Shear Fluctuations Research Articles

Diurnal Cycles of Cloud Properties and Precipitation Patterns over the Northeastern Tibetan Plateau During Summer

In the context of rising temperatures and increasing humidity in Northwest China, substantial gaps remain in understanding the mechanisms of land–atmosphere cloud–precipitation coupling across the northeastern Tibetan Plateau (TP), Loess Plateau (LP), and Huangshui Valley (HV). This study addresses these gaps by investigating cloud properties and precipitation patterns utilizing the Fengyun-4 Satellite Quantitative Precipitation Estimation Product (FY4A-QPE) and ERA5 datasets. We specifically focus on Lanzhou, a pivotal city within the LP, and Xining, which epitomizes the HV. Our findings reveal that diurnal variations in precipitation are significantly less pronounced in the eastern regions compared to northeastern TP. This discrepancy is attributed to marked diurnal fluctuations in convective available potential energy (CAPE) and wind shear between 200 and 500 hPa. While both cities share similar wind shear patterns and moisture transport directions, Xining benefits from enhanced snowmelt and effective water retention in surrounding mountains, resulting in higher precipitation levels. Conversely, Lanzhou suffers from moisture deficits, with dry, hot winds exacerbating the situation. Notably, precipitation in Xining is strongly correlated with CAPE, influenced by diurnal variability, and intensified by valley and lake–land breezes, which drive afternoon convection. In contrast, Lanzhou’s precipitation exhibits a weak relationship with CAPE, as even elevated values fail to generate significant cloud formation due to insufficient moisture. The ongoing trends of warming and humidification may lead to improved precipitation patterns, especially in the HV, with potential ecological benefits. However, concentrated rainfall during summer afternoons and midnights raises concerns regarding extreme weather events, highlighting the susceptibility of the HV to geological hazards. This research underscores the need to further explore the uncertainties inherent in precipitation dynamics in these regions.

Incremental increases in physiological fluid shear progressively alter pathogenic phenotypes and gene expression in multidrug resistant Salmonella

ABSTRACT The ability of bacteria to sense and respond to mechanical forces has important implications for pathogens during infection, as they experience wide fluid shear fluctuations in the host. However, little is known about how mechanical forces encountered in the infected host drive microbial pathogenesis. Herein, we combined mathematical modeling with hydrodynamic bacterial culture to profile transcriptomic and pathogenesis-related phenotypes of multidrug resistant S. Typhimurium (ST313 D23580) under different fluid shear conditions relevant to its transition from the intestinal tract to the bloodstream. We report that D23580 exhibited incremental changes in transcriptomic profiles that correlated with its pathogenic phenotypes in response to these progressive increases in fluid shear. This is the first demonstration that incremental changes in fluid shear forces alter stress responses and gene expression in any ST313 strain and offers mechanistic insight into how forces encountered by bacteria during infection might impact their disease-causing ability in unexpected ways.

Hybrid Computation of the Aerodynamic Noise Radiated by the Wake of a Subsonic Cylinder

The noise radiated by the flow around a cylinder in the subcritical regime at ReD=1×104 and at a subsonic Mach number of M=0.5 is here studied. The aerodynamic sound radiated by a cylinder has been studied with a wide range of Reynolds numbers, but there are no studies about how the Mach number affects the acoustic field in the subsonic regime. The flow field is resolved by means of large-eddy simulations of the compressible Navier–Stokes equations. For the study of the noise propagation, formulation 1C of the Ffowcs Williams–Hawkings analogy is used. The fluid flow results show good agreement when comparing the surface pressure coefficient, the recirculation length, the vortex shedding frequency and the force coefficients against other studies performed under similar conditions. The dynamic mode decomposition of the pressure fluctuations is used to relate them with the far-field noise. It is shown that, in contrast to what happens for low Mach numbers, quadrupoles have a significant impact mainly in the observers located in the streamwise direction. This effect leads to a global monopole directivity pattern as the shear fluctuations compensate for the lower value of the aeolian tone away from the cross-stream direction.

