Turbulent Air Flow Research Articles

Investigation of Flow Over an Airfoil Using a Hybrid Detached Eddy Simulation–Algebraic Stress Turbulence Model

Turbulent air flow over an NACA 4412 airfoil is investigated computationally. To overcome the near-wall inaccuracies of higher order turbulence models such as large Eddy simulation (LES) and detached Eddy simulation (DES), it is proposed to couple DES with algebraic stress model (ASM). Angles of attack (AoA) of 0 and 14 deg are studied for an airfoil subjected to flow with Re = 1.6 × 106. Distribution of the pressure coefficient at airfoil surface and the chordwise velocity component at four locations near the trailing edge are determined. Results of the baseline DES and hybrid DES–ASM models are compared against published data. It is demonstrated that the proposed hybrid model can slightly improve the flow predictions made by the DES model. Findings of this research can be used for the improvement of the near-wall flow predictions for wind turbine applications.

Effect of turbulent thermal plume on aspiration efficiency of micro-particles

Evaluating the potential health risks from exposure to the air contaminants and evaluating the inhalability of micro-particles through the human nose or mouth is a crucial subject in designing ventilation systems. Using a realistic human body with detailed facial features, the effects of turbulent airflow field generated by the human thermal plume on the aspiration efficiency of the nasal airway for different ambient conditions were investigated. For the rest breathing rate of 7.5 l/min, the airflow was first simulationed by solving the mean continuity, momentum and energy equations using the RNG k-ε turbulence model for the buoyancy-driven flow around the heated mannequin. Then a continuous random walk (CRW) stochastic model utilizing the normalized Langevin equation was used to investigate the effect of turbulence fluctuations on particle trajectories released in the domain around the mannequin. Particular attention was given to the aspiration of the inhaled micro-particles under various conditions. In addition to the common case that the body temperature is higher than the ambient condition, the special case that the situation is reversed is also studied. The simulations results showed that the thermal plume around the human body and stochastic behavior of particles due to turbulence significantly affect the aspiration of inhaled particles through nostrils.

Model-based experimental data evaluation of separation efficiency of multistage coarse particle classification in a zigzag apparatus

In most industrial processes dealing with particles, separation of fine (or light) and coarse (or heavy) particles in a fluid flow is an essential part to obtain the required product quality. These separation processes are typically designed based on the particles' terminal settling velocity as the dominant property. In this work, sand and gravel have been selected as suitable model grains representing mineral and agricultural raw materials and products. The terminal settling velocity depends on stochastically distributed physical and granulometric particle properties like size, density, and shape. Separation experiments in a pilot-scale zigzag separator have been performed using different channel velocities and solid mass loadings in order to improve the understanding of the separation process in this complex turbulent flow. Then, classification has been modeled as a multistage cross-flow separation process in a turbulent air flow. Performance has been analyzed and discussed with respect to separation functions and characteristic parameters like cut size, separation sharpness, separation stage utilization, and product quality, quantified by cumulative product purities of over- and underflow as well as specific energy consumption. Finally, a SMART analysis has been used to identify optimal process parameters. The obtained cut sizes differ significantly from those found in literature. Thus, a more detailed analysis accounting for aerodynamics and turbulence is developed to show the significant influence of channel geometry on separation efficiency.

The study of droplet‐laden turbulent airflow over waved water surface by direct numerical simulation

AbstractThe objective of the present paper is to elucidate possible effects of sea spray on the momentum transfer in marine boundary layer under strong wind forcing conditions by performing direct numerical simulation (DNS) of turbulent, droplet‐laden airflow over a waved water surface. Three‐dimensional, turbulent Couette airflow is considered in DNS as a model of a constant‐flux layer in the atmospheric surface layer. Two‐dimensional stationary waves at the water surface are prescribed and assumed to be unaffected by the airflow and/or droplets. Droplets are considered as nondeformable spheres and tracked in a Lagrangian framework, and their impact on the carrier flow is modeled with the use of a point‐force approximation. The results show that drops dynamics and their impact on the carrier airflow is controlled by the drops velocity at injection, the ratio of drops gravitational settling velocity versus the product of air friction velocity and Karman constant ( ), and the wave slope, ka. Drops injected into the flow with the surrounding airflow velocity reduce the turbulent air‐stress and increase mean air velocity as compared to the droplet‐free case. On the other hand, the opposite effect is observed for drops injected with velocity equal to the water surface velocity, which increase the turbulent air stress and reduce the mean wind velocity. This modification of the airflow by drops is most pronounced for the ratio , increases with drops mass loading, and is reduced for steeper waves and smaller settling velocity.

