Turbulence Spreading Research Articles

The Kuroshio flowing over seamounts and associated submesoscale flows drive 100-km-wide 100-1000-fold enhancement of turbulence

Although previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. Here, we conducted a series of intensive in-situ turbulence observations using a state-of-the-art tow-yo microstructure profiler in the Kuroshio flowing over the seamounts of the Tokara Strait, south of Kyusyu Japan, in November 2017, June 2018, and November 2019, and employed a high-resolution numerical model to elucidate the turbulence generation mechanisms. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. The observed turbulence could supply nutrients to sunlit layers, promoting phytoplankton primary production and CO2 uptake.

Hyperdiffusion of dust particles in a turbulent tokamak plasma

The effect of plasma turbulence on the trajectories of dust particles is investigated for the first time. The dynamics of dust particles is computed using the ad hoc developed Dust Injection Simulator code, using a 3D turbulent plasma background computed with the TOKAM3X code. As a result, the evolution of the particle trajectories is governed by the ion drag force, and the shape of the trajectory is set by the Stokes number St∝ad/n0, with ad the dust radius and n0 the density at the separatrix. The plasma turbulence is observed to scatter the dust particles, exhibiting a hyperdiffusive regime in all cases. The amplitude of the turbulent spread of the trajectories Δr2 is shown to depend on the ratio Ku/St, with Ku∝urms the Kubo number and urms the fluctuation level of the plasma flow. These results are compared with a simple analytical model, predicting Δr2∝(Ku/St)2t3, or Δr2∝(urmsn0/ad)2t3. As the dust is heated by the plasma fluxes, thermionic emission sets the dust charge, originally negative, to slightly positive values. This results in a substantial reduction of the ion drag force through the suppression of its Coulomb scattering component. The dust grain inertia is then no longer negligible and drives the transition from a hyperdiffusive regime toward a ballistic one.

Physics of turbulence spreading and explicit nonlocality

In this paper, we systematically derive a model for turbulence spreading from the basic kinetic equation. The model contains explicit nonlocal nonlinear diffusion and nonlocal growth. When the nonlocality scale parameter δ b (banana width) vanishes, this model reduces to the usual turbulence spreading model. We elucidate the mechanisms of nonlinear saturation and nonlocal growth. Results show that nonlocal effects, especially the nonlocal growth, thicken the turbulence spreading front and increase the speed of front propagation. More turbulence intensity penetrates the stable region when δ b increases. The penetration depth Δ p is proportional to , therefore the fraction of turbulence in the unstable region scales as 1 − δ b*. The transport scales the same way.

High-speed turbulent gas jets: an LES investigation of Mach and Reynolds number effects on the velocity decay and spreading rate

The aim of this work is the investigation of Mach and Reynolds numbers effects on the behaviour of turbulent gas jets in order to gain new insights into the fluid dynamic process of turbulent jet mixing and spreading. An in-house solver (Flow-Large Eddy and Direct Simulation, FLEDS) of the Favre-filtered Navier Stokes equations has been used. Compressibility has been analyzed by considering gas jets with Mach number equal to 0.8, 1.4, 2.0 and 2.6, and Re equal to 10,000. As concerns the influence of Re on gas jets, four cases have been investigated, i.e. mathrm{Re} = 2500, 5000, 10,000 and 20,000, with Mach number equal to 1.4. The results show that, in accordance with previous experimental and numerical studies, the potential core length increases with Mach number. As regards the velocity decay and the spreading rate downstream of the potential core, compressibility effects are not relevant except for the jet with Mach number of 2.6. The normalized turbulent kinetic energy along the centerline as a function of the normalized streamwise distance shows a similar peak at the end of the potential core for all jets, except for the case with Mach number of 2.6. By increasing Re, the length of the potential core decreases up to the same value for all Re higher than 10,000. In the region downstream of the potential core, the velocity decay decreases as Re number increases from 10,000 to 20,000, whereas, for lower values of Re, the influence is almost negligible.

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak

Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between an MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented further understanding. Here, we provide comprehensive observations such as turbulence spreading into an MI and turbulence enhancement at the reconnection site, elucidating intricate effects of plasma turbulence on the nonlinear MI evolution.

