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In order to study the vortex-induced vibration (VIV) characteristics of rigidly connected four-cylinder systems and the suppression of vortex-induced vibration by nonlinear energy sinks (NESs), a fluid–structure coupling dynamic simulation model of a two-degree-of-freedom rigidly connected four-cylinder system is established based on computational fluid dynamics, structural dynamics, and overset mesh technology. The accuracy of the numerical model established in this paper is verified by comparing with the experimental data of literatures. The results show that the dimensionless vertical amplitude of the four-cylinder system decreases with increase in the inflow angle, the reduced velocity advance of the maximum vertical amplitude moves forward and the frequency "lock-in" interval is shortened. Among them, the maximum amplitude at U r = 7 is 0.75 when the inflow angle is 0°, and the maximum amplitude is 0.54 at U r = 6.5 when the inflow angle is 45°. The corresponding frequency "lock-in" interval ranges from U r = 4.5 – 7 change to U r = 5 – 6.5. The NES can absorb the cylinder vibration energy, and when the NES parameter β = 0.1 , ξ = 0.8 , γ = 0.8 , the maximum vertical of the four-cylinder system with inflow angle of 0° can be reduced by 76%. [ABSTRACT FROM AUTHOR]

This paper uses a reactive flow large eddy simulation (LES) and decomposition techniques to study combustion instabilities in a methane–oxygen combustor. This work examines two case scenarios to elucidate the significance of injector–chamber frequency coupling as the cause of thermo-acoustic instability. Initial investigation in a well-known benchmark case of the continuously variable resonance combustor (CVRC) reports the potential instability mechanisms and the role of injector–chamber frequency coupling in thermo-acoustic instability. Subsequently, the multi-element rocket combustor case study identifies the critical resonant modes and highlights potential frequency coupling between the injector and the chamber region. The interplay between longitudinal pressure oscillations in the oxidizer post and transverse pressure waves in the chamber is responsible for the enhanced pressure dynamics in the combustor. The present work uses the dynamic mode decomposition (DMD) technique to reveal the evolution of acoustic modes in the injector and the chamber for CVRC and multi-element combustor. The dominant pressure mode forms found by DMD analysis also showcase the role of injector–chamber frequency coupling in amplified combustion dynamics. The results demonstrate how the predominant cause of combustion instability in rocket combustors can be effectively determined using the high-fidelity LES framework in conjunction with the modal decomposition technique. [ABSTRACT FROM AUTHOR]

Molecular-level liquid bridges at contact interfaces have a much more important impact on micro-/nanosystems and flexible wearable sensors. However, their dynamic behavior and mechanical properties are still vague due to the limitations of current characterization methods. Here, under the support of molecular dynamic simulations on smooth, pillared, and concave pillared plates, the heterogeneous anisotropic pressure distribution of layered liquids is found to be distinct from the isotropic pressure distribution in the macroscale liquid bridge. Oscillating interfacial contact forces appear in the period of layered liquid compression due to the oscillating system free energy, in which the peak forces greatly increase as the number of layers decreases. The layered liquids with impressively remarkable solid-like pressure-bearing capacity at the gigapascal level and exceptional fluidity along confined atomic layers can spontaneously strengthen adhesion vertically and facilitate pressure transport horizontally, respectively. The significance of layered liquids in contact systems can improve our understanding of liquid bridges at the molecular level, which is beneficial for interfacial mechanical regulation. [ABSTRACT FROM AUTHOR]

In the current study, a dynamic layer freeze desalination system is simulated numerically. The computational domain is a two-dimensional rectangular channel which contains inlet and outlet flows, and the simulation is performed using computational fluid dynamics. A pre-concentrated mixture is considered as the feed solution with an initial temperature and concentration of 257 K and 0.2 kg/kg, respectively. In this work, since the simulation parameters such as temperature and species mass fraction have high importance across the entire domain, the k–ω shear stress transport turbulence model was selected. A parametric study on the effect of the heat flux of the cold wall on the ice salinity and desalination rate is performed using three cases with heat fluxes of − 1000 , − 1500 , and − 3000 W m − 2 . It is observed that the ice generation speed in the case with a heat flux of − 3000 W m − 2 is 3.28 times greater compared to the case with − 1000 W m − 2 and its desalination rate is only 2.59 % lower. The effect of the inlet velocity on the mentioned parameters is also investigated. It is observed that the cases with turbulent flow have approximately 19 % and 23 % lower ice salinity compared to the case with laminar flow. [ABSTRACT FROM AUTHOR]