Airborne bacterial contamination during orthopedic surgery: A randomized controlled pilot trial

Study objectiveSeveral factors such as lack of unidirectional, turbulent free laminar airflow, duration of surgery, patient warming system, or the number of health professionals in the OR have been shown or suspected to increase the number of airborne bacteria. The objective of this study was to perform a multivariate analysis of bacterial counts in the OR in patients during minor orthopedic surgery. DesignProspective, randomized pilot study. SettingMedical University of Vienna, Austria. PatientsEighty patients undergoing minor orthopedic surgery were included in the study. InterventionsSurgery took place in ORs with and without a unidirectional turbulent free laminar airflow system, patients were randomized to warming with a forced air or an electric warming system. MeasurementThe number of airborne bacteria was measured using sedimentation agar plates and nitrocellulose membranes at 6 standardized locations in the OR. Main resultsThe results of the multivariate analysis showed, that the absence of unidirectional turbulent free laminar airflow and longer duration of surgery increased bacterial counts significantly. The type of patient warming system and the number of health professionals had no significant influence on bacterial counts on any sampling site. ConclusionORs with unidirectional turbulent free laminar airflow, and a reduction of surgery time decreased the number of viable airborne bacteria. These factors may be particularly important in critical patients with a high risk for the development of surgical site infections.

Numerical simulation of swirling airflow dynamics in vortex spinning

In this paper, a computational fluid dynamics model has been established and a three-dimensional numerical simulation of the flow characteristics of airflow in a vortex spinning nozzle has been carried out. A realizable k-ɛ model is used to simulate the turbulence of airflow in the nozzle. Unstructured tetrahedral grids which have good adaptability to complex boundaries are used in this paper to mesh the computational region. The computational model of the airflow field is solved and the characteristics of airflow in a vortex spinning nozzle are obtained. The results show that the flow principle of the airflow is determined by the pressure distribution of the airflow. The airflow field in the nozzle can be divided into external and internal areas, according to the flow characteristics and functions of airflow. The trajectory of swirling airflow from the jet orifices has a regular rotational motion within the twisting chamber, but the trajectory of swirling airflow from nozzle inlet is complex and backflow occurs; finally, these two strands of airflow are fused together as a regular rotational motion.

Convective drying of fruit: A deeper look at the air-material interface by conjugate modeling

A better physical understanding but also prediction of convective drying processes of fruit is essential for further process optimization. This study uses validated conjugate modeling to gain insight in how fruit drying kinetics are related to the convective heat and mass exchange with the surrounding turbulent airflow via the fruit surface. Conjugate modeling implies that the heat and mass transport in both air and fruit domains are solved simultaneously. We explore the impact of several model assumptions and different convective drying conditions. The conjugate model is inherently more accurate than the use of constant convective transfer coefficients (CTCs), so the non-conjugate approach. However the gain in accuracy was found to be limited in terms of overall fruit drying kinetics, such as total mass loss. Nevertheless, conjugate modeling allowed to identify spatial and temporal variability in CTCs, which locally affected drying rates and internal moisture content distribution. Thereby, we identified the occurrence of negative convective transfer coefficients, which led to rehydration at specific locations on the fruit surface, due to the surrounding high-humidity microclimate. The ability to identify the direct relation between non-uniformities in the airflow to those in the tissue is a unique trait of the conjugate approach. Furthermore, it was shown that isothermal modeling should not be used, even for near-isothermal conditions such as low-temperature drying, and that including thermal radiation exchange with the environment clearly affected the drying rates. Regarding the drying conditions, the impact of the air speed and approach flow temperature was found to be smaller compared to altering the approach flow humidity. When direct solar radiation was present, the presence of airflow provided significant cooling of the fruit, which is beneficial for preserving heat-sensitive nutritional compounds in the fruit, and also enhanced the drying rate. This study will aid drying technologists to define the required complexity of their model.

