Velocity-field Characteristics Research Articles

Monte Carlo transport analysis to assess intensity dependent response of a carbon-doped GaN photoconductor

Evaluation of the photoresponse in wurtzite GaN photoconductive switches is presented based on kinetic Monte Carlo simulations. The focus is on electron transport physics and assessment of high frequency operation. The roles of GaN band structure, Pauli exclusion, and treatment of internal fields based on the fast multipole method are all comprehensively included. The implementation was validated through comparisons of velocity-field characteristics for GaN with computational results in the literature. Photocurrent widths of less than ∼7 ps for the 1 μm device can be expected, which translates into a 100 GHz upper bound. Photocurrent pulse compression below the laser full width at half maxima at high applied fields are predicted based on the interplay of space-charge effects and the negative differential velocity characteristics of GaN.

Carrier Velocity in Amorphous Metal–Oxide–Semiconductor Transistors

A model for carrier velocity and velocity saturation in amorphous metal oxide thin-film transistors (TFTs) is presented and discussed. The charge transport in such TFTs involves the interplay between the trapping and extended state transport when the mobility is large enough. The model considers multiple trapping and release and band transport between the trapping events. The dominant scattering mechanisms are polar optical phonon scattering and trapped carrier scattering. Our model predicts that the velocity will saturate due to emission of optical phonons at high electric fields (>105 V/cm) with a maximum value near 107 cm/s. Trapping reduces the velocity and also imparts an additional electric field dependence in which the velocity increases with field. The model is compared with the experimental data from several groups. Experimental data are strongly affected by contact resistance effects, which must be corrected for. Procedures to correct for the contact resistance are described. The corrected velocity-field characteristics of six amorphous indium gallium zinc oxide (IGZO) TFTs are presented.

Numerical Simulation of NO x Emission Characteristics of a Cyclone Boiler with Slag-Tap Furnace.

In order to optimize the parameters of boilers and realize the burning of pure, high-alkali coal, the velocity field, temperature field, and component distribution characteristics of a new cyclone boiler with slag-tap furnace were numerically studied using ANSYS software. The influence law of the over-fire air rate on the NOx emission of the cyclone boiler with slag-tap furnace was established, and the optimal over-fire air rate was determined. The renormalization-group k–ε double equation model was used to simulate the gas phase flow, the discrete phase model was used to compute the gas–solid two-phase flow, and the high-alkali coal combustion model was revised based on experimental data. The results show that the overall aerodynamic field in the entire boiler with slag-tap furnace is favorable, the flue gas is completely formed, and the cyclone burners in a staggered and reversed arrangement can enhance combustion. The temperature near the wall of the cyclone can reach 1700–2100 K, which satisfies the requirements of a liquid slag discharge. The temperature under various over-fire air rate conditions can allow the high-alkali coal to burn normally and ensure fluidization of its ash. The greater the over-fire air rate, the lower the average temperature in the furnace and the lower the NOx concentration at the outlet of the furnace. Considering that it is not easy to fluidize the ash of high-alkali coal when the average temperature in the cyclone boiler with slag-tap furnace is very low, an over-fire air rate of 10% is selected for the optimal air-staged combustion scheme.

Electronic transport properties of hydrogenated and fluorinated graphene: a computational study

Hydrogenation and fluorination have been presented as two possible methods to open a bandgap in graphene, required for field-effect transistor applications. In this work, we present a detailed study of the phonon-limited mobility of electrons and holes in hydrogenated graphene (graphane) and fluorinated graphene (graphene fluoride). We pay special attention to the out-of-plane acoustic (ZA) phonons, responsible for the highest scattering rates in graphane and graphene fluoride. Considering the most adverse cut-off for long-wavelength ZA phonons, we have obtained electron (hole) mobilities of 28 (41) cm2 V−1 s−1 for graphane and 96 (30) cm2 V−1 s−1 for graphene fluoride. Nonetheless, for a more favorable cut-off wavelength of ∼2.6 nm, significantly higher electron (hole) mobilities of 233 (389) cm2 V−1 s−1 for graphane and 460 (105) cm2 V−1 s−1 for graphene fluoride are achieved. Moreover, while complete suppression of ZA phonons can increase the electron (hole) mobility in graphane up to 278 (391) cm2 V−1 s−1, it does not affect the carrier mobilities in graphene fluoride. Velocity–field characteristics reveal that the electron velocity in graphane saturates at an electric field of ∼4 × 105 V cm−1. Comparing the mobilities with other two-dimensional (2D) semiconductors, we find that hydrogenation and fluorination are two promising avenues to realize a 2D semiconductor while providing good carrier mobilities.

