Turbulent Acceleration Research Articles

Electromagnetic theory of turbulent acceleration of parallel flow and momentum conservation

Intrinsic flow in plasma physics is a long-standing puzzle, since it is difficult to understand its origin without contradiction to momentum conservation in conventional wisdom. It is proved that the electromagnetic turbulent acceleration as a candidate for intrinsic parallel flow generation driven by pressure gradient along the total magnetic field line does not contradict the momentum conservation. The conserved quantity corresponding to axial symmetry is the total gyrocenter parallel canonical momentum carried by both species or the total gyrocenter parallel momentum including the ion gyrocenter kinematic momentum and electromagnetic fields momentum, but not the ion kinematic momentum, or even the ion parallel flow. A conservation equation of total parallel momentum including the ion particles' kinematic momentum and electromagnetic fields momentum is also presented.

Intrinsic parallel rotation drive by electromagnetic ion temperature gradient turbulence

The quasilinear intrinsic parallel flow drive including parallel residual stress, kinetic stress, cross Maxwell stress and parallel turbulent acceleration by electromagnetic ion temperature gradient (ITG) turbulence is calculated analytically using electromagnetic gyrokinetic theory. Both the kinetic stress and cross Maxwell stress also enter the mean parallel flow velocity equation via their divergence, as for the usual residual stress. The turbulent acceleration driven by ion pressure gradient along the total magnetic field (including equilibrium magnetic field and fluctuating radial magnetic field) cannot be written as a divergence of stress, and so should be treated as a local source/sink. All these terms can provide intrinsic parallel rotation drive. Electromagnetic effects reduce the non-resonant electrostatic stress force and even reverse it, but enhance the resonant stress force. Both the non-resonant and resonant turbulent acceleration terms are also enhanced by electromagnetic effects. The possible implications of our results for experimental observations are discussed.

Turbulence and particle acceleration in giant radio haloes: the origin of seed electrons

About 1/3 of X-ray-luminous clusters show smooth, Mpc-scale radio emission, known as giant radio haloes. One promising model for radio haloes is Fermi-II acceleration of seed relativistic electrons by compressible turbulence. The origin of these seed electrons has never been fully explored. Here, we integrate the Fokker-Planck equation of the cosmic ray (CR) electron and proton distributions when post-processing cosmological simulations of cluster formation, and confront them with radio surface brightness and spectral data of Coma. For standard assumptions, structure formation shocks lead to a seed electron population which produces too centrally concentrated radio emission. Matching observations requires modifying properties of the CR population (rapid streaming; enhanced CR electron acceleration at shocks) or turbulence (increasing turbulent-to-thermal energy density with radius), but at the expense of fine-tuning. In a parameter study, we find that radio properties are exponentially sensitive to the amplitude of turbulence, which is inconsistent with small scatter in scaling relations. This sensitivity is removed if we relate the acceleration time to the turbulent dissipation time. In this case, turbulence above a threshold value provides a fixed amount of amplification; observations could thus potentially constrain the unknown CR seed population. To obtain sufficient acceleration, the turbulent magneto-hydrodynamics cascade has to terminate by transit time damping on CRs, i.e., thermal particles must be scattered by plasma instabilities. Understanding the small scatter in radio halo scaling relations may provide a rich source of insight on plasma processes in clusters.

Features of Heart Rate Variability in Patients with Coronary Heart Disease and Risk Factors for Cardiovascular Disease According to the SCORE Charts in the Presence of Heart Rate Turbulence

