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The air flow underneath high-speed trains have been measured at full-scale for three types of high-speed trains running on two types of ballasted tracks, and the characteristics of the flow have been investigated and comparatively analyzed related to the train and track types. 6 Kiel probe rakes vertically distributed in 40 mm interval from just above the track surface have been employed to measure the temporal histories of wind velocity during the passage of train. The results show that the flow under test trains reached fully-developed state before the end of the second vehicle and maintains until the start of the last vehicle, and average velocities over the fully-developed region show very good consistency for no more than 3 repeated samples. The vertical profile of average velocity over the fully-developed region has been suggested as a useful model to represent the characteristics of the flow underneath high-speed trains, and equivalent roughness of the test trains and tracks have been obtained by introducing the analytical solution of the turbulent Couette flow. The resulting values of equivalent roughness of Korean high-speed trains ranged from 50 mm to 150 mm, and that of the ballast track was 150 mm. • A comprehensive description and analysis have been made for full-scale test campaigns conducted in Korea over the years. • The vertical profile of average velocity has been suggested as a useful model for underfloor flow of high-speed trains. • Equivalent roughness of the test trains and tracks have been obtained by introducing the turbulent Couette flow analogy. • The resulting values of equivalent roughness ranged from 50 mm to 150 mm for trains, and was 150 mm for ballast track. [ABSTRACT FROM AUTHOR]

• New data on droplet velocities at supercritical turbulent air velocity are reported. • Effect of liquid properties is demonstrated via tests with water-MEG solutions. • Mechanistic models for critical air velocities and droplet velocities are established. • Droplet shape is different at subcritical and supercritical gas velocities. • Critical air velocity for displacing droplet exceeds that for keeping its movement. Experiments were performed to measure the velocities of droplets propagating on the bottom of an upward inclined pipe when exposed to air shear flow. Effects of the droplets' shape, liquid properties, air velocity, and the pipe inclination were examined. It was found that the droplet velocity increases with the air velocity and droplet length, and reduces with the pipe inclination and the liquid viscosity. A mechanistic model that considers the stretched shape and forces acting on a moving droplet was established. The model uses a correlation for the drag coefficient that was derived based on the experimental data of the droplet velocity. It enables the prediction of the measured droplets' velocities and the effect of the various parameters on the droplet motion. Based on data from the literature, a different drag coefficient correlation for a stationary droplet, yet unstretched, is also presented. The different models for stationary and moving droplets imply a hysteresis phenomenon regarding the air critical velocity. Namely, the air velocity required to displace a stationary droplet is larger than the air velocity below which the drop will stop its movement. However, the hysteresis becomes less notable with larger droplets. [ABSTRACT FROM AUTHOR]

The recent episodes of haze in Southeast Asia have caused some of the worst regional atmospheric pollution ever recorded in history. In order to control the levels of airborne fine particulate matters (PM) indoors, filtration systems providing high PM capturing efficiency are often sought, which inadvertently also results in high airflow resistance (or pressure drop) that increases the energy consumption for air distribution. A pre-conditioning mechanism promoting the formation of particle clusters to enhance PM capturing efficiency without adding flow resistance in the air distribution ductwork could provide an energy-efficient solution. This pre-conditioning mechanism can be fulfilled by acoustic agglomeration, which is a phenomenon that promotes the coagulation of suspended particles by acoustic waves propagating in the fluid medium. This paper discusses the basic mechanisms of acoustic agglomeration along with influencing factors that could affect the agglomeration efficiency. The feasibility to apply acoustic agglomeration to improve filtration in air-conditioning and mechanical ventilation (ACMV) systems is investigated experimentally in a small-scale wind tunnel. Experimental results indicate that this novel application of acoustic pre-conditioning improves the PM2.5 filtration efficiency of the test filters by up to 10% without introducing additional pressure drop. The fan energy savings from not having to switch to a high capturing efficiency filter largely outstrip the additional energy consumed by the acoustics system. This, as a whole, demonstrates potential energy savings from the combined acoustic-enhanced filtration system without compromising on PM capturing efficiency. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to propose a novel approach to analytical modelling of steady-state heat transfer from the exterior of totally enclosed fan-cooled induction motors. The proposed approach is based on the geometry simplification methods, energy balance equation, modified correlations for forced convection, the Stefan-Boltzmann law, air-flow velocity profiles, and turbulence factor models. To apply modified correlations for forced convection, the motor exterior is presented with surfaces of elementary 3-D shapes as well as the air-flow velocity profiles and turbulence factor models are introduced. The existing correlations for forced convection from a short horizontal cylinder and correlations for heat transfer from straight fins (as well as inter-fin surfaces) in axial air-flows are modified by introducing the Prandtl number to the appropriate power. The correlations for forced convection from straight fins and inter-fin surfaces are derived from the existing ones for combined heat transfer (due to forced convection and radiation) by using the forced-convection correlations for a single flat plate. Employing the proposed analytical approach, satisfactory agreement is obtained with experimental data from other studies. [ABSTRACT FROM AUTHOR]

Different variants of ventilation of the cab of a truck were investigated by numerical simulation methods. An unsteady viscous gas flow was numerically simulated using the VP2/3 package. In the process of numerical simulation of this flow, the Reynolds equations closed with the use of the differential k- ω model of turbulence in the SST modification of Menter were solved numerically. On the basis of analysis of the fields of the velocity, pressure, and turbulence characteristics of the air flows in the truck cab, obtained as a result of their numerical simulation, the efficiency of ventilation of this cab was estimated. [ABSTRACT FROM AUTHOR]