Investigating immiscible displacement in fractured porous media is essential for understanding the two-phase flow behavior within pores and fractures. In this work, a three-dimensional pore-fracture network model was developed to address the influence of fracture on flow patterns and to characterize fracture-matrix crossflow under different flow conditions. Sensitivity studies at a wide range of viscosity ratios and capillary numbers underscored that fracture significantly influenced flow patterns in the capillary fingering zone. Fracture with an advantageous path effect in the displacement direction caused a shift in the boundary of capillary fingering zone toward an increase in capillary numbers. As fracture aperture decreased and aspect ratio increased, there was a discernible decline in the crossflow rate. When fracture aperture equaled average matrix throat diameter, fracture lose advantageous path effect in compact displacement zone but retained it in viscous fingering and capillary fingering zones. Distinct matrix-fracture crossflow development processes were observed in different zones: in cross zone, following displacement breakthrough, the crossflow underwent a "long-term" process to attain stability. Viscous fingering zone promptly achieved stability post-breakthrough, whereas both capillary fingering and compact displacement zones had already reached a stable state before breakthrough. Nonlinear variations in breakthrough saturation were observed in the cross zone between compact displacement and capillary fingering zones. The control process of immiscible displacement exhibited variability under different flow conditions: compact displacement zone was characterized by matrix dominance, viscous fingering zone was jointly controlled by matrix displacement and fracture-matrix crossflow, and capillary fingering zone was primarily governed by fracture-matrix crossflow. These findings enhance scholarly comprehension of immiscible displacement behavior in fractured porous media. [ABSTRACT FROM AUTHOR]

Capillary wicking characteristics play an important role in two-phase thermal management devices including heat pipes and vapor chambers, yet three-dimensional (3D) pore-scale simulations of the dynamic capillary wicking process on various micro-structured surfaces have been rare. In this paper, we conduct 3D pore-scale simulations of capillary wicking on three commonly used micro-structured wicks including micro-pillar array, micro-channel, and sintered particles. The micro-scale liquid propagation dynamics and the "stick-slip" behavior of the propagating liquid front are captured using a 3D pseudo-potential multiple-relaxation-time lattice Boltzmann method. Based on the Lucus–Washburn approach and a work-energy approach, we theoretically analyze wickabilities of different micro-structured wicks. Effects of wick geometry and structural parameters on the capillary wicking characteristics are discussed. We demonstrate that an optimal pillar pitch distance exists, which maximizes the wickability of the micro-pillar array. We show that when the porosity is relatively low, the wickability of the micro-channel is higher than that of the micro-pillar array and the sintered particles. When the porosity is large, however, the sintered particles exhibit higher wickability than the micro-pillar array and the micro-channel. We also demonstrate that the capillary pressure of the sintered particles is always higher than that of the micro-pillar array and the micro-channel throughout the porosity range investigated. The numerical simulation results are compared with theoretical predictions. Findings in this work provide guidelines for the designs of porous wick in various two-phase thermal management systems for high heat flux devices. [ABSTRACT FROM AUTHOR]

In modern high-fidelity computational fluid dynamic simulations, the primary vortex system in hover often breaks down into secondary vortices. The sources of numerical error influencing the prediction of the vortex system were studied by performing high-fidelity simulations of the wake of a two-bladed rotor and comparing the predictions to stereoscopic particle image velocimetry measurements in different measurement planes. Various numerical inputs, including sub-iteration convergence, blade pitch offset, and grid resolution, were varied to resolve discrepancies between the measured and predicted vortex characteristics from a previous study done by the authors. A parametric study on near- and off-body solver sub-iteration convergence demonstrated that although the secondary vortex characteristics converged as the sub-iteration convergence of both solvers increased, a large discrepancy in the number of secondary vortices remained. This discrepancy was investigated by varying the thrust, where it was found that the breakdown of the primary vortex is directly linked to the number of secondary vortices. Dissimilarities in the blade pitch angle, which could not be avoided in the experiment, were modeled by intentionally using an offset in the blade pitch angle of the two blades. It was shown that as blade pitch angle offset increases, vortex pairing becomes more distinct. When vortex pairing occurred in both the experiment and simulation, the decay of secondary vortices in the experiment and simulation agreed best. To better match the experimental resolution, grid resolution was increased and comparing the two simulations, the finer mesh simulation agreed best with the measured primary and secondary vortex characteristics. [ABSTRACT FROM AUTHOR]

