Effect Of Relative Velocity Research Articles

On the effect of the approach velocity on the coalescence of fluid particles

The effect of the constant relative approach velocity of two Newtonian fluid particles on their coalescence is investigated via a film drainage model. The interfaces are deformable and allowed to have any degree of mobility. The thinning equation is derived based on the lubrication theory, and the tangential velocity of the interface is determined via the Boundary Integral Method (BIM). The details for the treatment of the inherent singularity in BIM are provided. As the approach velocity increases, regardless of the strength of the van der Waals forces and the value of the viscosity ratio (or the degree of the interfacial mobility), three types of behavior of the coalescence time are identified: the linear slow, the dimpled and the multiple-rim drainage regimes. In the first two regions the coalescence time decreases with the approach velocity, eventually passes through a minimum and starts to increase in the third region. The slope in the linear region and the critical velocities separating the regimes are used to quantify the behavior, and given as functions of the Hamaker constant and the viscosity ratio. The results are shown to be in good agreement with several recent experimental studies in the literature.

Biased tracers of two fluids in the Lagrangian picture

We explore Lagrangian perturbation theory (LPT) for biased tracers in the presence of two fluids, focusing on the case of cold dark matter (CDM) and baryons. The presence of two fluids induces corrections to the Lagrangian bias expansion and tracer advection, both of which we formulate as expansions in the three linear modes of the Lagrangian equations of motion. We compute the linear-order two-fluid corrections in the Zeldovich approximation, finding that modifications to the bias expansion and tracer advection both enter as percent-level corrections over a large range of wavenumbers at low redshift and draw parallels with the Eulerian formalism. We then discuss nonlinear corrections in the two-fluid picture, and calculate contributions from the relative velocity effect (∝ vr2) at one loop order. Finally, we conduct an exploratory Fisher analysis to assess the impact of two-fluid corrections on baryon acoustic oscillations (BAO) measurements, finding that while modest values of the relative bias parameters can introduce systematic biases in the measured BAO scale of up to 0.5 σ, fitting for these effects as additional parameters increases the error bar by less than 30% across a wide range of bias values.

Molecular dynamics simulations of friction forces between silica nanospheres

In this work, the interfacial friction forces between silica nanospheres are explored using molecular dynamics (MD) simulations through oblique impact. The effects of initial relative impact velocity, interaction path, contacting surfaces (relative orientation), impact angle (initial overlapping degree), and particle size on the interfacial forces are first studied. MD simulations demonstrate that the tangential friction forces depend sensitively on the impact velocity, interaction path and impact angle (initial overlapping degree), especially on the joint effect of interaction path and impact angle, but is less dependent on interacting surfaces and particle size. By increasing the initial overlapping degree, the relation between tangential friction forces and normal forces demonstrates rich and unusual behaviours in the approach and departure processes, ranging from hysteresis, incomplete anti-hysteresis, complete anti-hysteresis to negative friction coefficient, and then recovering to the usual hysteresis. Moreover, the classic Mindlin-Deresiewicz theory is still valid at small overlapping degrees, both qualitatively and quantitatively; with increasing overlapping degrees, the trend of rebounding prevails over relative sliding and the deviation from theory is weakened. The thermal vibration and dislocation of atoms during sliding and compression result in small fluctuations, especially in the departure process. This work offers atomic-scale insights into the friction characteristics between nanoparticles and consolidates our capability of modelling nanoparticle behaviours using advanced computing techniques, such as discrete element method in future.

Effects of the prevision relative velocity on traffic dynamics in the ACC strategy

This paper focuses on exploring the effects of the prevision relative velocity on the dynamics characteristic of the simulated traffic flow in the ACC strategy. Firstly, an improved car-following considering the prevision relative velocity is developed for numerical simulations. Secondly, an adaptive neural-fuzzy inference system is trained and employed to predict the relative velocity. Thirdly, a microscopic traffic simulation program is created to analyze the traffic flow evolution process, further to explore the effects of the prevision relative velocity on the dynamics characteristic. The results show that considering the prevision relative velocity in designing the ACC control strategy can significantly affect the dynamics characteristic of the simulated traffic flow, which can improve the traffic flow stability in the ITS environment.

