Local Density Gradient Research Articles

Experimental validation of momentum transport theory in the core of H-mode plasmas in the ASDEX Upgrade tokamak

This study employs the established momentum transport analysis at ASDEX Upgrade [Zimmermann et al., Nucl. Fusion 63, 124003 (2023)] to investigate the parametric variations of the momentum transport coefficients in the core of H-mode plasmas. These experimental results are compared to a comprehensive database of gyrokinetic calculations. Generally, good agreement between predicted and measured diffusive and convective transport coefficients is found. The predicted and measured Prandtl numbers correlate most dominantly with the magnetically trapped particle fraction. The experimentally inferred pinch numbers strongly depend on the logarithmic density gradient and magnetic shear, consistent with the theoretical predictions of the Coriolis pinch. The intrinsic torque from residual stress in the inner core is small, scales with the local logarithmic density gradient, and the data indicate a possible sign reversal. In the outer periphery of the core, the intrinsic torque is always co-current-directed and scales with the pressure gradient. This is consistent with prior experimental findings and global, non-linear gyrokinetic predictions. It suggests that profile shearing effects generate the intrinsic torque in the inner core. Toward the outer core, most likely, effects from E×B-shearing become more influential. These results offer the first comprehensive picture of this transport channel in the core plasma and contribute to validating the corresponding theoretical understanding. The derived scaling laws are used to construct a reduced momentum transport model, which has been validated against an additional dataset. This demonstrates that the model captures the essential contributions to momentum transport in the core of H-mode plasmas.

Generation Mechanism and Beaming of Jovian nKOM From 3D Numerical Modeling of Juno/Waves Observations

AbstractThe narrowband kilometric radiation (nKOM) is a Jovian low‐frequency radio component identified as a plasma emission produced in the region of the Io plasma torus. Measurements from the Waves instrument onboard the Juno spacecraft permitted to establish the distribution of nKOM occurrence and intensity as a function of frequency and latitude. We have developed a 3D geometrical model that can simulate at large scale the plasma emissions occurrence observed by a spacecraft based on an internal Jovian magnetic field model and a diffusive equilibrium model of the plasma density in Jupiter's inner magnetosphere. With this model, we propose a new method to discriminate the generation mechanism, wave mode, beaming and radio source location of plasma emissions. Here, this method is applied to the study of the nKOM observed from all latitudes by the Juno/Waves experiment to identify which conditions reasonably reproduce the observed occurrence distribution versus frequency and latitude. The results allow us to exclude the two main nKOM models published so far, and to show that the emission is produced at the local plasma frequency and then beamed anti‐parallel to the local density gradient into free space. We also propose that depending on its latitude, Juno observes two distinct kinds of nKOM: the low frequency nKOM in ordinary mode at high latitudes, and the high frequency nKOM on extraordinary mode at low latitudes. Both radio source locations are found to be distributed near the centrifugal equator ranging from the outer edge to the inner edge of the Io plasma torus.

Baroclinic factor in the stirring of waters of a sinking thermal with surrounding fresh water

The results of studying the process of stirring of a sinking thermal (small volume of denser water) with surrounding water are presented. The analysis of laboratory flows in a hydroflume has been allowed to identify the main peculiarities of the stirring of the thermal and the surrounding water. The essence of one of these features concluded in the presence of many high-gradient layers in the density field. The phase of an advective-rotational nature of the stirring of thermal waters with different density has been identified. Detailing of the evolution of the flow density structure was performed using the calculated flows on a 2d-nonlinear model of the dynamics of a fluid that is inhomogeneous in density. Features of the density structure are revealed using the values of local density gradients for the internal nodes of the computational grid. The density gradients along and across the streamlines were calculated. Quantitative estimates of the repeatability of high-gradient layers have shown significant variability in the flow density field. An analysis of the potential stability of the selected local regions of the flow has been carried out. The calculation of the Reynolds (Re) and Richardson (Ri) numbers has been allowed to assume a laminar nature of the flow, with the exception of two small zones in the back part of the sinking thermal. The role of the baroclinic mechanism of vorticity generation as a structure-forming factor in the formation of a distinguished stage of advective-vortex movement within interpenetration of thermal with surrounding water has shown.

