Deceleration Regime Research Articles

Plasma transport simulations of Rayleigh–Taylor instability in near-ICF deceleration regimes

Rayleigh–Taylor (R–T) instability between plasma species is examined in a kinetic test and near-inertial confinement fusion (ICF) regimes. A transport approximation to the plasma species kinetics is used to represent viscosity and species mass transport within a hydrodynamic fluid code (xRage). R–T simulation results are compared in a kinetic test regime with a fully kinetic particle-in-cell approach [vectorized particle-in-cell (VPIC)] and with an analytic model for the growth rate of R–T instability. Single-mode growth rates from both codes and the analytic model are in reasonable agreement over a range of initial wavelengths including the wavenumber of maximum growth rate. Both codes exhibit similar diffusive mixing fronts. Small code-to-code differences arise from the kinetics, while simulation-analytic model differences arise from several sources dominated by the choice of gradients establishing the hydrostatic equilibrium initial conditions. After demonstrating code agreement in the kinetic test regime, which is practically accessible to the VPIC code, then the xRage code, with the fluid plasma transport approximation, is applied to single mode R–T instability under deceleration conditions closer to an ICF implosion, approximated with a carbon (C) shell imploding on a deuterium (D) fuel. The analytic wavelength of maximum instability is limited by the kinetics, primarily in the viscosity, and is found to be ≈10 μm for an ion temperature near 1 keV at this C–D interface, with the most unstable wavelength increasing as temperature increases. The analytic viscous model agrees with simulation results over a range of initial perturbation wavelengths, provided the simulation results are analyzed over a sufficiently short duration (⪅0.2 ns in this case). Details of the fluid structure evolution during this R–T deceleration are compared between the inviscid Euler equations and cases, which include plasma transport over a range in initial wavelengths and initial perturbation amplitudes. The inviscid Euler solutions show a grid-dependent cascade to smaller scale structures often seen in the R–T instability, while simulations with plasma transport in this deceleration regime develop a single vortex roll-up, as the plasma transport smoothes all hydrodynamic fluid structures smaller than several micrometers. This leads to a grid-converged transient solution for the R–T instability when kinetic effects are included in the simulations, and thus represents a direct numerical simulation of the thermal ions during R–T unstable mixing in ICF relevant conditions.

Approach and breakup of Taylor bubble and Taylor drop in a Hele-Shaw cell

The collision dynamics of a Taylor drop and a Taylor bubble is investigated in an immiscible surrounding liquid. The interaction of both the fluidic entities is studied using experiments and simulation in a vertically aligned Hele-Shaw cell. The steady rise of the bubble and fall of the drop are followed by a deceleration regime where their velocity has decreased due to the pressure imposed by the leading interfaces, indicated by the change in the curvature of their tip. Subsequently, the rapid outward expansion of the bubble has caused the swelling of the tip of the drop. The drop swell has then grown exponentially similar to Rayleigh–Taylor instability and resulted in a split of the bubble into two volumes.

Experimental analysis on fluid flow and mass transfer characteristics of CMC solutions in wavy-walled tubes for steady and pulsating flow by PIV

This paper reports experimental results for fluid flow and mass transfer of sodium carboxymethyl cellulose (CMC) solution in wavy-walled tubes under both steady and pulsating flows where the PIV technique is employed. The rheological properties of the solution are revealed from the present experiment, and it is found that CMC solutions are featured with a shear- thinning behavior when the concentration is above 500 ppm regardless of temperature. The findings demonstrate that the increasing concentration makes the mass transfer performance degradation of CMC solutions. However, the mass transfer rate could be improved with the increasing amplitude of wavy-walled tubes in pulsating flow. The presence of reverse flow promotes the mass transfer and they are mostly distributed in the deceleration regime in one pulsating cycle. An optimal oscillatory fraction corresponding to the highest mass transfer enhancement is obtained. Furthermore, the phase shift phenomenon is observed in pulsating flow and the phase difference increases with the Strouhal number and amplitude of tube, but it is unaffected by the wavelength of the tube, the polymer concentration and the oscillatory fraction of the flow rate.