The objective of the research was to develop new approaches to the prediction, prevention and treatment of complications in patients at high and very high total cardiovascular risk based on the assessment of heart rate turbulence.Materials and methods. 319 patients were examined to study the role of heart rate turbulence in the formation and progression of sudden cardiac death in the cardiovascular continuum based on the Holter monitoring. All patients were divided into 4 groups: Group 1 included patients with coronary heart disease without concomitant risk factors such as smoking, obesity, metabolic syndrome; Group 2 consisted of patients smoking tobacco for more than 2 years (those at very high cardiovascular risk); Group 3 included patients with metabolic syndrome without existing coronary heart disease or arterial hypertension (those at high cardiovascular risk); Group 4 consisted of 50 patients with metabolic syndrome and arterial hypertension (the control group).Results. In practically healthy people with the phenomenon of heart rate turbulence, there was a high level of sympathetic activity of the autonomic nervous system, which was more pronounced in the passive period of the day. In people with postinfarction cardiosclerosis and the phenomenon of heart rate turbulence hypersympathicotonia being associated with high psycho-emotional stress was typical in the active as well as the passive period of the day; the central mechanisms of regulation prevailed over autonomous ones. In people with coronary heart disease (postinfarction cardiosclerosis) psycho-emotional stress and functional state of the cerebral cortex played an important role in the pathogenesis of the disease. In the passive period of the day, the tension value of the total body regulation decreased, mainly due to high-frequency component of heart rate variability. Hypersympathicotonia and high activity of the vasomotor center at night explain the high incidence of complications and exacerbations of coronary artery disease in the passive period of the day. In individuals at very high cardiovascular risk according to the SCORE charts with the phenomenon of heart rate turbulence the QT interval lengthening was not observed. The feature of this group was the moderate total tension of heart rate variability in the active period of the day and its rise to the highest limits during the passive period of the day, which was associated with low activity of all spectra. The stress index in patients at very high cardiovascular risk according to the SCORE charts in the passive period of the day reduced compared to the active period of the day. The indices of acceleration and deceleration of the sinus rhythm in patients at very high cardiovascular risk according to the SCORE charts were within normal limits. In individuals at high cardiovascular risk according to the SCORE charts with the phenomenon of heart rhythm turbulence the parameters of heart rate turbulence acceleration were below normal limits; the paramters of deceleration were normal; shortening of the QT interval was not observed.Conclusions. The determination of heart rate turbulence is quite simple, non-invasive, affordable screening method of forecasting the occurrence and development of cardiovascular diseases as well as early detection of patients predisposed to sudden cardiac death in the general population.

Experimental study of the relative effect of pressure drop and flow rate on the droplet size downstream of a pipe restriction

ABSTRACTAn experimental setup consisting of a 100 mm inner diameter pipeline, a butterfly valve with inner diameter of 100 mm, and oil and water pumping capacities of up to 20 m3/h were used to study droplet breakup in two-phase oil–water flow. The tests were performed at atmospheric pressure and under ambient temperatures. A particle-sizing camera was used to quantify droplet sizes. Combinations of different flow rates, water cuts, and pressure drops were tested to determine the relative effects of flow rate and pressure drop over a valve on the droplet breakup process. The test matrix was designed so that it should be possible to determine if the droplet sizes produced were independent of the flow rate. The fluid system consisted of a water phase and a mineral oil with viscosity of 4 mPa · s. Two different droplet breakup models were compared against the measured droplet sizes. The two models considered turbulence and droplet acceleration through the restriction respectively as the main contributor for droplet breakup.

Stone Stability under Stationary Nonuniform Flows

AbstractA stability parameter for rock in bed protections under nonuniform stationary flow is derived. The influence of the mean flow velocity, turbulence, and mean acceleration of the flow are included explicitly in the parameter. The relatively new notion of explicitly incorporating the mean acceleration of the flow significantly improves the description of stone stability. The new stability parameter can be used in the design of granular bed protections using a numerical model for a large variety of flows. The coefficients in the stability parameter are determined by regarding the measured low-mobility entrainment rate of rock as a function of the stability parameter. Measurements of flow characteristics and stone entrainment of four different previous studies and many configurations (uniform flow, expansion, contraction, and sill) are used. These configurations have different relative contributions of mean flow, turbulence, and stationary acceleration. The coefficients in the parameter are fit to all ...

