Air Velocity Components Research Articles

A Study of Turbulent Fluctuation of Three-Component Wind and Air Temperature in the Surface Layer of Baghdad Urban

Abstract In this study, the air temperature and velocity component means were also computed, as were the fluctuation values of the three component velocities (u, v, and w) and the air temperature (T). The instantaneous velocity can be broken down into its tally of varying and time-averaged contributions using a process known as Reynolds decomposition. The Reynolds decomposition depends on how you define the mean. The results show that the behaviors of the u, v, and w diurnal cycles are anticipated in ABL. The best fit is for air temperature because the correlation coefficient between time and temperature is large. The component fluctuated around zero, and the sum of the fluctuation averages equaled zero. The fluctuation of u was large, which increased because of the initial value of u and because of wind direction and friction velocity.

Comparative hydrodynamics study of fluidized bed gasifier incorporating static and rotating air distributor plates: A CFD approach

This paper studies hydrodynamics of bubbling fluidized bed gasifier using static and rotating perforated air distributor plates. Pressure drop across the bed is calculated numerically using a static distributor plate to validate the model results with available experimental data. CFD results are good enough within the acceptable range of 10% difference. Dynamic mesh methodology coupled with the siding mesh technique of ANSYS FLUENT is incorporated to rotate distributor plates for specified rotational velocity using a user-defined function (UDF) hooked to the solver. Hydrodynamics study of fluidized bed with varying operational parameters using rotating plate distributor is carried out extensively. It is revealed that the impact of tangential and radial velocities on the fluidized bed due to rotation of plate is highest for lower superficial velocities, shallow initial depth, and the maximum rotational velocity of the distributor plate. The tangential and radial velocities variations with column height up to 3Umf showed a similar trend that changed significantly for higher superficial velocities (4Umf, 5Umf), resulting in more chaotic solids. The comparative evaluation of static and rotating distributor plates results shows an appreciable difference. The pressure drop across the bed is increased about 6–7% and minimum fluidization velocity is decreased upto 10% when compared to static plate distributor. Moreover, bed height rise is on a higher side for static plate distributors due to the dominant axial component of air velocity compared to the rotating plate distributor. It is also revealed that the pressure fluctuations within the fluidized bed are significantly reduced (88%) when using a rotating distributor plate compared with static plate distributors.

Blade pitch and air-outlet width effects on the airflow generated by an airblast sprayer with wireless remote-controlled axial fan

The airblast sprayers are equipped with a fan generating an air stream that helps the sprayed droplets to reach out and penetrate the tree canopy. Recently an automatic air regulation system has been developed and integrated in an airblast sprayer equipped with a conventional axial fan (900 mm diameter). This system, thanks to a wireless connection between a dedicated tablet and sprayers actuators, allows to remotely control the blade pitch and the air-outlet section, varying the characteristics of airflow generated by the fan. Therefore, the present work aims to characterize the different air streams derived from the combination of two fan outlet section widths (110 and 150 mm) and three blade pitches (20°, 25° and 30°). For each combination, the three components of air velocity (m s−1) were measured, taking as reference the plane following the theoretical trajectory of the main current leaving the fan. In this plane, velocities were measured at 1.0, 3.0, 5.0 and 10.0 m from the outlet section on both sides of the sprayer. At each distance, the velocities from 0.25 to 4.00 m in height were recorded. This same procedure was repeated in two planes parallel to the reference plane, 0.30 m behind and after respectively. Additionally, the velocities in the fan outlet section were also measured to obtain the airflow rate. In general, the outlet section and the blade pitch had a significant effect on the velocity components. An outlet section of 110 mm meant a smaller airflow rate and a higher initial velocity, while with 150 mm the airflow rate reduced and the initial velocity decreased. Velocities could be bigger by enhancing the blade pitch. The turbulence intensity was similar at 1.0 m distance in all cases.

