Different Types Of Turbulence Research Articles

The effects of finite electron inertia on helicity-barrier-mediated turbulence

Understanding the partitioning of turbulent energy between ions and electrons in weakly collisional plasmas is crucial for the accurate interpretation of observations and modelling of various astrophysical phenomena. Many such plasmas are ‘imbalanced’, wherein the large-scale energy input is dominated by Alfvénic fluctuations propagating in a single direction. In this paper, we demonstrate that when strongly-magnetised plasma turbulence is imbalanced, nonlinear conservation laws imply the existence of a critical value of the electron plasma beta (the ratio of the thermal to magnetic pressures) that separates two dramatically different types of turbulence in parameter space. For betas below the critical value, the free energy injected on the largest scales is able to undergo a familiar Kolmogorov-type cascade to small scales where it is dissipated, heating electrons. For betas above the critical value, the system forms a ‘helicity barrier’ that prevents the cascade from proceeding past the ion Larmor radius, causing the majority of the injected free energy to be deposited into ion heating. Physically, the helicity barrier results from the inability of the system to adjust to the disparity between the perpendicular-wavenumber scalings of the free energy and generalised helicity below the ion Larmor radius; restoring finite electron inertia can annul, or even reverse, this disparity, giving rise to the aforementioned critical beta. We relate this physics to the ‘dynamic phase alignment’ mechanism (that operates under yet lower beta conditions and in pair plasmas), and characterise various other important features of the helicity barrier, including the nature of the nonlinear wavenumber-space fluxes, dissipation rates, and energy spectra. The existence of such a critical beta has important implications for heating, as it suggests that the dominant recipient of the turbulent energy, ions or electrons, can depend sensitively on the characteristics of the plasma at large scales.

Experimental investigation and performance prediction of SAH using different arc rib roughness geometries – A comparative study

This study aims to investigate and compare the thermal performance of a solar air heater using a passive technique to enhance heat transfer between the absorber plate and the flowing fluid. The technique involves generating turbulence near the heat transferring surface through the use of artificial rib roughness. The study focuses on two different novel roughness geome-tries: full symmetrical arc rib roughness and half symmetrical arc rib roughness. By introducing additional gaps and varying the number of gaps in the roughness geometries, the study examines their effects on the solar air heaters thermal performance. The artificially roughened surface creates different turbulent zones, which are essential to the development of different types of turbulence in the vicinity of the heat transferring surface. The study finds that an optimal escalation in Nusselt number and friction factor by 2.36 and 3.45 times, respectively, occurs at certain gap numbers as 6 and ng as 5 for full symmetrical arc rib roughness. The maximum thermal-hydraulic performance parameter of 1.66 is attained at a Reynolds number of 6 000. The study also conducts correlation, mathematical modeling, and performance prediction under different operating circumstances.

Langmuir turbulence in suspended kelp farms

This study investigates the influence of suspended kelp farms on ocean mixed layer hydrodynamics in the presence of currents and waves. We use the large eddy simulation method, where the wave effect is incorporated by solving the wave-averaged equations. Distinct Langmuir circulation patterns are generated within various suspended farm configurations, including horizontally uniform kelp blocks and spaced kelp rows. Intensified turbulence arises from the farm-generated Langmuir circulation, as opposed to the standard Langmuir turbulence observed without a farm. The creation of Langmuir circulation within the farm is attributed to two primary factors depending on farm configuration: (i) enhanced vertical shear due to kelp frond area density variability, and (ii) enhanced lateral shear due to canopy discontinuity at lateral edges of spaced rows. Both enhanced vertical and lateral shear of streamwise velocity, representing the lateral and vertical vorticity components, respectively, can be tilted into downstream vorticity to create Langmuir circulation. This vorticity tilting is driven by the Craik–Leibovich vortex force associated with the Stokes drift of surface gravity waves. In addition to the farm-generated Langmuir turbulence, canopy shear layer turbulence is created at the farm bottom edge due to drag discontinuity. The intensity of different types of turbulence depends on both kelp frond area density and the geometric configuration of the farm. The farm-generated turbulence has substantial consequences for nutrient supply and kelp growth. These findings also underscore the significance of the presence of obstacle structures in modifying ocean mixed layer characteristics.

Coexistence of acoustic waves and turbulence in low Mach number compressible flows

In this work, direct numerical simulations of the compressible fluid equations in turbulent regimes are performed. The behavior of the flow is either dominated by purely turbulent phenomena or by the generation of sound waves in it. Previous studies suggest that three different types of turbulence may happen at the low Mach number limit in polytropic flows: Nearly incompressible, modally equipartitioned compressible, and compressible wave. The distinction between these types of turbulence is investigated here applying different kinds of forcing. Scaling of density fluctuations with Mach number, comparison of the ratio of transverse velocity fluctuations to longitudinal fluctuations, and spectral decomposition of fluctuations are used to distinguish the nature of these solutions. From the study of the spatio-temporal spectra and correlation times, we quantify the contribution of the waves to the total energy of the system. Also, in the dynamics of a compressible flow, three associated correlation times are considered: the non-linear time of local interaction between scales, the sweeping time or non-local time of large scales on small scales, and the time associated with acoustic waves (sound). We observed that different correlation times dominate depending on the wave number (k), the Mach number, and the type of forcing.

