnetic fluctuations, which include current sheets and intermittent structures. Here, by using measurements from the WIND spacecraft, we present magnetic spectral features as derived by the fast Fourier transform method from six-minute time series of magnetic fluctuations with low relative amplitudes () of only 0.05–0.11. When comparing their spectra with those of moderate-amplitude fluctuations (), we find that for the low-amplitude fluctuations the averaged magnetic spectral indices are and , respectively, for small and large sampling angles, i.e., the angle between the Sun-to-Earth radial direction and the mean magnetic field direction. However, for the moderate-amplitude fluctuations, these two indices are and , respectively. This result of the moderate-amplitude fluctuations is consistent with that of previous analyses, which, by using the wavelet technique, revealed spectral anisotropy of magnetic fluctuations and yielded an index of in the parallel direction and in the perpendicular direction. However, the result found here for the low-amplitude fluctuations has not been reported before, and thus will probably initiate new studies aiming to better understand the turbulent nature of such low-amplitude fluctuations. Future studies will help us to understand why the spectral anisotropy differs for fluctuations of different amplitudes.

The appearance and propagation of filaments in the private flux region in Mega Amp Spherical Tokamak

The transport of particles via intermittent filamentary structures in the private flux region (PFR) of plasmas in the MAST tokamak has been investigated using a fast framing camera recording visible light emission from the volume of the lower divertor, as well as Langmuir probes and IR thermography monitoring particle and power fluxes to plasma-facing surfaces in the divertor. The visible camera data suggest that, in the divertor volume, fluctuations in light emission above the X-point are strongest in the scrape-off layer (SOL). Conversely, in the region below the X-point, it is found that these fluctuations are strongest in the PFR of the inner divertor leg. Detailed analysis of the appearance of these filaments in the camera data suggests that they are approximately circular, around 1–2 cm in diameter, but appear more elongated near the divertor target. The most probable toroidal quasi-mode number is between 2 and 3. These filaments eject plasma deeper into the private flux region, sometimes by the production of secondary filaments, moving at a speed of 0.5–1.0 km/s. Probe measurements at the inner divertor target suggest that the fluctuations in the particle flux to the inner target are strongest in the private flux region, and that the amplitude and distribution of these fluctuations are insensitive to the electron density of the core plasma, auxiliary heating and whether the plasma is single-null or double-null. It is found that the e-folding width of the time-average particle flux in the PFR decreases with increasing plasma current, but the fluctuations appear to be unaffected. At the outer divertor target, the fluctuations in particle and power fluxes are strongest in the SOL.

Discrete element modeling on the crack evolution behavior of brittle sandstone containing three fissures under uniaxial compression

Based on experimental results of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code $$(\hbox {PFC}^{\mathrm{2D}})$$ . Then, the validation of the simulated models were cross checked with the experimental results of brittle sandstone containing three parallel fissures under uniaxial compression. The simulated results agreed very well with the experimental results, including the peak strength, peak axial strain, and ultimate failure mode. Using the same micro-parameters, the numerical models containing a new geometry of three fissures are constructed to investigate the fissure angle on the fracture mechanical behavior of brittle sandstone under uniaxial compression. The strength and deformation parameters of brittle sandstone containing new three fissures are dependent to the fissure angle. With the increase of the fissure angle, the elastic modulus, the crack damage threshold, and the peak strength of brittle sandstone containing three fissures firstly increase and secondly decrease. But the peak axial strain is nonlinearly related to the fissure angle. In the entire process of deformation, the crack initiation and propagation behavior of brittle sandstone containing three fissures under uniaxial compression are investigated with respect to the fissure angle. Six different crack coalescence modes are identified for brittle sandstone containing three fissures under uniaxial compression. The influence of the fissure angle on the length of crack propagation and crack coalescence stress is evaluated. These investigated conclusions are very important for ensuring the stability and safety of rock engineering with intermittent structures.

Synthetic activity indicators for M-type dwarf stars

AbstractHere, we present a set of time-dependent 3D RMHD simulations of a M-dwarf star representative of AD Leo, which extend from the upper convection zone into the chromosphere. The 3D model atmospheres are characterized by a very dynamic and intermittent structure on small spatial and temporal scales and a wealth of physical processes, which by nature cannot be described by means of 1D static model atmospheres. Artificial observations of these models imply that a combination of complementary diagnostics such as Ca II lines and the continuum intensity from UV to millimeter wavelengths, probe various properties of the dynamics, thermal and magnetic structure of the photosphere and the chromosphere and thus provide measures of stellar activity, which can be compared to observations. The complicated magnetic field structure and its imprint in synthetic diagnostics may have important implications for the understanding and characterization of stellar activity and with it possibly for the evaluation of planetary habitability around active M-dwarf stars.