Comparative Analysis of Type III Radio Bursts and Solar Flares: Spatial Localization and Correlation with Solar Flare Intensity

We present a comprehensive study of type III radio bursts and their association with solar flares of magnitude M1.0 and larger, as observed by four widely separated spacecraft (Parker Solar Probe, Solar Orbiter, STEREO-A, and Wind). Our main focus is the introduction and validation of two methods for localizing radio bursts using the available multispacecraft data. The first method utilizes intensity fitting with a circular Gaussian distribution, while the second method is based on the time arrival of radio bursts. We demonstrate the effectiveness of these methods through the analysis of a single type III burst event and compare their results with the traditional radio triangulation technique. Furthermore, we conduct a statistical study of 17 type III bursts associated with M- and X-class solar flares in years 2020–2022. Our findings suggest a possible correlation between solar flare intensities and longitudes, with east limb flares tending to be weaker than west limb flares. We also observe a systematic drift of radio burst longitudes toward the east, potentially explained by a poleward component of the local density gradient. Our results suggest a strong correlation between solar flare intensities and radio burst properties, enhancing our understanding of the relationship between solar flares and type III radio bursts.

Adaptive marine traffic behaviour pattern recognition based on multidimensional dynamic time warping and DBSCAN algorithm

Identifying marine traffic behaviour patterns is important for intelligent maritime traffic management systems. The introduction of Automatic Identification System (AIS), which can record speed over ground (SOG) and course over ground (COG) data, makes these patterns accessible. Based on that, a novel adaptive marine traffic behaviour pattern recognition algorithm is proposed to explore potential traffic behaviour patterns. In the proposed recognition algorithm, multidimensional dynamic time warping is introduced to measure the similarity among trajectories; this process considers both the spatial and motion characteristics of trajectories. However, the computational complexity of multidimensional dynamic time warping is high. To reduce the number of required computations, a data simplification algorithm considering ship behaviours is adopted to compress trajectories and, therefore, to achieve an improved computation ability. The density-based spatial clustering of applications with noise (DBSCAN) algorithm is utilized to classify marine traffic behaviour patterns. Due to the poor adaptation of the DBSCAN algorithm, we design a local mean density to evaluate the trajectory density distribution in the given dataset and then determine the optimal parameters in DBSCAN according to the local mean density gradient to improve the adaptivity of the algorithm. The proposed approach is illustrated by numerical experiments involving U.S. east coastal waters. The experimental results indicate that our approach is more accurate and adaptive in terms of recognizing hidden traffic behaviour patterns than other methods.

The Potential Role of Bering Strait in the Dynamics of Multidecadal Variability in the North Atlantic: An Idealized Model Study

Abstract Multidecadal variability on time scales between 20 and 70 years have been observed in the time series of North Atlantic SST. Many mechanisms have been proposed to explain multidecadal variabilities in the Atlantic. Generally, it is the interaction between the meridional overturning circulation (MOC) and North Atlantic surface buoyancy distribution that sustains this variability, with buoyancy anomalies either due to ocean-only processes or to air–sea interactions. In this context, the role of the Arctic Ocean, especially its freshwater flux into the North Atlantic, has been underappreciated. Bering Strait, the only oceanic pathway between the Pacific Ocean and the Arctic Ocean, has been found important in Arctic Ocean freshwater budget and in modulating the time-averaged state and long-term response of the MOC to high-latitude buoyancy forcing anomalies, via freshwater transport between the Pacific and Atlantic Oceans. In this paper, we use idealized configurations that include a Pacific-like wide basin and an Atlantic-like narrow basin. The two basins are connected both in the south and north to longitudinally periodic channels, representing the Southern Ocean and the Arctic Ocean, respectively. The Pacific-like basin is opened to the north only through a shallow and narrow strait, while the Atlantic-like basin is fully open to the north. With the goal of studying the role of Bering Strait in the multidecadal variability, we find that the freshwater transport from the Bering Strait forms a tongue structure along the western boundary of the narrow basin, which enhances the local horizontal density gradient. The western boundary region becomes unstable to large-scale baroclinic anomalies, giving rise to multidecadal variability.

