Turbulent Air Flow Research Articles

The effects of small water surfaces on turbulent flow in the atmospheric boundary layer: URANS approach implemented in OpenFOAM

A 3-D numerical method is developed to investigate the spatial distribution of surface fluxes over heterogeneous surfaces in (semi-)arid regions. Quantifying the effects of changes in the momentum, thermal and moisture roughness lengths on the airflow and fluxes in the ABL is important for water resources management and local climate studies. The governing equations and turbulence models are modified to include the effects of atmospheric stability conditions on the airflow. The turbulent airflow in ABL is simulated based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach to understand the air flow over the non-homogeneous surfaces from dry land through the water surface and vice versa. The model can be used to study airflow in neutral and non-neutral ABL over complex and non-homogeneous surfaces. The model results were used to investigate the flow parameters and (heat) flux variations over small water surfaces considering its surrounding conditions.

Simulation of Airflow in the Burning Cave of an Auxiliary Heating System in a Greenhouse

Abstract. In this study, numerical simulations of airflow were carried out in the burning cave of an auxiliary heating system. Experimental measurements were also conducted to verify the performance of the numerical model, and turbulent airflow in the burning cave was considered. The numerical simulation in the burning cave was performed for three cases:(1) with a baffle at the bottom of the burning cave entrance, (2) without a baffle at the burning cave entrance, and (3) with a baffle at the top of the burning cave entrance. The turbulent airflow was modeled using the realizable k-e turbulence model as well as the non-equilibrium wall function. The airflow velocity was assessed in the burning cave, and some suggestions were given to improve the performance of the burning cave. The results showed that the airflow entering the burning cave differed due to different positions of the baffle. The smoldering combustion was more even and the burning rate could be controlled more easily when the baffle was placed at the top of the burning cave entrance, making the airflow enter the burning cave through the bottom of the baffle. The results also showed that the maximum airflow velocity in the burning cave increased with increased distance between the baffle and the bottom of the burning cave. Keywords: Airflow, Burning cave, Greenhouse, Simulation.

CFD Analysis and Electrical Efficiency Improvement of a Hybrid PV/T Panel Cooled by Forced Air Circulation

The thermal and electrical efficiency of a custom-designed PV/T panel cooled by forced air circulation was investigated by experimental and computational fluid dynamics (CFD) analysis. Experiments were carried out with four different array configurations, under constant irradiation of 1100 W/m2 and 3 different air velocities (3.3 m/s, 3.9 m/s, and 4.5 m/s). The heat transfer surface area and forced air circulation are known to positively affect the total heat transfer, and therefore, it is foreseeable that an increased number of fins and higher air velocities will help maintain the electrical efficiency of the panel at higher levels. The main objective of this study is to determine the critical threshold for the abovementioned parameters as well as to show how important parameters, such as fin arrangement and consequent turbulent air flows, are for satisfying the heat removal needs. Highest efficiency was achieved with a 108 pc type 1 arrangement at 12.02% as expected. Nevertheless, while the 108 pc type 2 arrangement could maintain the electrical efficiency at 11.81%, a close level of 11.55% could be obtained with a 54 pc type 2 arrangement. Experimental results are compared with ANSYS Fluent program, and the effect of the number and arrangement of the fins on the efficiency of the panel has been shown.