The size dependence of the surface tension of critical bubbles in a superheated (stretched) Lennard–Jones solution with complete solubility of the components is considered. Two approaches are used to determine this dependence. The first one is based on the van der Waals gradient theory, and the second one is based on molecular dynamic simulation results of nucleation in a solution. It is established that, unlike in a one-component liquid, where the surface tension of the equilibrium bubble is less than that for the flat interface, in solution, it can exceed the flat limit. The ranges of temperatures, pressures, and mixture compositions, where this effect occurs, are determined. The asymptotic behavior of the surface tension of vapor phase nuclei within the limits of zero and infinitely large curvature of the dividing surface is analyzed. [ABSTRACT FROM AUTHOR]

Static and dynamic fluid–structure interaction of a flexible plate behind a stationary cylinder in uniform flow is explored for a body-to-fluid mass ratio of 10. Steady flow-structure computations for Re = 150, based on the diameter of cylinder and free-stream speed, reveal four regimes with respect to increasing flexibility of the plate. The plate does not undergo any lateral deflection in regime 1 beyond which it undergoes a symmetry-breaking bifurcation causing it to spontaneously deflect laterally. The curvature is of the same sign along the entire length of plate in regime 2 while it changes sign along the plate in regime 3. The lateral deflection, however, is still maximum at the plate tip. The location of maximum deflection moves away from the plate tip in regime 4. The evolution of flow structures including the strength of various standing vortices, with flexibility, is studied. The role of reattachment of flow on the surface of the plate and the modification of the pressure distribution is explored. The critical Re, beyond which the splitter plate spontaneously achieves a deflected shape, decreases with increase in flexibility. It is estimated to be Re = 122.33 for the limiting case of an infinitely flexible plate. Computations for dynamic fluid–structure interaction reveal several regimes of lock-in with different natural vibration modes of the plate and related hysteresis. No lateral bias in the time-averaged deflection is found during lock-in; it occurs in the desynchronization regime that precedes the lock-in regime with second mode. For the mass ratio considered, the bias in the static and dynamic simulations start at the same flexibility. [ABSTRACT FROM AUTHOR]

Multi-stage centrifugal pumps with floating impellers provide a convenient means of adjustment to meet diverse head requirements. These pumps utilize sealing gaskets to minimize leakage losses and restrict impeller axial motion. However, the impact of sealing gasket breakage on hydraulic performance and internal flow remains uncertain. To investigate this, a novel dynamic mesh simulation method is proposed to enable simultaneous axial motion and rotation of the impeller. Validation of numerical simulations with experiments is performed. Transient hydraulic performance is influenced by impeller axial motion, with a lagging flow field response. Head coefficient and efficiency curves exhibit asymmetric triangular functions with higher fluctuations compared to impeller with fixed axial position. At the design condition, with the impeller floating, the head coefficient ranges from 0.7 to 1.9, while the efficiency ranges from 29.7% to 60.1%. Among the various loss ratios, the hydraulic loss ratio exhibits the highest magnitude, followed by the leakage loss ratio, with the shroud friction loss ratio being the smallest. Entropy production reveals the significant influence of impeller oscillation on local flow loss. The axial motion of the impeller causes drastic velocity and pressure fluctuations in both time and space. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the three-dimensional nature of dynamic stall. Conducting the investigation, the flow around a harmonically pitching National Advisory Committee for Aeronautics (NACA) 0012 airfoil is numerically simulated using Unsteady-Reynolds-Averaged Navier–Stokes (URANS) and multiple detached eddy simulation (DES) solvers: the Delayed-DES (DDES) and the Improved-DDES (IDDES). Two- and three-dimensional simulations are performed for each solver, and the results are compared against experimental measurements in the literature. The results showed that three-dimensional simulations surpass two-dimensional ones in capturing the stages of dynamic stall and predicting the lift coefficient values, with a distinguished performance of the DES solvers over the URANS ones. For instance, the IDDES simulations, as an inherently three-dimensional solver, predicted the necessary cascaded amalgamation process of vortices to form the adequate strength of the dynamic stall vortex. This vortex size and timing provided accurate and sufficient suction that resulted in identical matching of the numerical and experimental lift coefficients at the peak value. Hence, the hypothesis that dynamic stall has a three-dimensional nature is supported by the superiority of the three-dimensional simulation in all aspects. In conclusion, it is found that dynamic stall is intrinsically a three-dimensional phenomenon. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulation, we studied thermodynamic effects of a nanobubble's collapse-induced erosion occurring at different ambient temperatures. We analyzed the dynamics of a single nanobubble collapsing near an aluminum (Al) solid boundary immersed in water at temperatures ranging 10–60 °C (283 – 333 K). We used a momentum mirror protocol to investigate the nanobubble's collapse-induced shock wave as the associated nanojet formed and moved toward the solid boundary. The results showed that the nanojet was formed during the collapse process after the collision of the nanobubble with the shock wave. On the aluminum surface, the erosion at lower ambient temperatures was greater than at higher ambient temperatures. [ABSTRACT FROM AUTHOR]