Cooperative lane control application for fully connected and automated vehicles at multilane freeways

Advances in communications technology have made the development of connected and automated vehicle (CAV) applications that may potentially replace traditional, gantry operated lane control signals (LCSs) possible. This paper develops the concept of a prototype CAV-enabled LCS application and provides a preliminary assessment of the potential improvement it offers over traditional LCS. Real-world data obtained from an LCS on I-66 in Northern Virginia was used to calibrate and validate a baseline simulation model. Further, the current lane control scenarios utilized in the Northern Virginia LCS were identified and relevant data resulting from implementation of those scenarios were collected to model the LCS in a simulation environment. The performance of real-world LCS was then compared to a prototype CAV-enabled LCS application developed in this research. The CAV-enabled LCS application consistently outperformed the traditional LCS with increased throughput and speeds. On average, an increase of 18.4%, 9.6% and 12.8% in throughput was observed for three selected scenarios under the best case of a 1 sec headway. Furthermore, the CAV-enabled LCS application was also found to reduce volatility, represented by variation in acceleration and deceleration regimes, by an average of 25.6% and 49.6%. The reduction in turbulence in the traffic stream may indicate potential improved safety with the CAV system.

Analysis of volatility in driving regimes extracted from basic safety messages transmitted between connected vehicles

Driving volatility captures the extent of speed variations when a vehicle is being driven. Extreme longitudinal variations signify hard acceleration or braking. Warnings and alerts given to drivers can reduce such volatility potentially improving safety, energy use, and emissions. This study develops a fundamental understanding of instantaneous driving decisions, needed for hazard anticipation and notification systems, and distinguishes normal from anomalous driving. In this study, driving task is divided into distinct yet unobserved regimes. The research issue is to characterize and quantify these regimes in typical driving cycles and the associated volatility of each regime, explore when the regimes change and the key correlates associated with each regime. Using Basic Safety Message (BSM) data from the Safety Pilot Model Deployment in Ann Arbor, Michigan, two- and three-regime Dynamic Markov switching models are estimated for several trips undertaken on various roadway types. While thousands of instrumented vehicles with vehicle to vehicle (V2V) and vehicle to infrastructure (V2I) communication systems are being tested, nearly 1.4 million records of BSMs, from 184 trips undertaken by 71 instrumented vehicles are analyzed in this study. Then even more detailed analysis of 43 randomly chosen trips (N=714,340 BSM records) that were undertaken on various roadway types is conducted. The results indicate that acceleration and deceleration are two distinct regimes, and as compared to acceleration, drivers decelerate at higher rates, and braking is significantly more volatile than acceleration. Different correlations of the two regimes with instantaneous driving contexts are explored. With a more generic three-regime model specification, the results reveal high-rate acceleration, high-rate deceleration, and cruise/constant as the three distinct regimes that characterize a typical driving cycle. Moreover, given in a high-rate regime, drivers’ on-average tend to decelerate at a higher rate than their rate of acceleration. Importantly, compared to cruise/constant regime, drivers’ instantaneous driving decisions are more volatile both in “high-rate” acceleration as well as “high-rate” deceleration regime. The study contributes to analyzing volatility in short-term driving decisions, and how changes in driving regimes can be mapped to a combination of local traffic states surrounding the vehicle.

