Intermittent Structures Research Articles

Energy Dispersion in African Easterly Waves

Abstract The existence of an upstream (eastward) group velocity for African easterly waves (AEWs) is shown based on single-point lag regressions using gridded reanalysis data from 1990 to 2010. The eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors. A local eddy kinetic energy (EKE) budget reveals that, early in the life cycle of AEWs, growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates. Later in the life cycle, EKE decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth. A potential vorticity (PV) budget is used to diagnose tendencies related to group propagation. Although both upstream and downstream group speeds are possible because of the reversal in the mean meridional PV gradient, upstream propagation associated with the positive poleward PV gradient dominates wave packet evolution. Analogous to the concept of downstream development of midlatitude baroclinic waves, new AEWs develop preferentially upstream of the older ones, thus providing a mechanism for seeding new waves. It is suggested that these results are also relevant to AEW intermittency and storm-track structure.

0521 回転チャネル乱流中に発生する大規模間欠構造の強化と減衰(OS5-5 壁乱流:統計,構造,動力学の理解,OS5 壁乱流:統計,構造,動力学の理解)

0521 回転チャネル乱流中に発生する大規模間欠構造の強化と減衰(OS5-5 壁乱流:統計,構造,動力学の理解,OS5 壁乱流:統計,構造,動力学の理解)

Seismic Analysis on the Large Span Exchanging Building

The conference center of the original structure of the frame structure, the ground floors.Now transform the functional requirements to construction, pumping column to the top of the structure to form a large space,Causing the vertical elements intermittent floors stiffness mutation irregular structure system.The basis for reinforced superstructure using SATWE conducted a multi-case earthquake under Elastic Compute using SAP elastoplastic analysis for validation.The analysis results show that the irregular structure, when reasonable structural arrangement, to take appropriate structural measures, the seismic performance to meet the regulatory requirements.

Local flow structure of turbulence in three, four, and five dimensions

Probability density functions (PDF's) of the eigenvalues of the strain tensor of an incompressible isotropic turbulence in 3, 4, and 5 dimensions are computed by direct numerical simulation of Navier-Stokes equations. The PDF's of the smallest (negative) eigenvalue are found to be wider than those of the other ones in all dimensions and to be very insensitive to the dimension. In any dimension, the eigenvalues other than the lowest one increase as the lowest one decreases, so that they tend to be positive for the large magnitude of the lowest eigenvalue. In such a situation the flow comes in along a single direction and comes out in the other directions, which is consistent with the dynamics of the Burgers turbulence in d dimensions. It is suggested that a driving motor of most intermittent turbulent structure is the compression along a single direction. For the velocity 2 form the conditional averages of the enstrophy and the total squared strain in three dimensions are computed as functions of the smallest eigenvalue and found to be monotonically increasing as the magnitude of the smallest eigenvalue increases. Also, it is found that PDF of the source term of the Poisson equation for the pressure is positively skewed but tends to be symmetric with increase of the spatial dimension. Dimension effects on the dynamics of the most compressible eigenvalue are argued.

The role of power and magnetic connection to the active antenna in the suppression of intermittent structures by ion cyclotron resonance heating

The effect of the ion cyclotron resonance heating (ICRH) on the scrape-off layer (SOL) is still an open issue, where, lately it was shown on the ASDEX-Upgrade tokamak that turbulence large-scale structures, known as blobs or avaloids, are suppressed by ICRH (Antar et al 2010 Phys. Rev. Lett. 105 165001). Furthermore, it was shown that the edge localized mode-induced turbulent transport is also reduced significantly. However, the reasons behind this interaction remain unknown. On the Tore Supra tokamak, we confirm that the ICRH suppresses large-scale structures while small-scale structures are enhanced; overall, the turbulence level of fluctuations is reported to drop from about 40% to 25%. This confirms that the effects on turbulence are independent of the type of plasma confinement, L-mode on Tore Supra versus H-mode on ASDEX-Upgrade. The dependence on the ICRH power showed that if a threshold existed it would be below 500 kW and that above this power, no additional effects on the SOL turbulence are reported. The other study reported in this paper deals with the importance of the magnetic field line connections or, in other words, whether the interaction between ICRH and turbulence is global, affecting the whole plasma, or local, affecting regions that are magnetically connected to the active antenna. We found that the toroidal connection to the active antenna is not critical in the sense that turbulence in regions close to the antenna but not necessarily connected are affected. For regions that are not connected and far from the active antenna, turbulence does not change much when applying ICRH.