Experiment and modeling of liquid-phase flow in a venturi tube using stereoscopic PIV

Venturi tube is based on turbulent flow, whereby the microbubbles can be generated by the turbulent fragmentation. This phenomenon is common in several venturi bubblers used by the nuclear, aerospace and chemical industries. The first objective of this paper is to study the liquid-phase velocity field experimentally and develop correlations for the turbulent quantities. The second objective is to research velocity field characteristics theoretically. Stereoscopic PIV measurements for the velocity field have been analyzed and utilized to develop the turbulent kinetic energy in the venturi tube. The tracking properties of the tracer particles have been verified enough for us to analyze the turbulence field. The turbulence kinetic energy has a bimodal distribution trend. Also, the results of turbulence intensity along the horizontal direction is gradually uniform along the downstream. Both the mean velocity and the fluctuation velocity are proportional to the Reynolds number. Besides, the distribution trend of the mean velocity and the velocity fluctuation can be determined by the geometric parameters of the venturi tube. An analytical function model for the flow field has been developed to obtain the approximate analytical solutions. Good agreement is observed between the model predictions and experimental data.

Numerical simulation and experimental study on internal and external characteristics of novel Hydrocyclones

The design and optimization of the hydrocyclone inlet are an effective way to improve the overall performance of the hydrocyclone. In this paper, a set of novel hydrocyclones are designed by changing single to multiple inlets and narrowing the inlet width. The effects of inlet size and number of inlet on the particle separation efficiency, cut-size, the velocity field and pressure characteristics are discussed based on the same feed flow rates, with computational fluid dynamics (CFD) and experimental methods. The governing equations are coupled using the SIMPLE algorithm, while the Reynolds stress model (RSM) is employed for hydrocyclone turbulent model due to anisotropic nature. Particle trajectories are simulated with discrete phase model (DPM) due to low volume fraction of solid particles in the mixed fluid. The simulated particle separation efficiencies and cut-size approximately agree with experimental data. The results show that it is changing single inlet to multiple inlets and narrowing the inlet width that produce positive effects on the growing of tangential velocity of the hydrocyclone, increasing the centrifugal force of the discrete phase, reducing the resistance buoyance, prolonging the residence time of particles, which reduce the effective size of the cut size and make the total separation efficiency increase by 4.31%.

MHD slip flow of upper-convected Maxwell nanofluid over a stretching sheet with chemical reaction

The present study scrutinizes slip effects and stagnation point flows of upper-convected Maxwell fluid past a stretching sheet. The non-linear ordinary differential equations are obtained from the governing partial differential equations and solved using implicit finite difference method. The impacts of non-dimensional governing parameters such as Brownian motion parameter, velocity ratio, velocity slip parameter, suction/injection parameter, Lewis numbers, upper-convected Maxwell parameter, magnetic field, thermophoresis parameter, chemical reactions parameter, thermal slip parameter, solutal slip parameter, and heat source parameter on the velocity field, heat and mass transfer characteristics are discussed and presented through graphs. The values of local Sherwood number, local Nusselt number, and skin friction coefficient are discussed and presented through tables. The results indicate that when the magnetic field is intensified, it reduces velocity profiles and raises temperature and concentration profiles. Moreover, with an upsurge in velocity slip parameter, the local Nusselt number and local Sherwood number diminish.

Electronic Transport Properties of Silicane Determined from First Principles.