Transport of a partially wetted particle at the liquid/vapor interface under the influence of an externally imposed surfactant generated Marangoni stress

Marangoni flows offer an interesting and useful means to transport particles at fluid interfaces with potential applications such as dry powder pulmonary drug delivery. In this article, we investigate the transport of partially wetted particles at a liquid/vapor interface under the influence of Marangoni flows driven by gradients in the surface excess concentration of surfactants. We deposit a microliter drop of soluble (sodium dodecyl sulfate) aqueous surfactant solution or pure insoluble liquid (oleic acid) surfactant on a water subphase and observe the transport of a pre-deposited particle. Following the previous observation by Wang et al. [1] that a surfactant front rapidly advances ahead of the deposited drop contact line and initiates particle motion but then moves beyond the particle, we now characterize the two dominant, time- and position-dependent forces acting on the moving particle: (1) a surface tension force acting on the three-phase contact line around the particle periphery due to the surface tension gradient at the liquid/vapor interface which always accelerates the particle and (2) a viscous force acting on the immersed surface area of the particle which accelerates or decelerates the particle depending on the difference in the velocities of the liquid and particle. We find that the particle velocity evolves over time in two regimes. In the acceleration regime, the net force on the particle acts in the direction of particle motion, and the particle quickly accelerates and reaches a maximum velocity. In the deceleration regime, the net force on the particle reverses and the particle decelerates gradually and stops. We identify the parameters that affect the two forces acting on the particle, including the initial particle position relative to the surfactant drop, particle diameter, particle wettability, subphase thickness, and surfactant solubility. We systematically vary these parameters and probe the spatial and temporal evolution of the two forces acting on the particle as it moves along its trajectory in both regimes. We find that a larger particle always lags behind the smaller particle when placed at an equal initial distance from the drop. Similarly, particles more deeply engulfed in the subphase lag behind those less deeply engulfed. Further, the extent of particle transport is reduced as the subphase thickness decreases due to the larger velocity gradients in the subphase recirculation flows.

Bus Following Model: A Case Study in Edinburgh

A number of car following models have been developed and calibrated for the purpose of understanding and simulating the behaviour of driving under various traffic and transportation management systems. Such studies are useful for safety impact studies, network analysis and capacity analysis. However bus following is a special behaviour, which impacts on other traffic especially in the absence of dedicated bus lanes. This paper reports on a study in the City of Edinburgh which was undertaken to investigate and understand the behaviour of buses in the city. A commuter bus equipped with a data logging device was followed by an instrumented vehicle to collect data to study bus following behaviour. A model of distance difference as a function of the acceleration/deceleration, speed and speed difference has been developed for bus following behaviour. The GHR model was used to investigate and calibrate the parameters of the model of the car following a bus. The study findings show that the speed and acceleration of the following vehicle are influenced by the leading commuter bus in both acceleration and deceleration regimes. The results also show that a car following a bus behaves differently to a car following a car. This is because of the limited visibility, huge delays for buses on the corridor and the different driving characteristics of the bus with respect to the car.

Two-dimensional planar plumes and fountains

AbstractClosed-form solutions describing the behaviour of two-dimensional planar turbulent rising plumes and fountains from horizontal planar area and line sources in unconfined quiescent environments of uniform density are proposed. Extending the analysis on axisymmetric releases by van den Bremer & Hunt (J. Fluid Mech., vol. 644, 2010, pp. 165–192) to planar releases, the local flux balance parameter$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\varGamma =\varGamma (z)$is instrumental in describing the bulk behaviour of steady Boussinesq and non-Boussinesq planar plumes and the initial rise behaviour of Boussinesq planar fountains as a function of height$z$. Expressions for the asymptotic virtual source correction are developed and the results elucidated by ‘scale diagrams’ (cf. Morton & Middleton,J. Fluid Mech., vol. 58, 1973, pp. 165–176) showing certain characteristic heights for different source conditions. These diagrams capture all the different manifestations of plume behaviour, encompassing fountains, jets, source-momentum-dominated or ‘forced’ plumes, pure plumes and source-buoyancy-dominated or ‘lazy’ plumes, and their associated key features. Other flow features identified include a gravity-driven deceleration regime and a mixing-driven regime for forced fountains. Deceleration in lazy fountains is purely gravity-driven. The results can be shown to be valid for both Boussinesq and non-Boussinesq plumes (but not for non-Boussinesq fountains) thus resulting in universal solutions valid for both cases provided the entrainment velocity is unaffected by non-Boussinesq effects. This paper presents and explores these universal solutions. An accompanying paper (van den Bremer & Hunt,J. Fluid Mech., vol. 750, 2014, pp. 245–258) examines the implications for non-Boussinesq plumes. The existing solutions of Lee & Emmons (J. Fluid Mech., vol. 11, 1961, pp. 353–368) generalized herein are valid for a constant entrainment coefficient$\alpha $. New results for an entrainment coefficient that varies linearly with$\varGamma (z)$and thus captures experimental values far more realistically are presented for forced plumes.