Microbubbles and Microparticles are Not Faithful Tracers of Turbulent Acceleration.

We report on the Lagrangian statistics of acceleration of small (sub-Kolmogorov) bubbles and tracer particles with Stokes number St≪1 in turbulent flow. At a decreasing Reynolds number, the bubble accelerations show deviations from that of tracer particles; i.e., they deviate from the Heisenberg-Yaglom prediction and show a quicker decorrelation despite their small size and minute St. Using direct numerical simulations, we show that these effects arise due the drift of these particles through the turbulent flow. We theoretically predict this gravity-driven effect for developed isotropic turbulence, with the ratio of Stokes to Froude number or equivalently the particle drift velocity governing the enhancement of acceleration variance and the reductions in correlation time and intermittency. Our predictions are in good agreement with experimental and numerical results. The present findings are relevant to a range of scenarios encompassing tiny bubbles and droplets that drift through the turbulent oceans and the atmosphere. They also question the common usage of microbubbles and microdroplets as tracers in turbulence research.

Inertial-particle accelerations in turbulence: a Lagrangian closure

The distribution of particle accelerations in turbulence is intermittent, with non-Gaussian tails that are quite different for light and heavy particles. In this article we analyse a closure scheme for the acceleration fluctuations of light and heavy inertial particles in turbulence, formulated in terms of Lagrangian correlation functions of fluid tracers. We compute the variance and the flatness of inertial-particle accelerations and we discuss their dependency on the Stokes number. The closure incorporates effects induced by the Lagrangian correlations along the trajectories of fluid tracers, and its predictions agree well with results of direct numerical simulations of inertial particles in turbulence, provided that the effects induced by inertial preferential sampling of heavy/light particles outside/inside vortices are negligible. In particular, the scheme predicts the correct functional behaviour of the acceleration variance, as a function of $St$, as well as the presence of a minimum/maximum for the flatness of the acceleration of heavy/light particles, in good qualitative agreement with numerical data. We also show that the closure works well when applied to the Lagrangian evolution of particles using a stochastic surrogate for the underlying Eulerian velocity field. Our results support the conclusion that there exist important contributions to the statistics of the acceleration of inertial particles independent of the preferential sampling. For heavy particles we observe deviations between the predictions of the closure scheme and direct numerical simulations, at Stokes numbers of order unity. For light particles the deviation occurs for larger Stokes numbers.

Intrinsic rotation drive by collisionless trapped electron mode turbulence

Both the parallel residual stress and parallel turbulent acceleration driven by electrostatic collisionless trapped electron mode (CTEM) turbulence are calculated analytically using gyrokinetic theory. Quasilinear results show that the parallel residual stress contributes an outward flux of co-current rotation for normal magnetic shear and turbulence intensity profile increasing outward. This may induce intrinsic counter-current rotation or flattening of the co-current rotation profile. The parallel turbulent acceleration driven by CTEM turbulence vanishes, due to the absence of a phase shift between density fluctuation and ion pressure fluctuation. This is different from the case of ion temperature gradient turbulence, for which the turbulent acceleration can provide co-current drive for normal magnetic shear and turbulence intensity profile increasing outward. Its order of magnitude is predicted to be the same as that of the divergence of the residual stress [L. Wang and P. H. Diamond, Phys. Rev. Lett. 110, 265006 (2013)]. A possible connection of these theoretical results to experimental observations of electron cyclotron heating effects on toroidal rotation is discussed.