Microscopic dynamics at the Running of the Bulls (San Fermín Festival) in the context of the Social Force Model

This research explores the dynamics of the emergency evacuation during the “Running of the Bulls” festival (Spain, 2013). As people run to escape from danger, many pedestrians stumble and fall down, while others will try to pass over them. We carefully examined three specific recordings of the running, that show this kind of behavior. We developed a microscopic model mimicking the stumbling mechanism in the context of the Social Force Model (SFM). In our model, “moving” individuals can suddenly switch to a “fallen” state when they are in the vicinity of a fallen individual. We arrived to the conclusion that the presence of a fallen pedestrian increases dramatically the falling probability of the pedestrians nearby. Also, the product between the local density gradient and the velocity of each pedestrian appears as a relevant indicator for an imminent fall. We call this the pedestrian “falling susceptibility (fs)”.

A thick cellular structural adhesive: Identification of its behavior under shear loading

This study focuses on the link between the microstructure and the mechanical behavior under shear loading of a thick cellular structural adhesive (TCSA).X-ray microtomography and image post-processing were first used to perform 3D-quantitative microstructure analysis. The cells morphometric parameters and their orientations were studied. The foaming process boundary conditions seems to create local density gradients changing the cells dimensions and shape. The cells are more spherical in the core of the material whereas being more ellipsoidal close to the upper and lower faces of the samples creating a skin layer. The effect on the strain field of this skin layer has then been highlighted. Secondly, a shear test method using an Arcan set-up coupled with digital image correlation was used and allowed to observe the mechanical behavior of the material under shear loadings. Instead of the material being cellular and heterogenous, it has been found that the strain field can be considered homogeneous at macroscopic scale to extract the properties on a homogeneous equivalent material. Shear test on samples with different densities were performed. Using the relation developed by Gibson and Ashby linking the shear modulus to the density squared is a first approximation, at this scale, to predict and describe the mechanical behavior under shear loading.

How do the Intravenously Injection of Nanoparticle and Oxytatic Bacteria Affect Through Circular Cylinder Cell for Non-Newtonian Micropolar Fluid: Mathematical Approach

In this paper, we have researched the conduct of non-Newtonian micropolar nanofluid flow through horizontal circular under the impacts of nanoparticles, oxytatic bacterial and Hall current effects. The utilizations of the current investigation are exceptionally powerful in biomedical therapies, for example, obliteration of malignant growth over biological cells by utilizing drug conveyance of nanoparticles and oxytatic microscopic organisms. If there should arise an occurrence of biological nanofluid it’s accepted to concern variable physical parameters which rely on the nanofluid temperature. Implicit Chebyshev pseudospectral (ICPS) technique by helping MATHEMATICA software has been applied to governing nonlinear system of dimensionless partial differential equations (PDEs). The nanofluid velocity, microrotation angular velocity, temperature, motile bacterial density distributions, oxygen concentration, local skin friction coefficient, Nusselt number, and wall motile density gradient distributions are delineated graphically for various variable physical parameters, likewise comparison between certain results in literature and our current output is introduced and great arrangement is found.

The effect of Prandtl number on decaying stratified turbulence

The effects of the variation in the Prandtl number on turbulence in a stably-stratified fluid is investigated by direct numerical simulation. The results of simulations are presented of the homogeneous decay of turbulence for a given initial Froude number and three different initial Reynolds numbers of increasing values. For each of these cases results for two different Prandtl numbers, 1 and 7, are shown. Various statistics are put forward, including kinetic and potential energy decay rates, kinetic and potential energy dissipation rates, buoyancy fluxes, energy spectra, and statistics conditioned on the local value of the vertical density gradient. It is found that the effect of increasing the Prandtl number is to increase the kinetic energy dissipation rate, while decreasing the potential energy dissipation rate. There is a notable transfer of potential to kinetic energy for the higher Prandtl number case. Finally there is evidence, based upon the analysis of vertical planes and statistics conditional on the local density gradient, that most irreversible mixing of both density and momentum occurs in regions of stronger static stability.