Nonlinear generation of sound and mean pressure drop in shock–shear interaction

Interactions of shear fluctuations with shock waves are ubiquitous in high-speed flow applications from scramjet propulsion to cosmic events like supernova explosions. They also serve as fundamental building blocks for the study of shock–turbulence interaction. In this work, we study the nonlinear effects in pressure arising due to the interaction of a normal shock with a two-dimensional shear wave. It employs the weakly nonlinear framework (WNLF) developed recently for vorticity amplification by Thakare et al. [“A weakly nonlinear framework to study shock–vorticity interaction,” J. Fluid Mech. 933, A48 (2022)]. The analysis includes the effect of intermodal interactions that is neglected in the widely used linear interaction analysis (LIA) of shock–turbulence interaction. It is found that the deformation of the shock wave and the fluctuation mass flux normal to the shock contribute to the dominant physical mechanisms responsible for the observed nonlinearities. Interestingly, the WNLF predicts a drop in mean pressure behind the shock due to a second-order intermodal interaction, which is consistent with the well-established results by Lele [“Shock-jump relations in a turbulent flow,” Phys. Fluids A 4, 2900–2905 (1992)] at low Mach numbers and brings out additional effects of shock deformation that are important at higher Mach numbers. We extend the WNLF to three-dimensional interaction of homogeneous isotropic turbulence with a normal shock. Comparison with existing direct numerical simulation data shows good agreement for low turbulent Mach numbers, which is a significant improvement over the prediction capability of LIA. We also compute the dilatation fields from WNLF and use them to distinguish between the acoustic and non-acoustic components of the second-order pressure fluctuations generated by the shock wave.

Shear mechanical responses and cracking evolution mechanisms of flawed sandstone containing two circular holes

Shear tests on rectangular plate sandstone containing two pre-existing holes were performed to analyze the shear mechanical responses and cracking evolution mechanisms. With generation of the penetrating fracture surfaces, both shear stress and normal displacement show a dramatic drop. Then, during the shear slipping process, the shear stress increases once again due to wear and shear off of the surface asperities, and three different variation trends of the normal deformation including shear dilation, fluctuation, and shear contraction are respectively identified. The role of ligament length and normal stress on several key indices such as peak shear stress, peak normal displacement and residual shear stress was investigated. Violent AE (acoustic emission) responses occur along with penetration of the fracture surfaces, and in the shear slipping process, AE events continue to occur due to wear of surface asperities, while the increase rate of accumulative AE events declines. Cracks coalesced at the rock bridge and running through the rock matrix form the main failure planes of the samples. From the PFC2D simulation results, the crack clusters first initiate from hole-walls and coalesce at the rock bridge for a small ligament length, while first produce the main failure planes at the right/left rock matrix for a large ligament length. The total number of micro cracks shows a “calm → rapid increase → decreasing growth amplitude” variation during shearing, and the induced cracks become more concentrated for a larger normal stress, implying a cumulative damage of the failure surfaces.

Suppression of the turbulent kinetic energy and enhancement of the flame-normal Reynolds stress in premixed jet flames at small Lewis numbers

We report the effects of the Lewis number on the turbulent kinetic energy, Reynolds stresses, and scalar fluxes in premixed jet flames with strong mean shear. The direct numerical simulation (DNS) of piloted turbulent premixed jet flames is performed under the same condition except for different Lewis numbers to isolate the Lewis number effects. The turbulence intensity within the mean flame brush is found to decay more rapidly along the axial distance with a smaller Lewis number. This Lewis number dependence results from the coupling of the mean shear and the combustion heat release, where the mean shear is the primary source of turbulence production. We show that, under the strain imposed by the mean shear and turbulence fluctuation, the heat release of premixed jet flames is significantly enhanced at the Lewis number smaller than unity, which is represented by the rising flamelet consumption speed and flame surface area. As a consequence, the mean shear within the flame brush is suppressed at small Lewis numbers via the dilatation effect, and the weakened mean shear production causes the fast turbulence suppression along the axial direction. The enhanced heat release at small Lewis numbers also increases the flame-normal Reynolds stress, and leads to the counter-Boussinesq behavior of the shear Reynolds stress and the counter-gradient transport of scalar fluxes. We model the Lewis number effects on local turbulence and scalar transport using the stretched laminar flame speed and thickness, and achieve overall good estimations of the flame-normal Reynolds stress and scalar transport at small Lewis numbers.

On large-scale dynamos with stable stratification and the application to stellar radiative zones

Abstract Our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. Such magnetic fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution. A major difficulty is the effect of stable stratification, which generally suppresses dynamo action. We explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a non-helical large-scale dynamo (LSD) mechanism known as the magnetic shear-current effect. We find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). Both are plausibly sourced in the presence of differential rotation. Our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. Additionally, a scan over magnetic Reynolds number shows no change in the growth or saturation of the LSD, providing good numerical evidence of a dynamo mechanism resilient to catastrophic quenching, which has been an issue for helical dynamos. These properties – the absence of catastrophic quenching and robustness to stable stratification – make the mechanism a plausible candidate for generating in situ large-scale magnetic fields in stellar radiative zones.

