Steady-state Particle Research Articles

Collisionless Magnetorotational Turbulence in Pair Plasmas: Steady-State Dynamics, Particle Acceleration, and Radiative Cooling.

We present 3D fully kinetic shearing-box simulations of pair-plasma magnetorotational turbulence with unprecedented macro-to-microscopic scale separation. While retrieving the expected fluid behavior of the plasma at large scales, we observe a steepening of turbulent spectra at kinetic scales and substantial angular-momentum transport linked with kinetic processes. For the first time, we provide a definitive demonstration of nonthermal particle acceleration in kinetic magnetorotational turbulence agnostically of shearing-box initial conditions by means of a novel strategy exploiting synchrotron cooling.

Parameter identification of an open-frame underwater vehicle based on numerical simulation and quantum particle swarm optimization

Accurate parameter identification of underwater vehicles is of great significance for their controller design and fault diagnosis. Some studies adopt numerical simulation methods to obtain the model parameters of underwater vehicles, but usually only conduct decoupled single-degree-of-freedom steady-state numerical simulations to identify resistance parameters. In this paper, the velocity response is solved by applying a force (or torque) to the underwater vehicle based on the overset grid and Dynamic Fluid-Body Interaction model of STAR-CCM+, solving for the velocity response of an underwater vehicle in all directions in response to propulsive force (or moment) inputs. Based on the data from numerical simulations, a parameter identification method using quantum particle swarm optimization is proposed to simultaneously identify inertia and resistance parameters. By comparing the forward velocity response curves obtained from pool experiments, the identified vehicle model’s mean square error of forward velocity is less than 0.20%, which is superior to the steady-state simulation method and particle swarm optimization and genetic algorithm approaches.

On the second law of thermodynamics: A global flow spontaneously induced by a locally nonchaotic energy barrier

It is well known that, inherently, certain small-scale nonchaotic particle movements are not governed by thermodynamics. Usually, such phenomena are studied by kinetic theory and their energy properties are considered “trivial”. In current research, we show that, beyond the boundaries of the second law of thermodynamics where Boltzmann’s H-theorem does not apply, there may also be large-sized systems of nontrivial energy properties: when the system is isolated, entropy can decrease; from a single thermal reservoir, the system can absorb heat and produce useful work without any other effect. The key concept is local nonchaoticity, demonstrated by using a narrow energy barrier. The barrier width is much less than the nominal particle mean free path, so that inside the barrier, the particle-particle collisions are sparse and the particle trajectories tend to be locally nonchaotic. Across the barrier, the steady-state particle flux ratio is intrinsically in a non-Boltzmann form. With a step-ramp structure, the nonequilibrium effect spreads to the entire system, and a global flow is generated spontaneously from the random thermal motion. The deviation from thermodynamic equilibrium is steady and significant, and compatible with the basic principle of maximum entropy. These theoretical and numerical analyses may shed light on the fundamentals of thermodynamics and statistical mechanics.

Integrated control of edge localized modes and divertor flux using mixed toroidal harmonic resonant magnetic perturbations in EAST

Mixed harmonic resonant magnetic perturbations (RMPs) for integrated edge localized modes (ELMs) and divertor flux control are demonstrated in EAST target plasmas of low input torque and normalized beta βN∼ 1.7–1.9, which are close to the equivalent value in ITER high Q operation. The applied RMPs are designed to combine a static harmonic of the toroidal mode number n = 3 with a static or rotating harmonic of n = 2. ELM suppression is achieved without a drop of plasma energy confinement, and tungsten concentration is effectively reduced during the application of RMPs. With mixed harmonics, the toroidal varying steady state heat and particle fluxes on the divertor target can be modified with the rotating n = 2 harmonic, which agrees with the numerical modeling of three-dimensional magnetic topology, with plasma responses being taken into account. ELM suppression correlates with the times of larger n = 3 response with mixed n = 2 and n = 3 RMPs. The mixture of harmonics and the rotating n = 2 harmonic does not require additional coil current because the variation is only in the upper-lower coil current phase space. These results further affirm the effectiveness of integrated ELM and divertor flux control using RMPs with mixed harmonics and improve the understanding of the role of plasma responses in ELM suppression.