Airflow dynamics and potential sediment transport were measured and modelled across various barchan dune topographies and incident wind conditions. Modification of near surface flow was recorded simultaneously in three dimensions (3D) using dense arrays of high-resolution 3D ultrasonic anemometers. In situ measurements provided rigorous validation and calibration for computational fluid dynamics (CFD) modelling. Measured and modelled results show good agreement between flow velocity, directionality, and turbulence intensity. Modelling of characteristic airflow conditions and surface shear stress beyond the instrument locations, elucidated airflow dynamics across the entire landform surface at an unprecedented level of detail. Emergent turbulent airflow patterns were identified in the form of two counter-rotating vortices that converge at the dune centreline downwind of the dune crest. Integrating a sediment transport function with CFD surface airflow allows for the spatial mapping of flux patterns across the entirety of the dune and interdune surface. On the stoss slope and laterally along the outer barchan arms, there is strong potential sediment flux in response to increased streamwise stress. In lee-side locations, sediment transport remains at ‘above threshold’ conditions and is redirected in response to complex turbulent vortices identified in the overlying wake zone. The precision of the models allows for the identification of complex flow perturbations and associated surface stresses that prove difficult to measure in the field. CFD in combination with a sediment transport function is demonstrated to be a useful tool in investigating morphodynamics of mobile dune systems. [ABSTRACT FROM AUTHOR]

People spend approximately one third of their life sleeping. Exposure to pollutants in the sleep environment often leads to a variety of adverse health effects, such as development and exacerbation of asthma. Avoiding exposure to these pollutants by providing a sufficient air quality in the sleep environment might be a feasible method to alleviate these health symptoms. We performed full-scale laboratory measurements using a thermal manikin positioned on an experimental bed. Three ventilation settings were tested: with no filtration system operated, use of portable air cleaner and use of a temperature-controlled laminar airflow (TLA) device. The first part of the experiment investigated the air-flow characteristics in the breathing zone. In the second part, particle removal efficiency was estimated. Measured in the breathing zone, the room air cleaner demonstrated high turbulence intensity, high velocity and turbulence diffusivity level, with a particle reduction rate of 52% compared to baseline after 30 minutes. The TLA device delivered a laminar airflow to the breathing zone with a reduction rate of 99.5%. During a periodical duvet lifting mimicking a subject’s movement in bed, the particle concentration was significantly lower with the TLA device compared to the room air cleaner. The TLA device provided a barrier which significantly reduced the introduction of airborne particles into the breathing zone. Further studies should be conducted for the understanding of the transport of resuspended particles between the duvet and the laying body. [ABSTRACT FROM AUTHOR]

The combustion of spark ignition engines converted to bi-fuel CNG is unstable and proper air and fuel mixing strategy is a concern. Also, CNG fuel causes the coefficient of variation for indicated mean effective pressure (COV imep ) higher than 10% which is into the region of unstable combustion. In order to create stable combustion more turbulence is required. This paper studies the valve movement with dissimilar valve lift (DVL) to increase swirl in the engine. The intake valve is the last point of airflow entry into an engine and the modification of the movement can contribute to increase turbulence. Three DVL setting simulated via computational fluid dynamics (CFD) gave improvement in peak pressure by 4% and a 32.2% improvement in flame propagation speed compared to baseline CNG. Engine testing shows that, the engine COV imep improves up to 8.7%, while efficiency improves by 5.7% and BSFC is reduced by 5.4% respectively with the 1 mm DVL at 4000 rpm compared to baseline CNG. The rate of heat release (ROHR) also shows early heat release of the fuel. The novelty is better mileage for future CNG engine design. [ABSTRACT FROM AUTHOR]

Well-resolved direct numerical simulation (DNS) is applied to investigate strongly heated air flow in a vertical pipe ( L = 30 D ) at inlet Reynolds numbers of Re 0 = 4240 and 6020. The DNS is based on the experimental cases of Shehata and McEligot (1995) and shows excellent agreement with heat-transfer and flow statistical results. Flow relaminarization is observed in the strongly heated cases. We apply a new semi-local wall coordinate to replace the conventional one. With the semi-local wall coordinates, which considers the local property variation, both the velocity and temperature fields show the process of relaminarization. This relaminarization is also indicated by the significant decrease of turbulence intensity and Reynolds shear stress. In the quasi-laminar flow, the viscous sublayer becomes thicker. Turbulence in this layer shows a growing anisotropic character. And turbulence in the pipe center becomes approximately isotropic. This two-layer character is clearly displayed by flow visualization. [ABSTRACT FROM AUTHOR]

This work explores the methodology and errors involved in using a commercial sonic anemometer to study confined industrial swirling air flows, such as those in large cyclones or dryers in the order of hundreds of m 3 . Common sources of uncertainty in time-of-flight techniques and multiple-path anemometry are evaluated and corrections and methodology guidelines are proposed to deal with issues typical of full scale measurement. In particular, this paper focuses on quantifying the error associated with the disruption of the local flow caused by a HS − 50 horizontal sonic anemometer under a range of turbulence characteristic of industrial swirl towers. Under the guidelines proposed and the conditions studied here, the presence of the instrument originates a measurement error < 1 − 4 % in velocity, < 1 − 3 ° in direction and < 7 − 31 % in turbulent kinetic energy for an isothermal flow in the absence of solids. These ranges are above traditional uses of sonic anemometry in meteorology due to the limitations inherent to industrial units, but remain within reasonable margins for engineering applications. Laser diagnostic methods are widely used in laboratory and pilot scale cyclones or dryers but are rarely applicable to large production scales. In this context, the data collected with sonic anemometers render much lower resolution but appear in agreement with historical Particle Image Velocimetry. Methods such as the one proposed here can be a useful alternative to improve the level of detail of fluid dynamic studies in industrial units, which are often qualitative or with a limited validation. [ABSTRACT FROM AUTHOR]

A three-dimensional large-eddy simulation model is applied to the study of sea-spray aerosol transport, dispersion and settling in the coastal thermal internal boundary layer (IBL) formed by cool airflow from the open sea to the warm land. An idealized situation with constant inflow from the ocean and constant heat flux over the coastal land is considered. The numerical results confirm that the thickness of the coastal thermal IBL increases with the distance from the coastline until the outer edge of the IBL penetrates into the capping inversion layer. The thickness increases also with time until a fully-developed thermal boundary layer is formed. In addition, the thickness of the coastal thermal IBL increases more rapidly when the heat flux over the land is greater. Existence of large-scale eddies within the thermal IBL is identified and the turbulence intensity within the thermal IBL is also found to be significantly higher than that above. It is also indicated that the vertical position of the maximum concentration does not occur at the surface but increases as sea-spray aerosols are transported inland. The vertical position of the maximum flux of sea-spray aerosols within the coastal thermal IBL is shown to coincide with that of the maximum vertical velocity fluctuations when the coastal thermal IBL is fully developed with increased distance in the airflow direction. [ABSTRACT FROM AUTHOR]