Full Coverage Shaped-Hole Film Cooling in an Accelerating Boundary Layer With High Freestream Turbulence

Full coverage shaped-hole film cooling and downstream heat transfer measurements have been acquired in the accelerating flows over a large cylindrical leading edge test surface. The shaped holes had an 8 deg lateral expansion angled at 30 deg to the surface with spanwise and streamwise spacings of 3 diameters. Measurements were conducted at four blowing ratios, two Reynolds numbers, and six well documented turbulence conditions. Film cooling measurements were acquired over a four to one range in blowing ratio at the lower Reynolds number and at the two lower blowing ratios for the higher Reynolds number. The film cooling measurements were acquired at a coolant to free-stream density ratio of approximately 1.04. The flows were subjected to a low turbulence (LT) condition (Tu = 0.7%), two levels of turbulence for a smaller sized grid (Tu = 3.5% and 7.9%), one turbulence level for a larger grid (8.1%), and two levels of turbulence generated using a mock aerocombustor (AC) (Tu = 9.3% and 13.7%). Turbulence level is shown to have a significant influence in mixing away film cooling coverage progressively as the flow develops in the streamwise direction. Effectiveness levels for the AC turbulence condition are reduced to as low as 20% of LT values by the furthest downstream region. The film cooling discharge is located close to the leading edge with very thin and accelerating upstream boundary layers. Film cooling data at the lower Reynolds number show that transitional flows have significantly improved effectiveness levels compared with turbulent flows. Downstream effectiveness levels are very similar to slot film cooling data taken at the same coolant flow rates over the same cylindrical test surface. However, slots perform significantly better in the near discharge region. These data are expected to be very useful in grounding computational predictions of full coverage shaped-hole film cooling with elevated turbulence levels and acceleration. Infrared (IR) measurements were performed for the two lowest turbulence levels to document the spanwise variation in film cooling effectiveness and heat transfer.

TURBULENT COSMIC-RAY REACCELERATION AT RADIO RELICS AND HALOS IN CLUSTERS OF GALAXIES

Radio relics are synchrotron emission found on the periphery of galaxy clusters. From the position and the morphology, it is often believed that the relics are generated by cosmic-ray (CR) electrons accelerated at shocks through a diffusive shock acceleration (DSA) mechanism. However, some radio relics have harder spectra than the prediction of the standard DSA model. One example is observed in the cluster 1RXS J0603.3+4214, which is often called the ``Toothbrush Cluster''. Interestingly, the position of the relic is shifted from that of a possible shock. In this study, we show that these discrepancies in the spectrum and the position can be solved if turbulent (re)acceleration is very effective behind the shock. This means that for some relics turbulent reacceleration may be the main mechanism to produce high-energy electrons, contrary to the common belief that it is the DSA. Moreover, we show that for efficient reacceleration, the effective mean free path of the electrons has to be much smaller than their Coulomb mean free path. We also study the merging cluster 1E 0657-56 or the ``Bullet Cluster'', in which a radio relic has not been found at the position of the prominent shock ahead of the bullet. We indicate that a possible relic at the shock is obscured by the observed large radio halo that is generated by strong turbulence behind the shock. We propose a simple explanation if the morphological differences of radio emission among the Toothbrush, the Bullet, and the Sausage (CIZA J2242.8+5301) Clusters.

The challenge of turbulent acceleration of relativistic particles in the intra-cluster medium

Acceleration of cosmic-ray electrons (CRe) in the intra-cluster medium (ICM) is probed by radio observations that detect diffuse, megaparsec-scale, synchrotron sources in a fraction of galaxy clusters. Giant radio halos are the most spectacular manifestations of non-thermal activity in the ICM and are currently explained assuming that turbulence, driven during massive cluster–cluster mergers, reaccelerates CRe at several giga-electron volts. This scenario implies a hierarchy of complex mechanisms in the ICM that drain energy from large scales into electromagnetic fluctuations in the plasma and collisionless mechanisms of particle acceleration at much smaller scales. In this paper we focus on the physics of acceleration by compressible turbulence. The spectrum and damping mechanisms of the electromagnetic fluctuations, and the mean free path (mfp) of CRe, are the most relevant ingredients that determine the efficiency of acceleration. These ingredients in the ICM are, however, poorly known, and we show that calculations of turbulent acceleration are also sensitive to these uncertainties. On the other hand this fact implies that the non-thermal properties of galaxy clusters probe the complex microphysics and the weakly collisional nature of the ICM.

