Velocity-field Characteristics Research Articles

Numerical investigation on smoke control with new‐style horizontal smoke baffle in an immersed road tunnel

AbstractTo investigate the improvement induced by horizontal smoke baffles during lateral smoke exhaust, an immersed road tunnel with various horizontal smoke baffles positioned below the lateral exhaust vent was studied numerically. Together with the velocity field characteristics, the temperature distribution was investigated near the lateral smoke exhaust vent, followed by the analysis of lateral smoke exhaust efficiency under different horizontal smoke baffles. Results showed that after installing the horizontal smoke baffle, there was a significant decrease in the extracted cold air, while the high‐temperature smoke in the exhaust vent increases, indicating the plug‐holing is effectively suppressed. It is known that the efficiency of smoke exhaust increases when the length exceedance ratio of the horizontal smoke baffle is smaller than 100%, while it changes slightly when the baffle length continues to increase. When the width ratio of horizontal baffle is smaller than 40%, the efficiency of smoke exhaust increases with the baffle width and then changes slightly with a wider smoke baffle. With a larger aspect ratio, the wider and shorter lateral exhaust vent is beneficial for improving the lateral smoke exhaust. Under the current conditions, the case shows the optimal smoke exhaust performance with a horizontal baffle length exceedance ratio of 100%, a baffle width ratio of 40%, and exhaust vent aspect ratio of 3:1. Finally, an empirical model is developed to describe the improvement of smoke exhaust efficiency caused by horizontal smoke baffle. These outcomes are helpful to the design of lateral smoke extraction system in road tunnels.

Influence of side dilution jets on swirling and non-swirling pulverised biomass turbulent annular flows

The use of pulverised biomass in non-premixed combustion systems, featuring turbulent annular flows (swirling, non-swirling) with downstream air dilution jets, is of immense practical importance. The overall combustion performance and emissions heavily rely on the underlying flow dynamics, turbulence and dispersion behaviour of the pulverised biomass particles. Despite the practical significance of side dilution jets in aiding complete combustion and controlling pollutants, the fundamental understanding of side dilution jets influence on the biomass particle velocity field, turbulence, and dispersion characteristics is still lacking. However, the presence of a multi-phase (solid-gas) flow field, highly turbulent annular flows, swirl, and cross-wise turbulent dilution jets poses significant challenges to these investigations. This work employs a combination of two-dimensional planar Particle Image Velocimetry (PIV) experiments and three-dimensional multi-phase numerical modelling to resolve the influence of turbulent side dilution jets (Red = 18,000 and 27,000) on the particle flow and dispersion characteristics of turbulent annular jets equipped with a central pulverised biomass-laden jet. The numerical modelling uses Reynolds stress model (RSM) and discrete phase model (DPM) for the solution of gas and dispersed phases, respectively. Walnut flour based raw pulverised biomass is injected at a certain particle loading ratio (Φp=0.33 and 0.49) through a turbulent central jet (Rj = 4500) into non-swirling (S = 0) and swirling (S = 0.3) turbulent confined annular flows (Res = 35,500 and 17,800). Comprehensive Constant Temperature Anemometry (CTA) experiments are conducted to obtain well-defined boundary conditions for experiments as well as numerical simulations. Prior to numerical predictions, the models are duly validated against the experimental data.Results reveal that the characteristic flow feature of side dilution jets, i.e., peripheral recirculation zones (PRZ) significantly entrains pulverised biomass particles from the turbulent particle-laden (central jet) annular flows (both swirling and non-swirling) and alters the particle flow and turbulence characteristics. Under non-swirling conditions (S = 0), side dilution jets reduce mean particle axial velocity by 22.4% and increase turbulent fluctuating velocity by 52.6%. These figures elevate to 32.5% and 100% respectively for swirling conditions (S = 0.3), depicting enhanced turbulent mixing between solid particles and the gas phase. In terms of particle dispersion, side dilution jets significantly enhance lateral dispersion of biomass particles, particularly under swirling flow conditions. In the PRZ region, side dilution jets led to a 108% and 279% increase in normalised particle count for non-swirling and swirling cases, respectively. These findings suggest that swirl doubles the particle dispersion in the PRZ. Furthermore, a 50% increase in Red enables side dilution jets to entrain more biomass particles and disperse them into the PRZ. Conversely, a 50% increase in ṁp leads to a significant increase in particle volume fraction along the central axis of the nozzle.