Exploring the relationship between particle deposition and near-wall turbulence quantities in the built environment

Particle deposition in the built environment can cause discoloration or damage to indoor surfaces. To better understand the deposition mechanisms, this study explored the relationship between particle deposition and near-wall turbulence quantities by means of experimental measurements and numerical simulations, taking a multi-slot supply air nozzle in a commercial airplane as an example. Chamber experiments were conducted in a cabin mockup to measure the particle deposition velocity distribution on the multi-slot nozzle and the near-wall 3D air velocity components. The large-eddy simulation (LES)-Lagrangian approach was used to calculate the turbulent flow in the cabin and particle deposition on the nozzle. Correlations between the particle deposition velocity (vd) and the wall-normal turbulence kinetic energy (kw), the wall-normal turbulence intensity (TIw), and a new turbulence quantity defined in this study (TIw,towards−wall) were examined in the near-wall buffer, logarithmic, and outer layers. According to the simulation results, the particle deposition velocity was significantly correlated with all three turbulence quantities (TIw, kw, TIw,towards−wall) in the logarithmic and outer layers. Furthermore, both the experimental and simulation results indicate that the new turbulence quantity defined in this study (TIw,towards−wall) was most closely correlated with the particle deposition velocity (vd) among the three near-wall quantities.

Solution of the system of gas-dynamic equations for the processes of interaction of vibrators with the air

The modern practice of using vibratory machines involving small seeds of low weight faces such an undesirable phenomenon as the effect exerted on the kinematics of vibrational movement of particles of fractions of the seed mixture by the aerodynamic forces and momenta. The periodic movement of air relative to the working planes of a vibratory machine arises due to fluctuations in the packets of these planes, which form flat aerodynamic channels. Consequently, the issues of studying the processes of interaction between the working bodies of vibratory machines and the air environment, aimed to justify their structural improvements, appear relevant. Existing mathematical models, which assess the parameters of air movement relative to the working planes of vibratory machines, produce only a generalized pattern and are flat. This paper proposes a statement, as well as an estimated finite difference scheme, of solving a three-dimensional boundary value problem on calculating the field of velocities and pressures in the region of air, located between two parallel synchronously oscillating planes. The problem employs a system of differential equations to describe the flow of the perfect gas. The finite difference scheme has been solved by a sweep method.Using the sweep method to solve these kinds of problems makes it possible to ensure the convergence and stability of estimation schemes, regardless of the step and other parameters of the grid applied. A variant of the calculation has been given, which demonstrated the feasibility of the proposed method for the assigned boundary conditions and parameters of the vibrational mode of machine operation. It has been established that in the working space enclosed between two oscillating planes there are both vertical (transverse) and horizontal (longitudinal) components of air velocity, which change over time

Particle Image Velocimetry (PIV) Investigation of the Turbulent Airflow in Slot-Die Melt Blowing.

In order to explore the forming mechanism of the fiber whipping motion in slot-die melt blowing, the turbulent airflow in slot-die melt blowing was measured online with the approach of the Particle Image Velocimetry (PIV) technique. The PIV results visualized the structure of the turbulent airflow and provided the distributions of air velocity components (vx, vy, and vz). Moreover, the PIV results also demonstrated the evolutive process of turbulent airflow at successive time instants. By comparing the characteristics of the turbulent airflow with the fiber whipping path, the PIV results provide a preliminary explanation for the specific fiber whipping motion in slot-die melt blowing.

Airflow patterns and turbulence characteristics above the canopy of a tomato crop in a roof-ventilated insect-proof screenhouse

