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We present the development of a non-reflecting boundary condition, based on the Local One-Dimensional Inviscid (LODI) approach, for Lattice Boltzmann Models working with multi-speed stencils. We test and evaluate the LODI implementation with numerical benchmarks, showing significant accuracy gains with respect to the results produced by a simple zero-gradient condition. We also implement a simplified approach, which allows handling the unknown distribution functions spanning several layers of nodes in a unified way, still preserving a comparable level of accuracy with respect to the standard formulation.

Understanding the behavior of human crowds is a key step toward a safer society and more livable cities. Despite the individual variability and will of single individuals, human crowds, from dilute to dense, invariably display a remarkable set of universal features and statistically reproducible behaviors. Here, we review ideas and recent progress in employing the language and tools from physics to develop a deeper understanding about the dynamics of pedestrians.

Inkjet printing inks frequently contain polar liquids of low volatility such as glycerol or poly(ethylene glycols) in addition to the main solvent water. The deposition of these liquids on paper sheets induces swelling of the cellulose fibers, which leads to an overall, anisotropic deformation of the sheet. We characterized the corresponding strain components by means of a grid projection method and white light interferometry. For pure water, most of the hydroexpansion strain vanishes again after drying is complete. However, for aqueous solutions of non-volatile co-solvents, a large fraction of the deformation persists after the water has evaporated. Because swelling occurs only after liquid enters the cellulose fibers, monitoring the dynamics of expansion provides insight into the pore-fiber distribution of co-solvents. The corresponding timescales of pore-fiber transport strongly depend on the co-solvent concentration, as a sufficient quantity of water is needed to plasticize the fiber walls.

The immersed boundary method has emerged as an efficient approach for the simulation of finite-sized solid particles in complex fluid flows. However, one of the well known shortcomings of the method is the limited support for the simulation of light particles, i.e. particles with a density lower than that of the surrounding fluid, both in terms of accuracy and numerical stability. Although a broad literature exists, with several authors reporting different approaches for improving the stability of the method, most of these attempts introduce extra complexities and are very costly from a computational point of view. In this work, we introduce an effective force stabilizing technique, allowing to extend the stability range of the method by filtering spurious oscillations arising when dealing with light-particles, pushing down the particle-to-fluid density ratio as low as 0.04. We thoroughly validate the method comparing with both experimental and numerical data available in literature.

This work investigates the reactions occurring in K2CO3-H2O-CO2 under ambient CO2 pressures in temperature and vapor pressure ranges applicable for domestic thermochemical heat storage. The investigation shows that depending on reaction conditions, the primary product of a reaction is K2CO3·1.5H2O, K2CO3·2KHCO3·1.5H2O, or a mixture of both. The formation of K2CO3·1.5H2O is preferred far above the equilibrium conditions for the hydration reaction. On the other hand, the formation of double salt is preferred at conditions where hydration reaction is inhibited or impossible, as the thermogravimetric measurements identified a new phase transition line below the hydration equilibrium line. The combined X-ray diffraction, thermogravimetric analysis, and Fourier-transform infrared spectroscopy study indicates that this transition line corresponds to the formation of K2CO3·2KHCO3, which was not observed in any earlier study. In view of thermochemical heat storage, the formation of K2CO3·2KHCO3·(1.5H2O) increases the minimum charging temperature by approximately 40 °C. Nevertheless, the energy density and cyclability of the storage material can be preserved if the double salt is decomposed after each cycle.

Magnetic density separation (MDS) is a technique for separating granular materials based on mass density. MDS uses magnetically responsive fluids and engineered magnetic fields to create a vertical gradient of apparent mass density inside the fluid. We present a numerical study to investigate the effect of honeycomb wake turbulence on the motion of almost neutrally buoyant spherical particles in a magnetized fluid. A four-way coupled Euler-Lagrange approach is used where particles are effectively treated as finite-size point particles. It is shown that the honeycomb-generated wake turbulence increases the levitation time of the particles. The presence of particles results in an earlier break-up of the individual velocity profiles stemming from the honeycomb cells. The effects of honeycomb geometry and cell Reynolds number on the collective motion of particles and turbulence properties are investigated, where the choice for the best honeycomb is a trade-off between the separation error and the particle flux.

The modification of liquid flow induced during atmospheric plasma-water interactions when the liquid is changed from electrically floating to grounding (thus entering a regime of electrolysis) is described. A helium plasma jet is impinged upon a demineralised water target when a ground electrode is both absent from or submerged within the solution, and flow is recorded using particle image velocimetry. Modifications of the applied voltage and the salinity of the grounded solutions are made, and results are given a qualitative comparison to theory. An increase in upwards flow velocity was observed with both grounding and an increase of applied voltage, whereas downwards velocity increased in grounded solutions with increased salinity. This is believed to be due to the electrolysis exerting a net force on the flow, with the direction depending on the mobility coefficients of the ions present.

Mixing ventilation in airplane cabins is driven by overhead supply jets, which may be directed towards each other, interact and merge before entering the passenger area. Although many studies already investigated airplane cabin mixing ventilation, detailed knowledge on the fundamental flow components (i.e. supply jets, interaction zone, merged jet and secondary flows) is rather scarce, but can be useful to obtain new insights into mixing ventilation design optimisation. This paper presents time-averaged 2D particle image velocimetry (PIV) measurements of the flow driven by isothermal opposing plane jets in a reduced-scale water-filled generic airplane cabin. Two configurations of opposing jets, i.e. ceiling jets and lateral jets, are examined with inlet Reynolds numbers ranging from 3,450 to 4,650. The analysis focuses on the mean velocity and Reynolds stresses of the whole flow field and the aforementioned fundamental flow components. Decay rates of the supply jets and decay and growth rates of the merged jet are analysed and the size of the interaction zone is assessed. The supply jet velocity decays very fast due to the interaction with the opposing jet. Within the interaction zone, the turbulence levels are high and turbulence is highly anisotropic. A comparison between the merged jet and a self-similar turbulent plane free jet is presented as well. The measurement data is also valuable for (sub-configuration) computational fluid dynamics (CFD) validation purposes.