Restricted optimal paths to transition in a plane Couette flow

To identify laminar/turbulent transition paths in plane Couette flow, a variational formulation incorporating a restricted nonlinear (RNL) system that retains a single streamwise Fourier mode, is used. Considering the flow geometry originally used by Monokrousos et al. (2011) and Duguet et al. (2013) and the same Reynolds numbers (Re), we show that initial perturbations obtained by RNL optimizations exhibit spatial localization. Two optimal states are found with comparable initial energy levels above which the flow structure evolves to turbulence. It is found that this level is twice that of the minimal threshold energy which has been obtained using the full nonlinear equations (Duguet et al. (2013)). Especially, the Re dependence of energy thresholds is studied within a RNL optimization framework for the first time, with evidence for a ORe−2.65 scaling close to the one found using the full Navier–Stokes equations (ORe−2.7). The first state is obtained for a short target time. It is symmetric with respect to the mid-plane y=0 and spanwise localized. For a long target time, the optimal appears to be localized in both spanwise and wall-normal directions. The mechanisms highlighted within the scope of nonlinear nonmodal theory (Kerswell (2018)): Orr mechanism, oblique wave interaction, lift-up, streak breakdown, localized pocket of turbulence and turbulence spreading, are also observed in the RNL simulations. Although greatly simplified, the RNL system provides a good approximation of these different fundamental mechanisms. The analysis gives then some insight into the potential of RNL optimizations for estimating Re scaling laws and routes to turbulence for shear flows.

Effect of pressure gradients on the different stages of roughness induced boundary layer transition

The physical mechanisms of roughness-induced transition (RIT) in pressure gradient boundary layers are studied using direct numerical simulations. Recent investigations have examined RIT processes in zero-pressure-gradient boundary layers (Suryanarayanan et al., 2019). The present study uses a vorticity dynamics point of view to examine how these processes are altered by a locally accelerating or decelerating flow that strains the vorticity field and creates a net vorticity flux at the wall. Flow acceleration is imposed on specific streamwise extents of the flow. This provides an understanding about how the fundamental mechanisms in different stages of RIT are affected by pressure gradients. The present results suggest that both lift-up and subsequent amplification of the unsteady perturbations are mitigated by flow acceleration. The effect on lift-up is explained by the compression (i.e. large negative value of the stretching term) of the wall-normal vorticity by negative dv/dy. Consistent with earlier experimental observations on spots and wedges, favorable pressure gradients reduce turbulent wedge spreading and nearly arrest the spreading when sufficiently strong. This result is also explained in terms of vorticity dynamics.

Global gyrokinetic particle simulations of microturbulence in W7-X and LHD stellarators

Global gyrokinetic particle simulations of electrostatic ion temperature gradient (ITG) instability show that the most unstable eigenmode is localized to some magnetic fieldlines or discrete locations on the poloidal plane in the Wendelstein 7-X (W7-X) stellarator due to its mirror-like magnetic fields, which vary strongly in the toroidal direction and induce coupling of more toroidal harmonics (n) to form the linear eigenmode than in the Large Helical Device (LHD) stellarator. Nonlinear electrostatic simulation results show that self-generated zonal flows are the dominant saturation mechanism for the ITG instabilities in both the LHD and W7-X. Furthermore, radial widths of the fluctuation intensity in both the LHD and W7-X are significantly broadened from the linear phase to the nonlinear phase due to turbulence spreading. Finally, nonlinear spectra in the W7-X are dominated by low-n harmonics, which can be generated both by nonlinear toroidal coupling of high-n harmonics and by linear toroidal coupling with large amplitude zonal flows due to the 3D equilibrium magnetic fields.

Experimental Investigation on Electromagnetic Waves Transmitting Through Exhaust Plume: From Propagation to Channel Characteristics

Exhaust-plume-induced plasma from solid rocket engines absorbs electromagnetic (EM) waves and may cause communication interruptions. Previous articles have proposed both physical phenomena and propagation models for this problem, but few articles have analyzed random fluctuations of EM waves propagating through plumes from a communications perspective. This article attempts to attach these fluctuations, which are caused by the plume-induced turbulent plasma spread medium, to the communications channel characteristics. A multi-parameter stochastic model is established to characterize the exhaust plume channel in which the first-order and the second-order statistical properties are presented. Accordingly, a ground firing test was conducted to validate the EM wave propagation characteristics and the channel characteristics. The experimental results are in good consistence with the theoretical results. The results indicated that the time-varying envelope of the transmitted EM waves obeys a log-normal distribution, whereas the time-varying phase shift obeys a normal distribution. Moreover, the time–frequency analysis method was applied to evaluate the EM–plume interactions in the time–frequency domain. These results are critical for designing the rocket communication systems and evaluating the systems performance.