Ionanofluid flow through a triangular grooved microchannel heat sink: Thermal heightening

With recent technological advances, thermal transport from different electronic and electrical devices is the most vital concern. The microchannel heat sink (MCHS) of liquid cooling is a useful device to remove over thermal load. Ionanofluid is a brand new and super potential cooling fluid for its ionic conductivity, non-flammability, negligible volatility, and high-level heat stability. In this research, the ionanofluid's velocity and thermal field characteristics through a triangular grooved MCHS are investigated using numerical tools. The combination of ionic liquid (IL) 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide [C4mim]NTf2 and propylene glycol (PG) is used as base fluid whereas graphene (G) and single-walled carbon nanotube (SWCNT) are chosen as hybrid nanoparticles to make the working ionanofluid. The governing equations of nonlinear partial differential equations describing the physical phenomena along with proper border settings are resolved by applying the finite element method (FEM). Different ratios of hybrid nanoparticles (G: SWCNT) like (1: 0, 1/3: 2/3, 1/2: 1/2, 2/3: 1/3, 0: 1) are suspended in the base fluid mixture. In addition, the base fluid mixture is assumed in different combinations of (IL: PG) as (100: 0, 50: 50, 0: 100). The numerical results are displayed in the forms of streamlines, isothermal lines, and rate of thermal transfer for the pertinent parameters namely forced convection (Re = 100–900) and solid concentration (φ = 0.001–0.05). Also, pressure drop, field synergy number, relative fanning friction feature, relative Nusselt number, and temperature enhancement efficiency are calculated. The results indicate that a higher heat transport rate is found using the IL-based ionanofluid with the highest solid concentration. Moreover, the higher forced convection enhances the thermal efficiency of MCHS. Two linear regression equations along with very good correlation coefficients have been derived from the numerical results.

Experimental study on rockburst of surrounding rock in an elliptical tunnel with different axial ratios

The effect of the ratio of long and short axes on the rockburst of an elliptical tunnel structure was investigated by performing a series of true triaxial tests on red sandstone cuboid samples containing seven types of elliptical holes with different long–short axial ratios, using a self-developed true triaxial compression equipment. The entire process of spalling was monitored and recorded using a miniature camera. According to the test results, the failure stress, geometrical characteristics of the rockburst notch, fragment characteristics, and ejection velocity field characteristics of the rockburst are summarized, and the influence of the axial ratios of the elliptical hole on the failure characteristics and stability of the tunnel are discussed in detail. The test results indicate that the rockburst process of elliptical tunnels with different axial ratios experiences four processes; namely, calm period, small particle ejection period, spalling, and rockburst. The shape of the failure area after the rockburst presents a V-shaped feature. With the increase of the axial ratio, the area, angle, and depth of the V-shaped area show a decreasing trend, the initial failure stress of the cavern and the rockburst failure stress exhibit a trend of first decreasing and then increasing; the tangential stress of the cavern shows an increasing trend; the mass percentage of micro, fine, and medium grain decreases; and the average ejection velocity first decreases and then increases. Under the same stress conditions, rockbursts are more likely to occur on circular sections. Under the same section conditions, the ellipse can bear larger loads when its major axis is parallel to the disturbance stress. These conclusions are helpful to the efficient design of the tunnel structure.

