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Three‐dimensional hydrodynamic numerical simulation and laboratory fish‐passage experiment with juvenile grass carp (Ctenopharyngodon idella) were used to systematically estimate hydrodynamic parameters, fish passage efficiency, fish swimming behavior, and fish swimming trajectories in a vertical‐slot fishway (VSF) under five different flow discharges. The spatial‐mean time‐averaged velocity magnitude along the slot section determined passage success rate. The absolute value of the time‐averaged Reynolds‐shear‐stress component in the X–Y direction played a decisive role in selecting the swimming path. In addition, fish energy expenditure along the mean‐50% swimming route indicated that higher mainstream velocity did not necessarily signify larger energy consumption, because successfully‐migrating fish selected the relatively lower velocity zone during the migration process. Our findings for fish swimming behavior in response to velocity and turbulence can be used to develop high‐efficiency fishways for grass carps or other species with similar traits. [ABSTRACT FROM AUTHOR]

The flow characteristics and pressure distribution of air flow that is turbulent though a channel of rectangular cross section that fitted with three different configuration arrangements of solid vertical baffle plates are the main topics of this study's numerical analysis. The method of finite volume is utilized to solve two-dimensional differential equations in the analysis. The turbulence dynamics are elucidated by the k-e turbulence model, which is applied in Fluent software. This simulation employed the Finite Volume Method with the SIMPLE algorithm. To thoroughly assess the system, boundary conditions were changed, including the entrance velocity field. The objective of this research is to study the effects of three different configurations of solid vertical baffles in a rectangular duct at a constant Reynolds number of 52,000 on turbulent kinetic energy, pressure drop, temperature variation, friction coefficient, velocity profile, and thermal enhancement factor. In the current model, there are three different arrangements of the solid vertical baffles. Case (1) where there are only two vertical baffles; one is situated at each wall of the channel with height of 0.08 m. In case (2), each baffle is replaced by two ones. So, there are two baffles at the top and bottom surfaces of the duct each has height of 0.04 m with total height of 0.08 m. By keeping the same approach, each baffle in case (2) is replaced by two ones with a height of 0.02 m with total height of 0.08 m, case (3), where there are four baffles at the upper and lower walls of the channel. The model's validity was confirmed using experimental data showing excellent comparability. The main purpose of the research is to study the effects of these configurations on turbulent kinetic energy, pressure drop, temperature variation, friction coefficient, velocity profile, and the factor of thermal enhancement. The Reynolds number is kept constant at 52,000. According to the current study, installing vertical baffles improves mixing. In case (1), maximal axial velocities are obtained in the channel's center; in case (2), they are found above and below the top and bottom baffles. There are several recirculation zones visible at crucial positions, including behind, above, and below the baffles. Case (2) had a thermal enhancement factor maximum of 1.39. Thus, four vertical solid baffles of 0.04 m in height are utilized to optimize this system, two of which are positioned at the top and bottom walls of the channel. [ABSTRACT FROM AUTHOR]

Excessive flow turbulence poses a threat to the development of drifting fish eggs, leading to mortality or developmental malformations and ultimately depleting early fish resources. Currently, there is a scarcity of quantitative studies investigating the effects of flow turbulence on the entire process of drifting fish egg development, from fertilized egg division to hatching. In this paper, the effects of different flow turbulence conditions (FTCs), including turbulent kinetic energy and shear stress, and action times on different stages of fish egg development were quantitatively explored using a transverse-oscillating-grids turbulence tank. Empirical formulas were established to predict the proportion of normal fish egg development under different FTCs within a selected range. The research findings provide a quantitative basis for protecting early fish resources, mitigating the biological invasion of specific fish, constructing fish-breeding facilities, and ensuring safe transfer and transportation. [ABSTRACT FROM AUTHOR]

Purpose: The mixing of fuel and air plays a pivotal role in enhancing combustion in supersonic regime. Proper mixing stabilizes the flame and prevents blow-off. Blow-off is due to the shorter residence time of fuel and air in the combustor, as the flow is in supersonic regime. The flame is initiated in the local subsonic region created using a flameholder within the supersonic combustor. This study aims to design an effective flameholder which increases the residence time of fuel in the combustor allowing proper combustion preventing blow-off and other instabilities. Design/methodology/approach: The geometry of the strut-based flameholder is altered in the present study to induce a streamwise motion of the fluid downstream of the strut. The streamwise motion of the fluid is initiated by the ramps and grooves of the strut geometry. The numerical simulations were carried out using ANSYS Fluent and are validated against the available experimental and numerical results of cold flow with hydrogen injection using plain strut as the flameholder. In the present study, numerical investigations are performed to analyse the effect on hydrogen injection in strut-based flameholders with ramps and converging grooves using Reynolds-averaged Navier–Stokes equation coupled with Menter's shear stress transport k-ω turbulence model. The analysis is done to determine the effect of geometrical parameters and flow parameter on the flow structures near the base of the strut where thorough mixing takes place. The geometrical parameters under consideration include the ramp length, groove convergence angle, depth of the groove, groove compression angle and the Mach number. Two different strut configurations, namely, symmetric and asymmetric struts were also studied. Findings: Higher turbulence and complex flow structures are visible in asymmetric strut configuration which develops better mixing of hydrogen and air compared to symmetric strut configuration. The variation in the geometric parameters develop changes in the fluid motion downstream of the strut. The fluid passing through the converging grooves gets decelerated thereby reducing the Mach number by 20% near the base of the strut compared to the straight grooved strut. The shorter ramps are found to be more effective, as the pressure variation in lateral direction is carried along the strut walls downstream of the strut increasing the streamwise motion of the fluid. The decrease in the depth of the groove increases the recirculation zone downstream of the strut. Moreover, the increase in the groove compression angle also increases the turbulence near the base of the strut where the fuel is injected. Variation in the injection port location increases the mixing performance of the combustor by 25%. The turbulence of the fuel jet stream is considerably changed by the increase in the injection velocity. However, the change in the flow field properties within the flow domain is marginal. The increase in fuel mass flow rate brings about considerable change in the flow field inducing stronger shock structures. Originality/value: The present study identifies the optimum geometry of the strut-based flameholder with ramps and converging grooves. The reaction flow modelling may be performed on the strut geometry incorporating the design features obtained in the present study. [ABSTRACT FROM AUTHOR]

The push for decarbonization of internal combustion engines (ICEs) has spurred interest in alternative fuels, such as hydrogen and ammonia. To optimize combustion efficiency and reduce emissions, a closer look at the intake system and in-cylinder flows is crucial, especially when a hard-to-burn fuel, such as ammonia is utilized. In port fuel injection ICEs, airflow within cylinders profoundly affects combustion and emissions by influencing the air–fuel mixing phenomenon. Adjusting intake port openings is an important factor in controlling the in-cylinder airflow. In previous experiments with a transparent cylinder, tangential and helical ports demonstrated that varying the helical port's opening significantly impacts flow velocities, swirl ratios, and swirl center positions (SCPs). In this study, we used a particle image velocimetry technique to investigate how the tangential port's opening affects intake and in-cylinder flows. Flow velocities were assessed at different planes near the cylinder head, evaluating streamline maps, turbulent kinetic energy (TKE), and SCPs. Under the given experimental conditions, swirl flows were successfully generated early in the compression stroke when the tangential port opening exceeded 25%. Our findings emphasize the importance of minimizing TKE and SCP variation for successful swirl flow generation in engine cylinders equipped with both tangential and helical ports. [ABSTRACT FROM AUTHOR]