Atmospheric non-thermal plasmas have received much attention due to their unique properties of ion and reactive species generation and delivery within “standard every day“ conditions. Applications of these plasmas can be found in a wide range of fields such as medicine, agriculture and water treatment amongst many others. One factor which has recently gaining further interest is that of the induced flow during Atmospheric Pressure Plasma Jet (APPJ) liquid target impingement. Due to the presence of a liquid phase within many APPJ applications, and the governance of flow on ion and reactive species transmission, furthering the much lacking understanding of fluid mechanics within this phase is important in potentially finding new improvements (in e.g. the species transmission flux efficiency) of atmospheric plasma applications. The effect of grounding an APPJ liquid target on the induced liquid flow is described. The addition of the ground electrode within the solution allows current to pass through the solution and induce electrolysis on the system. A helium APPJ is impinged upon a water target for varying plasma applied voltage and solution salinity, and PIV results are compared for when this ground electrode is both absent from or submerged within the solution during these parameter investigations. The flow modifications observed included demineralised water targets having an upwards flow velocity increase with grounding, whereas in saline solutions the velocity instead increased downwards. This is believed to be due to the electrolysis exerting a net drag force on the flow, with the direction depending on the mobility coefficients of the ions present.

In the published article, in Section 3.1 ‘‘Thermal Marangoni flow’’, there is a typo in the estimation of DT proposed,† it should read DT = Hv/Cp. While this expression is dimensionally correct and gives rise to the definition of the Marangoni number in eqn (23), it does not give realistic values for evaporation-driven cooling. A better approximation would be: (Formula Presented). where Hv is the latent heat of vaporization, Cp is the specific heat capacity at constant pressure (all defined in Section 3.1), D is the diffusion coefficient of the vapor in air, r is the liquid density, Dc is the difference between the saturated vapor concentration at the interface and in the far field (all defined in Section 2.1) and DT is the thermal diffusivity of the liquid phase. The results and conclusions presented in the published article are unaffected. The authors would like to thank Jafar Farhadi for pointing out the typo. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

Future fusion reactors require a safe, steady-state divertor operation. With deep divertor detachment, which is typically induced by impurity seeding, the radiation concentrates in a small region at the X-point or on closed flux surfaces above the X-point. This so-called X-point radiator (XPR) moves further inside the confined region with increasing seeding and the location can be actively controlled. At AUG, the parameter space for operation with an XPR was significantly extended, using active feedback on the XPR location. The XPR is observed in nearly the whole operational space of AUG in the high-densities or high collisionality regime. ELM suppression is consistently observed in all cases where the XPR was moved to a significant height above the X-point. Direct measurements of density and temperature from the region around the XPR using the new divertor Thomson scattering system at AUG indicate that the temperature at the location of the XPR remains high (>30eV) and only the region towards the X-point cools down further. In this cold XPR core, the temperature reduces to about 1eV. An XPR is also observed in TCV by the injection of nitrogen as extrinsic impurity. This highlights that the wall material (W for AUG, C for TCV) or machine size does not play a significant role for the existence of the regime. However, the scenario appears to be less stable in TCV. First experiments show the necessity of an active control for the XPR: Depending on the wall conditions and the nitrogen wall storage, the required nitrogen seeding level to achieve an XPR changes. Both, the low temperatures measured radially outside of the radiation zone at AUG, and the lower stability of the XPR regime at TCV with the presence of carbon are consistent with the predictions of a one-dimensional model of the XPR. However, the model would predict the development of the cold XPR core, and significant radiation at the X-point might already exist before reaching this cold temperature solution.

Stabilised dense emulsions display a rich phenomenology connecting microstructure and rheology. In this work, we study how an emulsion with a finite yield stress can be built via large-scale stirring. By gradually increasing the volume fraction of the dispersed minority phase, under the constant action of a stirring force, we are able to achieve a volume fraction close to 80%. Despite the fact that our system is highly concentrated and not yet turbulent we observe a droplet size distribution consistent with the −10/3 scaling, often associated with inertial range droplets breakup. We report that the polydispersity of droplet sizes correlates with the dynamics of the emulsion formation process. Additionally, we quantify the visco-elastic properties of the dense emulsion finally obtained and we demonstrate the presence of a finite yield stress. The approach reported can pave the way to a quantitative understanding of the complex interplay between the dynamics of mesoscale constituents and the large-scale flow properties of yield stress fluids.

Flows in thin fluid layers, like in the Earth’s atmosphere or oceans, tend to behave as quasi-two-dimensional flows. Their dynamics is strikingly different from three-dimensional flows, and main features of the flow dynamics can be understood by considering two-dimensional (2D) fluid flows. Inviscid 2D flows are governed by conservation of vorticity due to absence of vortex stretching and tilting. Together with conservation of kinetic energy this results in the famous inverse energy cascade and the emergence and persistence of large-scale vortices. This also occurs in shallow fluid-layer flows even if they are neither purely inviscid nor perfectly two-dimensional. Basic phenomena for understanding the dynamics of 2D flows will be discussed and 2D flows on bounded domains, mainly dealing with the large-scale phenomenology of the flow, will be addressed: the self-organization of 2D turbulence in confined domains and the interaction of coherent structures with domain walls. This will be complemented with some observations from recent experiments on quasi-2D turbulence in shallow-fluid layers including the role and impact of bottom friction and out-of-plane motion on the flow evolution.