Inverse current induced thermoelectric conversion in a parallel-coupled double quantum dot system

We investigate the thermoelectric transport and conversion of a parallel-coupled double quantum dot system, which consisting of two capacitively coupled quantum dots in the Coulomb-blockade regime. We found that the system exhibits an unconventional thermoelectric conversion process induced by the inverse current effect, which is attributed to the increased Coulombic interaction between quantum dots, resulting in strong asymmetry in the system. We study the transport properties of steady-state particle current and heat current, and analyze the influence of Coulomb interaction on the thermodynamic characteristics of unconventional thermoelectric heat engines and refrigerators.

Impact of extended versus typical rapid mixing HRTs on continuous-flow coagulation-ultrafiltration

Although bench-scale coagulation-ultrafiltration (UF) studies are typically conducted using batch systems, continuous-flow systems allow operational parameters to be more readily evaluated and optimized over consecutive permeation cycles. When simulating coagulation-UF at bench-scale using continuous-flow systems, relevant reactor volumes require flowrates ≥1 L/min in order to achieve hydraulic retention times (HRTs) typical of full-scale rapid mixing (30 s to several minutes). In the present study, HRTs of 2 and 20 min were evaluated to determine their impact on particle properties (size, structure, and concentration) as well as UF performance (fouling resistance, reduction of natural organic matter) over a range of alum dosages. While not practical at full-scale, a 20 min HRT has been previously applied at bench-scale to reduce the overall volume of water required to conduct continuous-flow coagulation-UF studies. However, potential impacts on steady-state particle properties and subsequent UF performance not been clearly elucidated. Results from this study suggest that increasing HRT significantly increases steady-state particle size and concentration, as well as reduces total/hydraulically irreversible UF fouling resistance and improves hydraulic cleaning efficiency. Despite reduced fouling when considering a longer HRT, significant differences in particle properties and UF performance suggest that HRT values equivalent to those typically applied during full-scale rapid mixing (30 s to several minutes) must be considered in order to produce results relevant to full-scale.

Novel SOLPS-ITER simulations of X-point target and snowflake divertors

The design and understanding of alternative divertor configurations may be crucial for achieving acceptable steady-state heat and particle material loads for magnetic confinement fusion reactors. Multiple X-point alternative divertor geometries such as snowflakes and X-point targets have great potential in reducing power loads, but have not yet been simulated widely in codes with kinetic neutrals. This paper discusses recent changes made to the SOLPS-ITER code to allow for the simulation of X-point target and low-field side snowflake divertor geometries. Snowflake simulations using this method are presented, in addition to the first SOLPS-ITER simulation of the X-point target. Analysis of these results show reasonable consistency with the simple modelling and theoretical predictions, supporting the validity of the methodology implemented.

Flux in Tilted Potential Systems: Negative Resistance and Persistence

Many real-world systems are well-modeled by Brownian particles subject to gradient dynamics plus noise arising, e.g., from the thermal fluctuations of a heat bath. Of central importance to many applications in physics and biology (e.g., molecular motors) is the net steady-state particle current or “flux” enabled by the noise and an additional driving force. However, this flux cannot usually be calculated analytically. Motivated by this, we investigate the steady-state flux generated by a nondegenerate diffusion process on a general compact manifold; such fluxes are essentially equivalent to the stochastic intersection numbers of Manabe (Osaka Math J 19(2):429–457, 1982). In the case that noise is small and the drift is “gradient-like” in an appropriate sense, we derive a graph-theoretic formula for the small-noise asymptotics of the flux using Freidlin–Wentzell theory. When additionally the drift is a local gradient sufficiently close to a generic global gradient, there is a natural flux equivalent to the entropy production rate—in this case our graph-theoretic formula becomes Morse-theoretic, and the result admits a description in terms of persistent homology. As an application, we provide a mathematically rigorous explanation of the paradoxical “negative resistance” phenomenon in Brownian transport discovered by Cecchi and Magnasco (Phys Rev Lett 76(11):1968, 1996).

Experimental characterization of extreme temperature granular flows for solar thermal energy transport and storage

High-temperature, dense granular flows along an inclined plane were considered for solar thermal energy transport and storage with sintered bauxite particles. A series of experiments was performed for particle inlet temperatures of ∼ 200, 400, 600, and 800 °C to understand the mechanisms of granular flows at extreme temperatures. Mass flow rates were measured using a load cell and free-surface velocities were measured and computed using particle image velocimetry. Surface temperatures were measured using infrared cameras. A significant decrease in steady-state particle mass flow rate was observed with increasing temperature due to changing flow properties. A decrease in bulk particle free-surface velocities was observed at higher temperatures. Free-surface velocity measurement error between experiments were within 20% of the average. The particle surface temperatures decreased from inlet to outlet with larger gradients at higher temperatures observed due to increasing convection and radiative heat losses. A decrease in temperature was observed along the side walls due to a decrease in particle velocities. • Refractory granular media enable high temperature solar thermal energy storage. • Granular flow experiments on inclined plane explored at elevated temperatures. • Significant changes in flow behavior observed at elevated temperatures. • Larger heat loss to surroundings observed at side walls due to decrease in velocity.