Airflow patterns and turbulence characteristics inside a naturally ventilated screenhouse with tomato plants were experimentally investigated using 3D sonic anemometers simultaneously measuring air velocities at six horizontal positions in the space between the screened roof and the top of the canopy. The screenhouse had a flat roof and was ventilated through the roof only. Testing was carried out during about 3 weeks of crop development, under variable external conditions. The mean horizontal air velocity component above the canopy was generally lower than 0.20 Uoh (where Uoh is the mean horizontal external wind speed) and mostly in a direction opposite to the external wind. At low external windspeed, the airflow direction was not necessarily opposite to the wind direction. The vertical air velocity component was generally lower than 0.07 Uoh. The root mean square of the air velocity components, the turbulence intensity and the turbulence kinetic energy were much larger during the day than at night mainly due to the higher daytime effect of buoyancy and to a lesser extent due to the higher external wind speed. Spectral energy slopes of the velocity components were close to −5/3 during the day when wind speed was high. However, at night, when wind speed was low, the spectral energy slope of the velocity vector increased significantly. The integral length scales of turbulent eddies were greater in the horizontal than the vertical direction which is not surprising given the volume under consideration is bounded by the top of the canopy, the roof and the sidewalls.

Analysis of the Effect of the Swirl Flow Intensity on Combustion Characteristics in Liquid Fuel Powered Confined Swirling Flames

This article examines the implementation of CFD technology in the design of the industrial liquid fuel powered swirl flame burner. The coupling between the flow field and the combustion model is based on the eddy dissipation model. The choice of the LES (Large Eddy Simulation) turbulence model over standard RANS (Reynolds Averaged Navier-Stokes) offers a possibility to improve the quality of the combustion-flow field interaction. The Wall Adapting Local Eddy-Viscosity (WALE) sub-grid model was used. The reaction chemistry is a simple infinitely fast one step global irreversible reaction. The computational model was setup with the Ansys-CFX software. Through the detailed measurements of industrial size burner, it was possible to determine the natural operational state of the burner according to the type of fuel used. For the inlet conditions, axial and radial velocity components were calculated from known physical characteristics of both the fuel and air input, with the initial tangential velocity of the fuel assumed as 18% of the initial axial fuel velocity. Different swirl number (S) values were studied. Addition of a surplus (in comparison to conventional flame stabilization) of tangential air velocity component (W), the rotational component increases itself with a considerably high magnitude, contributing to the overall flame stabilization. The level of S especially influences the turbulent energy, its dissipation rate and turbulent (Reynolds) stresses. In the case of high swirl number values (S > 0,65) it is possible to divide the flow field in three principle areas: mixing area (fuel-air), where exothermal reactions are taking place, central recirculation area and outer recirculation area, which primarily contains the flow of burnt flue gases. The described model was used to determine the flow and chemical behavior, whereas the liquid atomization was accounted for by LISA (Linear Instability Sheet Atomization) model incorporating also the cavitation within injection boundary condition. The boundary conditions were determined based on the data from the experimental hot water system. Depending on system requirements, especially with continuous physical processes as well as the results of experimental measurements, the paper reports on determination of the mixing field and its intensity in the turbulent flow, the description of heat release and interaction of turbulent flow field and chemical kinetics in the case of confined swirling flames.

A study on tilted tunnel fire under natural ventilation

Tilted tunnel fire under natural ventilation has been discussed. Smoke movement in a scale tunnel model of length 8m, width 1.5m and height 1m was studied. Air temperature distribution and velocity components along the longitudinal axis at the tunnel opening were measured. Simulations using Computational Fluid Dynamics on smoke movement in the tilted tunnel fire were then carried out. Buoyancy of smoke layer in the tilted tunnel model was deduced by integrating experimental data with simulations results. Smoke velocity distributions in different tilted tunnels were studied numerically. For a horizontal tunnel, the smoke temperature decay rate along the longitudinal direction can be described by an exponential function. For tunnels tilted from 3° to 9°, smoke temperature decayed with different exponential functions on the two sides of the fire. The smoke velocity along the longitudinal axis was not symmetric about the fire source, but with a maximum value located on the leeward side in tunnels tilted at 3–9°. The neutral plane of flow disappeared at the lower opening of the tunnel when the angle was above 9°. Empirical expressions of smoke temperature and velocity decays along the longitudinal axis for a tilted tunnel were also derived.