When does turbulence spreading matter?

Few, if any, of the many papers on turbulence spreading address the key question of how turbulence spreading actually affects the profile structure. Here, we are using a reduced model to answer that question. Turbulence spreading is most relevant near regions where the profiles support a strong intensity gradient ∇I. One such case is at the edge of an L mode discharge, near a source of turbulence [i.e., either a localized source of edge turbulence or an influx of turbulence from the scrape-off layer (SOL)]. Another is in “No Man's Land” (NML), which connects the pedestal to the stiff core in H mode. In the case of L mode, without an edge intensity source, the turbulence intensity profile is nearly flat and spreading has a weak effect. An edge localized source increases the edge ∇I, which then drives inward spreading. Invasion of turbulence from the SOL to the edge softens the edge pressure gradient. In H mode, the strong shear suppression of pedestal turbulence necessarily forces a sharp ∇I in NML. This sharp ∇I drives a significant flux of turbulence from the core to the pedestal, where it is ultimately dissipated by shearing. Counter-intuitively, the results indicate that spreading actually increases the pedestal height and width and hence the energy content in H mode. This suggests that models of the pedestal structure should include NML turbulence spreading effects. The relation of avalanches to spreading is studied. Spreading weakly affects the avalanche distribution, but the spatiotemporal correlation of intensity increases with spreading.

Experimental study of a lenticular jet.

Non-circular jets, particularly elongated jets and jets with sharp corners, are of interest due to their enhanced large- and small-scale mixing and combustion stability. Non-circular jet geometries such as elliptic, triangular, rectangular, and square have been extensively studied. The non-uniform curvature and elongated design of elliptic and rectangular jets has been shown to facilitate axis switching. Additionally, jets with sharp corners have been shown to facilitate small-scale mixing and further vortex ring deformation, as has been observed in triangular, square, and rectangular jets. There are, however, currently no data on the flow characteristics of a jet that combines these two features, i.e., it enhances both large- and small-scale mixing. The jet is issuing from a nozzle shape defined by two arcs of a circle connecting at sharp corners in the shape of an eye (hereafter termed 'lenticular'). The current study uses experimental techniques to characterize the lenticular jet and compare its behavior to previously studied circular and non-circular jets. Additionally, snapshot POD analysis was used to identify coherent structures and their dynamic features. The lenticular jet was found to have higher entrainment and stronger mixing than a traditional round jet, and comparable mixing and entrainment characteristics as previously studied non-circular jet geometries. It is particularly interesting to compare the jet behavior of the lenticular jet to an elliptic jet. The geometry of these two jets is extremely similar, the difference being sharp corners along the major axis of the lenticular jet. By comparing the lenticular jet and the elliptic jet, the effect of sharp corners, which have been shown to increase small-scale mixing and vortex ring deformation, can be observed. It was found that along the minor axis, where geometry was similar, shear layer turbulence intensity and spreading rate of the jets were also similar. Along the major axis, however, introducing corner features in the lenticular jet lowered the spread rate, resulted in a faster breakdown of turbulence in the shear layer, increased exit turbulence, and resulted in anisotropic centerline turbulence.

A closer look at turbulence spreading: How bistability admits intermittent, propagating turbulence fronts

In magnetic fusion plasmas, mounting evidence suggests the possibility of sustained turbulence below the linear stability threshold or more generally global turbulence bistability. The usual reduced models for turbulence spreading are unistable/supercritical and incompatible with this result. The older models also cannot realistically support fronts connecting laminar and turbulent domains. In this work, a minimal model for “subcritical” turbulence spreading is introduced and analyzed. The model may be viewed as phenomenological or derived directly by considering the effect of profile corrugations in an E × B staircase. The model, which is related to the FitzHugh–Nagumo system, supports the robust coexistence of multiple turbulence levels via bistability. We show that this model predicts stronger penetration of turbulence into a linearly stable region as well as the formation of intermittent turbulence fronts that resemble avalanches. We derive the critical size that a localized slug of turbulence must exceed in order to spread. Finally, we make a prediction of global hysteretic behavior associated with the bistability, which should be testable via experiment.