Pattern formation and front stability for a moving-boundary model of biological invasion and recession

We investigate pattern formation in a two-dimensional (2D) Fisher–Stefan model, which involves solving the Fisher–KPP equation on a compactly-supported region with a moving boundary. The boundary evolves analogously to the classical one-phase Stefan problem, such that boundary speed is proportional to the local population density gradient. By combining the Fisher–KPP and classical Stefan theory, the Fisher–Stefan model alleviates two limitations of the Fisher–KPP equation for biological populations. Unlike the Fisher–KPP equation, solutions to the Fisher–Stefan model have compact support, explicitly defining the region occupied by the population. Furthermore, the Fisher–Stefan model admits travelling wave solutions with non-negative density for all wave speeds c∈(−∞,∞), and can thus model both population invasion and population recession. In this work, we investigate whether the 2D Fisher–Stefan model predicts pattern formation, by analysing the linear stability of planar travelling wave solutions to sinusoidal transverse perturbations. Planar fronts of the Fisher–KPP equation are linearly stable. Similarly, we demonstrate that invading planar fronts (c>0) of the Fisher–Stefan model are linearly stable to perturbations of all wave numbers. However, our analysis demonstrates that receding planar fronts (c<0) of the Fisher–Stefan model are linearly unstable for all wave numbers. This is analogous to unstable solutions for planar solidification in the classical Stefan problem. Introducing a surface tension regularisation stabilises receding fronts for short-wavelength perturbations, giving rise to a range of unstable modes and a most unstable wave number. We supplement linear stability analysis with level-set numerical solutions that corroborate theoretical results. Overall, front instability in the Fisher–Stefan model suggests a new mechanism for pattern formation in receding biological populations.

Stabilization of double tearing mode growth by resonant magnetic perturbations

It is well known that for non-monotonic profiles of the safety factor q with two q = m/n resonant surfaces inside the plasma (m/n being the poloidal/toroidal mode numbers), the low-m double tearing modes (DTMs) are usually unstable, especially for plasmas with a high bootstrap current fraction as required for the steady operation of advanced scenarios. The effect of applied resonant magnetic perturbations (RMPs) on the m/n = 2/1 DTM growth is investigated numerically in this paper using two-fluid equations. The DTM growth is found to be stabilized by moderate static m/n = 2/1, 4/2 or 6/3 RMPs below their penetration threshold if the distance between the two resonant surfaces and the local plasma rotation velocity at the outer resonant surface are sufficiently large. The outer magnetic island is stabilized due to the change of the local plasma current density gradient around the outer resonant surface caused by RMPs, while the inner island growth is stabilized by the bootstrap current perturbation in the negative magnetic shear region. The mode stabilization is more effective for a higher electron temperature, indicating a possible method to improve the DTM stability in a fusion reactor.

Effect of Phase-Selective Nanoscale Precipitates on the Bauschinger Effect in Austenitic–Ferritic Duplex Stainless Steels

The Bauschinger effect in austenitic–ferritic duplex stainless steel 1.4462 was investigated using tension–compression tests combined with electron backscatter diffraction (EBSD). A major focus was on the impact of phase-selective nanoscale α′ precipitates formed in the ferritic phase due to 475 °C embrittlement. Contrary to the general knowledge that coherent precipitates have only a short-range effect and thus no significant influence, a strong increase in the Bauschinger effect was detected. Based on EBSD data and cyclic micro-indentations in individual grains, it was demonstrated that α′ precipitates enhance the phase difference between austenite and ferrite and increase deformation incompatibility and local dislocation density gradients at phase boundaries. Thus, despite their small size and coherence, α′ precipitates lead to long-range back stresses that significantly enhance the Bauschinger effect. In addition, the influence of precipitation was shown to depend on the extent of pre-strain during initial loading. The insights demonstrate that the influence of precipitates on the Bauschinger effect is highly complex and always needs to be analyzed with respect to precipitation characteristics, microstructure, and external boundary conditions.