Silicane, a hydrogenated monolayer of hexagonal silicon, is a candidate material for future complementary metal-oxide-semiconductor technology. We determined the phonon-limited mobility and the velocity-field characteristics for electrons and holes in silicane from first principles, relying on density functional theory. Transport calculations were performed using a full-band Monte Carlo scheme. Scattering rates were determined from interpolated electron–phonon matrix elements determined from density functional perturbation theory. We found that the main source of scattering for electrons and holes was the ZA phonons. Different cut-off wavelengths ranging from 0.58 nm to 16 nm were used to study the possible suppression of the out-of-plane acoustic (ZA) phonons. The low-field mobility of electrons (holes) was obtained as 5 (10) cm2/(Vs) with a long wavelength ZA phonon cut-off of 16 nm. We showed that higher electron (hole) mobilities of 24 (101) cm2/(Vs) can be achieved with a cut-off wavelength of 4 nm, while completely suppressing ZA phonons results in an even higher electron (hole) mobility of 53 (109) cm2/(Vs). Velocity-field characteristics showed velocity saturation at 3 × 105 V/cm, and negative differential mobility was observed at larger fields. The silicane mobility was competitive with other two-dimensional materials, such as transition-metal dichalcogenides or phosphorene, predicted using similar full-band Monte Carlo calculations. Therefore, silicon in its most extremely scaled form remains a competitive material for future nanoscale transistor technology, provided scattering with out-of-plane acoustic phonons could be suppressed.

Empirical model for the velocity-field characteristics of semiconductors exhibiting negative differential mobility

An empirical model for the velocity-field characteristic of a semiconductor exhibiting a peak and a region of negative differential mobility is proposed. The adjustable parameters in this model are shown to be directly determinable from the form of the velocity-field characteristic itself, rather than being resolvable through the use of a trial-and-error approach. The model is then applied to a classical compound semiconductor of interest, namely gallium arsenide. Through a direct comparison with results acquired through the use of Monte Carlo electron transport simulations, the strengths and weaknesses of this model are critically assessed.

Tidal current energy potential assessment in the Avilés Port using a three-dimensional CFD method

Tidal energy is considered as an energy resource of maximum interest in both technical and research fields due to its largely unexploited energy potential. The use of hydrokinetic microturbines is now an attractive option with reduced environmental impact. The first step to evaluate the feasibility of a hydrokinetic microturbines installation is to perform a study of the velocity field characteristics and therefore the energy potential available. Up to now, different numerical models, of one, two and three spatial dimensions have been applied to evaluate the tidal potential in large areas. Due to the high computational resources needed, they include simplifications, like avoiding a precise study of the velocity in the vertical dimension, resulting in incomplete estimations of the available kinetic energy. To complete these estimations, the research presented sets out a methodology to evaluate the current effects, velocity profiles and the energy potential derived from tide movements in an estuary or port by solving the full Navier–Stokes equations. It also considers the water–air interface in the numerical scheme. The methodology is based, firstly, on the definition of a three-dimensional geometrical model of the geographical area of study, and then, the complete model is meshed with finite volumes, where the full three-dimensional Unsteady Navier–Stokes equations are solved. The methodology was applied and validated with a three-dimensional water–air numerical model of the port of Aviles (Spain). In conclusion, water surface elevations, averaged speed cycles, velocity profiles as a function of depth and tidal power and energy data have been obtained without the usual simplifications, which will mean an evaluation more accurate when assessing the implementation of a power generation system.

Analysis of Airflow Velocity Field Characteristics of an Oat Cleaner Based on Particle Image Velocimetry Technology

Abstract.To improve the cleaning performance of oat sorting devices with a single-fan three-cylinder screen system, this study investigated the airflow velocity field of a cleaning machine using particle image velocimetry (PIV) technology. Specifically, an optimal cleaning performance can be described as one with no material idling under the optimal operating parameters of the oat cleaning machine, the measured airflow velocity field space is divided into nine representative longitudinal planes that can reflect airflow velocity characteristics in the longitudinal plane. The results showed that a phenomenon of airflow backflow occurred in the radial direction above the medium cylinder screen, the airflow velocity in each longitudinal section of the airflow velocity field was less uniform in the transverse distribution, a. The movement of oat extracts in the cleaning room was recorded by a high-speed digital video camera. The phenomenon of oat impurities moving along the airflow backflow direction, and a transverse flow irregularity appearing in the radial direction above the medium cylinder screen were observed. The grain loss rate and impurity rate of the oat cleaner were 1.53% and 1.25%, respectively, under optimal conditions. Therefore, to improve the cleaning performance of the cleaner, it is necessary to theoretically analyze the phenomenon of airflow backflow in the cleaner, which can provide a reference for the follow-up design and optimization of the structure of the cleaner. Keywords: Airstream, Analysis, Cleaning, Particle image velocimetry, Test, Velocity field.