This chapter explores the shift in the balance of individual versus collective values instigated by the COVID-19 pandemic. The incredible viral spread rate among the population and its relatively high fatality rate has initially resulted in an assertion of the primacy of collective values (such as collective safety, collective responsibility, conformism). In contrast, individual rights and values (such as the individual counterparts of autonomy, freedom, responsibility, and privacy) took a ‘back seat’ for the good of the collective. However, as the pandemic extended over the months, there emerged a pressure to reject the primacy of collective values and restore the importance of individual values. If we are to return to a healthy and prosperous living within a well-functioning society, this balance shift between collective and individual values will have to be re-negotiated and resolved to reach a societally acceptable balance position.We conduct this ethical exploration, first, by following the ethico-philosophical discussion on the balance between individual and collective values generally, as well as in times of crisis, with special focus on the COVID-19 crisis. Second, we explore this topic through the lens of recent changes to how particular technologies were and are used before and during the COVID-19 pandemic. More precisely, we identify and explore broad trends we see relevant to ethics – with a particular focus on crowd management and nudging and on the balance shift between individual and collective values. Finally, by exploring findings from a sociophysics case study dealing with crowd management of people before and during the pandemic, we argue that some sociophysical phenomena can be used as proxies for ethical principles. Here, distance is used as a conceptual proxy for individual and collective responsibility, having in mind COVID-19 distancing rules and recommendation.With all the above, we identify several broad trends that have been instigated by the pandemic that are relevant to ethics. These changes relate to future crowd management, nudging, and control; individual (per-person) tracking; insistence on the importance of collective values in times of crisis, and the rejection of this infringement upon individual rights. In this regard, we assert that such value changes are an opportunity to rethink and (re)set balance points between individual and collective rights for each particular society.

Funder: Dutch NWO Aspasia program, Funder: University of Colorado Boulder; doi: http://dx.doi.org/10.13039/100007493, Funder: FSE/POPH, Women have made significant contributions to applied physics research and development, and their participation is vital to continued progress. Recognizing these contributions is important for encouraging increased involvement and creating an equitable environment in which women can thrive. This Roadmap on Women in Applied Physics, written by women scientists and engineers, is intended to celebrate women’s accomplishments, highlight established and early career researchers enlarging the boundaries in their respective fields, and promote increased visibility for the impact women have on applied physics research. Perspectives cover the topics of plasma materials processing and propulsion, super-resolution microscopy, bioelectronics, spintronics, superconducting quantum interference device technology, quantum materials, 2D materials, catalysis and surface science, fuel cells, batteries, photovoltaics, neuromorphic computing and devices, nanophotonics and nanophononics, and nanomagnetism. Our intent is to inspire more women to enter these fields and encourage an atmosphere of inclusion within the scientific community.

We study using three-dimensional kinetic Monte Carlo (KMC) simulations to what extent the formation of Schottky contacts between a metal electrode and a molecularly doped disordered organic semiconductor can be understood from the theory for crystalline inorganic semiconductors, adapted to include the effects of the localized nature of the states in which the charge carriers reside and the hopping transport in between these states. The thickness of the Schottky-contact depletion region is found to be significantly smaller than as expected when the energetical disorder is neglected. The presence of energetic disorder is also found to influence the voltage dependence of the width of the depletion regions near the contacts of single-layer double-Schottky-contact devices. The voltage drop over the two depletion regions and the remaining charge-neutral bulk layer is shown to be described successfully by a semianalytical model, based on an accurately parameterized bulk mobility function of the dopant concentration, energetic disorder, and the electric field. We furthermore find that the mobility in the depletion regions is drastically reduced. As a result, the depletion-region formation process can be ultraslow, with a characteristic time scale ranging from microseconds to beyond milliseconds.

Knowledge of the instantaneous flow behaviour of interacting opposing jets, in addition to knowledge of the mean flow, is important for science and practice. Whereas studies often focused on axisymmetric jets, analyses for plane jets are scarce in general and for plane jets in an enclosed domain (i.e. not a (semi-)open environment) in particular, as e.g. encountered in airplane cabin ventilation. In this paper, 2D particle image velocimetry measurements are performed to study isothermal interacting opposing plane wall jets and plane free jets in a generic empty reduced-scale water-filled enclosure. Inlet Reynolds numbers vary from 3450 to 4650. The analyses encompass an inspection of the global flow patterns (in the vertical midplane) and of the flow components (e.g. interaction zone, merged jet, return flows, recirculation cells), using distribution plots, correlation functions and fast Fourier transforms. Vortical structures are also visualised and tracked over time. It is shown that the transient interaction of the opposing wall jets drives a merged jet that resembles a flapping turbulent plane jet. Remarkable are the occasional deviating (more unstable) flow patterns that appear. Furthermore, many vortical structures are present that could enhance mixing within the enclosure. The opposing free jets mainly show quasi-periodic oscillations with a given frequency (Strouhal number around 3.3 × 10–3), comparable to opposing plane free jets mentioned in the literature. Also in this configuration, many different vortices are present that can grow considerably large while transported through the flow domain. Both configurations show a potential for contaminant lock-up (stagnation zones).