Migration of non-Brownian particles localized inside sheared droplets

• Particles inside sheared droplets migrate along the vorticity direction. • The migration direction depends on the viscosity ratio. • The migration kinetics depends on Ca, interfacial tension and particle concentration. • VOF simulations provide velocity and shear rate distribution in a particle-free droplet. • Shear-induced migration plays a major role. The migration of non-Brownian particles inside a droplet subjected to shear flow is investigated. The viscosity ratio, particle concentration, particle to droplet size ratio and capillary number are systematically varied. It was observed that particles migrate in the vorticity direction either towards or away from the equatorial plane. The main factor determining the migration direction is the viscosity ratio. The particle concentration and applied capillary number have no influence on the equilibrium distribution and only affect the time necessary to reach this distribution. In order to identify the flow profile, both velocity and shear rate magnitudes to which the particles inside the droplet are subjected, are numerically simulated using the Volume-of-Fluid method in a particle-free droplet/matrix system. The numerical predictions of the flow are used to evaluate the presence of possible driving mechanisms for cross-streamline particle motion, such as particle inertia and shear rate gradients. Depending on the conditions, the simulations reveal significant differences in the streamline profiles and shear rate gradients within the droplet. The observed steady state particle distributions can partially be explained by shear-induced migration towards zones of low shear rates and/or low streamline curvature. However, in complex flows, such as inside a deformed droplet, different migration mechanisms can occur simultaneously. Hence, confinement inside the droplet and potential secondary flows may also play a role.

Analysis of Smoluchowski’s Coagulation Equation with Injection

The stationary solution of Smoluchowski’s coagulation equation with injection is found analytically with different exponentially decaying source terms. The latter involve a factor in the form of a power law function that plays a decisive role in forming the steady-state particle distribution shape. An unsteady analytical solution to the coagulation equation is obtained for the exponentially decaying initial distribution without injection. An approximate unsteady solution is constructed by stitching the initial and final (steady-state) distributions. The obtained solutions are in good agreement with experimental data for the distributions of endocytosed low-density lipoproteins.

Fluctuations and first-passage properties of systems of Brownian particles with reset.

We study, analytically and numerically, stationary fluctuations in two models involving N Brownian particles undergoing stochastic resetting in one dimension. We start with the well-known reset model where the particles reset to the origin independently (model A). Then we introduce nonlocal interparticle correlations by postulating that only the particle farthest from the origin can be reset to the origin (model B). At long times, models A and B approach nonequilibrium steady states. In the limit of N→∞, the steady-state particle density in model A has an infinite support, whereas in model B, it has a compact support, like the recently studied Brownian bees model. A finite system radius, which scales at large N as lnN, appears in model A when N is finite. In both models, we study stationary fluctuations of the center of mass of the system and of the radius of the system due to the random character of the Brownian motion and of the resetting events. In model A, we determine exact distributions of these two quantities. The variance of the center of mass for both models scales as 1/N. The variance of the radius is independent of N in model A and exhibits an unusual scaling (lnN)/N in model B. The latter scaling is intimately related to the 1/f noise in the radius autocorrelation. Finally, we evaluate the mean first-passage time (MFPT) to a distant target in model A, model B, and the Brownian bees model. For model A, we obtain an exact asymptotic expression for the MFPT which scales as 1/N. For model B and the Brownian bees model, we propose a sharp upper bound for the MFPT. The bound assumes an evaporation scenario, where the first passage requires multiple attempts of a single particle, which breaks away from the rest of the particles, to reach the target. The resulting MFPT for model B and the Brownian bees model scales exponentially with sqrt[N]. We verify this bound by performing highly efficient weighted-ensemble simulations of the first passage in model B.