Scrape-off layer (SOL) power width scaling and correlation between SOL and pedestal gradients across L, I and H-mode plasmas at ASDEX Upgrade

A cross-regime (L-mode, I-mode and H-mode) database combining scrape-off layer (SOL) power decay length λq divertor measurements and upstream SOL electron pressure, temperature and density decay lengths has been assembled at ASDEX Upgrade. It is found that a cross-regime λq scaling is best described by a local edge quantity, such as the edge electron pressure evaluated at ρpol = 0.95. Furthermore, λq exhibits a clear correlation with edge electron pressure gradient lengths, no matter if taken inside or outside the separatrix. In addition, the database reveals that SOL and pedestal electron pressure gradients are remarkably well correlated across all confinement regimes. The physical interpretation of this observation is discussed with regard to an edge pressure critical gradient paradigm governing the edge physics and to a turbulence spreading in the SOL. Moreover, it is shown that the Spitzer–Härm electron conduction regime is a reasonable approximation to estimate λq across different confinement regimes. The main implication of these findings is that a widening of λq is linked to a reduction of edge electron pressure gradients.

Three-dimensional Simulation of Turbulent Rectangular Jet Spreading Technology of Carbon Fiber Bundle

In order to smoothly spread large-sized carbon-fiber bundle, hot-jet blower is added to a turbulent spreading platform to conduct pre-spreading process. To achieve the best spreading performance, CFD model of the blower are simulated based on the Reynolds average Naiver-Stokes equation. The influence of the relevant geometric parameters and process parameters on the internal flow field are analyzed. The distributions of streamline, surface temperature and air pressure under different working condition are characterized and compared parametrically. The effective area of the nozzle directly contacts with fiber wall surface. Simulating model has been built and crossover experiments have been carried out on the spreading platform to investigate and optimize the influence of jet distance, nozzle air pressure and hot-jet temperature on the spreading performance of fiber bundle. According to the experimental data and simulating results, relevant geometric parameters and process parameters of the nozzle are optimized.

Radiation thermo-chemical models of protoplanetary disks

Context. Disks around pre-main-sequence stars evolve over time by turbulent viscous spreading. The main contender to explain the strength of the turbulence is the magnetorotational instability model, whose efficiency depends on the disk ionization fraction. Aims. Our aim is to compute self-consistently the chemistry including polycyclic aromatic hydrocarbon (PAH) charge chemistry, the grain charging, and an estimate of an effective value of the turbulence α parameter in order to find observational signatures of disk turbulence. Methods. We introduced PAH and grain charging physics and their interplay with other gas-phase reactions in the physico-chemical code PRODIMO. Non-ideal magnetohydrodynamics effects such as ohmic and ambipolar diffusion are parametrized to derive an effective value for the turbulent parameter αeff. We explored the effects of turbulence heating and line broadening on CO isotopologue submillimeter lines. Results. The spatial distribution of αeff depends on various unconstrained disk parameters such as the magnetic parameter βmag or the cosmic ray density distribution inside the protoplanetary disk s. The inner disk midplane shows the presence of the so-called dead zone where the turbulence is almost inexistent. The disk is heated mostly by thermal accommodation on dust grains in the dead zone, by viscous heating outside the dead zone up to a few hundred astronomical units, and by chemical heating in the outer disk. The CO rotational lines probe the warm molecular disk layers where the turbulence is at its maximum. However, the effect of turbulence on the CO line profiles is minimal and difficult to distinguish from the thermal broadening. Conclusions. Viscous heating of the gas in the disk midplane outside the dead zone is efficient. The determination of α from CO rotational line observations alone is challenging.