Decay of grid turbulence in superfluid helium-4: Mesh dependence

Temporal decay of grid turbulence is experimentally studied in superfluid 4He in a large square channel. The second sound attenuation method is used to measure the turbulent vortex line density (L) with a phase locked tracking technique to minimize frequency shift effects induced by temperature fluctuations. Two different grids (0.8 mm and 3.0 mm mesh) are pulled to generate turbulence. Different power laws for decaying behavior are predicted by a theory. According to this theory, L should decay as t−11/10 when the length scale of energy containing eddies grows from the grid mesh size to the size of the channel. At later time, after the energy containing eddy size becomes comparable to the channel, L should follow t−3/2. Our recent experimental data exhibit evidence for t−11/10 during the early time and t−2 instead of t−3/2 for later time. Moreover, a consistent bump/plateau feature is prominent between the two decay regimes for smaller (0.8 mm) grid mesh holes but absent with a grid mesh hole of 3.0 mm. This implies that in the large channel different types of turbulence are generated, depending on mesh hole size (mesh Reynolds number) compared to channel Reynolds number.

The Different Types of Turbulence in Rotating Spherical Layers

Turbulent flows of a viscous incompressible fluid in a layer between rotating concentric spheres under the action of the modulation of the velocity of one of the spheres have been studied experimentally and numerically. The form of spectra of turbulent pulsations of the azimuthal velocity depends on the sphere whose rotational velocity is modulated, as well as on the amplitude and frequency of modulation. The possibility of the formation of turbulence with spectra qualitatively similar to spectra obtained in measurements in the upper atmosphere is established: with the slope close to –3 at low frequencies and close to –5/3 at high frequencies and with the negative longitudinal velocity structure function of the third order. It has been shown that such spectra are formed in the regions of a flow that are strongly synchronized under the action of the modulation of the rotational velocity.

Multi-Objective Optimization Algorithm Based on Sperm Fertilization Procedure (MOSFP)

In this paper, we propose an extended multi-objective version of single objective optimization algorithm called sperm swarm optimization algorithm. The proposed multi-objective optimization algorithm based on sperm fertilization procedure (MOSFP) operates based on Pareto dominance and a crowding factor, that crowd and filter out the list of the best sperms (global best values). We divide the sperm swarm into three equal parts, after that, different types of turbulence (mutation) operators are applied on these parts, such as uniform mutation, non-uniform mutation, and without any mutation. Our algorithm is compared against three well-known algorithms in the field of optimization. These algorithms are NSGA-II, SPEA2, and OMOPSO. These algorithms are compared using a very popular benchmark function suites called Zitzler-Deb-Thiele (ZDT) and Walking-Fish-Group (WFG). We also adopt three quality metrics to compare the convergence, accuracy, and diversity of these algorithms, including, inverted generational distance (IGD), spread (SP), and epsilon (∈). The experimental results show that the performance of the proposed MOSFP is highly competitive, which outperformed OMOPSO in solving problems such as ZDT3, WFG5, and WFG8. In addition, the proposed MOSFP outperformed both of NSGA-II or SPEA2 algorithms in solving all the problems.

Dynamics of quantum turbulence of different spectra

Turbulence in a superfluid in the zero-temperature limit consists of a dynamic tangle of quantized vortex filaments. Different types of turbulence are possible depending on the level of correlations in the orientation of vortex lines. We provide an overview of turbulence in superfluid (4)He with a particular focus on recent experiments probing the decay of turbulence in the zero-temperature regime below 0.5 K. We describe extensive measurements of the vortex line density during the free decay of different types of turbulence: ultraquantum and quasiclassical turbulence in both stationary and rotating containers. The observed decays and the effective dissipation as a function of temperature are compared with theoretical models and numerical simulations.

Reconnection studies under different types of turbulence driving

Abstract. We study a model of fast magnetic reconnection in the presence of weak turbulence proposed by Lazarian and Vishniac (1999) using three-dimensional direct numerical simulations. The model has been already successfully tested in Kowal et al. (2009) confirming the dependencies of the reconnection speed Vrec on the turbulence injection power Pinj and the injection scale linj expressed by a constraint Vrec ~ Pinj1/2linj3/4and no observed dependency on Ohmic resistivity. In Kowal et al. (2009), in order to drive turbulence, we injected velocity fluctuations in Fourier space with frequencies concentrated around kinj = 1/linj, as described in Alvelius (1999). In this paper, we extend our previous studies by comparing fast magnetic reconnection under different mechanisms of turbulence injection by introducing a new way of turbulence driving. The new method injects velocity or magnetic eddies with a specified amplitude and scale in random locations directly in real space. We provide exact relations between the eddy parameters and turbulent power and injection scale. We performed simulations with new forcing in order to study turbulent power and injection scale dependencies. The results show no discrepancy between models with two different methods of turbulence driving exposing the same scalings in both cases. This is in agreement with the Lazarian and Vishniac (1999) predictions. In addition, we performed a series of models with varying viscosity ν. Although Lazarian and Vishniac (1999) do not provide any prediction for this dependence, we report a weak relation between the reconnection speed with viscosity, Vrec ~ ν−1/4.