The objective of this study is to experimentally and numerically investigate the aerodynamic behaviour and the effectiveness of an air curtain confining cavity and subjected to external lateral flow. Experiments were carried out on a scale down model (1:5) under isothermal conditions representing a generic configuration of a display case, using LDV and PIV techniques to investigate the air flow characteristics. A numerical approach based on computational fluid dynamics (CFD) Fluent code was used with two turbulence models: the standard k–ε model and the RSM. Comparisons of experimental and numerical data obtained with and without external perturbation make it possible to quantify the effect of the perturbation on the air curtain characteristics such as airflow patterns, velocity profiles, jet decay, and its stability as well as the ambient thermal entrainment. These comparisons also allow a critical evaluation of the performance of the two turbulence models for free and perturbed jets. [ABSTRACT FROM AUTHOR]

• Disturbance airflow increases flame instability and turbulence intensity. • The flame is less resistant to disturbances during the acceleration phase. • Flame propagation velocity, overpressure maximum and overpressure rise rate are sensitive to the initial pressure of the disturbance airflow. To evaluate the effect of the disturbance airflow on the methane deflagration flame, a deflagration experiment is conducted in a closed duct. The effects of three disturbance airflow levels and two disturbance positions on the deflagration flame dynamics, including the variation of flame propagation behavior, disturbance mechanism, and deflagration overpressure development are investigated. The experimental results show that the vertical disturbance of the airflow disrupts the original structure of the deflagration flame, increasing the inherent instability and turbulence of the flame. The increase in the initial pressure of the disturbance airflow can increase the flame turbulence intensity, propagation velocity, the maximum value of the deflagration overpressure and the rate of overpressure rise. The contribution of the disturbance airflow at the initial acceleration position of the flame to the characteristic parameters of deflagration is more prominent. The experimental results complement and explain the propagation behavior of deflagration flame in a non-static environment. It provides suggestions on the prevention and protection of combustible gas deflagration risks in complex environments. [ABSTRACT FROM AUTHOR]

The focus of this paper is on a thermal performance of the window solar air collector with seven-moveable circular perforated absorbent plates that controlled manually. The proposed window solar air collector is a solar thermal system which can be employed to preheat the ventilation air supply and compromise between sunlight penetration to the desired buildings and hot air gain. In this work, five angles are selected (0 ° , 30 ° , 45 ° , 60 ° , and 90 °) and mathematical model and formulation based on the finite volume scheme (SIMPLE) algorithm has been implemented to solve the 3-D forced convection with turbulent flow. Different values of solar irradiance intensities (330, 530, and 730 W/m2) and air mass flow rates (0.0097, 0.0151, 0.0224, and 0.0298 kg/s) were adopted. The results found significant difference between air inlet and outlet temperatures are gained at angle 0 ° (vertical absorbent plates position) at irradiance 730 W/m2 and mass flow rate 0.0097 kg/s is 11.2 °C and the maximum thermal efficiency is 71% at a mass flow rate of 0.0298 kg/s. Also, flexibility between hot air from the collector and sunlight penetrating into the room is gained when the plate angles set on (30 ° , 45 ° , 60 ° , and 90 °). • A novel collector with 7 movable absorber plates with different opening angles. • Best thermal performance and minimum of sunlight were obtained at angle 0 °. • Lower thermal performance and maximum of sunlight were obtained at angle 90 °. • A flexibility between hot air and sunlight requirements for angles between 0 and 90 °. [ABSTRACT FROM AUTHOR]

• A novel ANN-CFD based modelling methodology reported for simulating flow in a batch vortex mixer. • Two-phase flow problem requiring interface tracking simulations reduced to a single-phase flow problem. • ANN model trained with a large number of experimental data is used to predict vortex shape. • Validation of ANN-CFD model with the PIV data reported in literature. • Velocity profiles obtained from ANN-CFD model show good match with the profiles obtained from VOF simulations. • Validated ANN-CFD model used to have detailed insights into the flow and turbulence structures. The study presents a novel ANN-CFD model for simulating flow in a vortex mixer (an unbaffled vessel stirred by a magnetic stirrer). Large eddy simulations (LES) are performed to simulate flow and turbulence considering presence of both air and liquid phases. The flow fields in air and liquid phases are coupled by applying suitable boundary conditions at the air–liquid interface represented by the vortex. The shape of the vortex is built-in in the computational domain and obtained by using an artificial neural network (ANN) model trained and validated with the data obtained from experiments carried out to capture the vortex shape under different parametric conditions. ANN-CFD model is validated with PIV data reported in literature. The ANN-CFD model reduces the computational time significantly by obviating the need of performing computationally very intensive interface tracking simulations. The model is used to understand hydrodynamics and turbulence characteristics of flow in the vortex mixer. [ABSTRACT FROM AUTHOR]