We have studied the imbibition and drying of water/co-solvent mixtures in paper and glass microfiber filters. The experiments reveal a rich interplay of solution imbibition, solvent evaporation and solvent-mediated pore-fiber transport. After deposition, liquids occupy the micron-scale inter-fiber pores of a paper sheet. In thermodynamic equilibrium, polar liquids such as water and co-solvents reside in the nm-scale intra-fiber pores of the cellulose fibers. The timescales for attaining equilibrium prove to depend sensitively on the water content, such that co-solvents can be temporarily trapped in a non-equilibrium configuration. The combination of two experimental methods allows the determination of both the overall co-solvent content and an estimation of what fractions reside in the pores and in the fibers of a paper sheet. The results are relevant to understanding the behavior of solutions in paper and provide insight into the dynamics of aqueous inkjet printing inks.

Surface silanols (SiOH) are important moieties on glass surfaces. Here we present a tag-and-count approach for determining surface silanol densities, which consists of tagging surface silanols with Zn via atomic layer deposition (ALD) followed by detection of the zinc by high sensitivity-low energy ion scattering (HS-LEIS). Shards of fused silica were hydroxylated with aqueous hydrofluoric acid (HF) and then heated to 200, 500, 700, or 900 °C. These heat treatments increasingly condense and remove surface silanols. The samples then underwent one ALD cycle with dimethylzinc (DMZ) or diethylzinc (DEZ) followed by water. As expected, fused silica surfaces heated to higher temperatures showed lower Zn coverages. When fused silica surfaces treated at 200 °C were exposed to DMZ for two different dose times, the same sub-monolayer quantity of Zn was obtained by X-ray photoelectron spectroscopy (XPS). Surface cleaning/preparation immediately before HS-LEIS, including atomic oxygen treatment and annealing, played a critical role in these efforts. Surfaces treated with DMZ generally showed slightly higher Zn signals by LEIS. Using this methodology, a value of 4.59 OH/nm2 was found for fully hydroxylated fused silica. Both this result and those obtained at 500, 700, and 900 °C are in very good agreement with literature values.

Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe$_3$GeTe$_2$ (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modelling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin-orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics., Supplementary information included

Solar-assisted water electrolysis is a promising technology for storing the energy of incident solar irradiation into hydrogen as a fuel. Here, an integrated continuous flow electrochemical reactor coupled to a monolithic perovskite-silicon tandem solar cell is demonstrated that provides light-driven electrochemical solar-to-hydrogen conversion with an energy conversion efficiency exceeding 21% at 1-Sun equivalent light intensity and stable operation during three simulated day-night cycles.

In this work, an experimental study investigating dielectric barrier discharge (DBD) plasma-assisted ignition in methane-air flows at near-atmospheric pressure is presented. Discharges are produced using 10 ns duration high voltage pulses at maximum 3 kHz pulse repetition rate. The goal is to check the feasibility of plasma as an ignition source for ignition-stabilized combustion in a wide range of equivalence ratio, pressure, and flow speed conditions. To that end, four important characteristics are investigated: 1) Ignition dynamics, 2) Minimum number of pulses required for ignition, 3) Plasma energy per pulse, gas temperature, and the effective reduced electric field, and 4) Plasma NOx production. In continuous plasma mode, we observe an elongated flame outside of the discharge region when a methane-air mixture flows through the discharge. The elongated flame is due to repetitively ignited kernels moving downstream with the flow. High-speed intensified imaging is used to explore plasma morphology and ignition dynamics. Plasma has a filamentary nature which is perceived as moving with the flow due to pulse-to-pulse memory effects. For fuel-air mixtures, ignition is followed by kernel expansion and splitting, flame propagation, and consecutive ignition. The time delay between consecutive ignition events is found to be a function of flow speed, because ignition is achieved only when a previously ignited kernel moves out of the plasma region, which happens faster at higher flow speeds. We performed optical emission spectroscopy in burst mode to further characterise plasma and ignition parameters. In air, plasma gas temperature and hence the reduced electric field increases because of pulse-to-pulse effects. By comparing the air plasma temperature with the methane auto-ignition temperature, we conclude that the observed ignition is low-temperature ignition. Parametric studies of NOx measurements suggest that plasma NOx emissions are higher for low flow speeds, high pulse repetition rates, and low-pressure conditions. Overall, our DBD filamentary plasma is found to be a fast, repetitive, and low-temperature ignition source that can be used for ignition-stabilized combustion at near atmospheric conditions.

In this work we provide analytic and numerical solutions for the Bjorken flow, a standard benchmark in relativistic hydrodynamics providing a simple model for the bulk evolution of matter created in collisions between heavy nuclei. We consider relativistic gases of both massive and massless particles, working in a ( 2 + 1 ) and ( 3 + 1 ) Minkowski space-time coordinate system. The numerical results from a recently developed lattice kinetic scheme show excellent agreement with the analytic solutions.

We report flow measurements in rotating Rayleigh--B\'enard convection in the rotationally-constrained geostrophic regime. We apply stereoscopic particle image velocimetry to measure the three components of velocity in a horizontal cross-section of a water-filled cylindrical convection vessel. At a constant, small Ekman number $Ek=5\times 10^{-8}$ we vary the Rayleigh number $Ra$ between $10^{11}$ and $4\times 10^{12}$ to cover various subregimes observed in geostrophic convection. We also include one nonrotating experiment. The scaling of the velocity fluctuations (expressed as the Reynolds number $Re$) is compared to theoretical relations expressing balances of viscous--Archimedean--Coriolis (VAC) and Coriolis--inertial--Archimedean (CIA) forces. Based on our results we cannot decide which balance is most applicable here; both scaling relations match equally well. A comparison of the current data with several other literature datasets indicates a convergence towards diffusion-free scaling of velocity as $Ek$ decreases. However, the use of confined domains leads at lower $Ra$ to prominent convection in the wall mode near the sidewall. Kinetic energy spectra point at an overall flow organisation into a quadrupolar vortex filling the cross-section. This quadrupolar vortex is a quasi-two-dimensional feature as it only manifests in energy spectra based on the horizontal velocity components. At larger $Ra$ the spectra reveal the development of a scaling range with exponent close to $-5/3$, the classical exponent for inertial-range scaling in three-dimensional turbulence. The steeper $Re(Ra)$ scaling at low $Ek$ and development of a scaling range in the energy spectra are distinct indicators that a fully developed, diffusion-free turbulent flow state is approached, sketching clear perspectives for further investigation., Comment: 18 pages, 6 figures

