Simulated Velocity Distributions Research Articles

Improved Multiphase Lagrangian Method for Simulating Sediment Transport in Dam-Break Flows

AbstractIn this study, sediment transport is considered as a typical multiphase flow in numerical simulation using a particle-based method and efforts are focused on reducing interface instability between phases. A new multiphase model, a particle-method based rheology model, and a higher order viscosity smoothening scheme are used in a particle-based method to reproduce the sediment transport. Model applications including open channel flow scouring on sand beds and various water-sediment dam break flows are simulated. This new particle-based multiphase method is able to predict both free surface and sediment profiles. Additionally, the simulated velocity distributions of water-sediment dam break flow also show good agreement with measured data, which has seldom been considered in previous studies using a particle-based method. The successful implementation of the sediment transport simulation confirms the strong capability of this particle-based Lagrangian method for predicting multiphase flow and provid...

Evaluation of diffusion coefficients by means of an approximate steady-state condition in sedimentation velocity distributions

This investigation examined the feasibility of manipulating the rotor speed in sedimentation velocity experiments to spontaneously generate an approximate steady-state condition where the extent of diffusional spreading is matched exactly by the boundary sharpening arising from negative s–c dependence. Simulated sedimentation velocity distributions based on the sedimentation characteristics for a purified mucin preparation were used to illustrate a simple procedure for determining the diffusion coefficient from such steady-state distributions in situations where the concentration dependence of the sedimentation coefficient, s = s0/(1 + Kc), was quantified in terms of the limiting sedimentation coefficient as c → 0 (s0) and the concentration coefficient (K). Those simulations established that spontaneous generation of the approximate steady state could well be a feature of sedimentation velocity distributions for many unstructured polymer systems because the requirement that Kcoω2s0/D be between 46 and 183 cm−2 is not unduly restrictive. Although spontaneous generation of the approximate steady state is also a theoretical prediction for structured macromolecular solutes exhibiting linear concentration dependence of the sedimentation coefficient, s = s0(1 − kc), the required value of k is far too large for any practical advantage to be taken of this approach with globular proteins.

IMPACT OF PICKUP IONS ON THE SHOCK FRONT NONSTATIONARITY AND ENERGY DISSIPATION OF THE HELIOSPHERIC TERMINATION SHOCK: TWO-DIMENSIONAL FULL PARTICLE SIMULATIONS AND COMPARISON WITHVOYAGER 2OBSERVATIONS

The transition between the supersonic solar wind and the subsonic heliosheath, the termination shock (TS), was observed by Voyager 2 (V2) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. The data reveal multiple crossings of a complex, quasi-perpendicular supercritical shock. These experimental data are the starting point for a more sophisticated analysis that includes computer modeling of a shock in the presence of pickup ions (PUIs). here, we present two-dimensional (2-D) particle-in-cell (PIC) simulations of the TS including PUIs self-consistently. We also report the ion velocity distribution across the TS using the Faraday cup data from V2. A relatively complete plasma and magnetic field data set from V2 gives us the opportunity to do a full comparison between the experimental data and PIC simulation results. Our results show that: (1) The nonstationarity of the shock front is mainly caused by the ripples along the shock front and these ripples from even if the percentage of PUIs is high. (2) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion dynamic energy is transferred to the thermal energy of PUIs instead of solar wind ions (SWIs). (3) The simulated composite heliosheath ion velocity distribution function is a superposition of a cold core formed by transmitted SWIs, the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. (4) The V2 Faraday cups observed the cool core of the distribution, so they saw only a tip of the iceberg. For the evolution of the cool core distribution function across the TS, the computed results agree reasonably well with the V2experimental results.

A CPFD model for a bubbly biomass–sand fluidized bed

This study reports a computational particle fluid dynamics model for the numerical simulation of a sand fluidized bed with 8wt.% and 16wt.% biomass loadings. The proposed CPFD model, based on a Barracuda framework, is validated using experimental data obtained in a 15.2cm diameter bed with a 22.5cm static height. This fluidized bed unit is equipped with a pair of fiber optic sensors allowing one to obtain both the bubble size and bubble velocity frequency distributions in an ample range of experimental conditions. In addition to this, the first-, second-, and third-order statistical moments of the bubble size and the bubble velocity distributions are calculated for each one of the operating conditions studied. On this basis, the present study demonstrates that the proposed CPFD model reproduces the asymmetric character of both bubble size and bubble velocity distributions. In addition, the simulated and experimental bubble size and bubble velocity distributions yield close first-, second-, and third-order statistical moments. Accordingly, one can ascertain that the proposed CPFD model is quite satisfactory for establishing fluid flow patterns in sand–biomass fluidized bed gasifiers.