Role of RpoS in Regulating Stationary Phase Salmonella Typhimurium Pathogenesis-Related Stress Responses under Physiological Low Fluid Shear Force Conditions.

The discovery that biomechanical forces regulate microbial virulence was established with the finding that physiological low fluid shear (LFS) forces altered gene expression, stress responses, and virulence of the enteric pathogen Salmonella enterica serovar Typhimurium during the log phase. These log phase LFS-induced phenotypes were independent of the master stress response regulator, RpoS (σS). Given the central importance of RpoS in regulating stationary-phase stress responses of S. Typhimurium cultured under conventional shake flask and static conditions, we examined its role in stationary-phase cultures grown under physiological LFS. We constructed an isogenic rpoS mutant derivative of wild-type S. Typhimurium and compared the ability of these strains to survive in vitro pathogenesis-related stresses that mimic those encountered in the infected host and environment. We also compared the ability of these strains to colonize (adhere, invade, and survive within) human intestinal epithelial cell cultures. Unexpectedly, LFS-induced resistance of stationary-phase S. Typhimurium cultures to acid and bile salts stresses did not rely on RpoS. Likewise, RpoS was dispensable for stationary-phase LFS cultures to adhere to and survive within intestinal epithelial cells. In contrast, the resistance of these cultures to challenges of oxidative and thermal stresses, and their invasion into intestinal epithelial cells was influenced by RpoS. These findings expand our mechanistic understanding of how physiological fluid shear forces modulate stationary-phase S. Typhimurium physiology in unexpected ways and provide clues into microbial mechanobiology and nuances of Salmonella responses to microenvironmental niches in the infected host. IMPORTANCE Bacterial pathogens respond dynamically to a variety of stresses in the infected host, including physical forces of fluid flow (fluid shear) across their surfaces. While pathogens experience wide fluctuations in fluid shear during infection, little is known about how these forces regulate microbial pathogenesis. This is especially important for stationary-phase bacterial growth, which is a critical period to understand microbial resistance, survival, and infection potential, and is regulated in many bacteria by the general stationary-phase stress response protein RpoS. Here, we showed that, unlike conventional culture conditions, several stationary-phase Salmonella pathogenic stress responses were not impacted by RpoS when bacteria were cultured under fluid shear conditions relevant to those encountered in the intestine of the infected host. These findings offer new insight into how physiological fluid shear forces encountered by Salmonella during infection might impact pathogenic responses in unexpected ways that are relevant to their disease-causing ability.

Experimental investigation of inertial fibres and disks in a turbulent boundary layer

We study experimentally the behaviour of negatively buoyant disks and fibres in a turbulent boundary layer. The regime is relevant to the transport of natural sediment or plastic particles in water, with density ratio $\rho _p/\rho _f \sim O(1)$ , major axis lengths $D_p^+ \sim 50$ , friction Stokes numbers $\mbox { {St}}^+ \sim O(10)$ and friction Reynolds number $\mbox { {Re}}_\tau = 620$ . The translational and rotational motion, as well as concentration and dispersion, are compared with those of spheres of similar inertia. Disks and fibres both oversample high-speed fluid near the wall, in agreement with particle-resolved numerical simulations. Fibres tend to orient mostly in the streamwise direction while disks maintain their symmetry axis quasi-normal to the wall. This alignment is more stable for disks than for fibres: the latter undergo strong tumbling near the wall in response to the mean shear and turbulent fluid velocity fluctuations, whereas the former wobble about their preferential wall-normal orientation. The translational and rotational accelerations indicate that, despite the nominal relaxation times being similar, the disks are slower than the fibres in responding to wall turbulence. For both, wall contact causes strong and intermittent tumbling. The concentration profiles follow Rouse–Prandtl theory over a limited portion of the boundary layer, deviating near the wall and in the outer region. This is largely due to the non-uniform settling velocity, which decreases steeply approaching the wall for all particle types. This is, in turn, a consequence of the reduced particle diffusivity, which closely matches the profile of the eddy viscosity.