This study presents a numerical analysis of turbulent air flow through a rectangular duct fitted with two perforated vertical baffle plates, focusing on flow characteristics and pressure distribution. The analysis employs two-dimensional differential equations, solved using the finite volume method. The k-e turbulence model, implemented in Fluent software, elucidates the turbulence dynamics. Boundary conditions, including the entrance velocity field and exit atmospheric pressure, were varied to comprehensively evaluate the system. The model's validity was established against experimental data at a Reynolds (Re) number of 87300, demonstrating excellent comparability at a distance of 0.525 m from the duct entrance. The research aims to investigate the effects of different Re numbers (10,000, 20,000, and 30,000) on various parameters in a duct equipped with two perforated, nozzleshaped vertical baffles, each comprising eight trapezoidal sections forming seven nozzles. Parameters analyzed include the velocity profile, pressure drop, turbulent kinetic energy, dynamic pressure coefficient, and skin friction coefficient. The study finds that the insertion of perforated vertical baffles enhances mixing, particularly at higher Re numbers. Axial velocities attain maximum values above the bottom baffle and below the top baffle, with multiple re-circulation zones observed at strategic locations including above, below, and behind the baffles. Significant findings include the occurrence of maximum vertical velocity values at Re=30000 upstream of the bottom baffle, and a substantial re-circulation zone immediately upstream of the upper baffle. The channel's thermal enhancement factor is noted to improve with decreasing levels of skin friction as the Re number increases. Pressure values rise at the junctions where the duct's inner walls meet the upper surfaces of the baffles. Turbulent kinetic energy across the duct is categorized into three distinct zones based on the Re number: Zone 1, between the entry and the first baffle, exhibits minimal turbulence influence; Zone 2, located between the baffles, shows a peak turbulent kinetic energy of about 3 m²/s² near the nozzle exits and the bottom of the top baffle; Zone 3, extending from the second baffle to the outlet, records the highest turbulent kinetic energy rate at a Re number of 30,000. [ABSTRACT FROM AUTHOR]

Abstract A semi-circular channel with connecting pipes is proposed to improve the overall heat transfer performance of the Printed Circuit Heat Exchanger (P CHE) used in the regenerator of the SCO2 Brayton cycle. Two semi-circular channels at the top and bottom, which are horizontally placed, were filled with hot and cold CO2, respectively. Thermophysical properties of SCO2 were solved by integrating NIST into the FLUENT 2020. A mesh number of 5.0M and the SST k-ω turbulence model were selected via comparison with experimental data. As results show, the overall heat transfer performance of the semi-circular channel with connecting pipes is improved by 1.1-2.5 times. The vortex structure induced by the connecting pipes increases turbulent kinetic energy and further enhances the heat transfer. This indicates that the connecting pipe proposed in this study is beneficial to activate heat transfer and reduce flow resistance. [ABSTRACT FROM AUTHOR]

Simulation of two-phase flows is relevant for reactor design and safety at normal operation or during accident scenarios. Often, the two-phase flow is in a regime in which slugs are formed or where the flow stratifies. Modeling such situations using standard single-phase Reynolds-averaged Navier-Stokes (RANS) turbulence models fails due to an overestimation of the eddy viscosity at the resolved two-phase interface. To solve this, an ad hoc turbulence damping term has been proposed in the literature that reduces the turbulence production locally at a two-phase interface, analogously to turbulence wall functions. However, this approach must be tailored to the specific setting and does not consider physical contributions such as surface tension or flow topology. Therefore, the problem of two-phase interfacial turbulence must be studied more in-depth. In this work, we consider co-current turbulent Taylor bubble flow using high-fidelity numerical simulation. The Basilisk code is used to simulate a Taylor bubble rising in a vertical pipe. By simulating the bubble in a moving frame of reference, we may study the turbulent kinetic energy (TKE) budgets ahead of the bubble, in its wake, and across the interface. The implementation of the TKE budget computation and the underlying averaging techniques are validated for the single-phase region ahead of the Taylor bubble using reference direct numerical simulation data. The analysis of the TKE budgets in the setting of Taylor bubble flow allows for the study of how turbulence behaves due to the presence of a two-phase interface and, in turn, supports the improvement of two-phase RANS models. [ABSTRACT FROM AUTHOR]

Turbulent flow features around three different objects of cylindrical shapes were investigated under identical flow conditions, using particle image velocimetry (PIV). Experiments were carried out for three in-line circular cylindrical objects (TCC), an oblong cylindrical object (OC), and a rectangular cylindrical object (RC) placed individually in the laboratory flume. The variations of turbulent parameters such as turbulent intensities, turbulent kinetic energy and Reynolds shear stress are presented on the symmetric plane along the object. Further, the contributions of burst–sweep cycles to the Reynolds shear stress and vorticity in terms of Q - criterion are also analyzed. Vorticity and turbulent intensity were found to be significantly higher within the gaps of TCC than observed in other objects. When compared with the sharp edges of RC, it is observed that the smooth edges of OC reduced turbulent intensities and Reynolds shear stress by almost half. The ejection and sweep events for RC are much more dominant than those of the other two objects along the longitudinal plane. [ABSTRACT FROM AUTHOR]

A scaled-model of a horizontal axis tidal current turbine (HATCT) is tested in the CNR-INM Circulating Water Channel. The experiments are designed to establish in the first place the performances of the turbine at different working settings. The second goal is to investigate the hydrodynamics generated by the turbine in the near wake using the Particle Image Velocimetry (PIV) technique. For this purpose, velocity measurements are performed in a longitudinal plane and phase-locked to the rotor angle in order to resolve the wake structure at different working regimes. The analysis of the axial and radial velocity fields reveals the flow features of the slipstream, as well as its expansion and dependence on the turbine operating parameters. Analysis of the non-diagonal terms of the Reynolds stress tensor provides insight into the onset of tip vortex pairing and of vortex instabilities. Furthermore the separate contributions of transport, production and dissipation to the turbulent kinetic energy in the wake field are discussed in detail. The vortex unsteadiness is captured and correlated with the evolution of the kinetic energy transport and production terms. Understanding these phenomenologies is an important step to develop computational tools able to estimate the radiated noise or the potential impact of turbulence on performances of further rotors placed in the wake, as in an array of tidal turbines. • Topology of rotor wake is revealed via in-phase Particle Image Velocimetry (PIV). • Slipstream expansion is largest at maximum power coefficient. • Production and transport of turbulent kinetic energy are dominant in tip vortices. • Dissipation and viscous effects are negligible in the near wake. • Vorticity skewness is a probe into the spatial fluctuations of the tip vortex. [ABSTRACT FROM AUTHOR]

This paper investigates the influences of the distribution of the grooves on the wall on the turbulent statistics, transport of turbulent kinetic energy, and flow structures in supersonic turbulent channel flows at the bulk Mach number of 3.0 by performing direct numerical simulations. It is found that the existence of the grooves leads to the enhancement of the turbulent kinetic energy close to the wall and the abatement thereof above the buffer layer. The density and temperature fluctuations are also enhanced, but only within the buffer layer, above which the influences of the grooves can be disregarded. The pressure fluctuations, however, are significantly increased, which is attributed to the radiated acoustic waves from the wall generated by the disturbances on the wall. Such inference is substantiated by the fact that the inclination angles of the phase averaged pressure are related to the Mach number. Nevertheless, the acoustic and dynamic processes seem to be decoupled, leading to insignificant pressure-dilatation terms in the transport of turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

Parameterizations for bottom shear stress are required to predict sediment resuspension from field observations and within numerical models that do not resolve flow within the viscous sublayer. This study assessed three observation‐based bottom shear stress (τb) parameterizations, including (a) the sum of surface wave stress and mean current (quadratic) stress (τb=τw+τc ${\tau }_{b}={{\tau }_{w}+\tau }_{c}$); (b) the log‐law (τb = τL); and (c) the turbulent kinetic energy (τb = τTKE); using 2 years of observations from a large shallow lake. For this system, the parameterization τb = τw + τc was sufficient to qualitatively predict resuspension, since bottom currents and surface wave orbitals were the two major processes found to resuspend bottom sediments. However, the τL and τTKE parameterizations also captured the development of a nepheloid layer within the hypolimnion associated with high‐frequency internal waves. Reynolds‐averaged Navier‐Stokes (RANS) equation models parameterize τb as the summation of modeled current‐induced bottom stress (τc,m) and modeled surface wave‐induced bottom stress (τw,m). The performance of different parameterizations for τw,m and τc,m in RANS models was assessed against the observations. The optimal parameterizations yielded root‐mean‐square errors of 0.031 and 0.025 Pa, respectively, when τw,m, and τc,m were set using a constant canonical drag coefficient. A RANS‐based τL parameterization was developed; however, the grid‐averaged modeled dissipation did not always match local observations, leading to O(10) errors in prediction of bottom stress. Turbulence‐based parameterizations should be further developed for application to flows with mean shear‐free boundary turbulence. Plain Language Summary: Bottom shear stress is the link between hydrodynamic motions and sediment resuspension, further relating to water quality in the lake. However, it is impractical to directly measure the bottom shear stress in the field. We assessed three observation‐based bottom shear stress parameterizations, using 2 years of observations from a large shallow lake, and found that the parameterization consisting of surface wave‐induced stress and bottom current‐induced stress is sufficient to capture major sediment resuspension events. In the numerical models, which averaged the turbulence dissipation, the parameterization based on modeled surface wave‐induced stress and bottom current‐induced stress was also assessed and compared against the values from observation‐based parameterizations. The usage of a constant, observed or literature‐based parameter in the model parameterization is recommended, and it should be calibrated to account for inaccuracies in modeled hydrodynamic variables (i.e., surface waves and bottom currents). Key Points: Three observation‐based bottom shear stress parameterizations were assessed in a large shallow lakeThe parameterizations of bottom shear stress in Reynolds‐averaged Navier‐Stokes equation models was assessed against the observations [ABSTRACT FROM AUTHOR]