Ultrafast laser-induced dynamics in a ferrimagnetic gadolinium iron cobalt (Gd/FeCo) multilayer with a magnetization compensation temperature of TM = 320 K is studied at room temperature as a function of laser-fluence and strength of the applied magnetic field. The dynamics is found to be substantially different from that in archetypical GdFeCo alloys, and depending on the laser fluence one can distinguish two different regimes. At low laser fluence (⩽1.6 mJ cm-2), ultrafast laser excitation of the medium triggers spin precession of an extraordinary large amplitude reaching over 30°. At high laser fluence (⩾2.2 mJ cm-2), the pump heats the medium over the magnetization compensation point, spin precession reduces significantly in amplitude and the process of field-assisted reversal of magnetization of Gd and FeCo is launched. It is argued that such a distinctly different laser-induced magnetization dynamics in the multilayers compared to the alloys is due to the symmetry breaking at the numerous interfaces, giving rise to additional surface anisotropy. The temperature dependence of the latter is found to be the key ingredient in the mechanism of ultrafast laser-induced magnetization dynamics in ferrimagnetic multilayers. Controlling the amount and properties of interfaces in multilayers can thus serve as a mean to achieve efficient ultrafast all-optical control of magnetism.

Abstract In this work, we explore the possibility of learning from data collision operators for the Lattice Boltzmann Method using a deep learning approach. We compare a hierarchy of designs of the neural network (NN) collision operator and evaluate the performance of the resulting LBM method in reproducing time dynamics of several canonical flows. In the current study, as a first attempt to address the learning problem, the data were generated by a single relaxation time BGK operator. We demonstrate that vanilla NN architecture has very limited accuracy. On the other hand, by embedding physical properties, such as conservation laws and symmetries, it is possible to dramatically increase the accuracy by several orders of magnitude and correctly reproduce the short and long time dynamics of standard fluid flows. Graphic abstract

In a typical laboratory nanodusty plasma, nanometer-sized solid dust particles can be generated from the polymerization of reactive plasma species. The interplay between the plasma and the dust gives rise to behavior that is vastly different from that of pristine plasmas. Two of the key parameters in nanodusty plasma physics are, among other things, the dust particle size and the dust density. In this work, we introduce a novel method for the determination of these two quantities from the measurement of the free electron density using microwave cavity resonance spectroscopy and laser light extinction measurements. When comparing these two measurements to theory, one can determine the best-fitting dust particle size and dust density. Generally, cyclic behavior of the dust particle size and dust density was observed, of which the trends were relatively insensitive to varying the most stringent input assumptions. Finally, this method has been used to explore the behavior of the dust particle size and dust density for varying plasma powers.

The leading edge bulge is a known defect in stereolithography with free-surface recoating. It is a resin excess that remains after deposition over a deep-to-shallow topographic transition, the leading edge of the cavity. In this paper, we explore an approach based on changing the gap between the recoating blade and the previously cured layer to minimize the local resin surface unevenness by suppressing resin excess. By means of numerical simulations, different blade trajectory configurations have been considered. The results show that the amplitude of the leading edge bulge can be reduced to a minimal value by a blade actuation protocol of lowering then raising the blade as it moves over the leading edge, without increasing the recoating process time.

We have studied the chromatographic separation of solvents and dyes after deposition of a dye solution on a paper substrate. Due to their larger molecular size, dyes typically exhibit a stronger interaction with the paper constituents. Consequently, the imbibition process of the dye is usually delayed compared to that of the solvent. This impacts the achievable resolution and color homogeneity in inkjet printing. We present experiments and a comprehensive numerical model to illustrate and quantify these effects. The model accounts for the solvent evaporation, heat transfer, multicomponent unsaturated flow, and dye adsorption, as well as the presence of permeable fibers in the paper substrate. We identify the key parameters that can be tuned to optimize the pattern fidelity of the printing process.

We consider packed-bed reactors with dielectric beads in a two-dimensional channel geometry, apply an electric field perpendicular to the walls, and explore numerically the sensitivity of reaction conversion efficiencies of a dissociation reaction on system parameters like shape, orientation, and size of the beads and porosity of packed-bed systems. We have developed a lattice Boltzmann (LB) model that allows for simultaneous simulation of the flow field, the electric field within fluid and (solid) beads, and transport of (charged) species, such as ions and reagents. It solves Navier–Stokes for the fluid flow and the concentration field for neutral and charged species by the advection–diffusion and Nernst–Planck equation, respectively, formulated in the LB framework. The model allows to compute electric field strengths in the fluid and in the beads, by solving the Poisson equation. The method is suitable for arbitrary geometries of the flow domain and does not require body-fitted meshes. Two important conclusions can be drawn. First, the proposed LB model enables simulation of a reactive electro-kinetic fluid in a reactor with dielectric packed beads of arbitrary shape, size, and orientation. The LB method is based on Cartesian meshes irrespective of the shape of the beads and is highly parallelizable and can be extended to three-dimensional packed-bed reactors. Second, we show that reactor conversion efficiency is sensitive to shape, orientation, and size of the beads and the porosity of the packed-bed reactor. Present observations will guide the parameter settings for the beads and packed-bed reactor of more realistic three-dimensional configurations.