Assessing the effectiveness of an empirical model for a compound semiconductor’s velocity-field characteristic

Empirical models for the velocity-field characteristic of a compound semiconductor, comprised of analytical expressions with nested adjustable parameters, are in common use within the electronic materials community. We focus on establishing means of quantitatively assessing the quality of a fit of such an empirical model with respect to a reference velocity-field characteristic, acquired either through experiment or simulation. We also develop a means of optimizing the adjustable parameter selection process, drawing upon the method of steepest descent. We apply this approach to the specific case of gallium arsenide, Monte Carlo simulations and experimental results providing us with the reference velocity-field characteristics needed for this analysis. The robustness of this approach is examined and commented upon.

The wideband tunability of double Gunn diodes

Gunn diodes use the Ridley–Watkins–Hilsum effect to generate low noise millimeter wave signals for communication and sensing applications. However, Monte Carlo calculations indicate that the velocity–field characteristics of compensated GaAs contain a second peak due to phonon scattering. A novel device that leverages this double Ridley–Watkins–Hilsum effect to achieve ultra-wideband tunability of low-noise millimeter waves is proposed in this article. The second region of negative differential mobility is gradually introduced in seven mobility profiles, which are each simulated in eight device lengths. It is found that a 2.5 μm Gunn diode with a compensation ratio of 0.6 operating at 160 K has a 700% increase in frequency tunability. An analysis of the domain formation indicates that the increase in tunability is due to significant distortions in domain shape. Gunn diodes with transit lengths of 2 μm and smaller show a 100% increase in tunability due to modulations in the effective transit length and absorption rate. These simulations indicate that compensated GaAs could provide linear, ultra-wideband frequency tunability.

The velocity field characteristics of high‐viscosity fluids falling film flow down clamped channels

AbstractThe complexity of falling film flow has been studied in many industrial applications. In this work, the velocity field of high‐viscosity fluids falling film flow down clamped channels was investigated numerically and experimentally. The results show that the numerical simulation results are consistent with the experimental results, and the characteristics of the velocity field are related to the fluid properties, operating conditions, and structure of the clamped channels. When the fluid viscosity is greater than or equal to 10 Pa ⋅ s, the type of velocity field changes into I shape, U shape, and V shape. While the fluid viscosity drops to 0.89 Pa ⋅ s, the viscous force cannot resist the inertial force and gravity, resulting in a cardioid velocity field. By adjusting the structure of the clamped channels and operating conditions, the tension of the liquid film can be changed, and the velocity distribution of the liquid film can be manipulated. Significantly, under the fluctuating curtain flow, the liquid film coalesces and breaks frequently, which enlarges the surface area of the liquid film and strengthens the surface renewal frequency. Hence, this form of falling film flow can be applied to process intensification of high‐viscosity materials.

Velocity-field characteristics of MgxZn1−xO/ZnO heterostructures

In this work, electron transport in MgxZn1−xO/ZnO heterostructures at room temperature is simulated by the ensemble Monte Carlo (EMC) method. Electron scattering mechanisms including acoustic deformation potential, piezoelectric acoustic phonon, polar optical phonon (POP), interface roughness (IFR), dislocation, electron escape (ESC) and capture (CPR) by optical phonons, and random alloy are considered in EMC. The electron drift velocity in MgxZn1−xO/ZnO heterostructures is calculated for various Mg mole fractions x (0.1–0.3) at electric fields up to 25 kV/cm. We find that no obvious velocity saturation occurs in the range of the electric field considered. The results show that ESC scattering is one of the main physical mechanisms limiting the drift velocity. On the other hand, the competition between IFR and intersubband POP scattering is found to play an important role in the change in electron drift velocity with the increasing Mg mole fractions.