Anomalous gravitomagnetic viscosity in accretion disks

We propose a viscosity prescription in a self-gravitating system based on the self-generated gravitomagnetic field. It has been proved that the self-generated gravitomagnetic field with very low frequency is modulational unstable, leading to a self-collapse, and resulting in the enhancement of perturbed matter density and gravitomagnetic field in a small region; the interactions between highly spatially intermittent gravitomagnetic structures and the macrofluid with differential rotation are responsible for the anomalous gravitomagnetic viscosity. The result indicates that, the gravitomagnetic viscosity would be nearly 108 stronger than the molecular viscosity, which maybe useful for investigating the structures and instabilities in the self-gravitating accretion disks; provide an explanation for the transportion of angular momentum and the formation of gravitomagnetic jets.

Statistical Analysis of the Magnetic and Velocity Field Fluctuations for the Solar Wind Turbulence

In this paper, the scaling properties of magnetohydrodynamic (MHD) turbulence fluctuations are studied for different data sets. The Data is collected in two different setups, namely in situ satellite observations of the solar wind and the direct numerical simulation of incompressible MHD equations. It is found that the probability density functions (PDFs) of inertial range energy fluctuations exhibit self-similarity and monoscaling in agreement with solar-wind measurements. Furthermore, the energy PDFs exhibit similarity over all scales of the turbulent system showing no substantial qualitative change of shape as the scale of the fluctuations varies. On the contrary, our results also exhibit that magnetic, velocity and Elsässer field fluctuations have an intermittent structure so that the associated PDFs change from Gaussian at large scales to leptokurtic (fat-tailed) at small scales.

Fouling Removal Technology with Intermittent Rotation Spiral-Insert

An intermittent rotation steel spiral automatic cleaning technology has been developed to address the fouling problem of the condenser tubes in small turbines. The intermittent rotation structure reduces steel spiral rotating speed to less than 1/10 of the continuous rotation spiral, which solves the wearing problems between the spiral and the tube. The results of a series of experiments showed that: the proposed technology meets the requirements of cleaning the hard fouling, and the heat transfer enhancement effect is significantly improved. The technology’s simple structure and low cost are suitable for heat exchangers with inside-tube velocity higher than 1 m / s, especially suitable for turbine condenser.

Magnetic Discontinuities in Magnetohydrodynamic Turbulence and in the Solar Wind

Recent measurements of solar wind turbulence report the presence of intermittent, exponentially distributed angular discontinuities in the magnetic field. In this Letter, we study whether such discontinuities can be produced by magnetohydrodynamic (MHD) turbulence. We detect the discontinuities by measuring the fluctuations of the magnetic field direction, Δθ, across fixed spatial increments Δx in direct numerical simulations of MHD turbulence with an imposed uniform guide field B(0). A large region of the probability density function (pdf) for Δθ is found to follow an exponential decay, proportional to exp(-Δθ/θ(*)), with characteristic angle θ(*)≈(14°)(b(rms)/B(0))(0.65) for a broad range of guide-field strengths. We find that discontinuities observed in the solar wind can be reproduced by MHD turbulence with reasonable ratios of b(rms)/B(0). We also observe an excess of small angular discontinuities when Δx becomes small, possibly indicating an increasing statistical significance of dissipation-scale structures. The structure of the pdf in this case closely resembles the two-population pdf seen in the solar wind. We thus propose that strong discontinuities are associated with inertial-range MHD turbulence, while weak discontinuities emerge from dissipation-range turbulence. In addition, we find that the structure functions of the magnetic field direction exhibit anomalous scaling exponents, which indicates the existence of intermittent structures.

Using the Stochastic Multicloud Model to Improve Tropical Convective Parameterization: A Paradigm Example

Abstract Despite recent advances in supercomputing, current general circulation models (GCMs) poorly represent the variability associated with organized tropical convection. A stochastic multicloud convective parameterization based on three cloud types (congestus, deep, and stratiform), introduced recently by Khouider, Biello, and Majda in the context of a single column model, is used here to study flows above the equator without rotation effects. The stochastic model dramatically improves the variability of tropical convection compared to the conventional moderate- and coarse-resolution paradigm GCM parameterizations. This increase in variability comes from intermittent coherent structures such as synoptic and mesoscale convective systems, analogs of squall lines and convectively coupled waves seen in nature whose representation is improved by the stochastic parameterization. Furthermore, simulations with a sea surface temperature (SST) gradient yield realistic mean Walker cell circulation with plausible high variability. An additional feature of the present stochastic parameterization is a natural scaling of the model from moderate to coarse grids that preserves the variability and statistical structure of the coherent features. These results systematically illustrate, in a paradigm model, the benefits of using the stochastic multicloud framework to improve deterministic parameterizations with clear deficiencies.