Bifurcations to turbulence in transitional channel flow

In wall-bounded parallel flows, sustained turbulence can occur even while laminar flow is still stable. Channel flow is one of such flows and displays spatio-temporal fluctuating patterns of localized turbulence along its way from/to featureless turbulence. By direct numerical simulation, we study the observed inconsistency between turbulence decay according to a two-dimensional directed-percolation (2D-DP) scenario and the presence of sustained oblique localized turbulent bands (LTBs) below the DP critical point. Above Reynolds number Reg \sim 700 sustained LTBs are observed; most LTBs have the same orientation so that the spanwise symmetry of the LTB pattern is broken below Re2 \sim 1000. The frequency of transversal splitting, by which an LTB generates another one with opposite obliqueness, so that turbulence spreading becomes intrinsically two dimensional, increases in the range Reg < Re < Re2. It reaches a critical rate at Re2 beyond which symmetry is restored. 2D-DP behavior is retrieved only above Re2. A mean-field model is proposed which qualitatively accounts for the above symmetry-restoring bifurcation by considering interactions between space-averaged densities of LTBs propagating in either direction.

On the interplay between MHD instabilities and turbulent transport in magnetically confined plasmas

The interplay between MHD and turbulence is an interesting topic in magnetically confined plasma and solar plasma. The experimental observations made recently, shown below, suggest coupling and interplay between MHD and turbulence in magnetically confined toroidal plasmas. (1) Turbulence spreading into the magnetic island, (2) there is a self-organized change in topology and turbulence in the magnetic island, (3) the flow is damped by a stochastic magnetic field, (4) the trigger mechanism for the MHD bursts, (5) MHD bursts have an impact on the ion velocity distribution and potential, and (6) turbulence exhausts are created at the MHD burst event. In this paper, experimental evidence for the interplay between MHD and turbulence in toroidal plasmas is reviewed. The physics mechanism of the interplay and a possible link to astrophysical plasma physics are also discussed.

Controlled neoclassical tearing mode (NTM) healing by fueling pellets and its impact on electron cyclotron current drive requirements for complete NTM stabilization

Controlled partial stabilization of core m/n = 2/1 neoclassical tearing modes (NTMs) by fueling deuterium pellets is reported in DIII-D and KSTAR H-mode plasmas (m/n are the poloidal/toroidal mode numbers). Analyses of DIII-D data exploring possible physics origins show that an explanation is offered by NTM-turbulence multi-scale interaction, triggered by a sudden increase of local gradients near q = 2 caused by the pellet. Pellet injection from the high-field side allows deep fueling which reaches the island region. In turn, low-k turbulent density fluctuations () increase by in the island region. This can drive transport across the island separatrices, reducing the pressure flat spot at the O-point and diminishing the NTM drive. The Mirnov probe array detects the reduction of the 2/1 magnetic amplitude by up to . Causality between elevated gradients outside of the island, turbulence spreading into the island and reduced NTM drive is qualitatively supported by non-linear gyrokinetic turbulence simulations. These show increased penetration of ion-scale from the background plasma to the O-point region when the background gradient is increased. This interaction has potentially far reaching consequences as it can lead to a reduction of the required electron cyclotron current density () for NTM suppression by 70%, as predicted by the modified Rutherford equation. This beneficial effect of fueling pellets can be important as is the anticipated active NTM control technique for ITER, but its efficiency will be lowered by third harmonic absorption in Pre-Fusion Power Operation-1 (PFPO-1) at half magnetic field.

Scale selection and feedback loops for patterns in drift wave-zonal flow turbulence

The scale selection and feedback loops for the formation and sustainment of a mesoscopic staircase profile structure are investigated for drift wave-zonal flow turbulence. A mean field model derived from the Hasegawa–Wakatani system and including the evolution of mean density, mean vorticity and perturbed potential enstrophy (PE) is used. It is found that a quasi-periodic zonal staircase forms from self-sharpening of modulation. The principle feedback loop is through the nonlinear dependence of mixing length on electron density gradient, which enters by way of the potential vorticity gradient. Counterintuitively, shearing is not effective. Moreover, the number of steps in the staircase is sensitive to both the drive (production rate of PE and initial density gradient) and damping (flow viscosity and collisional diffusivity) factors. The minimal step scale is selected by competition between the initial density gradient and diffusive dissipation. Finite turbulence spreading is necessary to form the staircase, but moderate enhancement of turbulence spreading tends to wash out the pattern. The staircase retains a memory of its initial state. Both the mean shear and zonal shear affect the staircase evolution. A strong mean shear quenches the pattern by suppressing the drift wave turbulence. The implications of these findings are also discussed.

Overview of recent TJ-II stellarator results

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components.As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presented.