In the current research, we studied the collapse mechanism of the nanobubble under mirror and real wall protocols using molecular dynamics simulation. Moreover, we analyzed reactive properties of the real wall during the collapse process. Towards this aim, an aluminum (Al) slab has been considered as a real wall, and its behavior after the formation of a nanojet has been investigated. The obtained results indicated that the dynamics of nanobubble collapse under the mirror and real protocols are similar. The collision between the shock wave and the nanobubble leads to the collapse of nanobubble, and the nanojet was formed during this collapse process. However, the nanobubble can collapse sooner when the Al slab is used as a real wall. Moreover, the surface roughness of the Al slab during the shock propagation and nanobubble collapse was increased due to the formation of the chemical reaction between Al and water under the real wall protocol, while the mirror wall has a roughness-free surface from the beginning to the end. The chemical reaction 2Al + H 2 O → AlOH + AlH creates the amorphous layer containing the AlOH and AlH species on the surface of the Al slab. This layer grows semi-smoothly during bubble shrinkage and collapse process, while the growth type was changed to an island shape after the complete collapsing. The island shape on the Al slab was formed behind the nanojet due to the water vortices that are formed after the nanobubble collapse. [ABSTRACT FROM AUTHOR]

Droplet impact dynamics on a superhydrophobic surface with a cubic protrusion was simulated by the lattice Boltzmann method and the contact time reduction mechanism due to the fact that the cubic protrusion was explored. In addition, the droplet bouncing behavior was analyzed with the effect of a wide range of Weber numbers (18.28–106.77). The simulated results showed three distinct bouncing modes, which are bouncing with no ring formation, bouncing with ring formation and disappearance, and bouncing with ring formation. The contact time can be sharply reduced by up to 58.41% as the We number exceeds the critical value 67.16, which is induced by the liquid ring bouncing generated by the collision between the inner and outer rims. In addition, no effect can be seen during the spreading stage, and hence, the liquid ring punctured by the cubic protrusion mainly reduces the retraction time of the droplet impact process. Moreover, the retraction distance can be shortened with the increase in We. Symmetrical dynamics during spreading and retraction due to the cubic protrusion can be seen, which is different from the asymmetric behavior on a macroridge. Discussions on the instantaneous velocity field further support the reduction mechanism of the contact time. [ABSTRACT FROM AUTHOR]