Relative drifts and temperature anisotropies of protons andαparticles in the expanding solar wind: 2.5D hybrid simulations

We perform 2.5D hybrid simulations to investigate the origin and evolution of relative drift speeds between protons and $\alpha$ particles in the collisionless turbulent low-$\beta$ solar wind plasma. We study the generation of differential streaming by wave-particle interactions and absorption of turbulent wave spectra. Next we focus on the role of the relative drifts for the turbulent heating and acceleration of ions in the collisionless fast solar wind streams. The energy source is given by an initial broad-band spectrum of parallel propagating Alfv\'en-cyclotron waves, which co-exists with the plasma and is self-consistently coupled to the perpendicular ion bulk velocities. We include the effect of a gradual solar wind expansion, which cools and decelerates the minor ions. This paper for the first time considers the combined effect of self-consistently initialized dispersive turbulent Alfv\'enic spectra with differentially streaming protons and $\alpha$ particles in the expanding solar wind outflows within a 2.5D hybrid simulation study. In the non-expanding wind, we find a threshold value of the differential streaming $V_{\alpha p} = 0.5 V_\mathrm{A}$, for which the relative drift speed remains nearly steady. For ions, streaming below the threshold value, the waves act to increase the magnitude of the relative drift speed. Ions, which stream faster than the threshold value become subject to nonlinear streaming instability and as the system evolves their bulk velocities decrease. We find that the solar wind expansion strongly affects the relative drift speeds and significantly slows down both ion species for all values of the relative drift speeds considered in this study.

THE SMARTS MULTI-EPOCH OPTICAL SPECTROSCOPY ATLAS (SaMOSA): AN ANALYSIS OF EMISSION LINE VARIABILITY IN SOUTHERN HEMISPHERE FERMI BLAZARS

We present multi-epoch optical spectroscopy of seven southern Fermi-monitored blazars from 2008 - 2013 using the Small and Medium Aperture Research Telescope System (SMARTS), with supplemental spectroscopy and polarization data from the Steward Observatory. We find that the emission lines are much less variable than the continuum; 4 of 7 blazars had no detectable emission line variability over the 5 years. This is consistent with photoionization primarily by an accretion disk, allowing us to use the lines as a probe of disk activity. Comparing optical emission line flux with Fermi $\gamma$-ray flux and optical polarized flux, we investigate whether relativistic jet variability is related to the accretion flow. In general, we see no such dependence, suggesting the jet variability is likely caused by internal processes like turbulence or shock acceleration rather than a variable accretion rate. However, three sources showed statistically significant emission line flares in close temporal proximity to very large Fermi $\gamma$-ray flares. While we do not have sufficient emission line data to quantitatively assess their correlation with the $\gamma$-ray flux, it appears that in some cases, the jet might provide additional photoionizing flux to the broad line region, which implies some gamma-rays are produced within the broad line region, at least for these large flares.

Linear vs. nonlinear acceleration in plasma turbulence. II. Hall–finite-Larmor-radius magnetohydrodynamics

The local k-space ratio of linear and nonlinear accelerations associated with a variety of initial conditions undergoing steady relaxation is investigated for the Hall–finite-Larmor-radius magnetohydrodynamics (MHD) system in the presence of a mean magnetic field. Building on a related study (Paper I) where it was shown that discrepancies exist between describing the global and local characterizations of the pure MHD system with mean magnetic field, we find regions of the Fourier space that are consistently dominated by linear acceleration and other regions that are consistently dominated by nonlinear acceleration, independent of the overall system's description as linear, weakly nonlinear, or turbulent. In general, dynamics within a certain angular range of the mean magnetic field direction are predominantly linear, while dynamics adjacent the Hall scales along the field-parallel direction and dynamics adjacent the finite Larmor radius scales in the field-perpendicular direction can become strongly nonlinear. The nonlinear influences are particularly significant as the plasma beta increases from unity to higher values.