Detailed measurements of interfacial dynamics in air-water pipe flow

Stratified air-water flow in a horizontal pipe is investigated experimentally using particle image velocimetry and conductance probes. This flow regime is characterized by a complex interplay between a turbulent airflow and propagating waves at the interface. The waves are generated by interfacial shear and pressure forces exerted by the faster flowing airflow. The goal of this study is to characterize the waves by means of statistical and spectral methods, and to explore the influence of different wave regimes on the airflow.Two cases in which the air bulk velocity increases from 2.4 m/s (case A) to 3.5 m/s (case B), while the liquid velocity remains constant at 0.26 m/s, are assessed in detail. Case A belongs to a region of flow conditions in which wave amplitudes grow as a consequence of increasing gas flow rates, i.e., wave growth regime. Meanwhile, case B is in a regime of saturated wave amplitudes. In the first case, the interface was populated by small amplitude 2D waves of relatively small steepness (ak ≈ 0.07). These waves obey Gaussian statistics and are thus considered to be linear. In the second case, the waves are larger, steeper (ak ≈ 0.13) and considerably more irregular. They display non-linear behaviour (steep crests and long troughs) and their exceedance probability distribution deviates significantly from Gaussian statistics. Bicoherence maps show evidence of both overtone and sub-harmonic interactions.Airflow velocity fields acquired by PIV were subjected to a conditional phase-averaging method based on a steepness criterion. The phase-averaged vorticity field shows evidence of shear-layer separation above the steeper waves of case B. Hence, in addition to non-linear mode interactions and micro-breaking, shear-layer separation may contribute to the transition from the growth regime to the saturation regime.

Influence of slit sizes on the interaction structure of supersonic turbulent air flow with a multi-component injection jet in a channel

The supersonic air flow in a flat channel with transverse injection of a turbulent hydrogen jet through a slit in the bottom wall is numerically simulated. The solution of the initial Favre-averaged Navier-Stokes equations closed by the k-ω turbulence model is performed using an algorithm built on the basis of the WENO scheme. The interaction of a shock-wave structure with boundary layers on the lower and upper walls under conditions of an internal turbulent flow is investigated, namely, the effect of the width of the jet slit is studied. It is found that, in addition to the known shock-wave structures arising from the interaction of the incident flow with the transverse jet and the interaction of the main shock wave with the boundary layers near the walls, there is an additional system of shock waves and separation of the flow on the bottom wall at some distance from the jet downwards flow. Comparison with experimental data showed satisfactory agreement. undefined undefined Звуковая функция ограничена 200 символами Настройки : История : Обратная связь : Donate Закрыть

Performance improvement in a tubular heat exchanger by punched delta-winglet vortex generators

A novel tubular heat exchanger incorporated with punched delta-winglet vortex generators (called perforated delta-winglet vortex generator, P-DWVG) is proposed for improving its thermal performance and energy saving. The P-DWVG elements are punched out from a straight tape having its width nearly equal to the tube diameter before insertion. The main aim at employing the P-DWVG insert is to produce counter-rotating vortices along the tube to promote turbulence intensity inside as well as to transport the cold fluid at the central core to the near-wall regions. The experiment was performed to study thermal behaviors in a uniform heat-fluxed tube inserted with P-DWVGs. The P-DWVGs with the attack angle of 45° were mounted periodically with three different blockage ratios (BR = 0.1, 0.2 and 0.3) and two pitch ratios (PR = 2 and 3). Air as the test fluid was varied to obtain turbulent airflow for Reynolds number (Re) in a range of 4,150-25,500. The experimental results show that the P-DWVG provides a considerable increase in the rate of heat transfer around 3.1-4.01 times whereas friction factor increases around 11.44- 34.23 times higher than the plain tube. To assess the real benefits of P-DWVGs, thermal performance factor (TEF) is examined and in the range of 1.39-1.48 where its maximum is at BR = 0.1 and PR = 2.

Applying CFD for Studying the Dynamic and Thermal Behavior of an Indirect Solar Dryer: Paramertric Analysis

Abstract In this work, an indirect solar dryer integrated thermal storage for drying figs. (Ficuscarica) is studied numerically. Unsteady turbulent airflow and heat transfer through a two-dimensional model is carried out for a typical day of August under the climatic conditions of Tlemcen (Algeria). Effects of air inlet size and thickness of the packed bed on the dynamic and thermal behaviors of the dryer with and without packed bed have been discussed. The study shows that: (1) Increase the inlet size from 0.04 m to 0.10 m can accelerate the extraction of air by about 13% and reduce the maximum crops temperature by about 14%. (2) The packed bed can reduce the mass flow rate extracted by 22% and the fluctuations of air temperature by 1.3%. (3) A packed bed with a thickness of 0.15 m can extend the operating time of the dryer up to 23%.