Computational Fluid Dynamics (CFD) modelling of transfer chutes: Assessment of viscosity, drag and turbulence models

Computational Fluid Dynamics (CFD) has been successfully applied to evaluate potential dust emissions from bulk material transfer chutes. The implementation of appropriate models and modelling parameters is shown to be critical to the overall accuracy of the simulation results. This paper presents the influence of different models on the CFD simulation of transfer chutes and follows from an earlier study that details the influence of model parameters. The aim of this paper is to offer guidance to select models, and provide a better understanding of their influence in order to evaluate the most appropriate viscosity model, drag model and turbulence model for this application. A two-phase three-dimensional Euler–Euler model in commercial CFD software ANSYS FLUENT has been selected to model the granular and air flow in the transfer chute. The simulated air velocity profiles are discussed by comparing with each other and against experimental data obtained from Particle Image Velocimetry (PIV) results. The simulated particle velocity distributions were compared with results obtained using a well-established continuum method which was developed by Roberts. Furthermore, the air mass flow rates were analysed to evaluate the influence of different models. The results show that the granular viscosity model had a strong influence on the predictions of both air and particle flow. It was observed that both the drag model and turbulence model had limited influence on the outlet air velocities. The results also indicate that the Di-Felice drag model and SST k–ω turbulence model provide solutions closer to the experimental values than the other models investigated.

Sinking/Floating Particles Solid Suspension Characteristics in Stirred Tank Based on CFD Simulation

Solid-liquid suspension characteristics for sinking and floating particles in a dual-impeller stirred tank are simulated numerically by using computational fluid dynamics(CFD). The multi-reference frame(MRF) method for treatment of impeller rotation and Euler-Euler two-fluid model based on the kinetics of granular are used in the simulation. Flow field and solid holdup distribution for sinking and floating particles in the agitated tank are predicted, and the effects of operating conditions on solid-liquid suspension are investigated. The numerical simulated liquid velocity distribution and flow pattern are good agreement with experimental data in literature. The simulated results indicated that solid-liquid suspension of sinking particles is quite different from that of floating particles. For sinking particles, solid holdup decreases along axial height as a whole, and the highest solid holdup is found in the centre of the bottom region. Local solid holdup in the centre of circulation loops, behind the blades and baffles was relatively low. Nevertheless, solid holdup distribution for floating particles is contrary to that of sinking particles as above-mentioned. The increase of impeller speed can cause local solid holdup in the centre of circulation loops decrease for sinking particle, while contrarily for floating particles. Decreasing solid content or smaller particles size is beneficial to the uniformity of solid-liquid suspension for the two types of particles. With increasing of particles content or particles size, sinking particles and floating particles are more prone to aggregate in the centre of bottom region and surface region, respectively.

Influence of Trash-Barriering on Operation of Pumping Station with Wet-Pits

Trash racks are set before inlet sumps of a pumping station with wet-pits in order to block trash. The flow fields before and behind the trash rack were numerically simulated and flow fields in the inlet sump were calculated using the simulated velocity distribution of the section behind the trash rack as the inlet boundary condition. Local head loss of trash-barriering increases with the increase of blockage ratio of the trash rack. When waterweeds block upper part of the trash rack, flow pattern behind the trash rack will be extremely turbulent, flow velocity of lower part of inlet section of inlet sump will increase, flow velocity of the upper part will decrease, and even backflow appears, which will deteriorate the flow pattern and increase head loss in the inlet sump. Trash-barriering has little effect on velocity distribution of impeller inlet section. Trash-barriering will not only increase head and power of the pump but also decrease the flow. For the example of the pumping station: when blockage ratios of the trash racks are 0.3, 0.5, and 0.7, operating cost will increase by 13%~43% than that of without trash racks. Therefore, the trash rack should be cleaned up timely.