Sand transportation and reverse patterns over leeward face of sand dune

Sand saltation has complex interactions with turbulent flow and dune form. Most models of wind-blown sand consider ideal circumstances such as steady wind velocity and a flat surface, and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter-dune space on the wind flow, neglecting the effect of morphology on sand saltation, particularly airflow and sand transportation over the leeward slope. Wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes. In order to understand sand dune dynamics, lee face airflow and sediment transportation should be paid more attention. Previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field. In addition, the reverse sand particles over leeward face could not be collected by sand trap in field. Numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field. In addition, the reverse transport of sand particles over leeward face could not be collected by sand traps in field. Therefore, this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope. A numerical model of sand saltation over slope terrain is constructed, which also considers the coupling effects between air flow and sand particles. The large eddy simulation method is used to model turbulent flow. Sand transport is simulated by tracking the trajectory of each sand particle. The results show that terrain significantly alters the turbulent air flow structure and wind-blown sand movement, especially over the leeward slope. Here, mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow, and the mass flux decreases with height in the reversed direction. The height of 0.5H is the height of vortex core in the reversed flow region. The vortex core is a critical point in the flow region where few particles are transited. In the reversed region, the reversed mass flux of sand particles is 25% of the mass flux in the flow direction. This research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control.

On the enhancement of particle deposition in turbulent channel airflow by a ribbed wall

The particle deposition at a vertical wall roughened by transverse square bars placed at a small spacing between them is investigated using large-eddy simulation of the turbulent flow in a ribbed channel with the gravity aligned in the flow direction together with Lagrangian particle-tracking. It is found that the particle deposition coefficient is substantially increased in the presence of roughness elements, exhibiting a weaker dependence on the variation of particle response time relative to the case of smooth channel. The enhancement ratio of particle deposition varies from three for the larger size particles to about 400 for the smaller particles examined here. The friction-weighted enhancement ratio of particle deposition is higher than unity for all particle sets, indicating that the present ribbed channel configuration efficiently increases particle deposition with respect to the increase in energy losses. The rise in the particle deposition coefficient at the rough surface is closely related to the direct inertial impaction and interception mechanisms. The population of particles depositing at the rear surface of the square bars is very small, revealing that an enlargement of the effective deposition area is not necessarily translated to a similar augmentation of particle removal.

Spatiotemporally resolved characteristics of a gliding arc discharge in a turbulent air flow at atmospheric pressure

A gliding arc discharge was generated in a turbulent air flow at atmospheric pressure driven by a 35 kHz alternating current (AC) electric power. The spatiotemporally resolved characteristics of the gliding arc discharge, including glow-type discharges, spark-type discharges, short-cutting events and transitions among the different types of discharges, were investigated using simultaneously optical and electrical diagnostics. The glow-type discharge shows sinusoidal-like voltage and current waveforms with a peak current of hundreds of milliamperes. The frequency of the emission intensity variation of the glow-type discharge is the same as that of the electronic power dissipated in the plasma column. The glow-type discharge can transfer into a spark discharge characterized by a sharp peak current of several amperes and a sudden increase of the brightness in the plasma column. Transitions can also be found to take place from spark-type discharges to glow-type discharges. Short-cutting events were often observed as the intermediate states formed during the spark-glow transition. Three different types of short-cutting events have been observed to generate new current paths between two plasma channel segments, and between two electrodes, as well as between the channel segment and the electrodes, respectively. The short-cut upper part of the plasma column that was found to have no current passing through can be detected several hundreds of microseconds after the short-cutting event. The voltage recovery rate, the period of AC voltage-driving signal, the flow rates and the rated input powers were found to play an important role in affecting the transitions among the different types of discharges.