Numerical investigation of supersonic flow breakdown at the inlet duct throttling

The work presents the results of investigating the process of supersonic flow deceleration in a duct of the two-dimensional inlet throttled by variation of the outlet cross-sectional area. An inlet with three external compression shock waves designed for the freestream Mach number Md = 7 was considered as an example for the investigation. A one-dimensional analysis of the conditions for realization of the supersonic flow deceleration regimes in the inlet duct with two throats — in the inlet entrance and at the inlet duct outlet, has been carried out. The parametric numerical computations of two-dimensional inviscid or turbulent flows in the inlet were performed with the use of the Euler and Navier—Stokes codes of the program package FLUENT. The critical conditions for the nonuniform flow in the outlet throat bringing to choking the inlet duct were determined.

Collective behavior and the identification of phases in bicycle pelotons

As an aggregate of cyclists, a peloton exhibits collective behavior similar to flocking birds or schooling fish. Positional analysis of cyclists in mass-start velodrome races allows quantitative descriptions of peloton phases based on observational data. Data from two track races are analyzed. Peloton density correlates well with cyclists’ collective power output in two clear phases, one of low density, and one of high density. The low density “stretched” phase generally indicates low frequency positional-change and single-file synchronization. The high density “compact” phase may be further divided into two phases, one of which is a laterally synchronized phase, and another is a high frequency and magnitude positional-change phase. Phases may be sub-divided further into acceleration and deceleration regimes, but these are not quantified here. A basic model of peloton division and its implications for general flocking behavior are discussed.

Controlling local packing and growth in calcium–silicate–hydrate gels

We investigate the development of gels under out-of-equilibrium conditions, such as calcium-silicate-hydrate (C-S-H) gels that form during cement hydration and are the major factor responsible for cement mechanical strength. We propose a new model and numerical approach to follow the gel formation upon precipitation and aggregation of nano-scale colloidal hydrates, whose effective interactions are consistent with forces measured in experiments at fixed lime concentrations. We use Grand Canonical Monte Carlo to mimic precipitation events during Molecular Dynamics simulations, with their rate corresponding to the hydrate production rate set by the chemical environment. Our results display hydrate precipitation curves that indeed reproduce the acceleration and deceleration regime typically observed in experiments and we are able to correctly capture the effect of lime concentration on the hydration kinetics and the gel morphology. Our analysis of the evolution of the gel morphology indicates that the acceleration is related to the formation of an optimal local crystalline packing that allows for large, elongated aggregates to grow and that is controlled by the underlying thermodynamics. The defects produced during precipitation favor branching and gelation that end up controlling the deceleration. The effects on the mechanical properties of C-S-H gels are also discussed.

The influence of sodium and potassium hydroxide on alite hydration: Experiments and simulations

The basic nature of alkali hydroxides (NaOH, KOH) when added to mixing water, increases the pH in proportion to the level of salt addition. For alite (impure tricalcium silicate; MIII-Ca3SiO5) hydration, this pH increase accelerates the rate of hydration and reduces the duration of the induction, acceleration and deceleration regimes. This study evaluates alite hydration in solutions of varying compositions and alkalinities (0.1M, 0.2M and 0.5M NaOH and KOH) in context of their heat release behavior and analysis of the solid/liquid phases. The modeling platform, μic, is used to simulate, describe and discriminate the impact of the pore solution chemistry and reaction product formation parameters on alite hydration (Bishnoi and Scrivener, 2009 [1]). Numerical predictions of the solid and liquid phase compositions and the heat release response show good agreement with experimental determinations. The simulations indicate that the effects up to the end of the induction period follow directly from a change in the pore solution composition under a solution controlled dissolution mechanism, which leads to the faster precipitation of portlandite. The changes in the main heat evolution peak appear to be related to an increase in the nucleation density of C–S–H in alkali hydroxide solutions. Examination under the SEM did not indicate significant difference in C–S–H morphology and composition in the presence of NaOH/KOH.