Characterizing airflow and turbulence patterns around real vegetation is essential for predicting aeolian erosion. However, few studies have explored the airflow field around live plants. In this field study, airflow field and turbulence patterns were investigated around a single Tamarix tree. Wind speeds were monitored at 34 locations around the tree at the heights of z/h = 0.13 h, 0.5 h, 1.1 h, and 1.8 h (where h is the height of the tree) in a wind speed of 12.5 m s−1. The results showed that a half-ellipse deceleration zone formed downwind and full-ellipse acceleration zones on the sides of the tree. The largest turbulence intensity was observed at lower heights and in the leeside of the tree where wind speed was highest, and the smallest was found on the sides where wind speed accelerated. On the sides of the tree the structure of wind speed and turbulence profiles was similar and no significant differences (P < 0.001) in wind speeds were found between identical locations. The mean wind velocities downwind of the tree at 0.5 h–7 h and DR1-DR4 (at the right side of the tree edge) were significantly different to the approach flow (wind speeds at −7 h) but no statistical differences were observed between wind speeds in other locations and the approach flow. The values of R c ∆ x , z (wind velocity reduction coefficient) were higher in the wake of the tree than upwind and on the sides of the tree indicating that the sheltering effect of the tree extended to the distance of seven times the height tree. This study represents an important investigation of the plant effect on airflow and the results can be used to inform Tamarix management and support parameterization of drag partition schemes to improve the accuracy of aeolian transport models in landscapes with Tamarix. [Display omitted] • Turbulence intensity was largest at lower heights and in the leeside of the tree. • Wind speed was lowest at lower heights and in the leeside of the tree. • Wind speed and turbulence profiles were similar on the sides of the tree. • Sheltering effect of the tree extended to a distance of 7 times the tree height. • The R c ∆ x , z values were higher in the wake than upwind and on the sides of the tree. [ABSTRACT FROM AUTHOR]

Visual understanding of air and particle flow is important and useful in guiding biomass combustor design. This study investigates the effects of declination angles of tangential and secondary air pipes in a low-density biomass combustor chamber to achieve perfect contact between air and rice husk particle. The simulation was aided by computational fluid dynamics (CFD) method using the standard k-ε turbulent model. According to the result, tangential air pipes with a declination angle of 75o provide the most intense air-particle contact which is maximal turbulent intensity of 168%, swirl amount of 3.0–3.5, particle path length of 5.1 m, and particle residence time of 2.3 s. Furthermore, four secondary air pipes with a declination angle of 60o lower the escaped particle amount to 11.45% from previously 71.24%. This configuration can also create recirculation zones in the bottom area of the combustor which elevates the particle residence time to 766.9 s. [Display omitted] • The air-particle turbulent flow behavior is complex phenomena. • CFD has been widely exploited to visualize the air-particle turbulent flow behavior. • Tangential air pipe declination of 75o gives the most intense air-particle contact. • Secondary air providence creates a recirculation zone in the combustor bottom area. • Perfect air-particle contact is expected to complete biomass combustion conversion. [ABSTRACT FROM AUTHOR]

The objective of this study is to experimentally and numerically investigate the aerodynamic behavior and the effectiveness of an air curtain confining cavity and subjected to external lateral stream. Experiments were carried out on a scale down model (1:5) representing a generic configuration of a display case, using LDV and PIV techniques to investigate the air flow characteristics. Two complementary modelling approaches were performed. The first is based on computational fluid dynamics (CFD) which enables to better understand the local air flow characteristics, while the second deals on the global fluxes exchanged between the air curtain, the ambient and the cavity. The good agreement obtained between CFD and experimental results allows to validate the numerical model and also to build the global model. Comparisons of experimental and numerical data obtained with and without external perturbation make it possible to quantify the effect of the perturbation on the air curtain characteristics such as airflow patterns, velocity profiles, maximum velocity decay, half-width jet growth and its stability as well as those related to the global fluxes exchanged between the air curtain and its surroundings. To evaluate the sealing performance of the air curtain with and without external perturbation, dimensionless parameters related to air tightness, thermal entrainment and thermal confinement efficiency were identified and analyzed. It was showed that for higher values of external lateral velocity, the performance of the air curtain was reduced consequently. [ABSTRACT FROM AUTHOR]

The dynamic characteristics of solution film flow with counter-current air flow, especially the two phase contact area, are important for the optimal design and operation of LDD (liquid desiccant dehumidifier). However, the unsteady interfacial information, including the interfacial pressure, velocity and film thickness, is hardly to be examined accurately by theoretical prediction or experimental measurement due to its complexity. In this study, CFD (computational fluid dynamic) models for the counter-current flow of the LDD were established based on the VOF (volume of fraction) and RNG (Renormalization group) k-ε turbulence model. Experimental research had been conducted to validate the liquid film thickness and typical interfacial waves obtained in simulation. With the established model, the dynamic formation process of unsteady counter-current flow is evaluated. Moreover, the effects of various parameters on the liquid film waves, pressure drop, liquid film thickness and interfacial information were obtained and analyzed by the built models. With the increase of liquid flow rate, the roll waves appear and the wave amplitudes are increased significantly, which may obviously enhance the performance in the LDD absorption and regeneration process due to the increased contact area. [ABSTRACT FROM AUTHOR]

Experiments in a stilling basin model with air injected at its downstream end exhibited plunging flow resulting in a downward roller, return flow along the bottom, and a downstream near-bed flow after the plunging flow impingement point. Turbulence measurements showed that air injection reduces turbulence both at the exit of the stilling basin and near the downstream bed. Thus the effect of air injection as a modifier of the turbulence structure of the flow downstream of the stilling basin is postulated as a plausible explanation for the observed scour reduction. [ABSTRACT FROM AUTHOR]

An advanced air cooling scheme that combines both active and passive cooling components is proposed and its thermal performance is demonstrated with single channel heat transfer experiments. The active cooling component, a piezoelectric translational agitator, generates strong air turbulence using a blade oscillating at a high frequency near either plain or micro pin-fin surfaces in the channel. The translational agitation of the blade is realized using an oval loop shell amplifier with a piezoelectric stack actuator. The micro pin-fin surfaces were fabricated by the LIGA photolithography technique. Single channel heat transfer experiments show promising results in the combined system with the micro pin-fin surface and the agitator. For instance, the combined system heat transfer coefficients were 250% of those on smooth surfaces without agitation. The channel flow rate was 40 LPM and the Reynolds number was 3300. Measurements are presented that assess, pin fin and agitation effects on thermal performance of the proposed active heat sink system for several channel flow rates. Based on these single channel test results, a multi-channel, full-size, active heat sink system utilizing micro pin fins and translational agitators is proposed, and its thermal performance is estimated. [ABSTRACT FROM AUTHOR]