Behind the iconic "pop!" accompanying the uncorking of a champagne bottle hides a gas flow of surprising complexity. Its modeling is made delicate by its supersonic nature, its interaction with the cork stopper, the eminently unsteady character of the flow escaping from the bottle, and the continuous change of the geometry of the computational flow domain due to the displacement of the cork. Computational fluid dynamics (CFD) simulations revealed the formation, evolution, and dissipation of shock wave patterns during the first millisecond following champagne cork popping. A first crown-shaped shock wave pattern develops radially, which is then followed by the formation of a detached shock wave, or bow shock, induced by the presence of the cork in the axial path of the supersonic gas flow. Moreover, the good agreement between the position of the bow shock previously observed through high-speed imaging and that determined through CFD simulations argues in favor of the numerical method used to describe the ejection of the gas mixture expelled from the bottleneck immediately after the cork popping process. [ABSTRACT FROM AUTHOR]

The behavior of droplets going through trans-critical conditions from subcritical to supercritical exhibits different features. The influence of multicomponent droplets on the relationship between supercritical transition and evaporation has not been determined. In the present work, molecular dynamic simulations for both single-component and two-component hydrocarbon droplets comprising alkanes (heptane, dodecane, and hexadecane, which are abbreviated to C7, C12, and C16, respectively, in present paper) and alcohol (n-butanol, which is abbreviated to BuOH), whose diameters are 30 nm, are conducted under various trans-critical conditions. To ensure efficiency and accuracy, the simulations are conducted under nitrogen atmosphere. The supercritical transition rate of droplets is quantified by the temporal declining of the maximum fuel mole fraction, and the evaporation rate is measured by the reduction of their radius. The inter-relationship between the supercritical transition and evaporation is mainly influenced by the ambient pressure, where the supercritical transition of droplets becomes more dominant under conditions with higher pressure. Furthermore, the separate phase-change processes and spatial stratification phenomena are discovered for trans-critical n-butanol/hexadecane droplets. Ultimately, by comparing the supercritical transition regions of the binary droplets and the corresponding unary droplets, the influence of intercomponent interaction on the phase-change process of binary droplets is discussed. [ABSTRACT FROM AUTHOR]

This study presents a framework to analyze the propulsive performance of a sail under oscillatory pumping motion, which should be modified from the analysis framework for conventional flapping wings. A new set of metrics, including the propulsive efficiency ηs and efficacy Δ C x ¯ , which depend not only on the aerodynamic coefficient but also on kinematic conditions of the sailing vehicle, are proposed to characterize the propulsive performance of sail pumping. The necessity of the proposed metrics is illustrated by studying the propulsive performance of a two-dimensional airfoil under different heaving and kinematic conditions through computational fluid dynamic simulations. It was found that the sail pumping typically improves the propulsive performance by lift enhancement instead of drag reduction. Thus, the formation of leading-edge vortices may not be detrimental to the propulsive efficiency of a pumping sail, contrary to that for flapping wings. A propulsive efficiency greater than unity is achievable under certain sailing conditions because a sailing vehicle can extract energy from the wind. [ABSTRACT FROM AUTHOR]

We report the dynamics of a viscoplastic (Bingham) torus suspended in an unbounded Newtonian medium. In this study, the immiscible ambient fluid is subjected to an axisymmetric compressional (biaxial-extensional) flow. Numerical simulations applying the boundary integral formulation to the Stokes flow are performed for the torus drop having initially a circular cross section. The quasi-stationary dynamic simulation reveals that depending on the initial conditions and the capillary number (Ca), the Bingham number (Bn), and the viscosity ratio (λ), three different scenarios of the drop evolution are obtained for a range of viscosity ratio and Bingham number: collapsing toward the axis of symmetry, expanding infinitely, and having a stationary toroidal shape. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) and volume of fluid (VOF) are powerful methods for the simulation of dynamic wetting at the nanoscale and macroscale, respectively, but the massive computational cost of MD and the sensitivity and uncertainty of boundary conditions in VOF limit their applications to other scales. In this work, we propose a multiscale simulation strategy by enhancing VOF simulations using self-consistent boundary conditions derived from MD. Specifically, the boundary conditions include a particular slip model based on the molecular kinetic theory for the three-phase contact line to account for the interfacial molecular physics, the classical Navier slip model for the remaining part of the liquid–solid interface, and a new source term supplemented to the momentum equation in VOF to replace the convectional dynamic contact angle model. Each slip model has been calibrated by the MD simulations. The simulation results demonstrate that with these new boundary conditions, the enhanced VOF simulations can provide consistent predictions with full MD simulations for the dynamic wetting of nanodroplets on both smooth and pillared surfaces, and its performance is better than those with other VOF models, especially for the pinning–depinning phenomenon. This multiscale simulation strategy is also proved to be capable of simulating dynamic wetting above the nanoscale, where the pure MD simulations are inaccessible due to the computational cost. [ABSTRACT FROM AUTHOR]