MHD stratified nanofluid flow by slandering surface

Current attempts focus on doubling the stratified effects of the magnetohydrodynamic flow of second grade nanoliquid over an impermeable surface stretching in a non-linear manner. The mathematical formulation is made using the boundary layer concept and assumptions of low magnetic Reynolds number. The non-linear stretching surface with variable thickness generates the flow. Incompressible non-Newtonian liquid is electrically conducted subject to a varying magnetic field. Heat transfer characteristics with heat generation/absorption, Joule heating and variable thermal conductivity effects are addressed. Novel features regarding Brownian motion and thermophoresis are also included. Mathematical representation of the considered aspects results in non-linear ODEs. The non-linear system is then computed and the derived solutions are adequate for convergence. The physical interpretation of quantities of interest are shown by graphical illustrations. The skin friction coefficient is also computed and inspected. Our computed results report that the influence of velocity ratio and Hartmann number result in similar velocity field characteristics. Furthermore, these aspects of nanoparticles, variable thermal conductivity and Hartmann number enhance the temperature distribution.

The effect of different structural parameters on wind supercharged solar chimney power plant combined with seawater desalination

Based on the mathematical model of the wind supercharged solar chimney power plant combined with seawater desalination (WSSCPPCSD), the velocity field characteristics of basic WSSCPPCSD were analyzed through three-dimensional numerical simulation. To investigate the influence of different structural parameters of system performance, systems with turbines of different blade numbers, installation angles and relative radial clearances, with guide cone or guide vane were also simulated. Unsteady numerical simulations of H-type wind pressure ventilator with different wind wheel blade installation angles were conducted, as well as another helix-type wind pressure ventilator. The results show that the increase of turbine blade number could lead to a reduced freshwater productivity, and the rotational speed corresponding to maximum turbine shaft power shifts in the direction of lower speed. Besides, with the increase of blade installation angle, both the freshwater productivity and the turbine shaft power increase first and then decrease. As the radial clearance of turbine blades increases, the system freshwater productivity increases and the noise decreases, and the turbine shaft power has a maximum value when the relative radial clearance is 2%. The addition of guide cone and guide vane can effectively improve the flow field. As for the wind pressure ventilator, system freshwater productivity and shaft power both increases first and then drops with the increase of wind wheel blade installation angle. In addition, the helix-type wind wheel can improve the self-starting performance of the wind pressure ventilator and decrease the fluctuation of blade torque at the same time, so as to improve the stability.

Passive Control and Enhancement of Low Reynolds Number Slot Jets Through the Use of Tabs and Chevrons

Liquid microjets are emerging as candidate primary or secondary heat exchangers for the thermal management of next generation photonic integrated circuits (PICs). However, the thermal and hydrodynamic behavior of confined, low Reynolds number liquid slot jets is not yet comprehensively understood. This investigation experimentally examined jet outlet modifications—in the form of tabs and chevrons—as techniques for passive control and enhancement of single-phase convective heat transfer. The investigation was carried out for slot jets in the laminar flow regime, with a Reynolds number range, based on the slot jet hydraulic diameter, of 100–500. A slot jet with an aspect ratio of 4 and a fixed confinement height to hydraulic diameter ratio (H/Dh) of 1 was considered. The local surface heat transfer and velocity field characteristics were measured using infrared (IR) thermography and particle image velocimetry (PIV) techniques. It was found that increases in area-averaged Nusselt number of up to 29% compared to the baseline case could be achieved without incurring additional hydrodynamic losses. It was also determined that the location and magnitude of Nusselt number and velocity peaks within the slot jet stagnation region could be passively controlled and enhanced through the application of outlet tabs of varying geometries and locations.

2D PIV measurement on the interaction zone between two parallel planar jets in a crossflow

This paper reports an experimental investigation on the zone between two parallel planar jets in a crossflow, termed as the interaction zone. The experiment was conducted using 2D PIV measurement, which involves a test matrix designed for a parametric study. Based on both mean and instantaneous results, effects of different parameters, i.e. front jet velocity, space between the two jets, and crossflow flow velocity, on the interaction zone were analysed. The two coupled recirculation structures, size, velocity field, and turbulence characteristics of the interaction zone are discussed. In addition, the large-scale vortices inside the interaction zone were recognised using Γ1 value. How these different parameters affect the evolution of these vortices was also studied.