Two-dimensional analytical model of dry air thermal convection

In the present work, the steady-state stationary dry air thermal convection in a lower atmosphere has been studied theoretically. The thermal convection was considered without accounting for the Coriolis force, and with only the vertical temperature gradient. The stream function has been analytically obtained within the framework of two-dimensional thermal convection model in the Boussinesq approximation with velocity divergence taken as zero. It has been shown that the stream function is symmetrical about the horizontal and vertical. The expressions for the horizontal and vertical air velocity components have been obtained. The maximal vertical velocities level is in the center of the convective cell where the horizontal air velocity component is equal to zero. It has been shown that the air parcel’s rotation period during the thermal convection is determined by the Brunt–Vaisala frequency. The expression for the maximal air velocity vertical component has been found. The dependence of the maximal air velocity vertical component on the overheat function at ground surface and on the atmosphere instability has been demonstrated. The expression for the pressure disturbance has been obtained. It has been demonstrated that at the points with maximal pressure disturbance the vertical velocity is equal to zero and the horizontal velocity is maximal. It has been found that the convection cell size depends on the atmosphere stability state.

New Method for Direct Numerical Simulation of Three-Dimensional Ice Accretion

A simple and reasonable mathematical model is developed to simulate ice accretions on three-dimensional bodies directly. Based on the extended heat-transfer model proposed by Myers, this paper brings forward the concept of critical ice thickness as a criterion for the existence of overflow in every icing control volume. Besides, the concept of inner time step is proposed. After each inner time step, the ice-layer thickness and associated icing properties including the ice accretion rate and overflow in all icing control volumes will be updated to make the model more close to the physical reality. Then, the method for deciding the outflow through boundaries of each icing control volume is given based on the contravariant air velocity components just above the icing surface. Ice accretions under typical glaze ice conditions on a GLC-305 swept-wing model are calculated, and the predicted sectional ice shapes are compared with the experimental data as well as LEWICE results. It indicates that the current results reflect some major characteristics of glaze ice and the spanwise overflow phenomenon. Despite some discrepancies between the numerical results and experimental data, the overall growth patterns are reproduced well.

Gusts caused by high-speed trains in confined spaces and tunnels

Little is known of the behaviour of transient air velocities and dynamic pressure loads generated by high-speed trains in confined spaces, or whether current methodologies for assessing transient gust loads in open spaces can be used in confined spaces. Experiments have been carried out in which a moving-model high-speed train passed walls, a partially-enclosed tunnel, and single-track tunnels with a variety of cross-sectional areas and lengths. An open air control experiment has also been carried out. The train model was a simplified 1/25 scale four-carriage ICE2 train travelling at 32m/s. Cobra Probes measured the three-dimensional air velocity components at various positions inside the structures. The results show that the peak gust magnitudes increase in all confined cases compared to the open air. In tunnels, a ‘piston effect’ appears to have been a dominant cause of the increases in the peak gust magnitudes, as well as prolonged winds occurring before and after the train passed the probes. The tunnel length impacted considerably on the flow characteristics, and the partially-enclosed tunnel showed further increases in the gusts due to high lateral and vertical velocities.

Characterization of turbulent airflow over evolving water‐waves in a wind‐wave tank

Results of an experimental investigation of the turbulent boundary layer in airflow over evolving young wind‐waves are presented. The experiments were conducted in a laboratory wind‐wave flume consisting of a closed‐loop wind tunnel capable of generating wind speed that may exceed 15 m/s, atop of a 5 m long wave tank. Simultaneous measurements of mean wind velocity and of instantaneous fluctuations of the horizontal and vertical air velocity components were carried out along the test section at different airflow rates and at numerous heights above the highest wave. Instantaneous surface elevation at the air sensors' location was simultaneously recorded. The friction velocities at all locations and for all airflow rates were determined by two independent methods: by fitting the logarithmic velocity profiles and by extrapolating the measured Reynolds shear stresses to mean water surface level. The variation with height and along the test section of the fluctuations of two velocity components, in the mean flow and in the vertical directions, was also studied and the results compared with flow behavior over rough and smooth plates. Wave‐induced airflow parameters were then investigated by application of cross‐spectral analysis. Results on the vertical extent of wave‐induced boundary layer, on the phase relation between the wave‐induced velocity fluctuations and the surface elevation, as well as on the wave‐induced Reynolds shear stress are reported.