Three-dimensional deformation velocity field and kinematic characteristic of the middle and east parts of Haiyuan fault zone from InSAR and GPS observations

Three-dimensional deformation velocity field can reveal fine characteristics of surface deformation, and provide data support for the research on crustal motion mechanism and seismic hazard. In this study, the ascending and descending InSAR deformation velocity fields in the middle and east parts of Haiyuan fault zone are firstly obtained with Sentinel-1A data and SBAS InSAR technology. Then, by combining with GPS observations, the three-dimensional deformation velocity field is extracted and analyzed. Finally, the locking depths and deep slip rates at different locations are inverted by using the classical two-dimensional screw dislocation model, and the maximum shear-strain rate is investigated. The results indicate that there are significant differences in the InSAR deformation velocity field between both sides of the middle and east parts of Haiyuan fault zone. The east–west deformation velocity field clearly reveals the left-lateral strike-slip characteristic of the fault zone. The northward movement gradually changes to southward movement from northwest to southeast. The vertical deformation velocity field displays significant differences on both sides of the section between MMSF and the middle of LHSF, and the middle and eastern of HYE fault. For LHSF, the locking depth and deep slip rate are 9.7 km and 4.9 mm/a; for HYW fault, they gradually decrease from west to east, with the locking depths of 6.4 km, 3.6 km and 3.2 km, respectively, and the deep slip rates of 5.5 mm/a, 4.2 mm/a and 3.8 mm/a, respectively; for HYM fault, they are 7 km and 4.7 mm/a; for HYE fault, they are 5.8 km and 3.6 mm/a. There are two obvious extreme regions of maximum shear-strain rate along the fault strike. This study can provide a reference for investigating the mechanism of regional uplift and extension and seismic hazard.

Effects of Turbulence Models on Flow Characteristics of a Vertical Fire Pump

The flow in the vertical long-axis fire pump exhibits complex, three-dimensional, unsteady flow features. In an attempt to understand the effects of turbulence models on the flow mechanism and performance characteristics of the pump, the ANSYS CFX software was used to carry out numerical studies on the vertical fire pump using URANS. The main objective of this study was to investigate the unsteady flow dynamics within the vertical fire pump and the influence of applying different computational turbulence models. The study then sought to conduct a brief analysis of the unsteady pressure pulsation characteristics of the pump. The reliability of the CFD model was validated with an external characteristic test. The transient pressure distribution, velocity field and external characteristics were analyzed. The results were compared to experimental results, where it was revealed that the SST k-ω model showed 1.82% and 0.81% improvements in efficiency and head, respectively, over the k-ε models. In terms of the power performance, however, the standard k-ε is less likely to over-predict the power used by the pump in overload conditions as compared to the other turbulence models. The pressure charts did not show significant reactions to varying turbulence models across all the studied flow rates. However, the velocity streamlines revealed that there were several disruptions in streamwise flow, where both the standard and RNG k-ε models exhibited more recirculation areas than the SST k-ω and standard k-ω models. Overall, for this type of application, SST k-ω was the best-performing turbulence model, while RNG k-ε showed the poorest performance. Nonetheless, the RNG k-ε also has its strengths. This investigation would serve as a theoretical reference for further research and development in fluid machinery.

Structure optimization of the organ-pipe cavitating nozzle and its erosion ability test on hydrate-bearing sediments

Cavitating jet is a promising drilling rate improvement technology in both the marine natural gas hydrate (NGH) fluidization exploitation method and the integrated radial jet drilling and completion method. In present study, we aim to improve the efficiency of jet erosion and extracting NGH. With a computational fluid dynamics (CFD) method, the pressure, velocity and cavitation field characteristics of organ-pipe cavitating jet (OPCJ) are analysed. The divergent angle, throat length, and divergent length of OPCJ nozzle are preferred to obtain stronger jet cavitation erosion effect. Laboratory experiments of gas hydrate-bearing sediments (GHBS) erosion by OPCJ and conical jet (CJ) are conducted to compare and validate the jet erosion performance. The impinging models of OPCJ and CJ are constructed to study the impact characteristics. Results show that the preferred values of divergent angle, throat length, and divergent length are 15°, 1d, and 3d, respectively, in present simulation conditions. For GHBS, the OPCJ possesses the advantages of high efficiency and low energy consumption. Moreover, the OPCJ has higher penetration efficiency, while showing equivalent penetration ability compared to CJ. During the impinging process, the OPCJ can induce stronger impact pressure and turbulence effect, and also shows stronger chambering effect and bottom cleaning ability compared to CJ. This study presents the erosion performance of OPCJ and CJ on GHBS, and provides preliminary insights on the potential field applications in NGH exploitation.