Observations of Intermittent Structures in the Periphery of a Cylindrical Linear Plasma in PANTA

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan 1)Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan 2)Itoh Research Center for Plasma Turbulence, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan 3)Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, Ibaraki 311-0193, Japan 4)Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Tokyo 277-8561, Japan 5)Japan Atomic Energy Agency, 2-166 Oaza-obuchi-aza-omotedate, Rokkasho, Kamikita, Aomori 039-3212, Japan 6)National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan

An Experimental Study of the Fracture Coalescence Behaviour of Brittle Sandstone Specimens Containing Three Fissures

To analyse the fracture coalescence behaviour of rock, rectangular prismatic sandstone specimens (80 × 160 × 30 mm in size) containing three fissures were tested under uniaxial compression. The strength and deformation behaviours of the specimens are first analysed by investigating the effects of the ligament angle β2 on the peak strength, peak strain and crack initiation stress of the specimens. To confirm the sequence of crack coalescence, a photographic monitoring technique is used throughout the entire period of deformation. Based on the results, the relationship between the real-time crack coalescence process and the axial stress–strain curve of brittle sandstone specimens is also developed, and this relationship can be used to evaluate the macroscopic deformation characteristics of pre-cracked rock. The equivalent strain evolution fields of the specimen, with α = β1 = 45° and β2 = 90°, are obtained using the digital image correlation technique and show good agreement with the experimental results of pre-cracked brittle sandstone. These experimental results are expected to improve the understanding of fracture mechanisms and be used in rock engineering with intermittent structures, such as deep underground excavated tunnels.

Analysis of coherent structures and atmosphere-canopy coupling strength during the CABINEX field campaign

Abstract. Intermittent coherent structures can be responsible for a large fraction of the exchange between a forest canopy and the atmosphere. Quantifying their contribution to momentum and heat fluxes is necessary to interpret measurements of trace gases and aerosols within and above forest canopies. The primary objective of the Community Atmosphere-Biosphere Interactions Experiment (CABINEX) field campaign (10 July 2009 to 9 August 2009) was to study the chemistry of volatile organic compounds (VOC) within and above a forest canopy. In this manuscript we provide an analysis of coherent structures and canopy-atmosphere exchange during CABINEX to support in-canopy gradient measurements of VOC. We quantify the number and duration of coherent structure events and their percent contribution to momentum and heat fluxes with two methods: (1) quadrant-hole analysis, and (2) wavelet analysis. Despite differences in the duration and number of events, both methods predict that coherent structures contribute 40–50% to momentum fluxes and 44–65% to heat fluxes during the CABINEX campaign. Contributions associated with coherent structures are slightly greater under stable atmospheric conditions. By comparing heat fluxes within and above the canopy, we determine the degree of coupling between upper canopy and atmosphere, and find that they are coupled the majority of the time. Uncoupled canopy-atmosphere events occur in the early morning (4–8 a.m. local time) approximately 30% of the time. This study confirms that coherent structures contribute significantly to the exchange of heat and momentum between the canopy and atmosphere at the CABINEX site, and indicates the need to include these transport processes when studying the mixing and chemical reactions of trace gases and aerosols between a forest canopy and the atmosphere.

Turbulence in fluid and plasma: search for a new paradigm

A first principle explanation of the origin of intermittency and nonlinear structure formation in the Lagrangian velocity increments of a turbulent flow is presented in the context of a scale-invariant analytical formalism that is being developed recently. The copious generation of power laws and nonlinear exponents in the structure functions are shown to follow quite naturally in the present formalism.

Investigation of turbulence in reversed field pinch plasma by using microwave imaging reflectometry

Turbulence in the reversed field pinch (RFP) plasma has been investigated by using the microwave imaging reflectometry in the toroidal pinch experiment RX (TPE-RX). In conventional RFP plasma, the fluctuations are dominated by the intermittent blob-like structures. These structures are accompanied with the generation of magnetic field, the strong turbulence, and high nonlinear coupling among the high and low k modes. The pulsed poloidal current drive operation, which improves the plasma confinement significantly, suppresses the dynamo, the turbulence, and the blob-like structures.