An electric power system is an important factor in national economic development. However, as an electric power system requires more electric equipment in its operation process, it is prone to short circuits, faults and other problems, which can lead to fires. To help prevent fires in such power systems, the hydraulic performance of the existing firefighting monitor should be optimized. A rectifier is an important structure which affects the performance of the firefighting monitor. In this paper, numerical simulations based on CFD (computational fluid dynamics) are carried out to analyze the fluid flow inside firefighting monitors with five different rectifier structures. In addition, the effects of rectifier structure on both the turbulent kinetic energy and axial velocity of the fluid inside the firefighting monitor are analyzed. The results show that rectifier installation can reduce the turbulent energy of the inlet and outlet of the firefighting monitor and improve the axial velocity distribution inside the firefighting monitor. Specifically, a forked row rectifier arrangement can significantly improve the effect of flow stabilization. However, there are limits to improving rectifier stabilization performance by changing the number of blades, as too many blades can cause reverse direction flow and large pressure losses. [ABSTRACT FROM AUTHOR]

The lance is a critical component of the bottom-blowing pool melting process, and its placement has an important impact on the pool's gas-liquid two-phase flow. In this study, a mathematical model of the bottom-blowing process is established, and the flow pattern, trajectory, wake vortex, and velocity of bubbles under four lance spacings are simulated. Results show that there are three basic bubble flow patterns appear in the flow field: bubbles coalesce before leaving the nozzle (Pattern II), bubbles coalesce after leaving the nozzle (Pattern I), and no coalescence during the rise of bubbles (Pattern III). The bubble pattern varies from Pattern I to Pattern III with the increase in lance spacing. The intensity of the influence of the wake vortex on the bubbles decreases. The Q (the Q is the second Galilean invariant of the velocity gradient tensor ∇v) value of the wake vortex is small, but the vortex structural distribution is complex. Moreover, there is a large velocity difference between gas and liquid at the beginning of gas injection, but the velocity difference between them decreases after gas injection, so the average turbulent kinetic energy in the pool initially increases sharply, and then approaches dynamic equilibrium. The top and bottom velocities of the bubbles are consistent, and the velocity fluctuation is orderly. Moreover, the greater the lance spacing is, the greater the disturbance in the pool is. The mixing effect of D = 0.2 m is the best among the four spacings. [ABSTRACT FROM AUTHOR]

About 20% of the atmospheric pollution variations are linked to local meteorology. This relationship may be more important in places with dense populations, such as Latin American cities since the topography and fast multiscale changes are part of the tropical climate. Even so, this possibility has not been addressed in previous studies. This research aims to characterize the relationship between tropical meteorological variables and PM2.5 levels during high pollution events. The relationship between wind field, Turbulent Kinetic Energy (TKE), radiation, temperature, relative humidity, boundary layer height, and atmospheric stability with PM2.5 concentration was investigated. Statistical correlations and the parcel method were used to analyze the relationship between vertical motions and PM2.5. Obtained results show that the stability and vertical velocity do not significantly affect pollution levels. We identified some signals that are strongly related to PM2.5 high concentrations: weaker than average horizontal wind speed and TKE throughout the day, easterly winds in the morning (associated with the transport of ashes from wildfires produced on the Eastern plains), combined with a higher-than-average radiation peak. These results lead to a better understanding of the PM2.5 variations, which can be applied for the improvement of air quality models and have the potential to be part of a novel policy to manage air quality risk. [ABSTRACT FROM AUTHOR]

A simple criterion for evaluating the grid resolution needed to obtain a grid-independent solution of turbulent flow problems in the framework of statistical approach to turbulence simulation is proposed. The criterion is derived on the basis of an a posteriori estimate of the local interpolation error of the field of turbulent kinetic energy. A good resolution should ensure a small local interpolation error of the discrete turbulent kinetic energy. The equation for the transport of turbulent kinetic energy and realizability conditions for the turbulent stress tensor made it possible to reduce the estimation of relative interpolation error to an explicit formula for the estimate of the maximal grid step required for obtaining a grid-independent solution. The proposed criterion is applied to a steady problem for a flow past a backward facing step and to the problem of unsteady flow around a half-circular profile arranged at a zero angle of attack for the Reynolds number Re = 45 000. A numerical study showed that the proposed criterion gives a good estimate of the grid resolution required for obtaining a grid-independent solution away from a wall. This criterion can be used both for estimating the grid independence of the solution and for adapting the computation grid. [ABSTRACT FROM AUTHOR]

By considering turbulence observations in the atmospheric stable surface layer over complex terrain, we study the effect of submeso motions on the budgets of the mean turbulent kinetic energy (TKE) and (half) the temperature variance. Different averaging times are considered (i.e., 100 s and 30 min), to filter out or retain the submeso contributions to second-order moments. Furthermore, results are interpreted by introducing four parameters that express the relative submeso contribution to the TKE, the temperature variance, and the vertical fluxes of heat and momentum. Four regimes are identified according to these four submeso parameters and the budgets are evaluated for these regimes. A balance among production, buoyancy (for the TKE) and dissipation occurs for the two regimes characterized by small submeso contribution to the fluxes, while an unbalance occurs for the other two regimes, where the submeso contribution to the fluxes is large. Instead, the budgets are independent of the magnitude of the submeso contribution to the TKE and the temperature variance. [ABSTRACT FROM AUTHOR]

Due to its simple structure and lack of moving parts, the supersonic air ejector has been widely applied in the fields of machinery, aerospace, and energy-saving. The performance of the ejector is influenced by the flow channel structure and the velocity of the jet, thus the confined jet is an important limiting factor for the performance of the supersonic air ejector. In order to investigate the effect of the confined jet on the performance of the ejector, an air ejector with a rectangular section was designed. The effects of the section width (Wc) on the entrainment ratio, velocity distribution, turbulent kinetic energy distribution, Mach number distribution, and vorticity distribution of the rectangular section air ejector were studied numerically. The numerical results indicated that the entrainment ratio of the rectangular section air ejector increased from 0.34 to 0.65 and the increment of the ER was 91.2% when the section width increased from 1 mm to 10 mm. As Wc increased, the region of the turbulent kinetic energy gradually expanded. The energy exchange between the primary fluid and the secondary fluid was mainly in the form of turbulent diffusion in the mixing chamber. In addition to Wc limiting the fluid flow in the rectangular section air ejector, the structure size of the rectangular section air ejector in the XOY plane also had a limiting effect on the internal fluid flow. In the rectangular section air ejector, the streamwise vortices played an important role in the mixing process. The increase of Wc would increase the distribution of the streamwise vortices in the constant-area section. Meanwhile, the distribution of the spanwise vortices would gradually decrease. [ABSTRACT FROM AUTHOR]