Dissipation range turbulent cascades in plasmas

Dissipation range cascades in plasma turbulence are described and spectra are formulated from the scaled attenuation in wavenumber space of the spectral energy transfer rate. This yields spectra characterized by the product of a power law and exponential fall-off, applicable to all scales. Spectral indices of the power law and exponential fall-off depend on the scaling of the dissipation, the strength of the nonlinearity, and nonlocal effects when dissipation rates of multiple fluctuation fields are different. The theory is used to derive spectra for MHD turbulence with magnetic Prandtl number greater than unity, extending previous work. The theory is also applied to generic plasma turbulence by considering the spectrum from damping with arbitrary wavenumber scaling. The latter is relevant to ion temperature gradient turbulence modeled by gyrokinetics. The spectrum in this case has an exponential component that becomes weaker at small scale, giving a power law asymptotically. Results from the theory are compared to three very different types of turbulence. These include the magnetic plasma turbulence of the Madison Symmetric Torus, the MHD turbulence of liquid metal in the Madison Dynamo Experiment, and gyrokinetic simulation of ion temperature gradient turbulence.

Transport properties of finite-β microturbulence

Via nonlinear gyrokinetic simulations, microturbulent transport is investigated for electromagnetic trapped electron mode (TEM) and ion temperature gradient (ITG) tokamak core turbulence with β up to and beyond the kinetic ballooning mode threshold. Deviations from linear expectations are explained by zonal flow activity in the TEM case. For the ITG scenario, β-induced changes are observed in the nonlinear critical gradient upshift—from a certain β, a strong increase is observed in the Dimits shift. Additionally, a Rechester–Rosenbluth-type model for magnetic transport is applied, and the amplitudes of magnetic field fluctuations are quantified for different types of turbulence.

Inter-arrival time patterns in manufacturing systems with main and side loops

Manufacturing systems with main and side loops that employ asynchronous, track-based workpiece holder transport with decentralised control generally allow arbitrarily small distances between workpiece holders, so that their state space is partly continuous. The considered systems show complex inter-arrival time distributions that can reduce system capacity when multiple routes share common tracks. This paper presents an analytical approach to modelling this inter-arrival time behaviour that distinguishes between different zones in parameter space by associating each zone with one specific queueing situation. For each queueing situation, a recursively defined sequence is set up that represents the inter-arrival times of workpiece holders. Two of these sequences yield inherently different types of turbulence that form characteristic patterns in phase diagrams. The emergence of these patterns is validated using a deterministic discrete-event simulation of an assembly system. A synchronisation methodology is introduced that limits the number of different inter-arrival times. The employed queueing–situational decomposition avoids sophisticated mathematical modelling and provides an easy explanation for the root causes of the emerging turbulence. It thereby helps the manufacturing system designer avoid the resulting decrease in capacity and reliability.

A three-wave heterodyne correlation reflectometer developed in the T-10 tokamak

A three-wave heterodyne O-mode reflectometer was developed and tested in experiments on the T-10 tokamak. Three launched waves are obtained by frequency splitting due to amplitude modulation with a p-i-n diode. Source frequencies from 36 to 78 GHz were used in the experiments. The ability of the reflectometer to measure simultaneously density profile characteristics, such as the time evolution of the relative phase and the unambiguous time delay of the reflected wave together with small-scale density fluctuation characteristics such as the radial and poloidal correlation lengths, was demonstrated with T-10 experimental data. The reflectometer can provide important information about fine changes in the density profile during sawteeth and low-m magnetohydrodynamics oscillations. Poloidal correlation measurements make it possible to measure transverse turbulence velocities and velocity shear. It was shown that both types of correlation measurements enable one to distinguish different types of turbulence according to the values of their correlation lengths, and that probing the plasma from several poloidal directions simultaneously greatly enhances the potential of reflectometry for the investigation of turbulence physics. In particular, it was found that different turbulence types may be either poloidally symmetrical or have much higher amplitude on the low field side of the discharge. Probing with several poloidal directions also enables one to make toroidal correlation measurements over long distances at certain resonant q values after one or two turns of magnetic field line around the major tokamak axis. In fact, correlations near 40% were observed experimentally. Such measurements may give additional information about turbulence characteristics on the one hand and allow radial q profile measurements on the other hand. In addition, the capabilities of reflectometry may be enhanced with the application of a proposed holography approach.

Experiments on potential gradients in a current-carrying plasma. II. Turbulence

Two different types of turbulence are observed in the presence of a large potential jump within a current-carrying plasma (vd<vte) and are investigated by means of correlation measurements. The electron beam on the high-potential side induced by the potential jump leads to the high-frequency turbulence ( f=fpe). The unstable waves follow the dispersion relation ω=kvb (vb is the electron beam velocity). The low-frequency turbulence ( f<fpi) observed on the low-potential side could be identified as the ion-acoustic turbulence and is dominated by low-frequency ( f<0.5 fpi ) waves propagating perpendicular to the electron drift.