Experimental determination of force and displacement of a car suspension

AbstractIn this paper, a dynamical laboratory testing of experimental car body connection with real elastic suspension system (ESS) is described. During the experiment a horizontal force in longitudinal direction was applied, in order to simulate an acceleration and deceleration regime of a vehicle. The experiment was carried out on the example of two car bodies, with identical geometric behavior but implemented engine compartment was made of different materials: in one case it was made of steel and in another one of Al-alloys. The obtained displacement results were compared.

Kick processes in the merger of two colliding black holes

We examine numerically the process of momentum extraction by gravitational waves in the merger of two colliding black holes, in the realm of Robinson-Trautman spacetimes. The initial data have already a common horizon so that the evolution covers the post-merger phase up to the final configuration of the remnant black hole. The analysis of the momentum flux carried out by gravitational waves indicates that two distinct regimes are present in the post-merger phase: (i) an initial accelerated regime, followed by (ii) a deceleration regime in which the deceleration increases rapidly towards a maximum and then decreases to zero, when the gravitational wave emission ceases. The analysis is based on the Bondi-Sachs conservation law for the total momentum of the system. We obtain the total kick velocity ${V}_{k}$ imparted on the merged black hole during the accelerated regime (i) and the total antikick velocity ${V}_{\mathrm{ak}}$ during the decelerated regime (ii), by evaluating the impulse of the gravitational wave flux during both regimes. The distributions of both ${V}_{k}$ and ${V}_{\mathrm{ak}}$ as a function of the symmetric mass ratio $\ensuremath{\eta}$ satisfy a simple $\ensuremath{\eta}$-scaling law motivated by post-Newtonian analytical estimates. In the $\ensuremath{\eta}$-scaling formula the Newtonian factor is dominant in the decelerated regime, that generates ${V}_{\mathrm{ak}}$, contrary to the behavior in the initial accelerated regime. For an initial infalling velocity $v/c\ensuremath{\simeq}0.462$ of each individual black hole we obtain a maximum kick ${V}_{k}\ensuremath{\simeq}6.4\text{ }\text{ }\mathrm{km}/\mathrm{s}$ at $\ensuremath{\eta}\ensuremath{\simeq}0.209$, and a maximum antikick ${V}_{\mathrm{ak}}\ensuremath{\simeq}109\text{ }\text{ }\mathrm{km}/\mathrm{s}$ at $\ensuremath{\eta}\ensuremath{\simeq}0.205$. The net antikick velocity (${V}_{\mathrm{ak}}\ensuremath{-}{V}_{k}$) also satisfies a similar $\ensuremath{\eta}$-scaling law with a maximum approximately $102\text{ }\text{ }\mathrm{km}/\mathrm{s}$ also at $\ensuremath{\eta}\ensuremath{\simeq}0.205$, qualitatively consistent with results from numerical relativity simulations, and post-Newtonian evaluations of binary black hole inspirals. For larger values of the initial data parameter $v/c$ substantial larger values of the net antikick velocity are obtained. Based on the several velocity variables obtained, we discuss a possible definition of the center-of-mass motion of the merged system.