Understanding the influence of vegetation on flow and sediment transport is essential to wetland protection and restoration projects. Laboratory experiments were conducted to investigate the sediment pickup (or entrainment) over bare and vegetated beds with model vegetation (i.e., cylinder arrays) of different solid volume fractions, ϕ $\phi $, and diameters, d $d$. For the vegetated beds, measured pickup rate was closely correlated with turbulent kinetic energy, kt ${k}_{t}$, and τ $\tau $‐based model developed for bare channels significantly underestimated the pickup rate measured in vegetated channels. Building on previous theoretical analyses and observations, this study proposed two kt ${k}_{t}$‐based predictions for pickup rate that include the impact of both bed‐ and vegetation‐generated turbulence. The new models improve the prediction of pickup rate within vegetated regions. Plain Language Summary: Aquatic vegetation abounds in rivers, lakes, and coastal regions. It plays an important role in ecosystems, for example, it enhances water quality, stabilizes riverbanks, and promotes soil carbon storage. During the past decades, however, the area of vegetated ecosystems has significantly decreased, leading to a growing interest in wetland restoration. To improve restoration outcomes, it is essential to understand how vegetation impacts flow and sediment motion. This study revealed that traditional models describing sediment erosion in bare channels underestimated by orders of magnitude the erosion rate in vegetated regions. This was attributed to the influence of vegetation‐generated turbulence on sediment erosion. New predictions for sediment erosion rate that incorporate the influence of turbulence were proposed and validated. The new description of erosion can improve the modeling of landscape evolution and marsh restoration. Key Points: Time‐averaged bed‐shear stress models significantly underestimated sediment pickup rate in vegetated channelsNew turbulence‐based models predicted sediment pickup rate in both bare and vegetated channelsPickup rate resulting in either suspended load or bedload can be described by the same law [ABSTRACT FROM AUTHOR]

Spark-ignited natural gas engines have received increasing attention in the heavy-duty market due to their low cost and reliability advantages. However, there are still some issues with natural gas engines retrofitted from 10 to 15 L diesel engines, which is a valuable medium-term goal for the automotive industry. In this work, the effect of intake port structure on the performance of a spark-ignited heavy-duty natural gas engine was investigated by multidimensional numerical simulations. A newly designed intake port was proposed, with strengthened in-cylinder turbulent kinetic energy and homogeneous air-fuel mixtures. Bench tests show that the proposed intake port has impressive thermal efficiency, cycle variation, and acceptable emissions performance. The effective thermal efficiency improves from 41.0% to 41.4%, and the cycle variation is 36% lower than traditional schemes. However, with the accelerated flame propagation, the in-cylinder temperature and NOx emission of the mixed-flow port increase while the CO emission decreases. In summary, a proper balance of in-cylinder swirl and tumble flow can significantly affect the economy and stability of natural gas engines. The proposed structure solves the inherent problems of slow natural gas flame propagation and harmful cyclic variations. [ABSTRACT FROM AUTHOR]

The interplay of centrifugal and buoyant forces on convective heat transfer in a vertical annulus formed by rotating adiabatic outer cylinder and stationary heated inner cylinder has been experimentally and numerically investigated. Experiments were performed for rotational speeds corresponding to the rotation parameter ζ in the range of 527 ≤ ζ ≤ 2860, maintaining the heat flux of the heated stationary inner cylinder as 80 W/m2, for radius ratio (η) and aspect ratio of the vertical annulus being 0.614 and 0.052, respectively. The problem was investigated numerically using the commercial computational fluid dynamics package, ANSYS CFX. The numerical methodology has been validated by comparing the numerically predicted average surface Nusselt number with experimentally obtained values. The comparison revealed an enhancement of the thermal performance of the heated stationary inner cylinder in the range 527 ≤ ζ ≤ 1190 due to the increase in turbulence intensity towards the heated inner cylinder. However, when the rotation parameter was increased further in the range 1190 ≤ ζ ≤ 2860, the thermal performance of the stationary heated inner cylinder showed only marginal improvement. The aforementioned thermal behavior of the inner heated stationary cylinder has been explored based on the flow statistics gathered from the numerical simulations. [ABSTRACT FROM AUTHOR]

The goal of the paper is to reveal discrepancies of turbulent variables over different surfaces (sea, island, land) based on the measurements taken on three towers during (including before and after) seven typhoon episodes from 2008 to 2018. The atmospheric stability, turbulent spectrum, friction velocity, turbulent kinetic energy, dissipative heating, and gust factor are examined. The similar turbulent characteristics over sea and on the island reinforce the previous conclusion that the turbulent measurements on the island mainly represent the sea surface. The turbulent characteristics over sea and on land are very different due to the different underlying surface roughness. The unstable (stable) condition dominates on the sea (land) surface. Turbulent spectra both over sea and on land follow the canonical Kolmogorov's power law with the −5/3 slope. The cospectra on land are more peaked than those over sea. All of the friction velocity, turbulent kinetic energy, and dissipative heating increase with increasing 10 m wind speed, and those on land are much larger than those over sea. The distributions of gust factors widen and shift to higher on land than those over sea. The distributions of gust factors at heights of 10 m and 40 m are biased to higher values than those at heights of 160 m and 320 m on land. [ABSTRACT FROM AUTHOR]

Gasoline direct injection (GDI) technology is conducive to the intake process of the engine, showing good NOx emission and fuel economy, and it is widely used in the field of passenger cars. The start of injection (SOI) seriously affects the uniformity of oil and gas mixing, which in turn affects the combustion and emission performance of the engine. In this paper, a high-speed GDI engine is simulated and analyzed by Converge software, and the performance changes of the engine under different SOI conditions are studied. The results show that as the engine runs at high speed, under the joint action of in cylinder air flow and fuel beam air flow, a strong tumble flow is formed on the left side of the cylinder, and the mixed oil and gas gather in the direction of the intake side. With the delay of SOI, the time of spray-wall interaction is delayed, and the fuel breaking is strengthened. However, it is not conducive to the evaporation of the oil film on top surface, and different concentrations of mixed oil-gas are formed at the spark plug. The 400 °CA is the best injection time, in which the fuel gas mixing uniformity is the best. At this time, the combustion center of gravity is advanced, and the maximum explosion pressure reaches 10.5MPa, the power performance and fuel economy are improved. The full combustion of fuel and the increase of combustion temperature in cylinder reduce CO and THC emissions, but NOx emissions increase. [ABSTRACT FROM AUTHOR]

Acoustic Doppler velocimetry profilers (ADVPs) are widely used in both experimental and field studies because of their robustness in velocity measurements. The acquired measurements do not only offer estimates of the local and instantaneous flow velocity at the interrogated measurement volume, but can also be further processed for the estimation of the bed surface shear stresses, thus they are finding a wide range of applications ranging from water engineering to geomorphology and eco-hydraulics. This study aims to evaluate the performance of an ADVP in obtaining hydrodynamics measurements under fixed flow conditions, with various probe configurations. To this goal, a robust search is conducted where ADVP probe settings are sequentially altered. A number of assessment criteria are used including qualitative observations, such as checking the shape of the velocity profile, as well as quantitative error metrics, including signal-to-noise ratio, correlations and number of spikes. Further, estimation of the bed shear stresses computed by means of using the log Law of the Wall and turbulent kinetic energy, allow obtaining a better understanding of the uncertainties involved and the importance of making a better informed choice with respect to the probe configuration settings. Thus, the methodology and performance metrics provided herein, although presented for a given flow, can generally be applied from practitioners and researchers alike. [ABSTRACT FROM AUTHOR]

Recent studies have indicated that low-slope bedrock reaches are more common in nature (the bedrock surface slope is slightly lower than 0.005). This paper focuses on the hydrodynamic structure and turbulent characteristics of low-slope and critically sharp bedrock bends under flood conditions. Three experiments under different flood frequencies were conducted to investigate the influence of the ratio of centerline curvature radius to depth (Rc/H) on the distribution and characteristics of the secondary flow, turbulence, and momentum transport measured by acoustic Doppler velocimeter (ADV). The results were then compared with the flow structure of a sharp bend. Our results indicated that the distribution area and the intensity of the secondary flow decreased with Rc/H , especially in the outer-bank cell and the center-region cell. The lower Rc/H was, the more obvious the flow separation at the inner bank was. The core area of the turbulent kinetic energy moved upstream gradually, and its intensity increased with Rc/H. The friction factor increased in the upstream bend and did not move further upstream when Rc/H was below 13.6. The intensity of the transverse bed shear stress decreased with Rc/H in a critically sharp bend, and the maximum value near the upstream centerline implied the potential erosion in the bedrock bend. These experimental results are conducive to understanding the flow process of erosion and deposition in the bedrock bend reach. [ABSTRACT FROM AUTHOR]