Numerical simulation of wave–current interaction using a RANS solver

A numerical model is developed to study the wave propagation in the presence of a steady current flow. This model is based on Reynolds-Averaged Navier–Stokes (RANS) equations with k−ε turbulence closure scheme. A novel volume of fluid (VOF) method is applied to accurately capture the water free surface. The current flow is initialized by imposing a steady inlet velocity on one domain end and pressure outlet on the other end, while the desired wave is generated by an internal wave-maker from mass source term of mass conservation equation. Simulated water surface profile and velocity distribution agree well with experimental measurements of Umeyama (2011), indicating that this model has a great ability in simulating wave–current interaction. The validated model is then used to investigate the effects of wave period and current velocity on regular wave–current induced water surface profile and velocity distribution. The propagation of a solitary wave traveling with a following/opposing current is also numerically investigated by this model.

Simulated ion velocity distribution and its incoherent scattering spectrum in the high-latitude ionosphere

If the relatively larger convection electric field and small the collision frequency between ions and neutrals in the high-latitude ionosphere are considered, the integral solution of the non-Maxwellian ion velocity distribution function for the relaxation collision model is simplified, and a torus distribution function of the ion velocity is obtained. Also, the feature of the torus velocity distribution function is analyzed and simulated with different parameters in high-latitude ionosphere. Furthermore, by using the electromagnetic radiation theory of Sheffield and taking into account the ion temperature anisotropy and its variation with the electric field, we simulate the incoherent scattering spectrum of ions in different directions. The research shows that the ionospheric convection electric field, the ion-neutral collision frequency and the ion anisotropic temperature distribution all have an effect on the incoherent scattering spectrum.

Ti–Ar scattering cross sections by direct comparison of Monte Carlo simulations and laser-induced fluorescence spectroscopy in magnetron discharges

A 3D Monte Carlo code (OMEGA) was developed to simulate the transport of sputtered atoms in a magnetron discharge operating in direct current mode. Collisions between the sputtered Ti atoms and the neutral process gas atoms (Ar) were modelled. Spatially resolved simulated velocity distributions of the sputtered particles parallel as well as perpendicular to the cathode surface for different operating pressures were recorded and benchmarked against experimentally obtained profiles using laser-induced fluorescence. New differential (angular and energy-dependent) cross sections for Ti–Ar elastic collisions were thereby obtained, which resulted in good agreement between modelled and experimental results. The differences with respect to commonly used extrapolated Ar–Ar cross sections to describe the Ti–Ar interaction are highlighted and discussed.

Vertical Velocity and Microphysical Distributions Related to Rapid Intensification in a Simulation of Hurricane Dennis (2005)

Abstract A 1-km Weather Research and Forecasting model simulation of Hurricane Dennis was used to identify precursors in vertical velocity and latent heating distributions to rapid intensification (RI). Although the observed structure qualitatively replicated data obtained during P-3 and Earth Resources-2 (ER-2) flights, the simulated reflectivity was overestimated. During the 6 h preceding RI, defined as 0000 UTC 8 July 2005 close to the time of simulated maximum central pressure deepening, the asymmetric convection transformed into an eyewall with the maximum 10-m wind speed increasing by 16 m s−1. Contour by frequency altitude diagrams showed unique changes in the breadth of simulated vertical velocity (w) distributions before and after RI. Outliers of w distributions at 14 km preceded RI onset, whereas the increase in w outliers at 6 km lagged it. Prior to RI there was an increase in the upward flux of hydrometeors between 10 and 15 km, with increased contributions from w > 6 m s−1. Increases in lower-level updraft airmass fluxes did not lead RI, but the 14-km positive w outliers were better indicators of RI onset than positive w averages. The area of convective bursts did not strongly increase before RI, but it continually increased after RI. Latent heating was dominated by contributions from w < 2 m s−1, meaning increases in positive w outliers before RI did not cause the increase in latent heating seen during RI. The location of convective bursts and outliers of positive and negative w distributions contracted toward the eye as the simulated Dennis intensified.