The Supersonic Project: rotational effects of supersonic motions on the first structures in the Universe

We introduce the "Supersonic Project," aimed at investigating the effects of the supersonic relative velocity between dark matter (DM) and baryons at high redshift using a combination of analytical calculations and cosmological simulations. In this paper, we study the effect of this stream velocity on the angular momentum of the first structures in the early Universe using simulations. We focus on DM haloes and their gas component as well as the recently predicted supersonically-induced gas objects (SIGOs) that arise as a result of the stream velocity phase shift. We find that the spin parameter of the gas component in these first haloes is increased with the stream velocity. Moreover, we find that when the stream velocity is taken into account, the angular momentum vectors of the DM component and the gas component are typically misaligned and this misalignment angle has a nearly isotropic distribution. The spin parameter value of the gas component is higher than in the no stream velocity case, which even in the absence of cooling, may result in more prolate objects. We also generalize the spin parameter to the SIGOs and find that they typically have a larger spin parameter with respect to their dark matter counterparts and that there is no correlation of the spin parameter and the prolateness of such structures. We speculate that SIGOs may be observed as very low luminosity objects in the early Universe and may serve as potential progenitors of Little Blue Dot-like systems.

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.

Numerical simulation of droplet evaporation characteristics of multi-component acetone-butanol-ethanol and diesel blends under different environments

Effects of ambient temperature, pressure and relative velocity between droplet and airflow on evaporation characteristics of multi-component droplet were investigated numerically. A recently proposed multi-component model was adopted in the present study by considering the pressure effects. The result shows that, present model predicts satisfactory results when compares with experimental data. Increasing ambient temperature especially promotes evaporation of low volatile components in ABE20. Ambient pressure influences ABE-diesel blends droplet evaporation differently under various ambient temperatures. For low ambient temperature environment, droplet evaporation rate decreases with ambient pressure increasing, especially for low diffusion coefficient components. However, evaporation characteristic shows an opposite tend under high ambient temperature environment, where evaporation rate increases with ambient pressure increasing. Additionally, increasing ambient pressure promotes maximum evaporation rate of high diffusion coefficient components. With temperature increasing, this promotion extends to low diffusion coefficient components. Finally, increase relative velocity between liquid and gas phase substantially enhances evaporation of high diffusion coefficient components in the mixture.

Research on the Time-Varying Properties of Brake Friction (September 2018)

In this paper, a dynamic model of the coefficient of friction (COF) is established from energy perspective and the principal friction mechanism of disc brake is investigated. The effects of temperature, normal pressure, and relative velocity between brake disc and pad on the COF are tested on a friction testing machine. The results show that the thermal effect is the main factor affecting the friction and wear process, and the thermal deformation and uneven heat distribution play key roles in the change of the COF. The proposed dynamic model can well describe the properties of the COF in friction process, and the work in this paper may provide some new insights into the brake friction mechanism and be helpful for the design of the brakes.

Neutron transport simulation in high speed moving media using Geant4

A method using Geant4 to simulate neutron transport in moving media is described. The method is implanted in the source code of the software since Geant4 does not intrinsically support a moving object. The simulation utilizes the existing physical model and data library in Geant4, combined with frame transformations to account for the effect of relative velocity between neutrons and the moving media. An example is presented involving a high speed rotating cylinder to verify this method and show the effect of moving media on neutron transport.