Dynamics of plasma jets and electrode spots in the pulsed source of a high-purity plasma obtained from a dielectric material at atmospheric pressure

The results of investigation of the distinctive features of the dynamics of outflow of plasma jets and of formation of electrode spots in the source of a high-purity plasma obtained from a dielectric material at atmospheric pressure by analogy with a capillary discharge have been given. It has been established that the geometry of the electrode-spot distribution is dependent on the height of the dielectric projection above the electrodes, cathode spots appear in the negative half-cycle of the current, the outflowing plasma jet is of an intricate shock-wave and intermittent structure, and the space-time distribution of the shock is determined by the energy stored in the storage system.

The effects of windscreen flow on noise in motorcycle helmets.

Vortex shedding from a motorcycle windscreen results in three flow regions into which the helmet of the rider may be immersed. First, the helmet may be fully in the free stream. Second, the helmet may be directly in the path of vortex shedding from the windscreen. Third, the helmet may be beneath the vortex shedding and shielded from the free stream by the windscreen. On-track tests were conducted and show a difference in sound pressure level of over 10 dB and a change in spectral content for different riding positions and helmet angle. Similar tests were then conducted in a wind tunnel, where simultaneous microphone and flow visualisation measurements allowed the identification and investigation of each flow region under controlled conditions. The contribution of vortex shedding to the noise was assessed using a combination of wavelet analysis and conditional averaging to identify intermittent structures.

Magnetic Fields in Cosmic Particle Acceleration Sources

We review here some magnetic phenomena in astrophysical particle accelerators associated with collisionless shocks in supernova remnants, radio galaxies and clusters of galaxies. A specific feature is that the accelerated particles can play an important role in magnetic field evolution in the objects. We discuss a number of CR-driven, magnetic field amplification processes that are likely to operate when diffusive shock acceleration (DSA) becomes efficient and nonlinear. The turbulent magnetic fields produced by these processes determine the maximum energies of accelerated particles and result in specific features in the observed photon radiation of the sources. Equally important, magnetic field amplification by the CR currents and pressure anisotropies may affect the shocked gas temperatures and compression, both in the shock precursor and in the downstream flow, if the shock is an efficient CR accelerator. Strong fluctuations of the magnetic field on scales above the radiation formation length in the shock vicinity result in intermittent structures observable in synchrotron emission images. Resonant and non-resonant CR streaming instabilities in the shock precursor can generate mesoscale magnetic fields with scale-sizes comparable to supernova remnants and even superbubbles. This opens the possibility that magnetic fields in the earliest galaxies were produced by the first generation Population III supernova remnants and by clustered supernovae in star forming regions.

Atmospheric wind field conditions generated by active grids

An active grid for turbulence generation of several rotatable axes with surmounted vanes that can be driven via stepper or servo motors is presented. We investigate the impact of different excitation protocols for the grid. Using such protocols that already have the intermittent structure of turbulence, higher intermittent flows can be achieved. This concept can also be used to generate turbulent flows of high turbulence intensities (>25%) exhibiting integral length scales beyond the typical size of the test section of the wind tunnel. Similar two-point correlations measured by the intermittent statistics of velocity increments that are characteristic for flows of high Reynolds number, i.e. in the atmospheric boundary layer, can be reproduced.

Spatial downscaling of precipitation from GCMs for climate change projections using random cascades: A case study in Italy

We present a Stochastic Space Random Cascade (SSRC) approach to downscale precipitation from a General Circulation Models (GCMs), developed for the assessment of water resources under climate change scenarios for the Oglio river (1440 km2), in the Italian Alps. The snow‐fed Oglio river displays complex physiography and high environmental gradient and statistical downscaling methods are required for climate change assessment. First, a back cast analysis is carried out to evaluate the most representative within a set of four available GCMs (R30, ECHAM4, PCM, HadCM3). Monthly precipitation for the window 1990–2000 from 270 gauging stations (one every 25 km2) in northern Italy is used and scores from objective indicators are calculated. The SSRC model is then tuned upon the Oglio river catchment for spatial downscaling (2 km2) of daily precipitation from the NCAR Parallel Climate Model, giving the comparatively best results for the area. Scale Recursive Estimation coupled with the Expectation Maximization algorithm is used for model estimation. The seasonal parameters of the multiplicative cascade are accommodated by statistical distributions conditioned upon the climatic forcing, based on a regression analysis. The SSRC approach reproduces well the spatial clustering, intermittency, self‐similarity, and spatial correlation structure of precipitation fields, with relatively low computational burden. Downscaling of future precipitation scenarios (A2 scenario from the Parallel Climate Model) is then carried out and some preliminary conclusions are drawn.