Airplane cabin ventilation is essential to ensure passengers' well-being. The conventional ventilation method is mixing ventilation with a statistically steady supply, which, according to former studies, has reached its limits regarding, for example, the ventilation efficiency. However, the effect of a statistically unsteady (time-periodic) supply on the mixing ventilation efficiency has remained largely unexplored. This research uses computational fluid dynamics (CFD) with the large eddy simulation (LES) approach to study isothermal time-periodic mixing ventilation in a section of a single-aisle airplane cabin model, in which the air exhaled by the passengers functions as (passive) contaminants. Two time-periodic supply strategies are evaluated. The induced time-periodic airflow patterns promote an efficient delivery of fresh air to the passenger zone and affect the passengers' expiratory plumes. This results in increased mean contaminant mass fluxes, causing a strong reduction of the mean contaminant concentrations in the passenger zone (up to 23%) and an increased contaminant extraction from the cabin. Mean velocities increase with up to 55% but remain within the comfortable range. It is shown that the ventilation efficiency improves; that is, the contaminant removal effectiveness and air change efficiency (in the full cabin volume) increase with up to 20% and 7%, respectively.

The alignment of a pair of spherical particles perpendicular to a horizontally oscillating flow is attributed to a non-zero residual flow, known as steady streaming. This phenomenon is the basis of complex patterns in denser systems, such as particle chains and the initial stages of rolling-grain ripples. Previous studies on such self-organization processes used two distinct systems: an oscillating box filled with viscous fluid and an oscillating channel flow, where the fluid oscillates relative to the bottom boundary. In this paper, we show that particle pair dynamics in these two systems are fundamentally different, due to the presence of a Stokes boundary layer above the bottom in the oscillating channel flow. The results are obtained from direct numerical simulations in which the dynamics of a pair of particles are simulated using an immersed boundary method. The oscillating box and the oscillating channel flow are only equivalent in a limited region of the parameter space, where both the normalized Stokes boundary layer thickness and the normalized relative particle excursion length are small. Overall, the particle dynamics in the oscillating channel flow, compared to the oscillating box, are governed by an additional dimensionless parameter, that is, the particle–fluid density ratio.

In this study we use an experimental investigation to shed light on the lightning inception problem. From atmospheric observations, it is known that the electric fields in thunderclouds are significantly lower than required for electric breakdown in air. One theory to explain lightning inception is that hydrometeors, i.e. any liquid or solid water particles formed in the atmosphere, greatly enhance the local electric field and can thereby initiate an electron avalanche leading to a streamer discharge. In this study, we investigate streamer initiation in the presence of artificial particles with different shapes. A metal or dielectric (TiO2) particle is suspended between a high-voltage and a grounded planar electrode which are separated by 16 cm in 50 mbar air. The particles are shaped as ellipsoids with a length of 8, 4, 2, and 1 cm and with different aspect ratios. A negative high voltage pulse is applied with a rise time of 30 ns, a pulse width of 1–10 μs, a repetition rate of 1 Hz, and a maximum voltage between 1 and 50 kV. Results show that the required background electric field for breakdown in the presence of a dielectric particle is decreased to 0.4 times the air breakdown field. Moreover, we observed bipolar streamer development from the particles where negative streamers are thicker and slightly slower than positive streamers. Finally, we found that streamers from longer particles are thicker and faster.

Electronic transport in transition metal spinel oxides is associated with small polaron hopping, either nearest-neighbor, resulting in Arrhenius activated conductivity, or variable energy, leading to a sub-Arrhenius relationship, with the conductivity logarithm being a convex function of inverse temperature. For the case of manganese spinel oxides alloyed with zinc and nickel, instances of super-Arrhenius behavior are measured, with the conductivity logarithm functional dependence on temperature deviating quadratically. Here, we study the transport in Zn0.5NixMn2.5−xO4 ternary oxide pellets, as a function of Ni content in the range 0 ≤ x ≤ 1.25, in combination with structural characterization and theoretical investigations of their electronic and structural properties using density functional theory. The coexistence of cubic spinel and tetragonal Hausmannite structures is revealed along with the presence of various magnetic conformations that are metastable at room temperature. For systems where metastable structures exist, having similar formation energy but different electronic structures, conductivity is a non-trivial function of temperature. Considering nearest-neighbor polaron transfer in such an energetically inhomogeneous landscape, a new hopping mechanism model is proposed which consistently describes the temperature dependence of conductivity in this ternary alloy spinel oxide system. Understanding the underlying physical transport mechanism is vital for sensor, electrochemical, and catalytic applications.

Enabling automated and non-intrusive management of pedestrian flows can help unlock intelligent public environments to maximize individual comfort and wayfinding efficiency. An effective option for this purpose is using dynamic nudging stimuli generated through the use of smart technologies. For this purpose, recent experimental studies have considered visual stimuli to steer pedestrians into alternative walking routes. This research lays the basis for using sonification as a pedestrian flow nudging mechanism. We have developed and deployed a spatialized dynamic sonification to nudge between alternative walking patterns in a real-life work environment endowed with highly accurate and automated pedestrian tracking. In this paper, we report on our preliminary nudging results showing that sound can alter the typical walking trajectories, yet counterintuitively.