Numerical study is made of heat transfer enhancement in the narrow channel with insulated walls in air steady flow around a heated spherical dimple, when its relative depth is varied from 0 to 0.26 (in terms of spot diameter) at a defined Reynolds number 4 × 10 4 based on bulk velocity and dimple spot diameter. The applicability of multiblock computational technologies for solution of Reynolds and energy equations with the implication of an implicit factorized finite-volume algorithm and overlapping different-scale structured grids of different topology, as well as the verification of the shear stress transfer model (SST model) modified with regard to the streamline curvature within the framework of Leschziner–Rodi–Isaev’s approach is assessed from the comparison of numerical predictions obtained by different SSTM versions. Flow regimes in a spherical dimple, as its depth is increased, are classified on the basis of the analysis of change in the jet–vortex flow structure in the dimple and its wake in the channel. In what follows, special attention is paid to asymmetric flow around a dimple with the greatest vortex heat transfer enhancement. [ABSTRACT FROM AUTHOR]

The effects of the radiative effects and the air variable properties (density, viscosity and thermal conductivity) on the buoyancy-driven flows established in open square cavities are investigated. Two-dimensional, laminar, transitional and turbulent simulations are obtained, considering both uniform wall temperature and uniform heat flux heating conditions. In transitional and turbulent cases, the low-Reynolds k − ω turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate have been obtained for a wide range of the Rayleigh number varying from 10 3 to 10 16 . The results obtained taking into account the variable thermophysical properties of air are compared to those calculated assuming constant properties and the Boussinesq approximation. In addition, the influence of considering surface radiative effects on the differences reached for the Nusselt number and the mass flow rate obtained with several intensities of heating is studied; specifically, the effects of thermal radiation on the appearance of the burnout phenomenon is analyzed. The changes produced in the flow patterns into the cavity when the radiative heat transfer and the effects of variation of properties are relevant, are also shown. [ABSTRACT FROM AUTHOR]

The influence of the roughness of the underlaying water surface on turbulence is studied in a stably stratified boundary layer (SSBL). Direct numerical simulation (DNS) is conducted at various Reynolds (Re) and Richardson (Ri) numbers and the wave steepness ka. It is shown that, at constant Re, the stationary turbulent regime is set in at Ri below the threshold value Ri depending on Re. At Ri > Ri, in the absence of turbulent fluctuations near the wave water surface, three-dimensional quasiperiodical structures are identified and their threshold of origin depends on the steepness of the surface wave on the water surface. This regime is called a wave pumping regime. The formation of three-dimensional structures is explained by the development of parametric instability of the disturbances induced by the surface water in the air flow. The DNS results are quite consistent with prediction of the theoretical model of the SSBL flow, in which solutions for the disturbances of the fields of velocity and temperature in the wave pumping regime are found to be a solution of a two-dimensional linearized system with the heterogeneous boundary condition, which is caused by the presence of the surface wave. In addition to the turbulent fluctuations, the three-dimensional structures in the wave pumping regime provide for the transfer of impulse and heat, i.e., the increase in the roughness of the water-air boundary caused by the presence of waves intensifies the exchange in the SSBL. [ABSTRACT FROM AUTHOR]

The wave generation model based on the rapid distortion concept significantly underestimates empirical values of the wave growth rate. As suggested before, inclusion of the aerodynamic roughness modulations effect on the amplitude of the slope-correlated surface pressure could potentially reconcile this model approach with observations. This study explores the role of short-scale breaking modulations to amplify the growth rate of modulating longer waves. As developed, airflow separations from modulated breaking waves result in strong modulations of the turbulent stress in the inner region of the modulating waves. In turn, this leads to amplifying the slope-correlated surface pressure anomalies. As evaluated, such a mechanism can be very efficient for enhancing the wind-wave growth rate by a factor of 2-3. [ABSTRACT FROM AUTHOR]

Four configurations of frustums (unrifled, inner-rifled, outer-rifled and two-faced rifled) were used to modulate the flow momentum. The rifle configuration transformed the axial flow velocity to radial direction for increasing the turbulence intensity ( T . I .). The increased T . I . improved the mixing between the air and fuel flow. Direct photography, Schlieren photography, hot-wire anemometry, thermocouple, and gas analyzer were utilized to measure the flame length, turbulence intensity, flame temperature, heat release, and exhaust-gas concentrations. The Schlieren flame structures show that the lifted flame is dismissed behind the unrifled, inner-rifled, outer-rifled, and two-faced rifled frustums; and therefore, the flame can be further stabilized. Behind the two-faced rifled frustum, the T . I . increased from 0.15% to 4.6% and the non-dimensional flame length ( H / D ) decreased from 24 to 14 ( i . e ., a flame shortening of around 42%). The low flame temperature approximately corresponded to the low concentration of nitride oxide ( C NO ). The high air–fuel mixing (caused by the high T . I .) corresponded to a high total combustion intensity and a low NO. [ABSTRACT FROM AUTHOR]

Cavities of various geometries occur in several technical applications and the knowledge about the heat transfer inside them is limited. For industrial steam turbines, this becomes critical since thermal casing distortion needs to be predicted reliably in the development process. A test rig concept is presented in this paper to investigate the forced convection heat transfer in cavities with various geometries where the enclosed fluid is driven indirectly by an external turbulent air flow. The dimensions are chosen to achieve similarity with side spaces in steam turbines. For estimating the flow conditions in the side spaces, a computational fluid dynamics analysis and experiments at a shallow water duct were conducted. Two different methods will be used to determine the heat transfer coefficient along the outer cavity wall of the compressed air test rig: the overtemperature method and the steady-state inverse method. In order to eliminate flow disturbances due to natural convection in the cavity and interference among the measurement systems, a parametrical finite element analysis was performed to determine an appropriate concept for the sensor arrangement. [ABSTRACT FROM AUTHOR]

