Changes In Turbulence Intensity Research Articles

Effect of turbulence intensity on bubble-particle detachment mechanism in a confined vortex

Bubble-particle detachment induced by turbulence is the primary factor for the low recovery of coarse particles, which has attracted much attention in recent years. The prevailing centrifugal detachment theory has achieved broad consensus. However, due to the complexity of bubble-particle aggregate motion in the turbulent field, the influence of turbulence intensity on bubble-particle detachment mechanism remains unclear. In this study, a controlled rotating vortex was generated using a custom-made fluid channel to systematically investigate the effect of turbulence intensity on bubble-particle centrifugal detachment. Firstly, detachment behaviours of bubble-particle aggregates were captured using high-speed cameras, followed by flow field visualization of bubble-particle detachment processes using Computational Fluid Dynamics (CFD). Experimental results reveal three typical detachment modes of bubble-particle aggregates in the confined vortex: fluid shear, bubble oscillation, and particle centrifugal motion, with particle centrifugal detachment becoming dominant with increasing turbulence intensity. This shift is attributed to changes in turbulence intensity directly affecting the trajectories of bubble aggregates and indirectly influencing the detachment mode of bubble-particle interaction. Compared to low turbulence intensity, bubble-particle aggregates exhibit smaller rotational radii under high turbulence intensity, reducing the probability of fluid shear and bubble oscillation detachment by mitigating the influence of high-speed shear zones at the top of the cavity. Conversely, particles located at the edge of aggregates are more susceptible to acceleration by sidewall flow fields, favouring detachment through particle centrifugal motion. Moreover, unlike traditional centrifugal detachment theories, particles do not merely undergo independent circumferential motion on the bubble surface but participate as a whole with bubbles in centrifugal motion governed by vortices. These findings are expected to provide fundamental insights into the bubble-particle detachment mechanism in turbulent fields.

A novel three-dimensional analytical model of the added streamwise turbulence intensity for wind-turbine wakes

Accurate prediction of the streamwise turbulence intensity in turbine wakes is of great significance to the layout of wind turbines. However, significant knowledge gaps exist on how the streamwise turbulence intensity changes in the turbine wakes, and we still lack an accurate model to predict the added turbulent intensity in the wakes. To this end, the wakes of a single turbine with different thrust coefficients and ground roughness heights are studied via large-eddy simulation under neutral atmospheric condition. To the best of our knowledge, it is the first time that we find that, like the velocity deficit of the wakes, the added streamwise turbulence intensity (ΔIu) also exhibits a self-similar characteristic and expands linearly with the downstream. Based on the findings, a novel three-dimensional analytical model of ΔIu is proposed. Considering the ground effect, a new correction function is also constructed to superpose on the proposed model. Data from both wind tunnel experiments and large-eddy simulations are introduced to validate the proposed model. The results demonstrate that the proposed model can accurately predict the variation of the streamwise turbulence intensity in both the vertical and horizontal directions, and the prediction accuracy is higher than that of the existing models.

Research on wind barrier of canyon bridge-tunnel junction based on wind characteristics

Due to the acceleration effect of the wind by the special geographical location of the canyon bridge-tunnel junction, the traffic safety and stability of this section are difficult to be guaranteed, resulting in frequent traffic accidents. In order to ensure the safety and comfort of vehicles driving on this section, the numerical simulation method based on CFD is adopted to establish the numerical model of the canyon bridge-tunnel junction. The acceleration of the incoming wind speed in the bridge-tunnel junction with a guardrail that is 0.8 m high is analyzed from different canyon spacings, wind directions and heights from the bridge deck. Based on the characteristics of wind field above the bridge deck, two kinds of gradient wind barriers—trapezoidal and stepped—are proposed, and their wind reduction effects and turbulence intensity changes are analyzed. Then the aerodynamic performances of running vehicle are compared. The results show that the stepped wind barrier with 50% porosity and rectangular section railings has the best wind reduction effects, and can noticeably improve the comfort of driving. The aerodynamic coefficients of vehicle are lower with stepped wind barrier.

Influence of Change in Turbulence Intensity on Suppression of Separated flow by DBD Plasma Actuator on a NACA0012 Airfoil

Influence of Change in Turbulence Intensity on Suppression of Separated flow by DBD Plasma Actuator on a NACA0012 Airfoil

Investigation of the impact of planetary boundary layer turbulence on the performance of onshore wind farm

Abstract In recent years, wind power has become the fastest growing renewable energy. In the wind power generation, wind energy assessment is also a crucial one, it is directly related to the layout and power generation forecast of a wind farm. Before a wind park is constructed, a site with high mean annual wind speeds in hub height has to be found. To analyse the wind resources at a potential wind park site the three-dimensional wind field has to be simulated. In this paper, accurate and reliable Computational Fluid Dynamics (CFD) simulations of wind flow over natural complex terrain in northern of Morocco have been performed. The Geographic Information System (GIS) is used to generate contour elevation data before can be directly imported into CAD to generate terraced three-dimensional topographic map of test area. The site is characterized regarding its flow features and the suitability for a wind turbine test field. An improved delayed detached eddy simulation (IDDES) was employed using measurement data to generate generic turbulent inflow. A good agreement of the wind profiles between the different approaches was reached. The terrain slope leads to a speed up, a change of turbulence intensity as well as to flow angle variations. For the numerical simulation the Open Source Computational Fluid Dynamics (CFD) Toolbox OpenFOAM (Open Field Operation and Manipulation) was used. It is a compressible finite volume flow solver using block-structured grids and featuring a wide range of turbulence models. This type of analysis may help in preliminary assessments and expedite the site selection process.