The growth and evolution of microbial populations is often subjected to advection by fluid flows in spatially extended environments, with immediate consequences for questions of spatial population genetics in marine ecology, planktonic diversity and origin of life scenarios. Here, we review recent progress made in understanding this rich problem in the simplified setting of two competing genetic microbial strains subjected to fluid flows. As a pedagogical example we focus on antagonsim, i.e., two killer microorganism strains, each secreting toxins that impede the growth of their competitors (competitive exclusion), in the presence of stationary fluid flows. By solving two coupled reaction-diffusion equations that include advection by simple steady cellular flows composed of characteristic flow motifs in two dimensions (2d), we show how local flow shear and compressibility effects can interact with selective advantage to have a dramatic influence on genetic competition and fixation in spatially distributed populations. We analyze several 1d and 2d flow geometries including sources, sinks, vortices and saddles, and show how simple analytical models of the dynamics of the genetic interface can be used to shed light on the nucleation, coexistence and flow-driven instabilities of genetic drops. By exploiting an analogy with phase separation with nonconserved order parameters, we uncover how these genetic drops harness fluid flows for novel evolutionary strategies, even in the presence of number fluctuations, as confirmed by agent-based simulations as well., Comment: 29 pages, 22 figures

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future opto-magnetic data storage applications, the role of non-local spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring THz standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current, but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the sub-ps timescale. We present modelling efforts using the $s$-$d$ model, which effectively reproduce our results and allow us to explain the behavior in terms of the temporal profile of the spin current., Supplementary information appended

Crowd accidents – defined as situations where mass gatherings of people lead to deaths or injuries – have become a frequent occurrence on a global scale. Given the recurring nature of these accidents, it is essential that their characteristics are analyzed. To this end, an important step would be documenting these records. Here, a database of crowd accidents is developed for the period of 1900–2019 through a comprehensive investigation of the press and media reports. The analyses focus mainly on temporal trends of their frequency and injury/casualty in each accident, as well as their geographical distribution and classification based on the purpose of gathering. Results show that the frequency of crowd accidents has been unambiguously on the rise over the last 120 years. Also, there was no indication that larger crowd sizes increase the risk of injury or death per person. In fact, the opposite was the case, although a causal relation between crowd size and risk of injury/death is impossible to establish. Over time, the share of sport events in crowd accidents has declined, and instead, religious gatherings have become more notably present in the statistics. An interesting observation is the association of accident rates to the income level of the countries where they happen, with low-and-middle-income countries being more represented in the records. India and (to a lesser extent) West Africa, in particular, appear to be hot spots for crowd accidents. Finally, it is argued that the exponential increase in crowd accidents of the last century was only partially real, with technology also playing a role in making information more accessible for recent accidents. After the internet (and SNS) became widespread, the trend for reported crowd accidents does not show anymore an exponential increase although it is difficult to conclude whether their frequency is stable or not. The insights obtained from this study can pave the way for developing diagnostic knowledge and raising awareness about the ubiquity of crowd accidents.

We experimentally investigate the speed of fragments produced by ligament breakup in the laser-induced deformation of tin microdroplets into axisymmetric sheets. The experiments were carried out covering a wide range of droplet diameters and laser-pulse energies. In addition to fragments produced by end-pinching, we also observe fragments shed via Rayleigh-Plateau breakup of long ligaments at late times. A double-frame backlit camera was used to obtain the speeds of the fragments u(f) and the time of their detachment t(d). We show that by normalizing u(f) to the initial expansion speed of the sheet R-0, all data collapse onto a single, universal curve that is a function of the dimensionless time t(d)/tau(c) only, where tau(c) is the capillary time. This universal curve is explicitly independent of the droplet's Weber number. The collapse of u(f) is supported by energy conservation arguments. Our findings enable the prediction of the instantaneous speed and position of the fragments shed from liquid tin targets used in state-of-the-art extreme ultraviolet nanolithography, facilitating the design of effective mitigation strategies against microparticulate debris.

The development of turbulence closure models, parametrizing the influence of small nonresolved scales on the dynamics of large resolved ones, is an outstanding theoretical challenge with vast applicative relevance. We present a closure, based on deep recurrent neural networks, that quantitatively reproduces, within statistical errors, Eulerian and Lagrangian structure functions and the intermittent statistics of the energy cascade, including those of subgrid fluxes. To achieve high-order statistical accuracy, and thus a stringent statistical test, we employ shell models of turbulence. Our results encourage the development of similar approaches for three-dimensional Navier-Stokes turbulence.

A pure spin current is predicted to occur when an external magnetic field and a linearly inhomogeneous spin-only field are appropriately aligned. Under these conditions (such as originate from nuclear contact hyperfine fields that do not affect orbital motion) a linear, spin-dependent dispersion for free electrons emerges from the Landau Hamiltonian. The result is that spins of opposite orientation flow in opposite directions giving rise to a pure spin current. A classical model of the spin and charge dynamics reveals intuitive aspects of the full quantum mechanical solution. We propose optical orientation or electrical polarization experiments to demonstrate this outcome., 5 pages, 3 figures. Accepted in Phys. Rev. B

This study explores the spatiotemporal variability in the residual circulation and its dependence on external forces in an interconnected system of intertidal basins. We focus on the Dutch Wadden Sea (DWS), where winds play a major role in water movements and storms significantly affect its short-term characteristics. We make use of realistic three-dimensional high-resolution numerical simulations to model the hydrodynamics in the DWS for the years 2005–2015. First, the ‘empirical orthogonal function’ method is applied to analyze systematically the relative contributions of various forcing mechanisms on the residual (i.e., tidally averaged) volume flow through the inlets and the shallow watersheds delimiting the system and each tidal basin. Then, we compute the tidally averaged flushing frequency of fresh water over the entire period of analysis, and we study its variations at short and long time scales. It is found that over 98% of the residual flow's spatiotemporal variations can be explained by the first three EOF modes, which are highly correlated with the cubic power of the vectorial wind components' speed and the amount of fresh water discharged into the back-barrier system. This study reveals that incidental short-duration events (e.g., storms) occurring at time scales of hours to days, albeit episodic and highly variable, strongly influence the residual transport's inter-annual variability and its long-term typical value in multiple-inlet coastal systems.