Quasi-linear approximation for description of turbulent boundary layer and wind wave growth

This study describes an approximate quasi-linear model for the description of the turbulent boundary layer over steep surface waves. The model assumes that wave-induced disturbances of the atmospheric turbulent boundary layer could be reasonably described in a linear approximation with the momentum flux from wind to waves retained as the only nonlinear effect in the model. For the case of periodic long-crested waves, the model has been verified with a set of the original laboratory and numerical experiments. The laboratory experimental study of the airflow over the steep waves was performed by means of the PIV technique. The numerical study was performed with direct numerical simulation (DNS) of the turbulent airflow over waved surface at Re=15,000 for quasi-homogeneous waves, wave trains and parasitic capillaries riding on the crest of a steep waves. Examples are given of the application of the quasi-linear approximation to describe the turbulent boundary layer over waves with the continuous spectrum under the assumption of random phases of harmonics. In the latter case the quasi-linear model provides the growth rate of surface waves in the inertial interval of the surface wave spectrum proportional to w7/3 in agreement with predictions in [1].

CFD simulation of pesticide spray from air-assisted sprayers in an apple orchard: Tree deposition and off-target losses

The ultimate goal of a pesticide spraying system is to provide adequate coverage on intended canopies with a minimum amount of spray materials and off-target waste. Better spray coverage requires an understanding of the fate and transport of spray droplets carried by turbulent airflows in orchards. In this study, an integrated computational fluid dynamics (CFD) model was developed to predict displacement of pesticide spray droplets discharged from an air-assisted sprayer, depositions onto tree canopies, and off-target deposition and airborne drift in an apple orchard. Pesticide droplets discharged from a moving sprayer were tracked using the Lagrangian particle transport model, and the deposition model was applied to droplets entering porous canopy zones. Measurements of the droplet deposition and drift in the same orchard were used to validate the model simulations. Good agreement was found between the measured and simulated spray concentrations inside tree canopies and off-target losses (ground deposition and airborne drifts) with the overall relative errors of 22.1% and 40.6%, respectively, under three growth stages. The CFD model was able to estimate the mass balance of pesticide droplets in the orchard, which was practically difficult to investigate by measurements in field conditions. As the foliage of trees became denser, spray deposition inside canopies increased from 8.5% to 65.8% and airborne drift and ground deposition decreased from 25.8% to 7.0% and 47.8% to 21.2%, respectively. Higher wind speed also increased the spray airborne drift downwind of the orchard. This study demonstrates that CFD model can be used to evaluate spray application performance and design and operate sprayers with increased spray efficiencies and reduced drift potentials.

Correlations for heat transfer coefficient and friction factor for turbulent flow of air through square and hexagonal ducts with twisted tape insert

The experimental and numerical Nusselt number and friction factor investigation for turbulent flow through a non-circular duct with twisted-tape inserts have been presented. The non-circular ducts include square, hexagonal duct. The results of non-circular ducts are compared with circular duct. All the ducts have same equivalent diameter. The twist ratios used for the experiment are Y = 3.5, 4.5, 5.5 and 6.5. Experiments were carried out on square duct, hexagonal duct and circular duct. The Reynolds number lied between 10,000 and 1, 05,000. The present study is restricted to the flow of air at Pr = 0.7 only and within a narrow temperature range of 40 to 75 ΟC, within which the compressible nature of air can be neglected. The results reveal that, both Nusselt number and friction factor increases as the side of non-circular duct increases. Maximum Nusselt number and friction factor is obtained in case of circular duct with twisted tape. Further the correlations of Nu and f are given for different non circular duct with twisted tape insert for engineering applications for the turbulent regime. Since the thermal performance factor (η) is observed to be within the range of 0.8 to 1.13 for both circular and noncircular ducts, the overall benefit of using twisted tape in the flow field shall nevertheless be marginal.