Determinación de los patrones de flujo de aire en un invernadero multitúnel mediante anemometría sónica

En este trabajo se ha desarrollado una metodología para estudiar la ventilación natural en invernaderos mediterráneos mediante anemometría sónica. Los ensayos se han realizado en un invernadero multitúnel de tres módulos situado en el Campo de Prácticas de la Universidad de Almería. Esta metodología nos ha permitido obtener los patrones de ventilación natural del invernadero experimental para los vientos más comunes de la región. También nos ha permitido describir la interacción entre el efecto eólico y térmico, así como detectar deficiencias en la ventilación natural del invernadero debidas al efecto barrera de otro invernadero adyacente (provocando una reducción media en la velocidad del aire en el exterior de la ventana de barlovento de hasta el 98% para u, 63% para uy y, más importante, 88% para ux, la componente de la velocidad del aire que es perpendicular a la superficie de ventilación lateral). Los conocimientos adquiridos en este trabajo nos permitirán mejorar los algoritmos de control del sistema de ventilación. En este trabajo se realizan una serie de propuestas que podrían mejorar sustancialmente la ventilación natural del invernadero. Por ejemplo, se proponen algunas posibles modificaciones de la configuración de las ventanas laterales, buscando una mayor captación del aire exterior: se podría optar por ventanas dotadas de alerones exteriores que dirijan al aire hacia el interior o ventanas en las que la malla no se coloque directamente sobre la superficie lateral del invernadero, diseñando una antesala donde el aire pueda entrar sin la oposición de la malla. Otra propuesta sería prolongar la apertura de las ventanas laterales hasta el suelo, para favorecer así la entrada de aire a nivel del cultivo.

The Impact of Insect Screens and Ventilation Openings on the Greenhouse Microclimate

The objective of this work was to experimentally investigate the influence of vent type (side, roof, or both) and of anti-aphid insect screens on airflow, air temperature, and air vapor pressure deficit distribution in a round arch, mono-span greenhouse with vertical side walls. The greenhouse was equipped with two side roll-up vents and a flap roof vent. A tomato crop planted in double rows was cultivated inside the greenhouse. The three components of air velocity were measured by a 3-D sonic anemometer, and the air temperature and relative humidity were simultaneously recorded at several positions inside the greenhouse. Concerning the effect of insect screens, it was found that the mean value of the normalized air velocity was 58% lower in the greenhouse with insect screens on the side vent openings than in the case of a greenhouse without screens. Furthermore, the spatial heterogeneity of the microclimate variables was reduced with screens in the vent openings. When the ventilation was provided by side openings only, the air velocity inside the greenhouse was characterized by a strong air current near the greenhouse ground and low air velocity near the roof; when the ventilation was provided by roof vents, a circulating current prevailing at the center of the greenhouse was observed. The combined use of roof and side openings increased air velocity and decreased air temperature inside the greenhouse but also increased the spatial heterogeneity of the greenhouse microclimate compared to the cases with side or roof vents only. The most homogeneous climate conditions were achieved with the use of roof openings only. The results of this study provide a better understanding of the plant environment behavior under different vent configurations and a high-resolution database for validating on-going efforts with computer simulations.

The Effect of Vent Configuration and Insect Screens on Greenhouse Microclimate

The influence of insect screens and vent configurations on airflow and temperature patterns in an arched plastic covered greenhouse used for cultivating tomatoes was analysed using a commercial computational fluid dynamics (CFD) code. The numerical model was first validated against experimental measurements which were carried out in an equivalent greenhouse incorporating continuous side openings. Measurement data included the three Cartesian components of air velocity, determined by means of a sonic anemometer, and the spatial distribution of air temperature measured using a fast response temperature sensor. The experimental data were used to define proper and realistic boundary conditions for use in the numerical model. Numerical results predicted a gradual increase of air temperature and a decrease of air velocity as the porosity of the tested insect screen was reduced. It was found that vent configuration affects the ventilation rate and the climate distribution inside the greenhouse.