The performances of the Color-Gradient (CG) and of the Shan-Chen (SC) multicomponent Lattice Boltzmann models are quantitatively compared side-by-side on multiple physical flow problems where breakup, coalescence and contraction of fluid ligaments are important. The flow problems are relevant to microfluidic applications, jetting of microdroplets as seen in inkjet printing, as well as emulsion dynamics. A significantly wider range of parameters is shown to be accessible for CG in terms of density-ratio, viscosity-ratio and surface tension values. Numerical stability for a high density ratio $\mathcal{O}(1000)$ is required for simulating the drop formation process during inkjet printing which we show here to be achievable using the CG model but not using the SC model. In terms of physical accuracy, the CG model shows good agreement with analytical solutions for droplet oscillation and ligament contraction test-cases. The SC model is effective in accurately simulating ligament contraction, but less accurate in handling droplet oscillations. Rayleigh-Plateau instability simulations show that both the CG and SC models give physically realistic results when breakup occurs, although smaller satellite droplets quickly vanish in the SC model due to droplet mass evaporating into the ambient phase. Our results show that the CG model is a suitable choice for challenging simulations of droplet formation, due to a combination of both numerical stability and physical accuracy. We also present a novel approach to incorporate repulsion forces between interfaces for CG, with possible applications to the study of stabilized emulsions. Specifically, we show that the CG model can produce similar results to a known multirange potentials extension of the SC model for modelling a disjoining pressure, opening up its use for the study of dense stabilized emulsions.

We model the behavior of a single colloid embedded in a cross-linked polymer gel, immersed in a viscous background fluid. External fields actuate the particle into a periodic motion, which deforms the embedding matrix and creates a local microcavity, containing the particle and any free volume created by its motion. This cavity exists only as long as the particle is actuated and, when present, reduces the local density of the material, leading to swelling. We show that the model exhibits rich resonance features, but is overall characterized by clear scaling laws at low and high driving frequencies, and a pronounced resonance at intermediate frequencies. Our model predictions suggest that both the magnitude and position of the resonance can be varied by varying the material's elastic modulus or cross-linking density, whereas the local viscosity primarily has a dampening effect. Our work implies appreciable free-volume generation is possible by dispersing a collection of colloids in the medium, even at the level of a simple superposition approximation.

Air infiltration and exfiltration that occur at open building entrances are of concern as these have been repeatedly associated with increased energy demand in buildings. Therefore, air curtains are often used to restrict air infiltration/exfiltration through busy building entrances, thereby limiting the exchange of heat and mass between indoor and outdoor environments. Air curtains are fundamentally turbulent plane impinging jets (TPIJs). This paper presents an experimental analysis of the fluid dynamics of TPIJs under parameters that are representative of air-curtain systems in buildings. Experiments are conducted at reduced scale in a water channel where measurements are taken via particle image velocimetry (PIV). Experimental data is subsequently used to validate computational fluid dynamics (CFD) simulations of the TPIJs based on steady Reynolds-averaged Navier-Stokes (RANS) approximation that is commonly used in air-curtain studies. The validation study shows that RANS simulations are capable of resolving the mean velocity field with reasonable accuracy, especially when employing the renormalization group k-ε turbulence model.

Atmospheric non-thermal plasmas have received much attention due to their unique properties of ion and reactive species generation and delivery within “standard every day“ conditions. Applications of these plasmas can be found in a wide range of fields such as medicine, agriculture and water treatment amongst many others. One factor which has recently gaining further interest is that of the induced flow during Atmospheric Pressure Plasma Jet (APPJ) liquid target impingement. Due to the presence of a liquid phase within many APPJ applications, and the governance of flow on ion and reactive species transmission, furthering the much lacking understanding of fluid mechanics within this phase is important in potentially finding new improvements (in e.g. the species transmission flux efficiency) of atmospheric plasma applications. The effect of grounding an APPJ liquid target on the induced liquid flow is described. The addition of the ground electrode within the solution allows current to pass through the solution and induce electrolysis on the system. A helium APPJ is impinged upon a water target for varying plasma applied voltage and solution salinity, and PIV results are compared for when this ground electrode is both absent from or submerged within the solution during these parameter investigations. The flow modifications observed included demineralised water targets having an upwards flow velocity increase with grounding, whereas in saline solutions the velocity instead increased downwards. This is believed to be due to the electrolysis exerting a net drag force on the flow, with the direction depending on the mobility coefficients of the ions present.

Rotating Rayleigh-Bénard convection is a simple model system used to study theinterplay of buoyant forcing and rotation. Many recent studies have focused on thegeostrophic regime of turbulent rotating convection where the principal balance offorces is between the Coriolis force and the pressure gradient. This regime is believed to be representative of conditions in geophysical and astrophysical flows. We hope to be able to extrapolate findings from laboratory experiments and numerical simulations towards these large-scale natural flows. In this paper I sketch the phase diagram of the geostrophic regime of rotating convection, put experimental and numerical studies in their place in these diagrams and discuss the partitioning into subranges characterized by different flow structures and heat transfer scaling. I also discuss some complications faced by experimentalists, such as constraints on the dimensions of the convection cell, wall modes near the sidewall and centrifugal buoyancy.