Controlling the Flow past a Semicircular Airfoil at Zero Angle of Attack Using Slot Suction in One or Two Vortex Cells for Attaining Extremal Lift

Calculations using multiblock computational technologies and a model of shear-stress transport modified with allowance for the curvature of streamlines in turbulent airflow were performed at a zero angle of attack for a semicircular airfoil containing one or two surface vortex cells with slot suction. The results showed evidence of stabilization of a nearly undetached flow and attainment of an extremal lift of C y = 5.2 and a lift-to-drag ratio of K = 24 with allowance for energy losses for suction in the vortex cells.

Ideal Particle Sizes for Inhaled Steroids Targeting Vocal Granulomas: Preliminary Study Using Computational Fluid Dynamics.

Objectives Vocal fold granulomas are benign lesions of the larynx commonly caused by gastroesophageal reflux, intubation, and phonotrauma. Current medical therapy includes inhaled corticosteroids to target inflammation that leads to granuloma formation. Particle sizes of commonly prescribed inhalers range over 1 to 4 µm. The study objective was to use computational fluid dynamics to investigate deposition patterns over a range of particle sizes of inhaled corticosteroids targeting the larynx and vocal fold granulomas. Study Design Retrospective, case-specific computational study. Setting Tertiary academic center. Subjects/Methods A 3-dimensional anatomically realistic computational model of a normal adult airway from mouth to trachea was constructed from 3 computed tomography scans. Virtual granulomas of varying sizes and positions along the vocal fold were incorporated into the base model. Assuming steady-state, inspiratory, turbulent airflow at 30 L/min, computational fluid dynamics was used to simulate respiratory transport and deposition of inhaled corticosteroid particles ranging over 1 to 20 µm. Results Laryngeal deposition in the base model peaked for particle sizes 8 to 10 µm (2.8%-3.5%). Ideal sizes ranged over 6 to 10, 7 to 13, and 7 to 14 µm for small, medium, and large granuloma sizes, respectively. Glottic deposition was maximal at 10.8% for 9-µm-sized particles for the large posterior granuloma, 3 times the normal model (3.5%). Conclusion As the virtual granuloma size increased and the location became more posterior, glottic deposition and ideal particle size generally increased. This preliminary study suggests that inhalers with larger particle sizes, such as fluticasone propionate dry-powder inhaler, may improve laryngeal drug deposition. Most commercially available inhalers have smaller particles than suggested here.

How tall buildings affect turbulent air flows and dispersion of pollution within a neighbourhood.

The city of London, UK, has seen in recent years an increase in the number of high-rise/multi-storey buildings ("skyscrapers") with roof heights reaching 150m and more, with the Shard being a prime example with a height of ∼310m. This changing cityscape together with recent plans of local authorities of introducing Combined Heat and Power Plant (CHP) led to a detailed study in which CFD and wind tunnel studies were carried out to assess the effect of such high-rise buildings on the dispersion of air pollution in their vicinity. A new, open-source simulator, FLUIDITY, which incorporates the Large Eddy Simulation (LES) method, was implemented; the simulated results were subsequently validated against experimental measurements from the EnFlo wind tunnel. The novelty of the LES methodology within FLUIDITY is based on the combination of an adaptive, unstructured, mesh with an eddy-viscosity tensor (for the sub-grid scales) that is anisotropic. The simulated normalised mean concentrations results were compared to the corresponding wind tunnel measurements, showing for most detector locations good correlations, with differences ranging from 3% to 37%. The validation procedure was followed by the simulation of two further hypothetical scenarios, in which the heights of buildings surrounding the source building were increased. The results showed clearly how the high-rise buildings affected the surrounding air flows and dispersion patterns, with the generation of "dead-zones" and high-concentration "hotspots" in areas where these did not previously exist. The work clearly showed that complex CFD modelling can provide useful information to urban planners when changes to cityscapes are considered, so that design options can be tested against environmental quality criteria.