Flame self-interactions with increasing turbulence intensity

The topology of flame-flame interaction is analysed for single turbulent premixed flames with increasing turbulence intensity. Morse theory for critical points is used for identifying the flame-flame interaction and characterising the local topology. The interactions have been categorised into four different groups, namely reactant pocket, tunnel formation, tunnel closure and product pocket. A histogram showing the frequency of occurrence of each of these groups is presented for single flames representative of hydrocarbon-air combustion and is compared with the results of colliding hydrogen-air flames. It is observed that most interactions for a single flame occur toward the leading edge. Also, more interactions are observed for higher intensity turbulence. The cylindrical topology types are found to dominate over spherical topology types. The relative frequency of occurrence of each type of topology is observed to change with changes in turbulence intensity. With increasing turbulence intensity, the fraction of product pockets and tunnel formation events increases whereas the fraction of reactant pockets and tunnel closure events decreases. The rise in product pockets is mirrored by the drop in reactant pockets, and likewise, the rise in tunnel formation events is mirrored by the drop in tunnel closure events.

Effect of Free-Stream Turbulence Intensity on Transonic Airfoil with Shock Wave

Airplanes regularly operate switching between various flight modes such as take-off, climb, cruise, descend and landing. During these flight conditions the free-stream approaching the wings undergo fundamental changes. In transonic flow conditions, typically in the military or aerospace applications, existence of nonlinear and unsteady effects of the airflow stream significantly alters the performance of an airfoil. This paper presents the influence of free-stream turbulence intensity on transonic flow over an airfoil in the presence of a weak shock wave. In particular, NACA 0012 airfoil performance at Ma∞ = 0.7 is considered in terms of drag, lift, turbulence kinetic energy, and turbulence eddy dissipation parameters under the influence of varying angle of attacks and free-stream turbulence. The finite volume method in a commercial CFD package ANSYS-CFX is used to perform the numerical analysis of the flow. Mesh refinement using a mesh-adaption technique based on velocity gradient is presented for more accurate prediction of shocks and boundary layers. A Shear Stress Transport (SST) turbulence model is validated against experimental data available in the literature. Numerical simulations were performed, with free stream turbulence intensity ranging from low (1%), medium (5%) to high (10%) levels. Results revealed that drag and lift coefficients are approximately the same at every aforementioned value of turbulence intensity. However, turbulence kinetic energy and eddy dissipation contours vary as turbulence intensity changes, but their changes are disproportionally small, compared with values adopted for free-stream turbulence.

Numerical simulations of wake characteristics of a horizontal axis tidal stream turbine using actuator line model

The wake of a laboratory scale tidal stream turbine in a shallow water channel with a turbulent inflow is simulated using the hybrid LES/ALM technique, which combines large eddy simulation with the actuator line method. The turbulent inlet conditions are generated using the mapping method to avoid a precursor running and large space for saving data. The numerical results demonstrated the usefulness of the mapping technique as well as some shortcomings that still remain to be addressed. Good agreement between numerical predictions and experimental data is achieved for both the mean and turbulent characteristics of the flow behind the turbine. The examination of changes in turbulence intensity and turbulent kinetic energy in the streamwise direction confirms the existence of a peak and transition to a highly turbulent flow about three diameters downstream of the turbine, which means that the distinct characteristics of the streamwise changes of turbulence intensity or turbulent kinetic energy may serve as an effective indicator for the flow regime transition and wake behaviour.

An experimental investigation of surface pressures in separated and reattaching flows: effects of freestream turbulence and leading edge geometry

Flow separation and reattachment over bluff bodies is frequently encountered in a wide range of engineering practice. This study presents an experimental investigation on the characteristics of surface pressures over bluff bodies in separated and reattaching flows, with emphasis placed on the effects of freestream turbulence and leading edge geometry. Measurements of mean, r.m.s and peak suction pressures on blunt flat plates with different leading edge shapes are conducted in both smooth and grid-generated turbulent flows in wind tunnel test, and comparative analyses are carried out to unveil the dependence of the distributions of mean, r.m.s and peak suction pressure coefficients on turbulence intensity, turbulence integral length scale and leading edge geometry. The experimental results show that the distributions of mean pressure coefficient respond strongly to changes of turbulence intensity, but with little effect on turbulence integral length scale except in the leading edge region for the cases there are large differences of the parameter. The distributions of r.m.s and peak suction coefficients demonstrate a close correlation on both the turbulence intensity and integral length scale. In addition, the distributions of surface pressures are also related evidently on the leading edge geometry. The mean reattachment length exhibits a monotonic increase as the leading edge angle increases, while the maximum magnitudes of r.m.s and peak suction pressure coefficients are found to increase with increasing leading edge angle.