Secondary Currents at Very Low and High Aspect Ratios of Open Channels

The present numerical study is an attempt to better understand the effect of the aspect ratio (AR) on the velocity field characteristics of the fully-developed turbulent flow in a straight open channel by contrasting a very low and very high aspect ratio cases (AR = 1 and 12). The AR is defined as the ratio of the width of the channel in a plane normal to the flow direction, to the flow depth. The bulk velocity and water depth considered in this study are 0.75 m/s and 30 mm, respectively, which yield a Reynolds number of ReH = 22,500. The transient three-dimensional Navier-Stokes equations were numerically solved using a finite-volume approach with improved-delayed detached-eddy simulation (IDDES) turbulence model. The results revealed that the normalized components of the normal stresses by the turbulent kinetic energy k are constant over most of the velocity field, although k varies in magnitude throughout the velocity field in both AR cases. In addition, the results also revealed a strong anisotropic flow which justifies the formation of secondary currents as a means for transporting the kinetic energy. The results also reveal that both vertical and transverse components of the normal stresses are correlated to the side-recirculation zone (SRZ) and bottom-recirculation zone (BRZ), respectively, which highlights the role of the mean recirculation zones (BRZ & SRZ) in the re-distribution of the turbulent kinetic energy k in the velocity field.

Stability criteria and convective mass transfer from the falling spherical drops, part II: Herschel–Bulkley fluids

AbstractThe combined effects of yield stress, shear‐thinning, and shear‐thickening fluid behaviour are investigated when a drop is falling in a Herschel–Bulkley fluid. The constitutive relation for Herschel–Bulkley fluids is regularized using the Papanastasiou regularization method. The governing partial differential equations for mass, momentum, and species transport are solved spanning a wide range of dimensionless numbers as Reynolds number, ; Schmidt number (10); Bingham number, ; viscosity ratio (0.1 and 10); and power‐law index, . The velocity field and mass transfer characteristics are expressed using streamlines, velocity contours, concentration contours, and sheared and un‐sheared regions, while the surface averaged gross engineering quantities are described as a drag coefficient, yield‐stress parameter, and Sherwood number. All else being equal, sheared regions in shear‐thinning fluids are observed to be larger with respect to the shear‐thickening fluids at finite Reynolds numbers. In the fully plastic flow limit, the yield stress effects dominate in the flow field, and therefore, the critical yield‐stress parameter is observed to be independent of shear‐thinning and shear‐thickening fluid behaviours. However, in the viscoplastic limit (finite Bingham number), shear‐thinning fluid always requires a larger value of yield stress to be static in the fluid with reference to shear‐thickening fluids. The new set of dimensionless parameters are defined based on the effective viscosity scales, and the predictive correlations are put forward for both drag coefficient and Sherwood number.

Numerical study on the force distribution on cylindrical structure by internal solitary wave and its prediction with Morison equation

The forces by internal solitary wave on cylinder structure are computed to understand their temporal distribution on different cylinder sections. The results confirm that the total forces could be decomposed into inertial and drag components approximately. Three empirical coefficients determination methods which are referred to as global coefficients, local coefficients, and coefficients based on regression models, are tested to analyze the dependence of the inertial and drag coefficients on the velocity field characteristics. The results show that the layer dependent empirical coefficients could improve the performances of Morison equation in the upper and lower layers, while its advantage in layers close to density interface is not obvious, which is related to the large vertical velocity components in this region. The regression models are derived by fitting selected coefficients samples, and the empirical coefficients predicted are very close to the results by Sarpkaya (1976) for oscillating flows in the same Reynolds number range.