Bifurcation structure of a car-following model with nonlinear dependence on the relative velocity

ABSTRACTUnderstanding the stability of solutions of mathematical models of traffic flow is important for alleviating jams as these are considered stable inhomogeneous solutions of traffic models. Traffic jams can be alleviated by destabilizing these solutions. Solution stability can be studied with the aid of bifurcation analysis, which has been used to describe the global bifurcation structure of a car-following model that exhibits bistable behavior and loss of stability due to Hopf bifurcations. However, previous studies on bifurcation analysis for traffic models have not considered the relative velocity effect, which is important in real-world traffic scenarios. This study analytically derives linear stability conditions and numerically investigates the global bifurcation structure of a car-following model with nonlinear dependence on the relative velocity (the STNN model), which exhibits multistable states. Moreover, the relative velocity drastically changes the bifurcation structure. This supports implementation of (semi-)automatic driving systems as a means to alleviate traffic jams.

Effect of Relative Velocity Between Rough Surfaces: Hydrodynamic Lubrication of Rotary Lip Seal

Since the sixties, most of numerical studies that model the rotary lip seal lubrication have been restricted by assuming that one of the two opposing surfaces is smooth: either the lip or the shaft. This hypothesis, although it is verified only for a shaft roughness ten times smaller than that of the seal, is the best solution to avoid the transient term “∂h/∂t” in the deterministic approach. Thus, the subject of the present study is twofold. The first part validates the current hydrodynamic model with the international literature by assuming the asperities on the lip and shaft as a two-dimensional cosine function. In the second part the Reynolds equation for rough surfaces with relative motion is solved. The numerical results show that the relative motion between rough surfaces impacts significantly the load support and the leakage rate, but affects slightly the friction torque.

Constraining the relative velocity effect using the Baryon Oscillation Spectroscopic Survey

We analyse the power spectrum of the Baryon Oscillation Spectroscopic Survey (BOSS), Data Release 12 (DR12) to constrain the relative velocity effect, which represents a potential systematic for measurements of the Baryon Acoustic Oscillation (BAO) scale. The relative velocity effect is sourced by the different evolution of baryon and cold dark matter perturbations before decoupling. Our power spectrum model includes all $1$-loop redshift-space terms corresponding to $v_{\rm bc}$ parameterised by the bias parameter $b_{v^2}$. We also include the linear terms proportional to the relative density, $\delta_{\rm bc}$, and relative velocity dispersion, $\theta_{\rm bc}$, which we parameterise with the bias parameters $b^{\rm bc}_{\delta}$ and $b^{\rm bc}_{\theta}$. Our data does not support a detection of the relative velocity effect in any of these parameters. Combining the low and high redshift bins of BOSS, we find limits of $b_{v^2} = 0.012 \pm 0.015\;(\pm 0.031)$, $b^{\rm bc}_{\delta} = -1.0 \pm 2.5\;(\pm 6.2)$ and $b^{\rm bc}_{\theta} = -114 \pm 55\;(\pm 175)$ with $68\%$ ($95\%$) confidence levels. These constraints restrict the potential systematic shift in $D_A(z)$, $H(z)$ and $f\sigma_8$, due to the relative velocity, to $1\%$, $0.8\%$ and $2\%$, respectively. Given the current uncertainties on the BAO measurements of BOSS these shifts correspond to $0.53\sigma$, $0.5\sigma$ and $0.22\sigma$ for $D_A(z)$, $H(z)$ and $f\sigma_8$, respectively.

Simulating the cold dark matter-neutrino dipole with TianNu

Measurements of neutrino mass in cosmological observations rely on two point statistics that are hindered by significant degeneracies with the optical depth and galaxy bias. The relative velocity effect between cold dark matter and neutrinos induces a large scale dipole into the matter density field and may be able to provide orthogonal constraints to standard techniques. We numerically investigate this dipole in the TianNu Simulation, which contains cold dark matter and 50 meV neutrinos. We first compute the dipole using a new linear response technique where we treat the displacement caused by the relative velocity as a phase in Fourier space and then integrate the matter power spectrum over redshift. Then, we compute the dipole numerically in real space using the simulation density and velocity fields. We find excellent agreement between the linear response and N-body methods. Utilizing the dipole as an observational tool will require two tracers of the matter distribution that are differently biased with respect to the neutrino density.