Experimental study on the effect of particle size on the shear characteristics of large-displacement soil exposed to heat treatment: Shear fluctuation and heat degradation

The shear characteristics of soil on the basal face of a landslide body have important implications on landslide dynamics. In this study, ring-shear tests were carried out on quartz sand to study the shear characteristics of soil particle materials on the basal facies. Quartz sand samples of different particle sizes (0.25–1 mm) were treated at high temperature (20–800 °C) to determine the influence of temperature and particle size distribution (PSD) on their shear characteristics at different shear speeds. The shear characteristics of the samples were analyzed, including the shear stress, shear stress fluctuation, density variation, and degree of fragmentation. The results show that the amplitude of shear stress fluctuation increases with increasing shear velocity. It can be determined that the smaller the particle size, the greater the residual shear strength. Elevated temperature has a certain influence on the quartz sand and the shear characteristics (e.g., shear stress) exhibit a low degree of deterioration. The dilatancy and compaction mechanism of the samples under large displacement shearing is further discussed.

An Integrated Spatio-Temporal Features Analysis Approach for Ocean Turbulence Using an Autonomous Vertical Profiler

Turbulent energy cascade and intermittency are very important characteristics in the turbulent energy evolution process. However, understanding the temporal–spatial features of kinetic energy transfer and quantifying the correlations between different scales of turbulent energy remains an outstanding challenge. To deeply understand the spatial–temporal features in the energy transfer process, an integrated features identification and extraction method is proposed to quantitatively investigate the correlations using the ocean shear turbulence measured by an autonomous vertical reciprocating profiler (AVRP). The proposed integrated method mainly contains two parallel features analysis modules: first, temporal multiscale features structures of the nonlinear and nonstationary turbulent cascade are identified by Variational Mode Decomposition (VMD); then, the ocean microstructure shear fluctuation data are decomposed into a series of intrinsic mode functions (IMFs), which are characterized by different time scales and frequency bandwidths. The local features of energy transfer are identified when the local intermittency peaks overlap and the phase-synchronization case occurs between two neighboring scales; second, the spatial statistical characteristics of the turbulent energy dissipation are quantitatively studied. The cumulative probability distribution functions (CPDFs) of kinetic energy dissipation are approximated well by a normal distribution, indicating that the turbulent dissipation process exhibits a robust spatial scaling correlation and a few intense dissipation locations dominate the integrated process. Finally, the proposed integrated method is evaluated through experiments using an autonomous vertical reciprocating profiler deployed in the South China Sea. Preliminary experimental results show that the proposed novel method is useful to improve our understanding of turbulent energy transfer and the evolution process in the ocean dynamic systems.

Understanding the Dyer-Roeder approximation as a consequence of local cancellations of projected shear and expansion rate fluctuations

It is shown with several concrete examples that the Dyer-Roeder approximation is valid in spacetimes which fulfill the condition that fluctuations in the expansion rate along a light ray locally cancels with the shear contribution to the redshift. This is the case for standard cosmological scenarios including perturbed FLRW spacetimes, N-body simulations and Swiss-cheese models. With another concrete example it is then illustrated that it is possible to construct statistically homogeneous and effectively statistically isotropic cosmological models which do not fulfill the condition. In this case, the Dyer-Roeder approximation is invalid. Instead, the mean redshift-distance relation can be described using a relation based on the spatial averages of the transparent part of the spacetime.

Deformation history of a foredeep basin during the incorporation of its deposits within an advancing orogenic wedge: The case of the Oligocene-Early Miocene Macigno Costiero Formation, southern Tuscany, northern Apennines, Italy

The Upper Oligocene Macigno Formation cropping out in the coastal section of SW Tuscany represents the sedimentary fill of a foredeep basin developed during the collisional stages that led to the construction of the Apennine mountain belt of peninsular Italy. The stratigraphic sequence consists of alternating sandstones and siltstones, that are affected by km-scale contractional structures, namely SW-dipping thrusts and related NE-verging folds. An original field survey carried out along a superb coastal exposure, integrated with analysis of mesoscopic fabrics and their overprinting relationships, makes it possible to unravel a complex deformation history. Four main deformation stages are recognized: i) layer-parallel shortening A; ii) top-to-the-foreland shear B; iii) folding C, comprising the fold nucleation sub-stage C1 and the fold amplification sub-stage C2; and iv) thrust propagation D. The sequence of recognized stages indicates a progressive deformation history of the Macigno Formation as it was incorporated within the evolving Apennine orogenic system. The kinematic history inferred from deformation fabrics and their overprinting relationships within Macigno Fm. deposits in SW Tuscany shows remarkable analogies with the structural evolution of foredeep sediments from other fold-and-thrust belts, yet with little deviations mainly consisting of pureshear vs. simple shear fluctuations in the early history of accretion. These provide original information on the modes of accretion of foredeep deposits within evolving collisional belts, thus contributing to an enhanced understanding of orogenic dynamics during mountain building with a classical example whose evolution is unravelled in detail.