Measured vorticity distributions in a model of tangentially fired furnace

Simultaneous three-component six-sensor hot-wire probe measurements were made in a model of tangentially fired furnace (TFF) to measure the flow properties of an isothermal flow field. The quantities measured including the spatially resolved isothermal air velocity components and their two higher order moment statistical properties, i.e., the skewness and flatness factors, and the vorticity components and their skewness and flatness factors. Detailed experimental data provided an improved understanding of the behavior of the turbulent transport in the burner zone and in the upper and central regions of the TFF. The measured velocity and vorticity fields showed the swirl characteristics of the fluid flow in the TFF interior as expected and that the flow has attached to the walls in the burner zone and central region which is not desired in an operating TFF. The solid walls in these domains, including the pendants located in the upper region, act as vortex generators. Oscillation of the velocity components, and of the other quantities measured in the experiment, may be derived from the fragmentation of the vortexes and the coherent structures existing in the furnace interior. This work helps clarity the fouling and slagging mechanisms and the flow attachment in the burner zone and central region, the flow deviation and residual swirling downstream the pendants in the upper furnace, by giving insight into the vorticity and the coherent structures in the TFF.

Quantifying acceptable mesh dependencies for computational investigations of airflows within rooms

Ignorance of mesh dependency can sometimes be an embarrassment in numerical calculations. Mesh structures need to be developed to eradicate mesh dependency without compromising the finite computing resource and/or incurring large computational expense. For the inexperienced, mesh deployment often degenerates into a time-consuming iterative procedure. To understand mesh influences, different mesh-structures have been applied to two simple, yet typical, room–air–ventilation configurations. The predictions, using various mesh regimes, have been compared with a datum result set: marks have been awarded according to the closeness of comparison to mesh-independent predictions. This mark, i.e. the ‘Mesh Independence Number’ (MIN), is a numerical indicator of the closeness to achieving a mesh in-dependent solution. A relationship for the MIN versus number of nodes is presented together with relationships for the average values of the air-velocity components. In many instances, high correlation-coefficients are established which suggests that this MIN methodology could reduce the need for iterative time-consuming mesh-dependency studies.

Lee-side Ventilation-induced Air Movement in a Large-scale Multi-span Greenhouse

Lee-side ventilation-induced air movement was experimentally investigated in a 1728 m2multi-span glasshouse. Air velocity measurements were performed by means of a customized multi-point two-dimensional sonic anemometer system which simultaneously measured the horizontal air velocity components parallel and normal to the ridge at 3 Hz frequency. The airflow pattern in the horizontal plane of a greenhouse was analysed qualitatively under different external wind conditions. The experimental results show both the vertical and the horizontal profiles of the normalized interior air speed as a function of the measurement height, the external wind speed and opening angle were acceptably presented by the simple linear functions. The airflow in the horizontal plane was of low turbulence with large eddies.

Measurements of air flow characteristics in a full-scale clean room

Detailed air turbulence characteristics in a full-scale and running fan—filter unit type clean room (4.20 m × 2.97 m × 2.70 m) with and without a clean bench loaded are investigated experimentally. The measurements of the three components of air velocity were conducted by means of a three-dimensional ultrasonic anemometer. Velocity vectors, turbulence intensity, turbulence kinetic energy, and probability density functions of velocity direction and velocity contours in the space domain are presented. The results show that the non-uniformity and deflection angle of air velocity are within 13% and 7.9 degrees, respectively. Moreover, the mean flow patterns, turbulence characteristics, and histogram features of the air flow are discussed. The detailed experimental data are not only useful for clean room design and equipment layout but also helpful for use in the evaluation of numerical modeling in clean room flow fields.