Ab initio velocity-field curves in monoclinic β-Ga2O3

We investigate the high-field transport in monoclinic β-Ga2O3 using a combination of ab initio calculations and full band Monte Carlo (FBMC) simulation. Scattering rate calculation and the final state selection in the FBMC simulation use complete wave-vector (both electron and phonon) and crystal direction dependent electron phonon interaction (EPI) elements. We propose and implement a semi-coarse version of the Wannier-Fourier interpolation method [Giustino et al., Phys. Rev. B 76, 165108 (2007)] for short-range non-polar optical phonon (EPI) elements in order to ease the computational requirement in FBMC simulation. During the interpolation of the EPI, the inverse Fourier sum over the real-space electronic grids is done on a coarse mesh while the unitary rotations are done on a fine mesh. This paper reports the high field transport in monoclinic β-Ga2O3 with deep insight into the contribution of electron-phonon interactions and velocity-field characteristics for electric fields ranging up to 450 kV/cm in different crystal directions. A peak velocity of 2 × 107 cm/s is estimated at an electric field of 200 kV/cm.

Cross‐shore transport of nearshore sediment by river plume frontal pumping

AbstractWe present a new mechanism for cross‐shore transport of fine sediment from the nearshore to the inner shelf resulting from the onshore propagation of river plume fronts. Onshore frontal propagation is observed in moorings and radar images, which show that fronts penetrate onshore through the nearshore and surf zone, almost to the waterline. During frontal passage a two‐layer counterrotating velocity field characteristic of tidal straining is immediately set up, generating a net offshore flow beneath the plume. The seaward flow at depth carries with it high suspended sediment concentrations, which appear to have been generated by wave resuspension in the nearshore region. These observations describe a mechanism by which vertical density stratification can drive exchange of material between the nearshore region and the inner shelf. To our knowledge these are the first observations of this frontal pumping mechanism, which is expected to play an important role in sediment transport near river mouths.

A plane turbulent wall jet on a fully rough surface

The mean velocity field and skin friction characteristics of a plane turbulent wall jet on a smooth and a fully rough surface were studied using Particle Image Velocimetry. The Reynolds number based on the slot height and the exit velocity of the jet was Re= 13,400 and the nominal size of the roughness was k = 0.44mm. For this Reynolds number and size of roughness element, the flow was in the fully rough regime. The surface roughness results in a distinct change in the shape of the mean velocity profile when scaled in outer coordinates, i.e. using the maximum velocity and outer half-width as the relevant velocity and length scales, respectively. Using inner coordinates, the mean velocity in the lower region of the inner layer was consistent with a logarithmic profile which characterizes the overlap region of a turbulent boundary layer; for the rough wall case, the velocity profile was shifted downward due to the enhanced wall shear stress. For the fully rough flow, the decay rate of the maximum velocity of the wall jet is increased, and the skin friction coefficient is much larger than for the smooth wall case. The inner layer is also thicker for the rough wall case. The effects of surface roughness were observed to penetrate into the outer layer and slightly enhance the spread rate for the outer half-width, which was not observed in most other studies of transitionally rough wall jet flows.

The electron transport that occurs within wurtzite zinc oxide and the application of stress

ABSTRACTWe examine how stress has the potential to shape the character of the electron transport that occurs within ZnO. In order to narrow the scope of this analysis, we focus on a determination of the velocity-field characteristics associated with bulk wurtzite ZnO. Monte Carlo simulations of the electron transport are pursued for the purposes of this analysis. Rather than focusing on the impact of stress in of itself, instead we focus on the changes that occur to the energy gap through the application of stress, i.e., energy gap variations provide a proxy for the amount of stress. Our results demonstrate that stress plays a significant role in shaping the form of the velocity-field characteristics associated with ZnO. This dependence could potentially be exploited for device application purposes.

Stochastic modeling of firebrand shower scenarios

Firebrand shower and its subsequent spot fires are responsible for more than half of the ignitions reported during wildfires, in particular at wildland urban interface (WUI) areas. The firebrand transport is a highly stochastic and nonlinear problem which directly influences the spotting distribution. Hence, a coupled stochastic model of firebrand showers, that is thoroughly and systematically validated against large scale wind tunnel experiments of lofting and downwind transport of model firebrands, is presented. It is shown that the developed model predicts the first and second order statistics of the flight accurately in relation to the experimental data. The sensitivity of the model to the initial conditions of the flight as well as the velocity field characteristics are examined.