Evaluation of bubble-induced turbulence using direct numerical simulation

The presented research evaluates the interaction between a single bubble and homogeneous turbulent flow using direct numerical simulation (DNS) approach. The homogeneous single-phase turbulence is numerically generated by passing a uniform flow through grid planes. The turbulence decay rate is compared with experiment-based correlation. The single phase turbulence is then used as an inflow boundary condition for a set of single bubble studies. By estimating the turbulent field around the fully resolved bubble, the effects of bubble deformability, turbulent intensity and relative velocity on the bubble-induced turbulence are investigated. The existence of bubble creates new vortices in the wake region and the enhancement of turbulence is observed in the region behind the bubble. The results show that the magnitude of the turbulence enhancement would increase as the bubble encounters larger liquid turbulent intensity or higher relative velocity. Set of bubble Weber numbers from 0.34 to 3.39 are used to investigate the effect of bubble deformability. The more deformable bubble is the higher the increase in the magnitude of the turbulence enhancement behind the bubble. This research provides systematic insight on the bubble-induced turbulence (BIT) mechanism and is important for multiphase computational fluid dynamics (M-CFD) closure model development.

Effect of different dust flow velocities on combined Kelvin-Helmholtz and Rayleigh-Taylor instabilities in magnetized incompressible dusty fluids

We investigate the effect of different dust flow velocities and two dimensional magnetic fields on the combined Kelvin-Helmholtz instability (KHI) and Rayleigh-Taylor instability (RTI) of two superimposed incompressible dusty fluids. A single fluid reduced dusty magnetohydrodynamic model is obtained for the three component magnetized incompressible dusty plasma, and it is assumed that a plane interface of infinite boundaries separates heavy and light dusty fluids at z = 0. The general dispersion relations for RT and KH configurations are obtained using appropriate boundary conditions and discussed in the case of equal and different dust fluid flow velocities. In the case of the stable RT configuration, we find that the condition of RTI depends upon both longitudinal and transverse magnetic field components and relative dust flow velocity. In the case of the KH configuration, the effect of magnetic field and relative dust flow velocity is observed and it is shown that dust flow velocity must be larger than a particular value of Alfven speed in order to excite KHI. Numerical calculations have been performed to illustrate the linear growth rates of RTI and KHI in both magnetized and unmagnetized dusty plasmas. We observe that the magnetic field has stabilizing whereas the dust Atwood number has destabilizing influence on the growth rate of RTI. The different dust flow velocities have destabilizing whereas magnetic field has stabilizing influence on the growth rate of KHI in dusty plasmas. The dust Mach number yields a stabilizing influence on the linear growth rate of KHI.

A methodology for assessment of erosive wear on a Francis turbine runner

This study addresses the problem of erosive wear of Francis turbine runner of a typical power station located in northern Himalayan belt of India. Wear damage is determined using the erosion models of IEC-2009, IEC-2013, empirical assessment of Thapa and a methodology proposed in this research work. The proposed model introduces effect of relative velocity of turbine instead of characteristic velocity and also the effect of curvature of the blades by incorporating a parameter referred to as geometrical factor. The results obtained from the proposed model of erosive wear of the Francis turbine have been calibrated against Jhimrukh Hydropower Centre(JHC).The study also includes petrographic analysis of the sediment taken from the site of power station to obtain the parameters related to concentration(C), size (Ksz), hardness (Khd) and shape (Ksh). Effect of these parameters have been introduced into the wear models used in this investigation. The erosive wear assessment reveals that damage is severe at the runner outlet due to the high relative velocity(45.5 m/s against 15.5 m/s at inlet) and presence of hard minerals. The average loss of thickness for runner is 1.3 mm/year of operation, which agrees within 10% of the measured values from site of the power station.