Distribution of indoor thoron in dwellings under normal and turbulent flow conditions using CFD simulation technique

Extensive work has been carried out on measurement of radon and thoron levels in indoor environment in last three decades. These studies are important from radiation protection point of view, if one considered the contribution of radon, thoron and their decay products to total inhalation dose. Numerous studies on radon measurement well established the behaviour of its dispersion in dwellings. But the short lives of thoron cause the difficulty to understand the distribution of thoron in dwellings. The problem becomes more complicated when thoron dispersion is studied under different inlet air flow rate. Different air flow pattern may cause different thoron level at different point in test dwellings causing uncertainty in the measurements. This work utilized the CFD simulation technique for study of indoor thoron dispersion in test dwellings under normal and turbulent flow of air. The simulation study for thoron distribution in a test room was performed for air velocities 0.10 ms-1, 0.25 ms-1, 0.50 ms-1, 1.0 ms-1, 1.5 ms-1, and 2.0 ms-1. The results show that the thoron distribution becomes uniform for the inlet velocity more than 0.5 ms-1 and appropriate to measure indoor thoron concentration. While in normal condition the measured thoron level varies depending upon the location of dosimeter. Thoron diffusion and migration length are also increased with air flow rate.

The effect of the nasal cavity form on air flow distribution during inhalation

The authors performed numerical modeling of convective air flow in three 3D-models of the nasal cavity during inhalation. The models of the nasal cavity had lepto-, meso- and platicavital forms. The simulation results revealed the specific features of aerodynamics of various forms of the nasal cavity. Platicavital nasal cavity is characterized by more turbulent air flow and leptocavital one – by more laminar. For all nasal cavity forms air flow rate reaches its maximum value in the lower two thirds of the front and middle part of nose. The same trend is observed in posterior part of plati- and mezocavital nasal cavities, and in leptocavital one the maximum values of the air flow rate are observed in the upper two thirds of the nasal cavity. In the direction from the nostrils to the choanas, pressure profile equalizes, and subsequently goes down. The highest pressure values during inhalation were observed in the lower and middle nasal passages. In the front part of the common nasal passage the maximum pressure value is observed in the upper two thirds of the cross-section for platicavital model form and in the lower two thirds of the cross-section for the meso- and leptocavital forms. During inhalation the greatest pressure values are observed in the upper third of the middle and posterior part for all the nasal cavity forms.

An approximate estimation of velocity profiles and turbulence factor models for air-flows along the exterior of TEFC induction motors

Compared to a number of other existing correlations for heat transfer, the empirical correlations for forced convection from a short horizontal cylinder in axial air-flows usually do not involve the effects of changes in air-flow velocity and/or air-flow turbulence. Therefore, a common analysis of the heat transfer by using only one energy balance equation for entire outer surface of a solid is considered insufficient for induction motor applications because it fails to include aforementioned effects. This paper presents a novel, empirically-based methodology to estimate approximately the values of air-flow velocities and turbulence factors, that is, velocity profiles and turbulence factor models for stationary horizontal cylinders with and without fins (frame and two end-shields) in axial air-flows. These velocity profiles and turbulence factor models can then be used in analytical modelling of steady-state heat transfer from the exterior of totally enclosed fan-cooled induction motors.

Numerical simulation of the turbulence effect on heat transfer between fluid and thin plates of a solid material

We present the results of numerical simulation of conjugate heat transfer between thin (100-1000 nm) solid plates and periodically heated air flow. Several air inflow conditions were studied with different turbulence intensity and integral scale. The thermal response in the plate was calculated showing that inflow turbulence interacting with the plate generates significant temperature fluctuations in the plate volume. These fluctuations amplitudes grow along the plate and with time for several periods, indicating the existence of some feedback process, with turbulent air flow acting as a medium, transferring thermal signals between different sections of the plate. The described spatial and temporal fluctuations may be used for pyroelectric energy harvesting applications.

High-Resolution LES of Terrain-Induced Turbulence around Wind Turbine Generators by Using Turbulent Inflow Boundary Conditions

We have developed an LES (Large-Eddy Simulation) code called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). The analysis do-main of this numerical model extends from several meters to several kilometers. The model is able to predict airflow over complex terrain with high accuracy and is also now able to estimate the annual power output of wind turbine generators with the use of field observation data. In the present study, a numerical simulation of turbulent airflow over an existing wind farm was performed using RIAM-COMPACT and high-resolution elevation data. Based on the simulation results, suitable and unsuitable locations for the operation of WTGs (Wind Turbine Generators) were identified. The latter location was subject to the influence of turbulence induced by small topographical variations just upwind of the WTG location.