In this study, three cylindrical combustion chambers with different diameter-to-depth ratios were designed to accelerate the flame propagation and enhance the combustion ratio of CH4 in a stoichiometric natural gas engine with exhaust gas recirculation (EGR). The effects of the diameter-to-depth ratio on the combustion and emission and the interaction between the flow field distribution and flame propagation were investigated numerically. The results showed that the value of the swirl ratio and turbulent kinetic energy (TKE) near the top dead center (TDC) could be increased continuously with a smaller diameter-to-depth ratio, which was conducive to promoting the uniform flame spread in the radial direction and enhanced the combustion efficiency. The peaks of pressure, heat release rate (HRR), and temperature dramatically increased by using the cylindrical chamber with a higher swirl ratio and higher TKE in the stoichiometric natural gas engines, thereby allowing more fuel energy to be released near the TDC in the chamber. The cylindrical chamber with the diameter-to-depth ratio of 2.36 displayed a higher peak value of combustion pressure and temperature, smaller CH4 and CO emissions, but more NOx emission, compared to other chambers. Moreover, the raised bottom bulge of the piston distorted the flame front, which accelerated the flame speed in the vertical direction. The CA50 was therefore advanced to the TDC. Thus, the cylindrical chamber with the increased squish area and the raised bottom bulge was conducive for the stoichiometric natural gas engine with EGR. [ABSTRACT FROM AUTHOR]

Hyporheic exchange, or the exchange of water and solutes between surface and subsurface water at the sediment‐water interface, regulates water quality and biogeochemical cycle in aquatic ecosystems. Vegetation, which is ubiquitous in nature, is known to impact hyporheic exchange; yet how vegetation impacts hyporheic exchange remains to be characterized. Here, we show that at the same spatially and temporally‐averaged flow velocity U $U$, vegetation increases the rate of hyporheic exchange by a factor of four. By tracking the movement of fluorescent dye in a flume with the refractive‐index‐matched sediment and translucent vegetation dowels, we demonstrate that the vegetation‐induced hyporheic exchange can be characterized by an effective hyporheic exchange velocity, VH ${V}_{H}$. We further demonstrate that VH ${V}_{H}$ correlates with the total near‐bed turbulent kinetic energy kt ${k}_{t}$ rather than U $U$, when kt<6×10−4m2/s2 ${k}_{t}< 6\times {10}^{-4}\,{\mathrm{m}}^{2}/{\mathrm{s}}^{2}$, indicating that turbulent kinetic energy is a better metric than flow velocity for predicting hyporheic exchange in regions with vegetation. Plain Language Summary: The exchange of contaminants and nutrients between surface and subsurface water in the hyporheic zone of rivers and wetlands controls water quality as well as the metabolism of benthic microbes and the associated biogeochemical cycle. Vegetation, which is ubiquitous in aquatic ecosystems, has been found to affect the surface‐subsurface exchange and as such impact water quality and stream biogeochemical cycle. However, how vegetation impacts this exchange remains unclear, making it difficult to predict the contaminant transport and biogeochemical cycle in streams, lakes, and coastal areas with vegetation. In this study, we directly visualized the release of a fluorescent dye from the transparent sediment into the surface water in a water‐recirculating tank filled with translucent vegetation. We discovered that vegetation can significantly increase the exchange in the hyporheic zone. Furthermore, we proposed a model to predict the impacts of vegetation on hyporheic exchange. We believe that this finding will help improve predictions of contaminant transport and biogeochemical cycle in streams and other aquatic ecosystems. The results of this study will also help ecologists design stream restoration projects that use vegetation to increase the retention and degradation of contaminants in sediment. Key Points: Emergent vegetation increases the exchange of water and solutes between surface and subsurface water in the hyporheic zoneThe release of solute from the sediment bed can be approximated by a mass transfer equation with an effective hyporheic exchange velocityThe impacts of vegetation on the hyporheic exchange velocity can be characterized by the near‐bed turbulent kinetic energy kt [ABSTRACT FROM AUTHOR]

The main and branch channels are positioned vertically in irrigated areas, and damage occurs at the side wall structure near the bleeder, owing to scouring and deposition of sediment particles. The divergence and confluence angles of natural rivers typically acute. In this paper, the open channel at a 30° water separation angle was used as the research object. The three-dimensional instantaneous velocity was measured using an acoustic Doppler velocimeter, and the distribution of the average velocity, circulation intensity, turbulence intensity, turbulence kinetic energy and bed shear stress at typical cross-sections were analyzed. The experimental results showed that for the 30° water separation angle, the lateral velocity of the water flow at the water outlet varied significantly along the canal width, and easily formed circulations, causing local erosion and deposition. The turbulent intensity near the branch channel changed sharply and irregularly, and the maximum value was observed at the forefront section of the bleeder. The kinetic energy of the lower flow significantly exceeded that of the upper flow at the water outlet, and the higher value was mostly concentrated near the water inlet of the side channel. The bed shear stress at the end of the water diversion section of the main channel was high, eroding the side wall and reducing stability. Compared to the right bleeder, the 30° water separation angle adversely caused the sediment particles to enter the side channel and decreased the erosion rate of the water flow to the bottom and side walls of the channel. This study provides a reference for channel design and operation in irrigation areas. [ABSTRACT FROM AUTHOR]

Ventilated cavitation is utilized for reducing drag and improving the headway of underwater vehicles. The ventilated cavitating flow around the axisymmetric body is simulated using the Large Eddy Simulation (LES) method in this study. Three cavity morphologies with different ventilation rates (C q) are observed: Foamy Cavity (FC), Continuous Transparent and Foamy Cavity (CTFC) and Super Cavity (SC). The cavity shedding characteristic of the three different cavity morphologies is revealed. For partial cavity, the periodic cavity shedding is dominated by the local high pressure, re-entrant jet and vortex. The distributions of the turbulent kinetic energy and the pulsation enstrophy exhibit different migration mechanisms during the cavity shedding. The energy characteristic is reflected by three terms of the entropy generation. And the entropy production rate by direct dissipation occupies a dominant position, accounting for about 80% of the total entropy production, while the entropy production rate by viscous dissipation only accounts for 4%. For super cavity, gas leakage is dominated by a re-entrant jet induced by the intense adverse pressure gradient. And the results also demonstrated the velocity and vorticity pulsations around the test body wrapped in a super cavity are not intense. • The typical ventilated cavitating flow structures at different ventilated rates have been studied. • The pulsation characteristics and the energy characteristics during the cavity shedding are revealed. • The energy loss composition during the cavity shedding is quantified. [ABSTRACT FROM AUTHOR]

In this paper, a self‐consistent model for the average radial turbulent transport of heat and particles in 2D anisothermal interchange‐dominated electrostatic ExB drift turbulence in sheath‐limited scrape‐off layers is presented. Diffusion models are used for the turbulent particle and heat fluxes, in which the transport coefficients scale with the square root of the turbulent kinetic energy (k⊥). The interchange drive proves to be the main source of turbulent kinetic energy, while the sheath term provides the main sink. These observations are qualitatively the same as for the isothermal case studied earlier. An analytical relation for the interchange term is derived, showing that the turbulent ExB thermal energy flux is the direct driver for k⊥ through the interchange source. A series decomposition of the sheath term in the k⊥ equation shows it includes a sink contribution that was also present in the isothermal case, but also a new source due to the sheath‐driven conducting‐wall (SCW) instability caused by electron temperature fluctuations. Using a simple regression model to close the total sheath term, a closed model for the turbulent transport is obtained. The interchange drive due to the ExB energy flux leads to a self‐saturation mechanism in the model, where the gradients and turbulence intensity increase until a sufficient transport level is reached to carry the power and particles across the field lines to find a balance with the sheath sink. An implementation of this model in a 1D mean‐field code approximates the profiles of the turbulence simulations with remarkable accuracy in part of the parameter space, while the error increases in parameter regimes where the relative importance of the SCW term is varied. [ABSTRACT FROM AUTHOR]