Horizontal Stirring Over the Northeast U.S. Continental Shelf: The Spatial and Temporal Evolution of Surface Eddy Kinetic Energy

AbstractThis study examines the spatial and temporal variability of eddy kinetic energy over the Northeast Shelf using observations of surface currents from a unique array of six high frequency radar systems. Collected during summer and winter conditions over three consecutive years, the horizontal scales present were examined in the context of local wind and hydrographic variability, which were sampled concurrently from moorings and autonomous surface vehicles. While area‐averaged mean kinetic energy at the surface was tightly coupled to wind forcing, eddy kinetic energy was not, and was lower in magnitude in winter than summer in all areas. Kinetic energy wavenumber spectral slopes were generally near k−5/3, but varied seasonally, spatially, and between years. In contrast, wavenumber spectra of surface temperature and salinity along repeat transect lines had sharp k−3 spectral slopes with little seasonal or inter‐annual variability. Radar‐based estimates of spectral kinetic energy fluxes revealed a mean transition scale of energy near 18 km during stratified months, but suggested much longer scales during winter. Overall, eddy kinetic energy was unrelated to local winds, but the up‐ or down‐scale flux of kinetic energy was tied to wind events and, more weakly, to local density gradients.

Proper Orthogonal Decomposition for a Turbulent Boundary Layer with Aero-Optic Distortion

The velocity field associated with the deflection of a laser beam passing through a heated incompressible boundary layer in the wall-normal direction was studied using variants of the classical proper orthogonal decomposition (POD) technique. Due to the heating of the boundary layer, temperature, density, and index of refraction fields were variable in the boundary layer and were instantaneously affected by turbulent coherent structures. The time-varying index of refraction field led to a time-varying beam path of a laser traversing the boundary layer; the final angle of the laser beam was measured simultaneously with particle image velocimetry. The relationship between coherent structures in the velocity field and the final beam angle was studied using conditional POD and space-time POD. Conditional POD was performed on the wall-normal velocity field using conditions of large upstream and downstream laser deflections. The modes from conditional POD were compared to modes from POD without conditions. Physical features within some conditional POD modes were found to be shifted in the streamwise direction compared to unconditioned modes, suggesting that the streamwise position of velocity features was correlated to laser deflection. Space-time POD resulted in the same modes as the conditional POD and showed the modes convecting in the streamwise direction without a significant change in the size of the structures. This suggested a hypothesis that the laser deflects when coherent structures reach particular positions relative to the laser position as the structures convect, aligning the largest local velocity and density gradients with the laser beam path.

A novel compressible enstrophy transport equation-based analysis of instability during Magnus–Robins effects for high rotation rates

The effects of compressibility on the instability of a two-dimensional flow past a rotating cylinder executing high rotation rates are investigated, in detail, using a novel analysis based on the compressible enstrophy transport equation (CETE). Accurate analysis of the instability necessitates the generation of high fidelity numerical solutions, and this is achieved by employing optimized numerical methods that enable high accuracy direct numerical simulation of compressible flows. To study the effects of compressibility induced by rotation alone, a low free stream Mach number and two high rotation rates are considered, as compared to that reported in the literature. Results demonstrate single-sided vortex shedding, the presence of significant compressibility in the flow field confirmed by local Mach number, and temperature and density gradient fields with transient formation of supersonic pockets noted for the higher rotation speed cases. The temporal instability is studied by analyzing the relative contributions of different terms in the CETE to the growth of enstrophy. As per the authors' knowledge, this is the first such research effort demonstrating an application of the CETE for instabilities. Analysis shows that viscous diffusion is the dominant mechanism in creating the flow instability with a secondary role played by the baroclinic mechanism.