Airflow measurements at a wavy air\u2013water interface using PIV and LIF

Physical phenomena at an air–water interface are of interest in a variety of flows with both industrial and natural/environmental applications. In this paper, we present novel experimental techniques incorporating a multi-camera multi-laser instrumentation in a combined particle image velocimetry and laser-induced fluorescence system. The system yields accurate surface detection thus enabling velocity measurements to be performed very close to the interface. In the application presented here, we show results from a laboratory study of the turbulent airflow over wind driven surface waves. Accurate detection of the wavy air–water interface further yields a curvilinear coordinate system that grants practical and easy implementation of ensemble and phase averaging routines. In turn, these averaging techniques allow for the separation of mean, surface wave coherent, and turbulent velocity fields. In this paper, we describe the instrumentation and techniques and show several data products obtained on the air-side of a wavy air–water interface.

Ensuring safe descend of reusable rocket stages – Numerical simulation and experiments on subsonic turbulent air flow around a semi-circular cylinder at zero angle of attack and moderate Reynolds number

Two-dimensional flow around semi-circular cylinder at zero angle of attack and at Re = 50000 during the self-oscillatory regime has been extensively studied within the URANS method with the use of different-structure grids (multiblock, structured overlapping, unstructured composite), the SST turbulence model and its versions (1993) and (2003) considering the streamline curvature influence modified within the Rodi–Leschziner–Isaev approach and numerical different-approximation methods realized in two codes (VP2/3, Fluent). Experiments have been made on flow around a semi-circular cylinder in the wind tunnel of the Lomonosov Moscow State University, Institute of Mechanics to obtain data for verification of numerical predictions. The double-mode character of a periodic time history of a drag force caused by a periodically forming and disappearing jet flap and acting upon a body is explained. With increasing compressibility at a Mach number ranging from 0 to 0.5, it is observed that periodic flow around the semi-circular cylinder is restructured, and the time history of the drag force acting upon it is described by a dependence close to a sinusoidal one. It is found that, as the Mach number is increased, pressure field distortions in the form of concentric cylindrical waves propagating from the semi-circular cylinder and the vortex street behind it grow over the infrasonic range.

Temperature-controlled airflow ventilation in operating rooms compared with laminar airflow and turbulent mixed airflow

To evaluate three types of ventilation systems for operating rooms with respect to air cleanliness [in colony-forming units (cfu/m3)], energy consumption and comfort of working environment (noise and draught) as reported by surgical team members. Two commonly used ventilation systems, vertical laminar airflow (LAF) and turbulent mixed airflow (TMA), were compared with a newly developed ventilation technique, temperature-controlled airflow (TcAF). The cfu concentrations were measured at three locations in an operating room during 45 orthopaedic procedures: close to the wound (<40cm), at the instrument table and peripherally in the room. The operating team evaluated the comfort of the working environment by answering a questionnaire. LAF and TcAF, but not TMA, resulted in less than 10cfu/m3 at all measurement locations in the room during surgery. Median values of cfu/m3 close to the wound (250 samples) were 0 for LAF, 1 for TcAF and 10 for TMA. Peripherally in the room, the cfu concentrations were lowest for TcAF. The cfu concentrations did not scale proportionally with airflow rates. Compared with LAF, the power consumption of TcAF was 28% lower and there was significantly less disturbance from noise and draught. TcAF and LAF remove bacteria more efficiently from the air than TMA, especially close to the wound and at the instrument table. Like LAF, the new TcAF ventilation system maintained very low levels of cfu in the air, but TcAF used substantially less energy and provided a more comfortable working environment than LAF. This enables energy savings with preserved air quality.