Magnetic field and activity phenomena of the K2 dwarf V471 Tau

ABSTRACT We analyse spectropolarimetric data of the pre-cataclysmic variable binary system V471 Tau obtained with ESPaDOnS at the Canada–France–Hawaii Telescope in two observational campaigns (in 2004 November/December and 2005 December). Using Zeeman–Doppler imaging, we reconstruct the distribution of brightness map and large-scale magnetic field of the K2 dwarf at both epochs, as well as the amount of differential rotation by which surface maps are sheared. We detect significant fluctuations in the surface shear between the two campaigns. It goes from about twice the solar differential rotation rate to less than the solar value in a 1-yr interval. We conclude that the differential rotation fluctuations obtained for the K2 dwarf resemble those detected on the single-star analogue AB Dor, although even larger amplitudes of variation are seen in the K2 dwarf of V471 Tau. Finally, we show that the differential rotation results obtained in this work do not favour an Applegate mechanism operating in the V471 Tau system, at least in its standard form, but leave room for explaining the observed orbital period fluctuations with exotic forms of similar phenomena based on dynamo processes operating within the convective zone of the K2 star.

Aerodynamic wall shear fluctuation in sand-laden flow in a turbulent boundary layer.

To improve our understanding on the aerodynamic wall shear fluctuation in particle-laden flow, we perform statistical and spectral analyses of aerodynamic wall shear fluctuation in sand-free and sand-laden flows in a turbulent boundary layer based on the results of a wind tunnel experiment. An increase in the skewness of the probability density function of aerodynamic wall shear fluctuation is found in the sand-laden flow. The turbulent intensity of aerodynamic wall shear stress increases rapidly with the sand mass flux. The decreased convective velocity indicates blocking effects of sand particles in the near-wall region. The power of aerodynamic wall shear fluctuation in the sand-laden flow increases at low frequencies corresponding to the duration of sand streamers. The results of a superstatistical analysis of skin friction velocity show that spatio-temporal fluctuation in the local energy dissipation rate is enhanced in the sand-laden flow. Finally, the local and spatio-drifting force acting on the stochastic system of wall shear fluctuation in sand-laden flow are different, providing a proof for the significant variation in flow condition caused by sand streamers near the wall.

Relation between dilation and stress fluctuations in discontinuous shear thickening suspensions.

We investigate dilation-induced surface deformations in a discontinuous shear thickening (DST) suspension to determine the relationship between dilation and stresses in DST. Video is taken at two observation points on the surface of the suspension in a rheometer while shear and normal stresses are measured. A roughened surface of the suspension is observed as particles poke through the liquid-air interface, an indication of dilation in a suspension. These surface roughening events are found to be intermittent and localized spatially. Shear and normal stresses also fluctuate between high- and low-stress states, and surface roughening is observed frequently in the high-stress state. On the other hand, a complete lack of surface roughening is observed when the stresses remain at low values for several seconds. Surface roughening is most prominent while the stresses grow from the low-stress state to the high-stress state, and the roughened surface tends to span the entire surface by the end of the stress growth period. Surface roughening is found only at stresses and shear rates in and above the shear thickening range. These observed relations between surface roughening and stresses confirm that dilation and stresses are coupled in the high-stress state of DST.

Behavior of viscoelastic models with thermal fluctuations

.Fluctuating viscoelasticity for conformation-tensor-based models is studied at equilibrium, in simple-shear deformation, and in uniaxial extension. The models studied are the upper-convected Maxwell model, the FENE-P model with finite chain-extensibility, and the Giesekus model with anisotropic drag. Using numerical simulations, the models are compared in detail both with each other and with analytical predictions for the Maxwell model. At equilibrium, the models differ only marginally, both in terms of static and dynamic characteristics. When deformed, the average mechanical response of the Maxwell model is unaffected by the strength of thermal fluctuations, while the mechanical response of the FENE-P and Giesekus models show a slight decrease the stronger the fluctuations in simple shear, whereas the decrease in uniaxial extension is marginal. For all models, the standard deviation of the mechanical response increases with increasing strength of fluctuations, and the magnitude of the standard deviation relative to the average for given fluctuation strength generally decreases the stronger the deformation, this effect being stronger for uniaxial extension than for simple-shear deformation.Graphical abstract