Hydrodynamics of an Undulating Fin for a Wave-Like Locomotion System Design

Motivated by the interest to develop an agile, high-efficiency robotic fish for underwater applications where safe environment for data-acquisition without disturbing the surrounding during exploration is of particular concern, this paper presents computational and experimental results of a biologically inspired mechanical undulating fin. The findings offer intuitive insights for optimizing the design of a fin-based robotic fish that offers several advantages including low underwater acoustic noise, dexterous maneuverability, and better propulsion efficiency at low speeds. Specifically, this paper begins with the design of a robotic fish developed for experimental investigation and for validating computational hydrodynamic models of an undulating fin. A relatively complete computational model describing the hydrodynamics of an undulating fin is given for analyzing the effect of propagating wave motions on the forces acting on the fin surface. The 3-D unsteady fluid flow around the undulating fin has been numerically solved using computational fluid dynamics method. These numerically simulated pressure and velocity distributions acting on the undulating fin, which provide a basis to compute the forces acting on the undulating fin, have been experimentally validated by comparing the computed thrust against data measured on a prototype flexible-fin mechanism.

Experimental studies and numerical model validation of overflowing 2D foam to test flotation cell crowder designs

A computational fluid dynamics model of froth motion has been developed to assess different flotation cell designs. This work presents an implementation of the model in a 2D case, to compare the simulated bubble velocity distribution and streamlines to an experimental foaming system. The model uses finite elements to solve Laplace's equation for a potential function from which the foam velocity can be obtained. It requires the air recovery, or the amount of air that overflows a flotation cell as unburst bubbles, as an input parameter to calculate the foam velocity distribution and bubble streamlines. The air recovery was obtained by image analysis from a vertical, overflowing monolayer of foam (2D) created in a Hele-Shaw column, which mimicked important flowing properties of flotation froths such as coalescence. Inserts were included in the foam column to represent potential crowder designs for industrial flotation cells. Three different designs were chosen to compare the effect of insert depth and shape, including rectangles and a triangle. The effect of the insert design on the overflowing foam is obvious from visual assessment of the bubble streamlines and velocity distribution, which were closely agreed by both the experiment and model.

Modified two‐stream instability at perpendicular collisionless shocks: Full particle simulations

A full particle simulation study is carried out for studying microinstabilities generated in self‐consistently excited perpendicular collisionless shocks. The present two‐dimensional simulation with a high ion‐to‐electron mass ratio shows that the modified two‐stream instability can be generated by the interaction of incoming and reflected ions with electromagnetic whistler mode waves propagating in the direction quasi‐perpendicular to the ambient magnetic field. Properties of the excited whistler mode waves are consistent with the linear dispersion analysis based on the simulated velocity distribution functions. It is also found that the excited waves have an electron‐scale wavelength in the shock‐normal direction and an ion‐scale wavelength in the shock‐tangential direction, suggesting that the electron‐scale microturbulences and the ion‐scale shock structures are coupled with each other through the modified two‐stream instability.

An alternative approach to quantifying fluid flow uniformity based on area-weighted average velocity and mass-weighted average velocity

Fluid flow uniformity is an important index in many research fields including artificial environment in the room. In order to facilitate quantifying the multi-section flow uniformity directly by simulation results and to simplify the computation process, a correlation based on area-weighted average velocities and mass-weighted average velocities is proposed in this paper. To test the proposed approach to quantifying flow uniformity, the correlation is applied in two studies – one based on assumed velocity distribution, another based on the simulated velocity distribution – in comparison with two existing formulae from prior studies. The results show that the evaluation results of the proposed correlation are in complete agreement with that of the two formulae from prior studies in the overall trend. The correlation may be used in a wide range of contexts without strict requirement for uniformity to quantify the fluid uniformity in a relatively simple manner.

Comparison of Three Constitutive Models for the Analysis of Laminar Magnetorheological Fluid Flows

In the current paper, three different constitutive models are employed for the analysis of laminar MRF (MagnetoRheological Fluid) flows between two parallel plates. The capability of a recently constructed non-convex constitutive model is compared with those of widely used Bingham model and Herschel-Bulkley model. The comparison among simulated flow velocity distributions and shear stress - shear strain rate characteristics based on different models is presented. It is shown that the non-convex consitutive model outperformance the other two models.