A hydraulic jump is the sudden transition from a high-velocity impinging flow into a turbulent roller in an open channel. Substantial amounts of air are entrapped at the impingement point, and significant free-surface fluctuations take place above the roller. In the present study, some physical modelling was conducted in a relatively large sized facility. The flow conditions included a wide ranges of inflow Froude numbers and Reynolds numbers (3.8 < Fr1<  10.0, 2.1 × 104< Re<  1.6 × 105). The fluctuating features of free-surface and roller position were investigated non-intrusively with a series of acoustic displacement meters. The characteristic frequencies of the fluctuating motions were documented, and some major roller surface deformation patterns were revealed. The air-water flow properties were investigated with an intrusive phase-detection probe. The void fraction and bubble count rate data were documented in the jump roller, together with the interfacial velocity distributions. The rate of air entrainment was estimated based upon the void fraction and interfacial velocity distribution data. Some simultaneous measurements of instantaneous void fraction and free-surface fluctuations as well as longitudinal jump front oscillations were conducted. The relationship between the rate of air entrainment and turbulent fluctuations is discussed. Both the turbulent fluctuation and aeration properties are basic design parameters in urban water systems in which a hydraulic jump may take place. The present work provides relevant information for water systems including covered channels and partially-filled pipes. [ABSTRACT FROM AUTHOR]

In this work, the effects of corona discharges on a wing and flap system are studied through numerical simulation. The mathematical model, which is used, reproduces the overall phenomenon of boundary layer acceleration and does not intend to model the plasma itself. The model integrates an electro hydrodynamic force, which is a function of the electric charge density of the plasma to the Navier–Stocks equations. The CFD code FLUENT associated with a UDF module is used to solve the equations associated with the mathematical model and perform the simulations. Computations are validated through the turbulent air flow over a flat plate system. Numerical results of the high lift system show a significant increase of the lift coefficients between the angles of attack of 1.8° and 6° and a delay of the boundary layer separation. [ABSTRACT FROM AUTHOR]

This work studies the air flow in a large swirl counter-current dryer using sonic anemometry. Air velocity and turbulence fields are reported at isothermal conditions and in the absence of particles. In a tall-form unit the structure of the flow is largely influenced by the design of the exit. A contraction originates a central jet and suppresses the formation of recirculation zones despite the vortex acquires a high swirl intensity Ω (i.e. 1 < Ω < 2). Access to a full scale tower has permitted to: (a) identify asymmetries owed to the design of inlet and exhaust ducts, (b) present the first detailed turbulence data in production units, characterized by a highly anisotropic field and the axial decay of the turbulence kinetic energy, (c) study the flow stability, identifying the precession of the vortex core and oscillations at a constant Strouhal number and (d) study the impact that a rough wall has in the strength of the swirl. This work presents the first clear evidence of significant friction in spray dryers. The swirl intensity Ω decays exponentially in the dryer at a rate between 0.08 and 0.09, much higher than expected in pipe flow and independent of Re in the range 10 5 –2.2 ⋅ 10 5 . Production dryers have a large characteristic wall roughness due the presence of deposits, which explains the stronger friction and the discrepancies found in the past between data at full scale or clean laboratory or pilot scale units. It is essential to address this phenomenon in current numerical models, which are validated on laboratory or pilot scale facilities and ignore the role of deposits, thus causing an overprediction of the tangential velocity above 30–40%. [ABSTRACT FROM AUTHOR]

A numerical investigation of the influence of the opening angle of a circular cavity in an U-shaped channel and the Reynolds number of a fluid fl ow in this channel on the local characteristics and turbulence of this fl ow has been performed based on the solution of the Reynolds equations, closed by the old and new Menter shear-stress transfer models and two variants of this model accounting for the curvature of streamlines, with the use of multiblock computational technologies realized in the VP2/3 package. The results of calculations were compared with each other and with experimental data of I. Castro and R. Savelsberg. This comparison has shown that the best agreement between the numerical predictions and experiments is obtained in the case where calculations are performed within the framework of the Leshtsiner-Rody-Isaev approach with correction for the eddy viscosity of the fluid fl ow. It was established that with increase in the Reynolds number and in the opening angle of the cavity the circulation flow in the near-wall layer of the vortex trapped in the cavity intensifies at a practically constant vorticity in the core of the vortex. [ABSTRACT FROM AUTHOR]

Wind tunnel experiments are conducted on a single zone building with a two-sided windcatcher and a window. Mean and fluctuating surface pressures are measured to investigate the effect of the turbulent onset flows on building ventilation. Additionally, an analytical model is developed to estimate the unsteady ventilation flow rates through the building. The results are presented at different wind incidence angles and relative to the room window condition (opened versus closed window), showing the performance behaviour of the windcatcher. It is found that the mean flows can indicate generalised ventilation aspects for designing considerations. Meanwhile, the unsteady analyses should be considered when the building openings face indirect wind streams and when both a windcatcher opening and the (opened) window are presented as windward openings. Furthermore, the room window is essential when implementing windcatcher ventilation systems, as it defines their performance and dominates the interior flow behaviour. • Windcatcher performance analyses through steady and unsteady flow perspectives. • The unsteady flows are presented through frequency domains and spectra plots. • Mean steady ventilation flows are effective at direct wind incidences. • Unsteady ventilation flows are dominant at indirect wind incidences. • Considering turbulent airflows could provide better performance designs. [ABSTRACT FROM AUTHOR]