In order to analyze the effect of blade tip-gap size on the tip leakage vortex (TLV) dynamics and TLV-induced cavitation, a scaled axial flow pump model was created and numerically studied by the combination of an improved SST k-w turbulence model and a homogeneous cavitation model. The trajectories of TLV core was obtained by using the swirling strength method at different tip-gap sizes vary significantly. The scale of TLV increases as the tip-gap size increases, and the starting point of TLV is sliding further downstream along the blade chord. The angle between the blade suction surface and the TLV also presented an increasing trend with the tip-gap size. The statistics of the velocity normal to the tip chord, as well as the turbulent kinetic energy (KTE) distributions were employed to illustrate a more disordered flow field, which was generated in the tip clearance in a larger amount of leaking flow due to the increased tip-gap size. The in-plain static pressure and vapor volume fraction distributions at different blade chord sections, coupled with three-dimensional cavitation patterns among three tip gaps, are further analyzed to verify the wandering motion of TLV, which shows good agreement with the visualization experiment. Considering the adverse effect of the TLV cavitation, a small tip gap is recommended for improving the axial flow pump performance. [ABSTRACT FROM AUTHOR]

We present results for a direct numerical simulation study of isothermal incompressible flow in a regularly packed pebble-bed domain with a bounding wall. We focus specifically on the near-wall behavior of the flow. Our simulation is carried out at a Reynolds number of 9308 to facilitate cross verification with available high-fidelity data. To reduce the required time to achieve statistically stationary results, we implemented an ensemble-averaging scheme that allowed for multiple simulation runs to be carried out concurrently. The close packing of the spheres in the domain causes significant acceleration effects in the domain, which result in boundary layer detachment and reattachment. Presented results include selected first- and second-order turbulence statistics, as well as selected terms of the turbulent kinetic energy transport equation. The acceleration effects in the near-wall region of the domain cause negative production of turbulent kinetic energy. The presented data may be useful for benchmarking Reynolds-averaged Navier-Stokes–based simulations of pebble beds. [ABSTRACT FROM AUTHOR]

A simple set of equations, capable of quantifying and predicting the spatial decay of freestream turbulence (FST) is derived in the current study. The prediction equations are based on the inviscid estimate of the turbulent kinetic energy (TKE) dissipation rate. The new set of model equations includes the integral length scale and the turbulent kinetic energy as variables and is superior to the previous set of decay equations because, unlike those, they are not dependent on any physical grid parameters (b or M). This new set of equations, when compared and validated against 17 sets (2 active grids, 2 multi-scale grids, 9 square-cross-sectioned grids and 4 circular cross-sectioned grids) of previous, well-accepted, experimental data, including those relating to grid-generated turbulence and covering a wide range of turbulent Reynolds number (ReLu0) (7.5 × 101 to 6.9 × 104), where Lu0 is the initial integral length scale, showed very good agreement (within ±15%). This set of correlation equations can be used to estimate the local and/or initial turbulent kinetic energy and integral length scale (Lu) in an FST flow and to locate the region within a flow domain where nearly-constant turbulence conditions are expected to prevail. [ABSTRACT FROM AUTHOR]

This study presents experimental investigations of entrainment events of a single sediment particle resting in a small pocket on a smooth bed in an open channel flow. The data were acquired with a tomographic particle tracking velocimetry system. The shake the box algorithm, a spatially high resolved Lagrangian tracking method, was applied to determine time-resolved and three-dimensional flow velocities in a volume during particle entrainment. The proper orthogonal decomposition (POD) method was applied to identify motions in the flow field carrying the most turbulent kinetic energy (TKE). Based on the POD method, the most energetic flow structures were linked with the occurring quadrant events at particle entrainment. It was shown that particle entrainment follows TKE peaks based on low-order POD modes caused by large sweeps. Two streamwise elongated counter rotating vortices with emerging sweeps in between were observed during particle entrainment. Here, most of the TKE exists in the first POD mode. This signature is suggested to be part of very large-scale coherent motions. [ABSTRACT FROM AUTHOR]

Low-slope bedrock meandering rivers widely exist in areas controlled by geological structures (the bed slope is less than 5‰). Flood has a great influence on riverbed sedimentation and erosion in bedrock meandering rivers, however, we have a poor understanding of dynamic structure of flood for bedrock meandering rivers. By geometrically generalizing bedrock meandering reaches, considering the Froude similarity and side wall roughness, a generalized model of a bedrock meandering channel is established in order to analyze the water surface profile distribution, time average flow fields and turbulence structure characteristics for bends under flood. The results show that under the flood flow, the minimum water surface transverse gradient, separation of flow on the convex bank, development of double circulation on the concave bank, velocity-dip-phenomenon and maximum secondary circulation intensity are caused at the upstream of the bend apex, and the maximum water surface transverse gradient and circulation splitting are caused at the downstream of the bend apex; the bed shear stress is distributed in the flow separation area at the convex bank as well as the center of the upstream of the bend apex, and the transverse momentum transport is concentrated at the upstream of the bend apex. The experiment results show an explanation in view of hydrodynamics in respect to bed erosion and sedimentation in bedrock meandering rivers. [ABSTRACT FROM AUTHOR]

Based on the RANS equations and applying the waves2Foam solver in OpenFOAM, the study numerically investigated the turbulent kinetic energy (TKE) generated due to emergent rigid vegetation in water waves. The numerical results show that the TKE generated due to emergent vegetation in water waves increases with decreasing water depth and increasing wave height and period, as well as vegetation density and stem diameter. Moreover, compared to wave height, the influence of wave period on the stem-generated TKE in waves is more significant. Furthermore, the ratio of the amplitude of wave motion trajectory to stem spacing has a significant impact on the stem-generated TKE in waves. When the amplitude of wave motion trajectory is less than half of the stem spacing, the stem-generated TKE in waves is similar to the TKE of the bare bed in waves. In the case where the amplitude of wave motion trajectory is greater than half of the stem spacing, the stem-generated TKE in waves in the front of the vegetation will be significantly enhanced. Based on the study, an improved formula is proposed for predicting the stem-generated turbulence in waves. • Stem-generated turbulent kinetic energy in waves increases with decreasing water depth and increasing wave height and period. • Stem-generated turbulent kinetic energy in waves increases with increasing plant density and stem diameter. • Influence of the ratio of wave excursion to stem spacing on stem-generated TKE in waves. [ABSTRACT FROM AUTHOR]

We study a simplified equation governing turbulent kinetic energy k in a bounded domain, arising from turbulence modeling where the eddy diffusion is given by ϱ (x) + ɛ , with ϱ representing the Prandtl mixing length of the order of the distance to the boundary, and a right-hand side in L 1. We obtain estimates of ϱ ∇ k in L q spaces and we establish the convergence toward the formal limit equation in the sense of the distributions as ɛ goes to 0. [ABSTRACT FROM AUTHOR]