Experimental and numerical analysis of indoor air quality affected by outdoor air particulate levels (PM1.0, PM2.5 and PM10), room infiltration rate, and occupants' behaviour

This study conducted an experimental analysis of how indoor air quality (IAQ) is influenced by the outdoor air pollutants levels, infiltration rate, and occupants' behaviours. The impacts of these factors on IAQ were analyzed using on-site measurements and numerical simulations. The results contribute to a better understanding of how to control the Indoor Particulate Level (IPL) for the specific conditions of the studied building. Results showed that occupant behaviour was the primary factor in determining the IPL, significantly changing the number of outdoor particles introduced to the building. Moreover, it was found that the IPL was exponentially correlated to the Outdoor Particulate Level (OPL). Based on numerical simulations, this study concluded that smaller particles do not always have more chance than larger particles of accessing the indoor environment through the building envelope. Meanwhile, a steady-state indoor particle concentration numerical model was established and verified using the 4-fold cross-validation method. Finally, simulation results identified that the room infiltration rate had a positive linear impact on IAQ if the OPL was under 30 μg/m3. This is because the increased air exchange rate can help to dilute indoor air pollutants when the outdoor air is relatively clean.

A rapid method for prediction of airborne disease infection risks in an intercity bus

In an intercity bus, respiratory infectious diseases put passengers at high risk of getting infected by the droplets exhaled by an infected person, and the risk increases when exposed to more droplets. Here, to quickly determine the concentration distribution of droplets, to predict the infection risks in a closed space, and to enhance the reliability of the conventional steady-state particle tracking method for predicting the trajectory of droplets released by coughing or sneezing, an improved steady-state particle tracking method is proposed. In it, the momentum of released droplets previously ignored in the conventional steady-state particle tracking method was specifically incorporated using experimental data. Then, the improved method was combined with a random walk model and applied to investigate all possible trajectories of droplets released by different passengers inside a bus. Consequently, the concentration distribution of droplets was obtained from the trajectory information. Finally, the Wells–Riley equation was used to predict the infection risk of every passenger based on the evaluated number of droplets inhaled per passenger. The results show that the improved steady-state tracking method performs more accurately at predicting the concentration field of droplets and associated infection risk than the conventional steady-state particle tracking method. Furthermore, the relative cost of the improved steady-state tracking method is just 1% of the transient calculation method currently considered the most accurate.

Nonperturbative renormalization of quantum thermodynamics from weak to strong couplings

By solving the exact master equation of open quantum systems, we formulate the quantum thermodynamics from weak to strong couplings. The open quantum systems exchange matters, energies and information with their reservoirs through quantum particles tunnelings that are described by the generalized Fano-Anderson Hamiltonians. We find that the exact solution of the reduced density matrix of these systems approaches a Gibbs-type state in the steady-state limit for both the weak and strong system-reservoir coupling strengths. When the couplings become strong, thermodynamic quantities of the system must be renormalized. The renormalization effects are obtained nonperturbatively after exactly traced over all reservoir states through the coherent state path integrals. The renormalized system Hamiltonian is characterized by the renormalized system energy levels and interactions, corresponding to the quantum work done by the system. The renormalized temperature is introduced to characterize the entropy production counting the heat transfer between the system and the reservoir. We further find that only with the renormalized system Hamiltonian and other renormalized thermodynamic quantities, can the exact steady state of the system be expressed as the standard Gibbs state. Consequently, the corresponding exact steady-state particle occupations in the renormalized system energy levels obey the Bose-Einstein and the Fermi-Dirac distributions for bosonic and fermionic systems, respectively. Thus, the conventional statistical mechanics and thermodynamics are thereby rigorously deduced from quantum dynamical evolution.

Characteristics of indoor human-induced particle resuspension under different ventilation conditions

This study performed a series of experiments to study the characteristics of human-induced particle resuspension under different ventilation conditions. Four modes of supplied airspeed were evaluated: no wind, low wind (2.742 m/s), medium wind (3.849 m/s) and high wind (4.422 m/s). Results show that the steady-state particle concentration can be approximated as a linear function of the airspeed. Human walking and ventilation conditions can have a significant and coupling role in particle behaviours. In particular, human walking provides initial energy for resuspension by disturbing the local fields, and thus increases the resuspension, especially for large particles of 5–10 μm in size. The wind has two competitive effects, that is, supplying energy to keep particles suspended and pushing the particles downward, and the latter becomes dominant when the airspeed was too high. Consequently, intermediate airspeeds can intensify the effect of human walking, and introduce higher peaks of particle concentration in the human-walking stages. As for the particle number density, similar conclusions can be drawn that the effect of ventilation conditions is more pronounced on small particles less than 1 μm while that of human walking is on large particles. This work lays a foundation for revealing the particle resuspension mechanism.