Large eddy simulations of air flow in a vertical heated pipe using unstructured Cartesian grids with local refinement

This paper presents the results of modeling of fully developed turbulent gas flow in a uniform heated vertical tube by using the LES method and Smagorinski model. Unstructured Cartesian grids with local anisotropic refinement were used for modeling hydrodynamics and heat transfer in cylindrical pipes. Such meshes unlike structured meshes in cylindrical coordinates allow to construct more detailed meshes for the LES method. In addition, such meshes allow to simulate flows in channels of arbitrary cross-section. To verify the proposed methodology the results of direct numerical simulation of turbulent air flow in a vertical heated tube for forced and mixed convection were used. Thus, for upward flows the most interesting case with significant laminarization flow due to the buoyancy influence was studied.

Potential of turbulence interference in rough rice bin drying and storage systems fitted with cabling technology

Modern on-farm in-bin drying and storage systems are equipped with networks of sensors to monitor air and grain conditions throughout the bins. However, the performance of sensors may be impaired by turbulent flow of the air through the grain mass. Computational fluid dynamic (CFD) simulations allow a more detailed view of fluid flow around the sensors to be obtained. The objectives of this work are to investigate the potential of turbulence generation around the sensors and the potential of turbulence to impair sensor performance. A finite volume method with porous media formulation was employed to simulate the turbulent airflow. The turbulence intensities predicted from different turbulence models (K−ɛ model, k−ω model and the Reynolds stress model (RSM)) were investigated and the result of airflow rates of 1.1 m3 min−1 [air] t−1 [rice] is presented. The results revealed that the turbulence models predicted a low to medium turbulence with intensities ranging between 1% and 2%. Sensors closer to the plenum experienced about 100% increase in turbulence intensity compared to the sensors closer to the outlet. Comparing the turbulence models used, RSM model require the least computational duration with average of 5 h simulation time compared with K–ɛ model and k –ω model requiring 9–42 h and 40 h, respectively. It can be concluded that sensors closer to the plenum should be built to cope with medium scale turbulent flow.

Improving the Airflow Distribution Within an Indirect Solar Dryer by Modifications Based on Computational Fluid Dynamics

Successful drying requires a uniform heating of the product along the trays. This condition is a real challenge in the design of solar dryers. Studies show that a conventional solar dryer provides a nonuniform temperature distribution at the trays level. This paper seeks to remedy this problem by adding a second air inlet to the cabinet dryer in order to get a more homogeneous temperature. Unsteady turbulent airflow and heat transfer through a two-dimensional model is carried out for a typical day of August under the climatic conditions of Tlemcen (Algeria). Figs fruit was selected to be dried. The effect of the proposed configuration on the dynamic and thermal behaviors of the solar dryer has been discussed. The results revealed that using two opposite inlets can improve the drying process by reducing the fluctuation of the temperature to about 67% and increasing the mass flow rate by about 18%.

Influence of the Rotor Configuration on the Electrostatic Charging of Helicopters

A helicopter flying through an atmosphere containing particulates may accumulate high electrostatic charges that can challenge its operational safety. In this paper the influence of the helicopter configuration on its electrification is reported. The current study is based on a recently developed numerical approach in which the turbulent airflow around the rotorcraft is estimated via large-eddy simulations, whereas the particulate flow is computed via Lagrangian particle tracking. Also, this approach incorporates a model for the triboelectric charge transfer during particle–helicopter collisions that is briefly described herein. The configurations examined in the study include rotor systems equipped with two, three, or four blades. Also investigated are the effects of the rotor size and its rotational frequency. The current results predict that a helicopter with fewer blades accumulates less electric current, even though the charge on each individual blade is higher. Furthermore, the location of the charge buildup on the rotor disk depends strongly on the number of blades. Also, according to current computations, a reduction of the rotor size leads to a reduction of its electrification, even if the blade tip velocity is kept constant.