• Laminar flame velocities of n-dodecane and ammonia mixtures were experimentally determined. • The presence of ammonia in the gas mixture diminishes the laminar flame velocity. • The proposed coupling mechanism accurately predicts the laminar flame velocity development. • The presence of ammonia in the gas mixture results in an increase in the thickness of the laminar flame. • The application of coupling mechanism in CFD serves as a valuable reference for the advancement of novel compound combustion engines. Laminar burning velocity is a crucial characterization parameter in fuel combustion progress. Investigating the combustion properties is significant to improve combustion efficiency. This paper employs a constant volume combustion bomb and high-speed ripple shadow camera technology to explore the influence of equivalence ratios (0.8–1.4) and ammonia energy shares (0%–40%) on the laminar burning velocity of n-dodecane/ammonia mixture at an ambient temperature of 400 K and a pressure of 1 bar. The results illuminated that the unstretched laminar burning velocity of n-dodecane increased from 46 cm/s at ϕ = 0.8 to 61 cm/s at ϕ = 1 and then decreased to 32 cm/s at ϕ = 1.4. When the ammonia energy ratio reaches 40%, the maximum laminar burning velocity experiences a reduction of 44% because the blended ammonia inhibited the formation of H and O radicals based on kinetic analysis in the fast reaction zone. With the increase of ammonia ratio, the Markstein length of mixed burned gas gradually increases. Particularly under a 1.4 equivalent ratio condition, the Markstein length increases from 0.2 mm to 0.8 mm, indicating that ammonia components significantly enhance flame stability. To gain a deeper understanding of the influence mechanisms of laminar burning velocity, a simplified Yao-Otomo (Y-O) model with 92 species and 355 reactions is proposed, coupling the mechanisms of n-dodecane and ammonia. The Y-O model has demonstrated a predictive trend that is consistent with more detailed WUT-NH 3 model (176 species and 2839 reactions), and the Y-O model reduces the amount of computation to save calculation time, which has a higher engineering applicability. The turbulent kinetic energy (TKE) induced by premixed fuel in engine cylinders is estimated through comprehensively analyzing the laminar burning velocity and flame thickness characteristics. The findings indicate a 45% reduction in the peak value of TKE as the ammonia energy ratio increases. [ABSTRACT FROM AUTHOR]

Double-tube heat exchangers are widely used in many fields. A novel double-tube heat exchanger with a perforated helical fin on the annulus side is proposed in this paper for thermal performance promotion. The effect of different perforation diameters ranging from 0 to 12 mm on the fluid flow and heat transfer are numerically studied under a turbulent state with Re ranges between 4000 and 16,000. The results showed that perforation diameter has a significant influence on the thermal performance. There exists an optimum perforation diameter for the best thermal performance. The largest value of Nu in the helical finned annulus increases by 20.2% by perforation, while the corresponding f only increases by 11.0% compared with the normal helical finned configuration without perforation. The thermal performance evaluation factor JF is also improved by up to 16.1% compared with the helical finned configuration without perforation, and is increased by 48.6% compared with the smooth annulus. Correlation formulas of f, Nu, and JF are fitted and the deviations are within ±4%, ±9%, and ± 5% for f , Nu, and JF , respectively. Perforated helical fin is an efficient annular-side heat transfer enhancement technology that can be applied to the optimized design of double-tube heat exchangers. • A novel double-tube heat exchanger with perforated helical fin is studied. • Heat transfer is significantly improved by perforations on the helical fin. • Nu increases by 20.2% with f increases by 11.0% compared with the normal helical fin. • JF increases by 16.1% and 48.6% compared with helical finned and smooth annular tube • Correlations of Nu , f and JF are proposed to facilitate the application of the results. [ABSTRACT FROM AUTHOR]

Velocity fluctuations caused by unsteady flows substantially increase mechanical loads on tidal turbine blades and represent a major challenge to tidal energy converters. Overcoming the challenges posed by high levels of turbulence and large waves is a significant concern for turbine developers. Here we quantify turbulence and characterize wave-turbulence interaction in two prospective tidal energy sites in Australia: Banks Strait and Clarence Strait. Turbulence characterization was derived from deployments lasting up to 3 months. Our datasets represent the longest turbulence measurements at tidal sites published to date, setting a benchmark in tidal energy site characterization. Banks Strait data was affected by wave orbital velocities at mid-water column for nearly the entire period, whilst Clarence Strait data rarely presented significant wave-turbulence interactions. Turbulence intensities varied mostly between 12% and 17% in both sites. Higher turbulent kinetic energy density was observed in Banks Strait, although one of Clarence Strait stations revealed faster current speeds exceeding 2.8 m/s. Investigation of strong wind events suggests turbulence parameters were not majorly affected. Our findings are valuable to obtain more realistic turbine performance and loadings estimates from numerical models and considerably contribute towards efforts to establish best-practice turbulence assessment methods in tidal energy sites. [ABSTRACT FROM AUTHOR]

• A CFD scrutiny has been done to find the THP of novel geometry in a triangular SAH duct. • Flow of air is shown by Axial Velocity contours. • Friction and Heat transfer attributes have been discussed with contours and plots. • The highest THPF value of 2.54 is found at e = 9 mm and β = 20 %. In the current analysis, the effect of perforated baffles in V-down pattern as artificial roughness on the thermal and hydraulic response of a solar air heater having equilateral triangular passage has been examined. A thorough numerical investigation has been done using ANSYS FLUENT-19.0. Effect of change in roughness parameters, viz., baffle height (e) and open area ratio (β) on thermo-hydraulic performance has been explored. Flow Reynolds number is systematically varied. Study reveals that upon varying the baffle height from 5 mm to 11 mm and open area ratio from 20% to 29%, maximum heat transfer rate is attained at e = 9 mm and β = 20%, respectively. Axial Velocity and Turbulence Kinetic Energy have been plotted for clear view of change in variables. Also, the correlations have been developed for Nusselt number and Friction factor. The thermo-hydraulic performance factor of proposed perforated baffle geometry has been observed to be 2.54 at e = 9 mm and β = 20%. Performance of the SAH with suggested roughness is found to be advantageous than other type of roughness elements studied in recent times. [ABSTRACT FROM AUTHOR]

Large-eddy simulation is performed to study the turbulence statistics and flow structures of the water past a rotating axial-flow pump under different flow-rate working conditions. A novel sharp-interface level-set based immersed boundary method is applied to capture the complex geometry of the pump. An unstructured triangular mesh is used to discretize the complex surface geometry of the pump, and a ray-tracing method is employed to classify the computational domain into fluid and solid regions. Turbulence statistics, including the mean velocity, turbulent kinetic energy (TKE), turbulence production, and turbulence dissipation, are analyzed under five different flow-rate working conditions around the designed condition. The results show that unsteady wake, tip leakage flow, and flow separation are accompanied by a high TKE magnitude. For the high turbulence intensity under off-designed working conditions, the tip leakage flow plays a leading role at low flow-rates, and flow separation dominates at high flow-rates. • LES study of detailed flow structures around a rotating axial-flow pump. • A novel immersed boundary method is applied to capture the complex pump geometry. • The additional efficiency loss is found to be correlated to the large TKE magnitude. • The TKE transport is analyzed to study the mechanism underlying the efficiency loss. [ABSTRACT FROM AUTHOR]

In this study, numerical simulation was conducted on the 3401 fully mechanized mining face in Xin'an Coal Mine regarding its serious dust pollution and complex dispersion law, based on which the spatial distribution of dust concentration and particle size was analyzed. The aim is to provide theoretical guidance for accurate and efficient spray dust reduction the 3401 mining face or similar working environment. The results show that the dust concentration in the area 23 m on the leeward side of the front drum center is about 1220–2620 mg/m3. Near the floor of the coal mining area, dust particles with the sizes of over 15.3 μm account for about 70% of the particle group due to the settlement of large-size dust. In the high-concentration dust-polluted area of the non-mining area, those with the sizes of over 12 μm account for about 40% of the particle group. [Display omitted] • A fully mechanized mining face mode that meets regulations was established. • The law of dust migration and diffusion under the influence of airflow was revealed. • The dust data of the fully mechanized mining face within 5 s were captured. • The dust particle size distributions in different areas were analyzed quantitatively. [ABSTRACT FROM AUTHOR]

In order to reveal the operating characteristics of the pumpjet propulsor, standard k – 𝜀 , standard k – ω , RNG k – 𝜀 and SST k – ω turbulence models were used to conduct steady calculation for the whole flow channels. By comparing the calculation results with experimental data, it was found that the calculation errors were very large in some operating conditions. Therefore, the uncertainty analysis was carried out at all operating conditions of the pumpjet propulsor and the error source was finally determined that it is mainly derived from the model error. Then, the applicability of different turbulence models was analyzed to numerical simulation for the pumpjet propulsor by comparing the internal and external characteristics. It can be seen that the strong turbulent kinetic energy in the guide vane will inevitably cause energy loss, but not necessarily in the impeller. In this area, the increase of turbulent kinetic energy will enhance the mixing and transport of fluids, and the impeller makes the fluids get more energy. In addition, a modified hybrid Reynolds Average Numerical Simulation/Large Eddy Simulation (RANS/LES) model was proposed for unsteady calculation, and the performances, internal flow states and the interaction between the pump and the outer region were further revealed under various conditions of the pumpjet propulsor, which provides some references for predicting accurately and selecting conditions optimally in the future. [ABSTRACT FROM AUTHOR]