The phase change process of a single hydrocarbon droplet in a supercritical environment has different features compared with subcritical evaporation. In this study, droplets surrounded by supercritical nitrogen are investigated by molecular dynamic simulation. Dodecane droplets are considered in order to reproduce the phase change process of actual fuel, and the radius ranges from 10 nm to 25 nm under various subcritical, transcritical, and supercritical conditions. The emphasis is to describe the entire phase change process and distinguish its different stages. Under supercritical conditions, traditional D2-law theory is invalid and the radius of droplets increases dramatically accompanied by huge fluctuation after a special time. Based on analysis toward fluid properties crossing this time point, the phases classification for supercritical mixture is provided and three phase change stages are distinguished in order, which can be divided by the onset time of supercritical transition, the disappearance of surface tension, and one-phase diffusion, respectively. Additionally, gas/liquid-like features have been observed in the supercritical binary dodecane–nitrogen coexisting system and the defined pseudo-boiling mole fraction (Xpb) is introduced to clarify such deviation in properties of supercritical mixture. At last, by judging whether supercritical transition and undroplet phenomenon will happen, three phase transition modes are summarized for droplets under supercritical conditions. [ABSTRACT FROM AUTHOR]

Multispecies mixing processes play an important role in many engineering, biological, and environmental applications. Since simulating mixing flows can be useful to understand its physics and to study industrial issues, this work aims to develop the basis of a methodology able to simulate the physics of multiple-species mixing flows, using a hybrid large eddy simulation/Lagrangian filtered density function (FDF) method on an adaptive, block-structured mesh. A computational model of notional particles transport on a distributed processing environment is built using a parallel Lagrangian map. This map connects the Lagrangian information with the Eulerian framework of the in-house code MFSim, in which transport equations are solved. The Lagrangian composition FDF method, through the Monte Carlo technique, performs algebraic calculations over an ensemble of notional particles and provides composition fields statistically equivalent to those obtained by finite volume numerical solution of partial differential equations. Finally, to maintain high accuracy in the system of stochastic differential equations solver when an adaptive mesh refinement environment is used, a methodology for ensuring mass conservation is developed to preserve at least the statistical moments up to order two, even in the case of annihilation or cloning of a large number of notional particles in one time step, ensuring the applicability of Lagrangian FDF methods in dynamically adaptive grid refinement. [ABSTRACT FROM AUTHOR]

When sedimenting in a viscous fluid under gravity, a cloud of particles undergoes a complex shape evolution due to the hydrodynamic interactions. In this work, Lagrange particle dynamic simulation, which combines the Oseen solution for flow around a particle and a Gauss–Seidel iterative procedure, is adopted to investigate the effects of the particle inertia and the hydrodynamic interactions on the cloud's sedimentation behavior. It is found that, with a small Stokes number (St), the cloud evolves into a torus and then breaks up into secondary clouds. In contrast, the cloud with a finite Stokes number becomes compact in the horizontal direction and is elongated along the vertical direction. The critical St value that separates the breakup mode and the vertical elongation mode is around 0.2. The cloud response time ( t ̂ r ) and the maximum settling velocity ( V ̂ max ) are measured at different Stokes numbers, particle Reynolds numbers, and particle volume fractions. A linear relationship, t ̂ r = a St , is found between t ̂ r and the Stokes number and the correlation between V ̂ max and St can be well described by an exponential function V ̂ max = b 1 exp − b 2 S t + b 3 . At last, the chaotic dynamics of the sedimentation system are discussed. A small difference between the initial configurations diverges exponentially. The sedimentation system containing particles with larger inertia has a lower divergence rate. [ABSTRACT FROM AUTHOR]