Measurement of velocity in swirling flows of liquid metals in the presence of a magnetic field

Studies of the dynamics of a round submerged jet of liquid metal in a transverse magnetic field. The experiments were performed in the range of Reynolds numbers up to 1000 and Hartmann numbers up to 1300 using a probe technique for measuring velocity using potential sensors. Detailed three-dimensional flow structures are visualized using a direct numerical simulation method independent of the experiment. The paper presents data on the averaged velocity field and statistical characteristics of the flow, reproducing the complex nature of a substantially unsteady flow, in which zones of reverse fluid flows are detected and a significant heterogeneity of the velocity field is observed.

CFD-DEM study of fluid flow and particle behaviors in a baffled deduster channel

One problem in the industrial dedusting process is the removal of fine particles, such as PM2.5. The gas flow features and subsequent particle behavior in a designed baffled deduster channel were numerically investigated in this study. By means of the coupling of computational fluid dynamics (CFD) and discrete element method (DEM), the key fluid flow patterns, vortex characteristics, and particle behaviors around the baffles were successfully captured. Effects of the baffle inclined angle on the velocity field, pressure drop, and vortex characteristics were analyzed. The results showed that the “dead zone” with low flow velocity was significantly improved with the baffle inclined angle approaching 90°. In addition, small-size particles were found to tend to enter the vortex region while large-size particles were more likely to collide with the walls or baffles. The outcome would provide references and inspirations for the development of novel dedusting technologies.

Numerical Simulation of Oil-Water Flow Velocity Field and Flow Pattern in Horizontal Wells and Near Horizontal Wells

Oil-water flow widely exists in oilfield development. Due to the gravity differentiation, the oil-water flow in low-flow horizontal wells has a clear characteristics of stratified flow. With the increase of flow rate, the stratified characteristics are not obvious, which leads to the difficulty of multiphase flow phase separation flow interpretation in oilfield. In this paper, the oil-water flow in horizontal wells is taken as the research object. The VOF model of Fluent software is used to study the relationship between velocity field and flow pattern distribution characteristics with water cut, well deviation angle and total flow. The results show that with the increase of water cut, the oil-water separation level gradually moves up, and the velocity of water phase is greater than that of oil phase. When the well deviation angle changes slightly, the flow stratification of oil-water changes sharply, and the flow velocity in the pipeline also changes. When the total flow rate is lower than 200 m3/d, the oil-water phases have obvious stratified flow characteristics. With the increase of flow rate, the oil-water interface fluctuates. The average velocity of oil and water is not much different.

Effect of the aspect ratio on the velocity field of a straight open-channel flow

This study investigates numerically the effect of the aspect ratio (AR) on the velocity field characteristics of the turbulent flow of a straight open-channel flow. Five aspect ratio cases (AR = channel width/flow depth) are investigated ranging from a narrow case of AR = 1 to a wide case of AR = 9. The transient three-dimensional Navier–Stokes equations were numerically solved using a finite-volume approach with an improved–delayed detached-eddy simulation turbulence model. The objective of this study is to enhance our understanding of the effect of AR on the formation of secondary currents in a channel flow. The results revealed the formation of a pair of counter-rotating recirculation zones near the bottom corners of the channel, whose axes are aligned with the main flow direction. Each pair consists of a mean recirculation zone that resides near the bed (bed recirculation zone), and another one that resides near the sidewall (side recirculation zone). The AR appears to significantly influence the size and strength of the recirculation zones. With decreasing the AR, the momentum transport by the streamwise-vertical Reynolds shear stress in the horizontal direction appears to be inhibited at higher vertical locations, the momentum transport by streamwise-transverse Reynolds shear stress is considerably enhanced, and the momentum transport by vertical-transverse Reynolds shear stress is slightly enhanced.

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.