Coarse-grained models for local density gradients

Coarse-grained (CG) models provide superior computational efficiency for simulating soft materials. Unfortunately, CG models with conventional pair-additive potentials demonstrate limited transferability between bulk and interfacial environments. Recently, a growing number of CG models have supplemented these pair potentials with one-body potentials of the local density (LD) around each site. These LD potentials can significantly improve the accuracy and transferability of CG models. Nevertheless, it remains challenging to accurately describe interfaces where the LD varies rapidly. In this work, we consider a new class of one-body potentials that depend upon the square of the LD gradient around each site. We investigate the impact of this square gradient (SG) potential upon both top-down dissipative particle dynamics (DPD) models and also bottom-up multiscale coarse-graining (MS-CG) models. We demonstrate that SG potentials can be used to tune the interfacial properties of DPD models without significantly altering their bulk properties. Moreover, we demonstrate that SG potentials can improve the bulk pressure-density equation of state as well as the interfacial profile of MS-CG models for acetic acid. Consequently, SG potentials may provide a useful connection between particle-based top-down models and mean-field Landau theories for phase behavior. Furthermore, SG potentials may prove useful for improving the accuracy and transferability of bottom-up CG models for interfaces and other inhomogeneous systems with significant density gradients.

Numerical study of instabilities of particle-laden fronts in continuously stratified environments

Numerical simulations are conducted to study instabilities and the associated convective motion of particle-laden layers settling in continuously stratified environments. We show that when the background density stratification is insignificant relative to the bulk excessive density of the particle-laden layer, the unstable motions of the particle-laden interface are mainly driven by Rayleigh–Taylor instability but become double-diffusive convection when the background stratification is relatively significant. Our results agree with theoretical prediction based on linear stability analysis. However, in the Rayleigh–Taylor instability regime, the motion of particle-laden plumes can be further suppressed by the background density stratification while the plumes reach the height of neutral buoyancy. This leads to the second stage of flow development, in which secondary instability occurs at the plumes' tip in the form of double-diffusive convection. Due to the change in the background density gradient within the plumes' head, the occurrence of secondary instability is accompanied by a shift of the dominant mode, which is particularly significant in cases with a high background Prandtl number (i.e., salinity-induced stratification). The theoretical argument on the mode shift is based on previous linear stability analysis for the two-layer structured background density gradient provided. The ratio between the particles' settling velocity and velocity scaling for the developed local density gradient at the plumes' tip then allows us to distinguish and predict whether the final convective motion is driven mainly by double-diffusive or settling-driven buoyancy-dominant convection.

Superadiabaticity in Jupiter and Giant Planet Interiors

Abstract Interior models of giant planets traditionally assume that at a given radius (i.e., pressure) the density should be larger than or equal to the one corresponding to a homogeneous, adiabatic stratification throughout the planet (referred to as the “outer adiabat”). The observations of Jupiter’s gravity field by Juno combined with the constraints on its atmospheric composition appear to be incompatible with such a profile. In this Letter, we show that the above assumption stems from an incorrect understanding of the Schwarzschild–Ledoux criterion, which is only valid on a local scale. In order to fulfill the buoyancy stability condition, the density gradient with pressure in a nonadiabatic region must indeed rise more steeply than the local adiabatic density gradient. However, the density gradient can be smaller than the one corresponding to the outer adiabat at the same pressure because of the higher temperature in an inhomogeneously stratified medium. Deep enough, the density can therefore be lower than the one corresponding to the outer adiabat. We show that this is permitted only if the slope of the local adiabat becomes shallower than the slope of the outer adiabat at the same pressure, as found in recent Jupiter models due to the increase of both specific entropy and adiabatic index with depth. We examine the dynamical stability of this structure and show that it is stable against nonadiabatic perturbations. The possibility of such an unconventional density profile in Jupiter further complicates our understanding of the internal structure and evolution of (extrasolar) giant planets.

Simulation and flow physics of a shocked and reshocked high-energy-density mixing layer

Abstract