Identifying economically viable intervention measures to reduce COVID-19 transmission on aircraft is of critical importance especially as new SARS-CoV2 variants emerge. Computational fluid-particle dynamic simulations are employed to investigate aerosol transmission and intervention measures on a Boeing 737 cabin zone. The present study compares aerosol transmission in three models: (a) a model at full passenger capacity (60 passengers), (b) a model at reduced capacity (40 passengers), and (c) a model at full capacity with sneeze guards/shields between passengers. Lagrangian simulations are used to model aerosol transport using particle sizes in the 1–50 μm range, which spans aerosols emitted during breathing, speech, and coughing. Sneeze shields placed between passengers redirect the local air flow and transfer part of the lateral momentum of the air to longitudinal momentum. This mechanism is exploited to direct more particles to the back of the seats in front of the index patient (aerosol source) and reduce lateral transfer of aerosol particles to other passengers. It is demonstrated that using sneeze shields on full capacity flights can reduce aerosol transmission to levels below that of reduced capacity flights without sneeze shields. [ABSTRACT FROM AUTHOR]

Vortex shedding of freely rotating hydrofoils and the fluctuations in hydrodynamic loads are typical problems in marine engineering. Hence, the hydrodynamic mechanism should be investigated in detail. In this study, the Reynolds-averaged Navier–Stokes method is used to analyze the unsteady flow characteristics of a two-dimensional freely rotating hydrofoil in uniform flow at different Reynolds numbers. The accuracy of the numerical simulation method is verified through convergence analysis of the simulation results. According to the mechanical characteristics and flow field distributions, the effects of three Reynolds numbers from 5 × 104 to 1.2 × 106 and five rotation centers from 0.2c to 0.4c on the dynamic stall of the hydrofoil are analyzed. The results show that the rotation center considerably influences the dynamic stall characteristics of the hydrofoil. As the rotation center approaches 0.4c, the amplitudes of the drag and lift coefficients and the rotation angle of the hydrofoil clearly increase by at least 206%, 10.5%, and 185%, respectively, along with the vortex shedding frequency, which also leads to the increase in the Strouhal number by at least 17.3%. Furthermore, the recovery of the drag and lift coefficients is delayed, resulting in an evident hysteresis effect. Simultaneously, this dynamic stall results in the decrease in the velocity distribution amplitude in the wake field and the increase in the pressure difference between the upper and lower surfaces. The continuous shedding of strong vortices from the trailing edge also leads to more complicated flow field characteristics. [ABSTRACT FROM AUTHOR]

This paper develops a homogenization approach, based on the introduction of exact local and integral moments, to investigate the temporal evolution of effective dispersion properties of point-sized and finite-sized particles in periodic media. The proposed method represents a robust and computationally efficient continuous approach, alternative to stochastic dynamic simulations. As a case study, the exact moment method is applied to analyze transient dispersion properties of point-sized and finite-sized particles in sinusoidal tubes under the action of a pressure-driven Stokes flow. The sinusoidal structure of the tube wall induces a significant variation of the axial velocity component along the axial coordinate. This strongly influences the transient behavior of the effective axial velocity V z (t) and of the dispersivity D z (t) , both exhibiting wide and persistent temporal oscillations, even for a steady (not-pulsating) Stokes flow. For a pointwise injection of solute particles on the symmetry axis, many interesting features appear: negative values of the dispersion coefficient D z (t) , values of D z (t) larger than the asymptotic value D z (∞) , and anomalous temporal scaling of the axial variance of the particle distribution. All these peculiar features found a physical and theoretical explanation by adopting simple transport models accounting for the axial and radial variation of the axial velocity field and its interaction with molecular diffusion. [ABSTRACT FROM AUTHOR]

This article shows the capability of using a higher order dynamic mode decomposition (HODMD) algorithm both to identify flow patterns and to extrapolate a transient solution to the attractor region. Numerical simulations are carried out for the three-dimensional flow around a circular cylinder, and both standard dynamic mode decomposition (DMD) and higher order DMD are applied to the nonconverged solution. The good performance of HODMD is proved, showing that this method guesses the converged flow patterns from numerical simulations in the transitional region. The solution obtained can be extrapolated to the attractor region. This fact sheds light on the capability of finding real flow patterns in complex flows and, simultaneously, reducing the computational cost of the numerical simulations or the required quantity of data collected in experiments. [ABSTRACT FROM AUTHOR]