• Experiments on bubble breakage in turbulent flow performed. • An evaluation of existing breakage models presented. • New models for cascade breakage time and daughter size distribution proposed. • Constant parameters of breakage models are not universally usable. The breakage of bubbles in a turbulent fluid flow is studied to gain more insight into the gas bubbles breakage mechanisms and assist evaluating the breakage models used in Population Balance Modelling (PBM) for more accurate predictions. Experimental tests were performed for air bubbles in flows with different levels of turbulence intensity. A few of the existing models were evaluated against the experimental results and the measurements were used for determination of constant parameters of breakage functions. Moreover, new modifications to existing models were proposed to improve the prediction of breakage functions for the case of cascade breakage. The new functions proposed for the case of cascade breakage of bubble were also tested for liquid drop breakage. A discussion regarding the practicality of the conventional breakage models is presented. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies on the air-injection drag reduction (AIDR) of the ship model are conducted. For the experiments, the ship model and air-injection device are specially designed and the AIDR of the ship model is investigated considering one draft and three speeds. Then, considering two ship speeds, the AIDR of the ship model is simulated using two numerical models, namely the Euler-Euler two-fluid model and VOF model. It is found that for the surface ship there are still three drag reduction regions. Microbubble drag reduction (MBDR) is dominant for low air flow rates and air layer drag reduction (ALDR) is dominant for high air flow rates, and the transitional region exists for the medium air flow rates. For MBDR, the mechanism is related to the reduction of both effective dynamic viscosity and normal velocity gradient. Bubble size is the key influencing factor. Smaller bubbles are more favorable for drag reduction. Large millimeter bubbles can increase drag, which is owing to the extra turbulence viscosity. For ALDR, the mechanism is mainly owing to the loss of wet surface area and the key influencing factor is the air cover area. In general, ships with higher speed are more favorable for AIDR. • The surface ship model experiment is specially designed to investigate the air-injection drag reduction (AIDR). • Two CFD numerical models are applied to simulate the AIDR of the ship model. • For surface ships, three drag reduction regions still exist in AIDR. • For MBDR, bubble size is the key factor, and for ALDR the air cover area is the key factor. • Large millimeter bubbles can increase drag, which is associated with extra turbulence kinematic viscosity. [ABSTRACT FROM AUTHOR]

New design of system for providing cold air within the building was recommended in this study. Mixture of TiO 2 (with fraction of 0.04) and RT25 was applied as NEPCM and three-lobed ducts were filled with them. Air become warmer outside the building owing to sun radiation and with moving along the channel, its temperature reduces because of melting procedure. Homogeneous model for NEPCM has been incorporated in equations of paraffin zone which contain laminar flow. Also, the range of inlet velocity contains the turbulent flow at air flow with Re = 6e3 to 1e4. The temperature of inlet air (T in) was assumed as another active parameter. Irreversibility of system has been analyzed, too. Testing the exactness of procedure of modeling was applied according to previous published work and good accommodation has been reported. To decline the computing price, size of grid and time step were analyzed to find the optimized values. With adding TiO 2 particles, the temperature of paraffin enhances and speeds of charging increases. At t = 5hr, dispersing nano-powders makes temperature and LF to grow around 0.19% and 0.79%, respectively. The required time declines around 31.26% and 40.78% with rise of Re and T in. • New design of system for providing cold air within the building was studied. • Mixture of TiO 2 (with fraction of 0.04) and RT25 was applied as NEPCM. • The temperature of inlet air (T in) was assumed as another active parameter. • The required time declines around 31.26% with rise of Re. • At t = 5hr, dispersing nano-powders makes temperature to grow around 0.19%. [ABSTRACT FROM AUTHOR]

In this study, airflow and temperature distributions in the well-known International Energy Agency (IEA) Annex 20 room are predicted numerically to investigate the effects of the inlet turbulence intensity and the length scale on the flow characteristics, while considering the possibility of solution multiplicity related to chaos theory. The flow is considered to be turbulent, steady, incompressible, and two-dimensional. Computations are performed using the standard k – ɛ , RNG k – ɛ , standard k – ω , and shear stress transport k – ω turbulence models with scalable and automatic wall functions, and the results are compared with numerical and experimental results from the literature. The validated turbulence model is then used to investigate the effects of the turbulence intensity and the length scale. At a low inlet turbulence intensity value (Tu = 0.01), the length scale variation has no influence on the flow pattern. However, the length scale affects the flow pattern at a high inlet turbulence intensity value (Tu = 0.4). At constant low and medium length scale values, an increase in the inlet turbulence intensity from 0.01 to 0.4 affects the flow pattern. However, an increase in the turbulence intensity has no influence on the flow pattern at a constant high length scale value. [ABSTRACT FROM AUTHOR]

This paper reports a numerical investigation of the buoyancy induced turbulent air flow in an inclined passive wall solar chimney (IPWSC) attached to a room (ventilated space) for ventilation applications over a range of controlling parameters. The standard k − ɛ turbulence model has been employed to model air turbulence in the solar chimney system. With an isoflux heating condition specified on the absorber wall, the ventilation performance of the IPWSC design has been examined over the Rayleigh number range of 1.36 × 10 13 ≤ Ra ≤ 1.36 × 10 16 and inclination angles from 0̊ to 6̊. The present numerical result shows that the turbulent kinetic energy and turbulent intensity in the solar chimney decrease with the increase of the inclination of the passive wall. The effectiveness of the IPWSC design for enhancing the thermally driven ventilation has been confirmed. Additional simulations have been carried out with a standalone solar chimney model. The result shows that the standalone model over-predicts the mass flow rate compared to that predicted by the attached model. The average difference in the predicted mass flow rates between the standalone model and the attached model with inclination angles from 0̊ to 6̊ at a representative Rayleigh number of Ra = 1.36 × 10 14 is around 10%. [ABSTRACT FROM AUTHOR]

In this work, a comprehensive CFD model was developed for pyrolysis to provide significant emission reductions. In wood stoves with natural draft, the airflow is driven by buoyancy to overcome the resistance to flow within the stove. This combustion model is tested to reproduce at best the physical and chemical phenomena taking place in a natural draft wood log stove for low power (18 kW) residential heating. The RANS approach was used to solve the aerothermodynamic equations. A body fitted hexahedron mesh was adopted and the k-ε turbulence model was used to ensure closure of the Navier-Stokes equations. The chemical reaction for combustion was modeled using species transport and rate of species production formulate d as the EDDY DISSIPATION CONCEPT. The key parameters for validation are based on temperature inside the combustion chamber at the center plane and the [CO]/[CO2] ratio for emissions at the exit plane. Therefore, the developed modelling approach can be use d for engineering analysis and optimization of existing stoves and for relatively-quick evaluations of new stove designs. [ABSTRACT FROM AUTHOR]