Dynamic measurement of impurity ion transport in runaway electron plateaus in DIII-D

The first dynamic (time-dependent) measurements of impurity ion radial (cross field) and parallel (along-field) diffusion coefficients for post-disruption runaway electron plateaus are presented. Small (∼1 mm diameter) carbon or silicon pellets are fired into the edge of steady-state runaway electron (RE) plateaus, and the resulting radial and toroidal transport of singly charged impurity ions (C+ or Si+) is monitored with spatially distributed visible spectrometers. Consistent with previous steady-state particle balance estimates of Ar+ radial transport, radial (cross field) diffusion coefficients D⊥≈2–5 m2/s are obtained, about 10× larger than expected from neo-classical theory. Parallel diffusion coefficients D∥≈30–80 m2/s are estimated, also much (≈50×) larger than classical. It is speculated at present that these large diffusion coefficients may be due to turbulent transport. Indications of fairly significant (almost 2×) toroidal variation in electron density are seen in the RE plateaus, and this appears to cause some toroidal variation in impurity radial diffusion rates. Indications of slow (≈1 Hz) toroidal rotation in the impurity ions are observed, although the uncertainty in this measurement is large.

Dynamics and instabilities of the free boundary of a two-dimensional dry active nematic aggregate

The dynamics of a two-dimensional aggregate of active rod-shaped particles in the nematic phase with a free boundary is considered theoretically. The aggregate is in contact with a hard boundary at y = 0, a free boundary at y = H(x, t), and in the x-direction the aggregate is of infinite extension. By assuming fast relaxation of the director field, we find instabilities driven by the coupling between the deformation of the free boundary and the active stress in parameter regimes where bulk systems are stable. For a contractile aggregate, when the particles are aligned parallel to the boundaries, the system is unstable in the long wavelengths at any strength of contractility for any H, and the critical wavelength increases as H increases; when the particles are aligned perpendicular to the boundaries, the system acquires a finite-wavelength instability at a critical active stress whose strength decreases as H increases. The behavior for an aggregate with steady-state particle density ρ s , strength of active stress χ, bulk modulus ρ s β, and particles aligned perpendicular to the boundaries can be mapped to one with active stress strength − χ, bulk modulus ρ s (β − χ), and particles aligned parallel to the boundaries. The stability of an extensile aggregate can be obtained from the analysis for contractile aggregates through this mapping as well, even though the corresponding physical mechanisms for the instabilities are different. In the limit H → ∞ , the free boundary is unstable for any contractile or extensile systems in the long-wavelength limit.

Combined high fluence and high cycle number transient loading of ITER-like monoblocks in Magnum-PSI

It is highly desirable to understand the long term evolution of the divertor material under the extreme steady-state and transient heat and particle loads expected during ITER operation. Here the impact of ELM-like transient loading under combined high-flux plasma and transient ELM-like heat loading in Magnum-PSI was explored to determine how plasma affects the fatigue cracking threshold of tungsten due to ELMs. Mock-ups consisting of five ITER-like monoblocks in a chain were simultaneously exposed to high flux plasma and a high power pulsed laser which closely simulated the ELM impact in terms of heat flux and duration. Loading conditions were chosen to enable comparison to existing data from electron-beam loading, while the influence of surface base temperature (750 °C, 1150 °C or 1500 °C) and impurity seeding (addition of 6.5 ion% He+ and/or 8 ion% Ne+) were also investigated. The plasma loading leads to differences in surface morphology and indicates synergistic effects on the extent of the surface damage. Base temperatures at or above 1150 °C are found to lead to a significant reduction in the fatigue cracking threshold by a factor of two or more compared to at 750 °C. Cracked surfaces are found to be more than ten times rougher than the original microstructure, and additionally when seeding impurities are added surface roughness can be significantly increased by up closely factor of two compared to roughening using pure H plasma. Overall the results indicate that avoiding fatigue cracking in ITER will be very challenging, and that understanding the level to which this can therefore be tolerated is vital for anticipating divertor lifetime and reliability.

Self-reliant cooling of ultracold atoms

A self-reliant quantum cooler without an external control is proposed. To understand the thermal transport properties of the quantum system interacting with the baths described by the grand canonical ensemble, a two-level system with a single energy transport channel and a two-level coupled system with multi-channels are established. By considering the temperature dependence of the chemical potentials of the ultracold atom baths, the steady-state heat flows, particle currents, and entropy productions of these systems are derived. Results show that a steady heat current flows against the thermal bias for a nonequilibrium system coupled with ultracold quantum gases and the cooling rate can be enhanced by quantum coherence.