A new method for estimating turbulent kinetic energy (TKE) from acoustic Doppler current profiler (ADCP) measurements is presented. Current methods of estimating TKE from ADCP measurements require assumptions regarding the flow being measured. Combining the geometric orientation of ADCP sampling beams and the relative contributions of root mean square (RMS) velocities to the total TKE results in a formula that can be written for any ADCP. Data from a direct numerical simulation (DNS) are used with a virtual ADCP to assess the success of this method, and laboratory experiments are used to test the new method in a physical setting. The proposed approach measures TKE with an average RMS error of 15.2%, which is less than that from previous methods. Error caused by the ADCP sampling in different spatial locations contributes most to overall error; commercially available ADCPs typically sample at a high enough frequency and fine enough resolution to measure the RMS velocities needed to calculate TKE. Guidelines for measuring TKE with an ADCP are proposed, and published data on rivers are used to estimate the applicability of this method in practice. • Turbulent kinetic energy (TKE) is measured with acoustic Doppler current profilers • The method reproduces TKE from a numerical simulation within 15% • Measuring in spatially different locations contributes the most to overall error • Instrument resolution contributes the second most to overall error • We recommend root-mean-squared velocities be at least 8 times the resolution [ABSTRACT FROM AUTHOR]

Net-type spacers are used to promote hydrodynamic mixing in the feed channel of crossflow membrane filtration. The induced fluid flow mitigates concentration and temperature polarization effects on the membrane surface, leading to the enhancement of heat/mass transfer across the membrane. The objective of this research is to study the effect of spacer geometry on fluid flow and heat/mass transfer in the feed channel. The particle image velocimetry (PIV) technique was used to measure flow velocity in the feed channel with net-type spacers. Flow characteristics at a wide range of Reynolds numbers were described in terms of time-averaged mean velocity and turbulence kinetic energy (TKE). The PIV result revealed distinct profiles of mean velocity and TKE at different channel heights with bi-planar spacers. The spacer with the largest hydrodynamic angle of 120° (Net120) was found to be the most efficient in promoting flow unsteadiness because it led to the largest channel-averaged TKE among the selected spacers. The unsteady flow regime on Net120 was accompanied by the greatest pressure drop over the feed channel. Moreover, the effect of spacer-induced mixing on heat/mass transfer across the membrane was further evaluated via the flux measurement with vacuum membrane distillation experiments. In comparison with other spacers, Net120 resulted in a relatively higher flux because of the improved hydrodynamic mixing. These findings reveal the validity of the experimental approach in identifying the impact of key spacer design parameters on hydrodynamic mixing. Nusselt number was calculated to relate the hydrodynamic flow and thermal flow in the feed channel of membrane distillation, and a modified Nusselt number correlation was proposed to better predict convective heat transfer in the spacer-filled channel in VMD. • PIV revealed flow characteristics in channels with net-type spacers. • A large hydrodynamic angle can promote flow turbulence more effectively. • TKE can be a metric of hydrodynamic mixing for spacer performance evaluation. • Spacer with the largest hydrodynamic angle was the best turbulence promotor. • Modified Nu number proposed better predict heat transfer for VMD. [ABSTRACT FROM AUTHOR]

Currently demand of Indian budget cars also called people's car in India is in great demand and around 1 million cars have been sold in the last financial year with 12% increase of sales in every forthcoming year. These are categorized in sub 4m category of sedan, hatchbacks and medium size SUVs' and their price ranges between 7 to 11 lakhs. The aerodynamics significantly affects the performance of the vehicle particularly at high speed. The manufactures are more focused on styling and giving a luxury look and other features of the car which sometimes make them to compromise on its aerodynamic design. This may lead to increase in fuel consumption at Indian road conditions. A cost-effective way to reduce fuel consumption, drag coefficient, lift force is to improve aerodynamic behavior and reduce the aerodynamic drag. The software used in this work is Solidworks, Ansys Fluent and commercial CFD post. Consequently, of using this software, it allows us to apply, learn and link technical knowledge of aerodynamics and computer knowledge. [ABSTRACT FROM AUTHOR]

One of the main issues in environmental hydraulics is pollutant release from sediments. For instance, the strong affinity between phosphorus and sediment permits most of the phosphorus to be adsorbed on the surface of the sediment particles in rivers or lakes. Post sediment resuspension, phosphorus is desorbed from the sediment to the overlying water. The release of phosphorus from the resuspended sediment is an important process in the secondary pollution of water. Herein, a coupled mechanical model of the overlying water, sediment, and pollutant was established based on the experimentally gathered data. Two types of sediment with different adsorption and desorption characteristics were selected to simulate the process of sediment resuspension and phosphorus release under different hydrodynamic conditions. The simulation results were subsequently used to analyze the relationship between the flow field characteristics and phosphorus concentration, from which the relationships between velocity, particle volume fraction, turbulent kinetic energy, total phosphorus concentration, desorbed phosphorus concentration, and time were elucidated. Based on the results, phosphorus is rapidly released into the overlying water from the resuspended sediment, and it reaches a peak value in a short duration. Unlike the release process of non-adsorption pollutants, hydrodynamic conditions and sediment properties play a crucial role in the phosphorus release process. The turbulent kinetic energy rapidly increases with the flow velocity, whereas the desorbed phosphorus concentration exhibits a certain relationship with the particle volume fraction and turbulent kinetic energy. In particular, the turbulent kinetic energy increases the desorbed phosphorus concentration per unit time. Additionally, the time taken by the total phosphorus concentration to attain its peak value is closely related to the characteristics of the flow field, whereas the amount of phosphorus is closely related to sediment properties. Post sediment resuspension, the release of phosphorus shows the characteristics of a centralized and massive release, which suggests that the total phosphorus concentration in the overlying water would change in a short duration and cause secondary pollution in the water environment. [ABSTRACT FROM AUTHOR]

In view of the heat transfer and ventilation characteristics of the Insulated Gate Bipolar Transistor (IGBT) in the Electric Multiple Unit (EMU), the authors aim to improve the traditional fin structure of the air-cooled heat exchanger (ACHE) and propose the use of corrugated fin structure. Using the computational fluid dynamics (CFD) technology, the ACHE with new fin structure was numerically simulated, and the temperature field, velocity field and pressure field at different fin corrugation angles and the synergy between the fields were analyzed. The results show that the improved fin structure can make the fluid flow in a corrugated flow channel, effectively increase the flow distance, and thus significantly enhance the turbulence performance of the fluid in the ACHE; the fin corrugation angle of 120 degrees is the key design point; compared with the traditional fin structure, the new fin structure improves the synergy between the various fields and increases the heat exchange efficiency. The research findings provide new ideas for the design of this type of ACHE. [ABSTRACT FROM AUTHOR]

The equations for double-mean, form-induced and spatially averaged turbulent energy budgets are employed to analyse data from direct numerical simulations of turbulent open-channel flows over transitionally rough mobile beds with intermediate flow submergence. Two scenarios were considered related to (i) near-critical bed condition, and (ii) fully mobile bed condition. The bed was composed of a layer of mobile spherical particles moving on the top of one layer of fixed particles of the same size. Data analysis showed the leading energy exchanges between double-mean, form-induced, turbulent flow field contributions as well as particle motions. Above the fixed particles tops, the turbulent flow receives kinetic energy directly from the mean flow as well as from moving bed particles, which in turn also receive energy from the mean flow. For near-critical bed condition, particle aggregations enhanced mean-flow heterogeneity, strengthened turbulent stresses and their effects on the flow, while at increased bed-mobility, energy transport mechanisms became weaker and conversions induced by viscous stresses and pressure became stronger. [ABSTRACT FROM AUTHOR]