Effects of Relative Velocity on Grinding Performances under Si Wafer Rotary Grinding

For the backgrinding of semiconductor devices, a rotary grinding process is indispensable for achieving the required wafer thickness. The relative velocity between the grinding wheel and the wafer is maximum at the periphery of the wafer and minimum at the center of wafer. Generally, the grinding performances are discussed in terms of the ratio of the rotational speeds of the grinding wheel and the wafer. However, it is not possible to use this ratio to determine the grinding conditions for different wafer sizes grinding as this ratio does not show the difference in relative velocity. Therefore, a new relative velocity ratio was defined in this study. Then, the Si wafer grinding was performed to investigate the effect of the surface roughness and the power consumption of the grinding wheel spindle on the relative velocity ratio.

Effect of relative velocity and density perturbations between baryons and dark matter on the clustering of galaxies

Pre-recombination acoustic oscillations induce non-adiabatic perturbations between baryons and dark matter, corresponding to a constant relative-density $\delta_{bc}$ and decaying relative-velocity perturbation $\vec{v}_{bc}$. Due to their significant large-scale correlations and prominent baryon acoustic oscillation (BAO) features, these modes are potentially important for the use of the BAO as standard ruler. We present a complete treatment of the effects of the baryon-CDM perturbations on galaxy clustering in the context of a rigorous perturbative bias expansion. The leading effects are proportional to $\delta_{bc}$ and $\theta_{bc} = \partial_i v_{bc}^i$. We estimate the magnitude of these terms through the excursion set approach. The contribution from $v_{bc}^2$, which has attracted significant attention recently, contributes at subleading (1-loop) order. The relative-density contribution $\delta_{bc}$ is expected to be by far the largest contribution. We also point out contributions to the galaxy velocity bias, the largest of which is simply $v_{bc}$, leading to a term $\propto \mu^2\theta_{bc}$ in the redshift-space galaxy power spectrum $P_g^s(k,\mu)$. Complete expressions of the galaxy power spectrum at 1-loop order are given, which contain several new terms.

Simulation on heat and mass transfer for a subcooled droplet falling freely in saturated steam by the lattice Boltzmann method

In this paper, the growth and falling process of a single droplet in saturated steam is studied via the lattice Boltzmann method. The propagation of temperature field and flow field are directly simulated and the mechanism of heat and mass transfer is discussed. Moreover, the effect of relative velocity between droplet and steam on the growth and falling process of the droplet is also simulated. The results indicate the mean temperature of droplet with vertical velocity of steam is highest, while that with horizontal velocity is lowest. Furthermore, better condensation effect of droplet is obtained with smaller velocity of steam.

The BOSS–WiggleZ overlap region – I. Baryon acoustic oscillations

We study the large-scale clustering of galaxies in the overlap region of the Baryon Oscillation Spectroscopic Survey (BOSS) CMASS sample and the WiggleZ Dark Energy Survey. We calculate the auto-correlation and cross-correlation functions in the overlap region of the two datasets and detect a Baryon Acoustic Oscillation (BAO) signal in each of them. The BAO measurement from the cross-correlation function represents the first such detection between two different galaxy surveys. After applying density-field reconstruction we report distance-scale measurements $D_V r_s^{\rm fid} / r_s = (1970 \pm 47, 2132 \pm 67, 2100 \pm 200)$ Mpc from CMASS, the cross-correlation and WiggleZ, respectively. We use correlated mock realizations to calculate the covariance between the three BAO constraints. The distance scales derived from the two datasets are consistent, and are also robust against switching the displacement fields used for reconstruction between the two surveys. This approach can be used to construct a correlation matrix, permitting for the first time a rigorous combination of WiggleZ and CMASS BAO measurements. Using a volume-scaling technique, our result can also be used to combine WiggleZ and future CMASS DR12 results. Finally, we use the cross-correlation function measurements to show that the relative velocity effect, a possible source of systematic uncertainty for the BAO technique, is consistent with zero for our samples.