Electron acceleration and synchrotron radiation in decelerating plasmoids

An equation is derived to calculate the dynamics of relativistic magnetized plasma which decelerates by sweeping up matter from the ISM. Reduction to the non-radiative and radiative regimes is demonstrated for the general case of a collimated plasmoid whose area increases as a power of distance x from the explosion, and in the specific case of a blast-wave geometry where the area increases quadratically with x. An equation for the evolution of the electron momentum distribution function in the comoving fluid frame is used to calculate the observed synchrotron radiation spectrum. The central uncertainty involves the mechanism for transfering energy from the non-thermal protons to the electrons. The simplest prescription is to assume that a fixed fraction of the comoving proton power is instantaneously transformed into a power-law, “shock”-like electron distribution function. This permits an analytic solution for the case of a relativistic plasmoid with a constant internal magnetic field. Effects of parameter changes are presented. Breaks in the temporal behavior of the primary burst emission and long wavelength afterglows occur on two time scales for external ISM density distributions n ext α x − η . The first, as emphasized by Mészáros & Rees, represents the time when the outflow sweeps up ≈ M th/Γ 0 of material from the ISM, where M this the mass in the outflow ejecta and Γ 0 is the initial Lorentz factor of the plasmoid. The second represents the time when synchrotron cooling begins strongly to regulate the number of nonthermal electrons that are producing radiation which is observed at a given energy. Results are applied to GRB and blazar variability. The slopes of the time profiles of the X-ray and optical light curves of the afterglows of GRB 970228 and GRB 970508 are explained by injection of hard electron spectra with indices s < 2 in a deceleration regime near the radiative limit. We also propose that blazar flares are due to the transformation of the directed kinetic energy of the plasmoid when interacting with the external medium, as would occur if relativistic outflows pass through clouds orbiting the central supermassive black hole.

Converters for ultrahard x-rays

A linear theory is constructed to explain the operation of single- and multipass converters for transforming the energy of beam electrons into bremmstrahlung within the spectral interval 10–100 keV (ultrahard x-rays, or UHX-radiation) in a cold-target approximation. Expressions are obtained that make it possible to calculate the spectral and energy characteristics of such radiation. The completed calculations make it possible to give recommendations on choosing target material and thickness and deceleration regimes from the viewpoint of maximizing the output of UHX-radiation energy.

Non-linear optimization of cylindrical electrostatic lenses

A method and computer code to design high-transmission cylindrical electrostratic lenses has been developed. Optimized geometries and voltages are determined using the CYLENS program enhanced with the simplex method of non-linear optimization. The possibility of improvements over unipotential lenses used in commercial instruments is predicted. The design of high-transmission unipotential lenses is described for such special purposes as time-of-flight mass spectrometry of organic ions or reflection geometry laser ionization mass spectrometry. Low accelerating voltage, in the first case and large working distance, in the second case have been realized, keeping the objective of high transmission. Asymmetric potential lenses were also investigated and, in the deceleration regime, they turned out to be superior to low accelerating voltage unipotential lenses. Misalignment and stability studies show that lens optimization provides less vulnerability towards anomalies in manufacturing or in operating conditions.

Unsteady effects in a thin viscous shock layer near a three-dimensional stagnation point of a body moving with given acceleration and deceleration

An unsteady viscous shock layer near a stagnation point is studied. The Navier-Stokes equations are analyzed in the limit γ → 1, Re0 → ∞, df/dt = αn-mF(t/αm). The Reynolds number Re0 is defined in the paper by Eq. (1.3) (df/dt is the velocity of the body with respect to an inertial frame of reference moving with the original steady velocity −V't8, α2 = (γ − 1)/(γ + 1)). Various flow regimes in the case α ≪ 1, ɛ ≪ l, n ≥ max(2m, m + 1), m ≥ 0, where ɛ2 = 1/Re0 are analyzed. Equations are derived that generalize the asymptotic analysis to the case of a viscous unsteady flow of gas in a thin three-dimensional shock layer. The problem of a thin unsteady viscous shock layer near the stagnation point of a body with two curvatures is formulated. Examples of numerical solution are given for different ratios of the principal curvatures of the body, the wall temperature, the parameters of the original steady flow, and the acceleration and deceleration regimes.