Supercooled drizzle development in response to semi-coherent vertical velocity fluctuations within an orographic-layer cloud

Abstract. Observations of supercooled liquid water are nearly ubiquitous within wintertime orographic-layer clouds over the Intermountain West; however, observations of regions containing supercooled drizzle drops (SCDDs) are much rarer and the factors controlling SCDD development and location less well understood. As part of the Seeded and Natural Orographic Wintertime clouds – the Idaho Experiment (SNOWIE) and its goal of improving understanding of natural cloud structure, this study examines the role of fine-scale (sub-kilometer) vertical velocity fluctuations on the microphysical evolution and location of SCDDs within the observed mixed-phase, wintertime orographic clouds from one research flight in SNOWIE. For the case examined, SCDDs developed in an elevated, postfrontal-layer cloud with cold cloud tops (T<-30 ∘C) and low number concentrations of both ice (less than 0.5 L−1) and cloud droplets (less than 30 cm−3). Regions of supercooled drizzle at flight level extended more than a kilometer along the mean wind direction and were first located at and below layers of semi-coherent vertical velocity fluctuations (SCVVFs) embedded within the cloud and subsequently below cloud top. The microphysical development of SCDDs in this environment is catalogued using size and mass distributions derived from in situ probe measurements. Regions corresponding to hydrometeor growth are determined from radar reflectivity profiles retrieved from an airborne W-band cloud radar. Analysis suggests that SCVVF layers are associated with local SCDD development in response to the kinematic perturbation pattern. This drizzle development and subsequent growth by collision–coalescence is inferred from vertical reflectivity enhancements (−20 dBZ km−1), with drizzle production confirmed by in situ measurements within one of these SCVVF layers. The SCDD production and growth occurs embedded within cloud over shallow (km or less) layers before transitioning to drizzle production at cloud top further downwind, indicating that wind shear and resultant vertical velocity fluctuations may act to enhance or speed up SCDD development compared to classic cloud top broadening mechanisms in orographic (or similarly sheared) cloud environment(s).

Turbulence and flow field alterations inside a fish sea cage and its wake

Measurements were performed inside and in the wake of a commercial salmon sea cage. The key instrumentation included the following: sea bottom-mounted acoustic Doppler current profilers providing continuous concurrent flow velocity and turbulence information about the water columns; vessel-mounted acoustic current profilers mapping the flow pattern around the wake of the cage in a selected incoming flow; a microstructure profiler measuring the fluctuations in vertical shear in the dissipation range; an acoustic Doppler velocimeter measuring the velocity inside the sea cage; dissolved oxygen sensors and echosounders measuring the distribution of fish inside the cage. The measurements have performed with stocked and emptied sea-cage. The results showed simultaneous strong flow reductions in the wake near the cage and high turbulence in the upper part of the water column, both of which were generated by the sea cage. Measurements inside the cage showed that although the schooling fish reduced the flow, there was no evidence that they generate secondary radial and vertical flows.

Large enhancement of conductivity in a strongly layered type-II superconductor with an artificial pinning array

Abstract We use the time-dependent Ginzburg–Landau equation to describe a type-II superconductor in a magnetic field in the presence of both strong thermal fluctuations and an artificial pinning array. Thermal fluctuations are represented by the Langevin white noise. The layered structure of the superconductor is taken into accounted with the Lawrence–Doniach model. The self-consistent Gaussian approximation is used to treat the nonlinear interaction term in the time-dependent Ginzburg–Landau equation. In the case of the $\delta $-function model for the pinning centers and the matching field, analytic expressions for the fluctuation electrical and thermoelectric conductivity are obtained. It is found that the fluctuations in electrical and thermoelectric conductivities increase with increasing pinning strength, and when the pinning strength comes near a critical value, the fluctuation conductivity is greatly enhanced. Our result shows that if a pinning array is added to a mixed state superconductor, the original properties of the superconductor are recovered. Physically, in the presence of thermal fluctuations, when the energy scale of the vortex lattice shear fluctuations becomes comparable to the pinning energy scale there is a large enhancement of the fluctuation conductivity in the presence of pinning.