Analysis of rapid flow of particles down and from an inclined chute using small scale experiments and discrete element simulation

In blast furnaces with bell-less charging systems, the particles enter the furnace from a hopper and are distributed on the burden surface by a rotating chute. The velocity distribution of the particles in the chute is known to affect the burden distribution and particle segregation on the burden surface. This paper investigates by simulating the translational and rotational velocity distributions of uniform spherical particles for the simplified cases where they are discharged from a hopper into a non-rotating chute set at different angles with respect to the vertical plane. The simulations, which are based on the discrete element method, are partly validated by small scale experiments, where particle velocities are estimated from observed trajectories of the particle stream leaving the chute, as well as by sampling particles in boxes to estimate the width of the stream. The simulated translational and rotational velocity distributions at different cross-sections of the chute for different tilting angles are also reported and discussed. The results show that the average velocity at the chute tip depends only on the chute angle for the bulk of the material flow. The angular velocity distribution of the particles along the cross-section of the chute exhibits a V shaped profile, while the opposite holds true for the translational velocity distribution. As the particles flow down the chute, both velocity distributions become more uniform.

Intensification of co-current gas–liquid reactors using structured catalytic packings: A multiscale approach

Cross-flow structured packings for tubular fixed bed reactors operated with co-current gas–liquid flow perform significantly better in terms of overall heat transfer than a randomly packed bed (glass beads), knitted wire packing, and Al-foam structure at a range of gas and liquid flow rates. This is mainly the result of radial directed convective transport of heat, realized by the channel structure of the cross-flow packings. A pseudo-homogeneous 2D (two-dimensional) plug flow model is found inadequate to quantify variations in overall heat transfer for experiments with different packing orientations and different gap sizes between the packing structure and the cooling wall, because it does not take directed convective transport of heat into account. Numerical simulations are performed to investigate in detail the fluid flow inside the packing. The geometry is defined as sheets separating layers of diagonal channels. For simplicity we assume no fluid exchange between different layers and we approximate the flow in the individual layers as 2D. The simulated velocity distribution inside the packing is highly inhomogeneous, showing regions of almost zero flow. These inhomogeneities in the flow are shown to depend on geometrical parameters, such as the packing width-to-height ratio and can have strong impact on reactor efficiency.

Microstructure patterning on glass substrate by imprinting process

This paper proposed a thermal imprinting process for transferring microscale patterns from a silicon mold to a glass substrate. Both finite element simulation and experiment were conducted to investigate the formability of microscale glass patterns. Herein, K-PG375 was used as the glass substrate. An FEM model of the periodic microstructures with prescribed boundary conditions was first established. Then, the geometrical parameters, such as aspect ratio, duty ratio, and substrate ratio, process parameters including imprinting temperature, velocity, holding time, etc., as well as the performance index, filling factor, were defined. From the simulated pressure and velocity distributions of glass flow, the mechanism of glass forming was obtained, which provided a guideline for performing the following experimental study. The experiments were conducted by using a homemade thermal imprinting machine. The process parameters were tuned according to the simulation conditions and an SEM was used for observing the geometries of the mold and the transferred patterns. Good agreement between simulation and experimental results were achieved and the 2 μm grating patterns were successfully imprinted on a 12.5 mm square glass substrate.

On the Mean Free Path for Backscattering in $k_{B}T$ Layer of Bulk Nano-MOSFETs

We perform Monte Carlo particle simulations on a silicon conductor for the purposes of reexamining the channel backscattering in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bulk</i> nano-MOSFETs. The resulting mean free path lambda <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</i> for backscattering in a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">long</i> and near-equilibrium conductor is constant, regardless of the potential profile. However, the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">apparent</i> mean free path lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> in a local quasi-ballistic <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kBT</i> layer depends on the curvature of the potential profile. In a linear potential profile, the lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> extracted in a wide range of the conductor length (15 to 100 nm) and lattice temperature (150 to 300 K) is found to fall below lambda <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</i> . The carrier heating as the origin of reduced mean free path is inferred from the simulated carrier velocity distribution near the injection point. Strikingly, the mean free paths in a parabolic potential profile remain consistent: lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> = lambda <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</i> . This indicates the absence or weakening of the carrier heating in the layer of interest, valid only for the parabolic potential barrier.