Simulating fiber motion in the airflow in a rotor spinning unit

In this study, a numerical model for simulation of fiber motion in the transport channel of a rotor spinning unit has been developed. A chain represents the fiber consisting of a series of cylindrical rods connected with beads. The fiber model is applied on the turbulent airflow field of the transport channel to present fiber configuration at different times. Results show that the airflow field in the transport channel has a great impact on fiber configuration. The vortex generated in the channel inlet tends to cause a hook on the fiber leading end. As the leading end escapes from the vortex and moves downwards the transport channel, the accelerating air will facilitate fiber straightening.

Fifteen-minute consultation: investigation and management of an infant with stridor

Stridor is an abnormal sound caused by turbulent airflow through the airway. In an infant under 3 months of age presenting with stridor, there are many different underlying aetiologies. Some...

Computational fluid dynamics analysis of the effect of topographic obstacles on extractable power by horizontal axis wind turbines

Wind velocity profile varies in the boundary layer according to a complex pattern that depends hugely on the surface roughness and local Reynolds number. The presence of a macroscopic obstacle on the ground surface modifies considerably the flow characteristics of wind speed profile. In this study, the effect on wind speed resulting from local circulatory motion induced by the existence of an obstacle is analyzed in stationary conditions under the assumption of two-dimensional approximation of the problem. Computational fluid dynamics is used to solve the turbulent air flow equations that consist of Navier–Stokes equations coupled to a K-ε turbulence model. A bounded domain having a rectangular form was introduced in order to schematize the atmospheric region containing the obstacle and wind turbine. The boundary conditions at ground surface were fixed by applying a modified wall law. The other boundary conditions included a logarithmic velocity profile at the input, a uniform speed applied on the upper edge of the rectangular domain and a uniform pressure in the outlet area. To solve the obtained equations, Comsol Multiphysics software package was used. The obtained results have shown that the presence of an obstacle has a huge effect on the wind profile pattern and affects largely the extractable power from wind by the wind turbine system.

Relative imbalance as etiology of laryngomalacia – A new theory

Laryngomalacia literally means weak larynx. It is the most common cause of noisy breathing in infants and children constituting around 70% of cases. Its aetiology is not clear and various theories are proposed. Treatment remains following the child with regular weight monitoring in view of expected spontaneous resolution. However we cannot predict which child will resolve spontaneously and which child may need surgical intervention. We propose a new theory based on relative imbalance of demand supply of air, suggesting the increase in demand causing turbulent airflow, increasing suction pressure and causing collapse of laryngeal structures. This theory also helps us in predicting early, which child will resolve spontaneously and which child will need surgery. The methodology to evaluate hypothesis along with techniques and tools are also suggested.

Analysis of protein enrichment during single- and multi-stage tribo-electrostatic bioseparation processes for dry fractionation of legume flour

Abstract The potential of a dry fractionation approach for the production of native plant protein concentrates has been validated for navy bean ( Phaseolus vulgaris ) flour as a model system utilizing a lab-scale tribo-electrostatic separator. For this approach, protein and carbohydrate particles in pin-milled flour were tribo-charged to different levels before being fractionated according to their acquired charge under the influence of an applied electric field. In the present study, the location and distribution of the charged protein-rich particles along the surface of the electrode plate was investigated as a function of air flow rate (laminar vs . turbulent) and electric field strength. A turbulent air flow rate at a variety of electric field strengths resulted in the formation of protein-rich particles and starch granule agglomerates affecting the production of high-purity protein concentrates. Charging the flour particles at a laminar air flow rate followed by separation under a low electric field strength enabled the production of fine protein-rich fractions with considerably higher protein contents (38.4–46.5%) but lower protein separation efficiency. The combination of a laminar air flow rate and high electrode voltage slightly reduced the protein content of the fraction (39.4–42.9%), but significantly improved the protein separation efficiency to ∼45%. To further improve the separation efficiency of the navy bean protein concentrate without compromising its protein content, a two-stage tribo-electrostatic separation approach was evaluated at a laminar air flow but different plate voltages. The combined protein-rich fraction produced by the two-stage approach had a protein content of ∼38% accounting for 60% of the total protein that was significantly higher than that of the optimized single-stage tribo-electrostatic separation, facilitating the scale-up of this dry fractionation technique for pilot-plant applications.