Linear vs. nonlinear acceleration in plasma turbulence. I. Global versus local measures

Magnetized turbulent plasmas are generally characterized as strongly or weakly turbulent based on the average relative strengths of the linear and nonlinear terms. While this description is useful, it does not represent the full picture and can be misleading. We study the variation of linear and nonlinear accelerations in the Fourier space of a magnetohydrodynamic system with a mean magnetic field and broad selection of initial states and plasma parameters. We show that the local picture can show significant departures from what is expected from the general global picture. We find that high cross helicity systems that are traditionally believed to have relatively weaker nonlinearities, compared to low cross helicity systems, can show strong nonlinearities in parts of the Fourier space that are orthogonal to the mean magnetic field direction. In some cases, these nonlinearities can exceed in strength the level of nonlinearities recovered from low cross helicity systems.

Intrinsic rotation in tokamaks: theory

Self-consistent equations for intrinsic rotation in tokamaks with small poloidal magnetic field Bp compared to the total magnetic field B are derived. The model gives the momentum redistribution due to turbulence, collisional transport and energy injection. Intrinsic rotation is determined by the balance between the momentum redistribution and the turbulent diffusion and convection. Two different turbulence regimes are considered: turbulence with characteristic perpendicular lengths of the order of the ion gyroradius, ρi, and turbulence with characteristic lengths of the order of the poloidal gyroradius, (B/Bp)ρi. Intrinsic rotation driven by gyroradius scale turbulence is mainly due to the effect of neoclassical corrections and of finite orbit widths on turbulent momentum transport, whereas for the intrinsic rotation driven by poloidal gyroradius scale turbulence, the slow variation of turbulence characteristics in the radial and poloidal directions and the turbulent particle acceleration can be become as important as the neoclassical and finite orbit width effects. The magnetic drift is shown to be indispensable for the intrinsic rotation driven by the slow variation of turbulence characteristics and the turbulent particle acceleration. The equations are written in a form conducive to implementation in a flux tube code, and the effect of the radial variation of the turbulence is included in a novel way that does not require a global gyrokinetic formalism.

1008 水素混合気の乱流燃焼速度特性と火炎伝ぱ加速特性との関係に関する実験的研究(燃焼・エンジンシステムII)

1008 水素混合気の乱流燃焼速度特性と火炎伝ぱ加速特性との関係に関する実験的研究(燃焼・エンジンシステムII)

Flat-plate film cooling from a double jet holes: influence of free-stream turbulence and flow acceleration

Results of an experimental study of flat-plate film cooling effectiveness achieved with an inlet double jet scheme are reported. At low (m = 0.5) and medium (m = 1.0) blowing ratio the average film cooling effectiveness is about 20 % greater of the traditional two-row scheme of round holes data, while at higher m = 1.5 it is close to it. The free-stream turbulence (≈ 7 %) influences weekly on the average flat-plate film cooling effectiveness. The flow acceleration decreases the film cooling effectiveness down to 25 % when the pressure gradient parameter K is ranged from 0.5·10−6 to 3.5·10−6.

Onsager's symmetry relation and the residual parallel Reynolds stress in a magnetized plasma with electrostatic turbulence

The physics of the residual parallel Reynolds stress in a rotating plasma with electrostatic turbulence is explicitly identified by using the transport formulation of the gyrokinetic turbulence. It is clarified that the residual stress consists of four terms, among which are the cross terms due to the pressure gradient and the temperature gradient and the terms related to the turbulent acceleration impulse and the turbulent heating rate. The last two terms are identified for the first time, and are shown to cause analogous residual term in the heat flux. Meanwhile, the transport matrix reveals diffusion in the phase space. The transport matrix is demonstrated to satisfy the Onsager's symmetry relation.