In this paper, two- and three-dimensional direct numerical simulations (DNS) of autoignition phenomena in stratified dimethyl-ether (DME)/air turbulent mixtures are performed. A reduced DME oxidation mechanism, which was obtained using rigorous mathematical reduction and stiffness removal procedure from a detailed DME mechanism with 55 species, is used in the present DNS. The reduced DME mechanism consists of 30 chemical species. This study investigates the fundamental aspects of turbulence-mixing-autoignition interaction occurring in homogeneous charge compression ignition (HCCI) engine environments. A homogeneous isotropic turbulence spectrum is used to initialize the velocity field in the domain. The computational configuration corresponds to a constant volume combustion vessel with inert mass source terms added to the governing equations to mimic the pressure rise due to piston motion, as present in practical engines. DME autoignition is found to be a complex three-staged process; each stage corresponds to a distinct chemical kinetic pathway. The distinct role of turbulence and reaction in generating scalar gradients and hence promoting molecular transport processes are investigated. By applying numerical diagnostic techniques, the different heat release modes present in the igniting mixture are identified. In particular, the contribution of homogeneous autoignition, spontaneous ignition front propagation, and premixed deflagration towards the total heat release are quantified. [ABSTRACT FROM AUTHOR]

In this paper, a bluff-body flame structure has been modeled using conditional moment closure model (CMC) to study the effect of airstream dilution with CO 2 and H 2 O on nitric oxide (NO) production. This scenario would mimic the effect of exhaust gas recirculation (EGR) in an internal combustion (IC) engine or in a gas turbine. A 2D in-house code, employing a k–ε turbulence model with modified constant C ε 1 , has been used to calculate the flow and mixing field. Methane (CH 4 ) and di-methyl-ether (DME) have been separately used as fuels to assess the performance. Fuel jet velocities are varied to investigate the effectiveness of these dilutions at different levels of turbulence–chemistry interactions. The presence of both CO 2 and H 2 O in the airstream reduced the NO quantity for both the fuels. The effectiveness of the H 2 O diluent on mass basis is more prominent in reducing NO. With the increased fuel velocity, a situation similar to increased injection pressure in IC engines, higher amounts of diluents caused near extinction of the flame. For the same amount of diluents, global NO reduction in DME is higher compared to that for methane. [ABSTRACT FROM AUTHOR]

A dynamic and thermal simulation for two-dimensional model is developed on air flow and heat transfer control by chicanes in solar air collectors. New chicane form is adopted with two parts: the first is orthogonal to the air flow direction and the second is titled (α=60°). It is apparent that the turbulence created by introducing chicanes, resulting in greater increase in heat transfer inside the dynamic air vein with a rise of 23%. The effect of the variation of the Reynolds number in the range of 100

Comparative analysis of the results of solution of the steady-state Reynolds equations closed with the use of the shear-stress transfer model for the air fl ow in a divergent channel with suction of the air from the surface of the cylindrical central body positioned in the circular vortex cavity built in the lower wall of the channel with the corresponding experimental data has been performed. [ABSTRACT FROM AUTHOR]

Increase in the speed of train leads to aerodynamic problems. These problems occur due to turbulent behaviour of air flow around the train. So it is of great concern to study the dynamics of fluid flow. This paper deals with the dynamic analysis of empty freight wagons due to air turbulence using computational fluid dynamics. The effect of important aerodynamic coefficients such as aerodynamic drag, pressure distribution and velocity stream line has been studied on realistic and simplified freight train geometries. The geometry is a scaled model of freight open wagon type'BOXN25' from Indian Railways. The aerodynamic analysis for empty wagons shows that the critical stress zone and the air drag effect are comparable between the two considered geometries. [ABSTRACT FROM AUTHOR]

Transition of free convection flow of air over an inclined heated surface is investigated numerically by using a large eddy simulation method. In particular, we focus on how inclination angle of an upward-facing heated plate affects flow transition. Special attention is paid to the development of the thermal boundary layer and the transition from the laminar to turbulent stage. Results show that the transition occurs early when the plate is moved from its vertical position due to the rapid growth of both the velocity and thermal boundary layers. As a consequence, the critical Grashof number drops. Effects of the inclination of plate on the turbulent velocity fluctuations are also investigated, and the predicted results are in very good agreement with various experimental data available in the literature. [ABSTRACT FROM AUTHOR]

The 2013 devastating tornadoes in Oklahoma, Illinois and other states in Tornado Alley raise an important question: Can we do something to eliminate the major tornado threats in Tornado Alley? Violent tornadoes in Tornado Alley start from the clash-between northbound warm air flow and southbound cold air flow. As there is no mountain in Tornado Alley ranging from west to east to weaken or block the air flows, some clashes are violent, creating vortex turbulence called supercells. These supercells are initially in horizontal spinning motion at the lower atmosphere and then tilt as the air turns to rise in the storm's updraft, creating a component of spin around a vertical axis. About 30% of supercells develop into tornadoes, causing tremendous damages. Here we show that if we build three east-west great walls in the American Midwest, 300 m high and 50 m wide, one in North Dakota, one passing Oklahoma to east and the third one in the south Texas and Louisiana, we will weaken or block such air mass clashes and therefore diminish the major tornado threat in the Tornado Alley forever. We may also first build such great walls locally at some areas with frequent devastating tornado outbreaks and then gradually extend them. [ABSTRACT FROM AUTHOR]

Highlights: [•] Turbulent non-premixed swirling flames were studied upon the oxygen enrichment. [•] Flame stability is enhanced with oxygen enrichment even for low rates. [•] Reverse flow velocities near the burner exit lead to improve flame stabilization. [•] Oxygen enrichment decreases CO emissions but increases NOx emissions. [•] Increasing swirl intensity has noticeably effects on NOx emission reduction. [ABSTRACT FROM AUTHOR]