Fractional Flow Speed-Up from Porous Windbreaks for Enhanced Wind-Turbine Power

The potential for porous windbreaks to enhance wind-turbine power production is studied using linearized theory and wind-tunnel experiments. Results suggest that windbreaks have the potential to substantially increase power production, while lowering mean shear, and leading to negligible changes in turbulence intensity. The fractional increase in turbine power output is found to vary roughly linearly with windbreak height, where a windbreak 10% the height of the turbine hub increases power by around 10%. Wind-tunnel experiments with a windbreak imposed beneath a turbulent boundary layer show the linearized predictions to be in good agreement with particle-image-velocimetry data. Power measurements from a model turbine further corroborate predictions in power increase. Moreover, the wake of the windbreak showed a significant interaction with the turbine wake, which may inform windbreak use in large wind farms. Power measurements from a second turbine downwind of the first with its own windbreak show that the net effect for multiple turbines is dependent on windbreak height.

Comparison of Different Measurement Techniques and a CFD Simulation in Complex Terrain

This paper deals with a comparison of data collected by measurements and a simulation for a complex terrain test site in southern Germany. Lidar, met mast, unmanned aerial vehicle (UAV) measurements of wind speed and direction and Computational Fluid Dynamics (CFD) data are compared to each other. The site is characterised regarding its flow features and the suitability for a wind turbine test field. A Delayed-Detached-Eddy- Simulation (DES) was employed using measurement data to generate generic turbulent inflow. A good agreement of the wind profiles between the different approaches was reached. The terrain slope leads to a speed-up, a change of turbulence intensity as well as to flow angle variations.

Spatially-averaged turbulent flow over cubical roughness in wave-current co-existing environment

The paper describes an experimental study carried out in a laboratory flume to investigate the interaction of surface-wave with unidirectional current over cube mounted rough-bed (3D flow). The cubes were made of wood and were positioned at the channel-bed with different spacing. The spacing was chosen to represent different roughness types under different spacing conditions as: isolated roughness flow, wake interference flow and skimming flow. The three-dimensional velocity field was measured by an acoustic Doppler velocimetre (ADV). The study particularly focused on the changes induced in the spatially-averaged velocity profiles, turbulence intensities and Reynolds shear stress due to the superposition of waves of different frequencies on current-induced flow. Spatially-averaged velocity profiles show two distinct distributions: linear or exponential distribution below the roughness crest and logarithmic distribution above the roughness crest. It is found that the shift of the velocity profile depends strongly on the spacing of roughness elements. The profile of stream-wise turbulence intensity changes significantly below the roughness crest due to presence of surface wave, whereas bottom-normal spatially-averaged turbulence intensity does not change much with increase in wave frequency. Within the roughness crest near the bottom, the form-induced shear stress changes significantly from negative to positive value with change in roughness spacing, but is not affected much due to the presence of surface-wave.

입자영상유속계를 이용한 가두리 망지의 난류강도 계측

This study is aimed to analyze the hydrodynamic characteristics of the cage net in the circulating water channel. It visualized wake flows using a PIV (paricle imaging velocimetry) and analyzed the flow velocity distribution. In addition, the vorticity and turbulence intensity were analyzed from the wake flow distribution and compared changes by flow velocity. Results showed that the average turbulence intensity in the circulating water channel was very stable showing less than 1% in the range between 0.2 and 0.8 m/s. The drag coefficient affecting to the netting was estimated to be 1.35. The flow decreasing rate of the wake in the middle of the netting was 2.1% at the range of 0.2 m/s and it was constant at 6.6-6.9% over the range of 0.4 m/s irrespective of velocity increases. Finally, the change of turbulence intensity by netting and knot mesh could be confirmed. These results can be utilized as a basic information for the future research of flow characteristics by fishing nets and meshes.

Analysis on the Effects of Atmospheric Turbulence on Line-of-Sight Ultraviolet Communication System Performance

In the ultraviolet communication system, optical signals being transmitted are not only attenuated in energy, but also affected by the atmospheric turbulence. The effects of atmospheric turbulence intensity and information transmission rate on the BER performance of UV communication system are analyzed and simulated in this paper. It can be found that when the atmospheric turbulence intensity changes from weak to strong, the BER performance deteriorates along, and under the certain atmospheric turbulence intensity, information transmission rate will also have an impact on the system BER performance, the information transmission rate becomes faster, the BER performance gets worse.

Effect of swirling inlet condition on the flow field in a stenosed arterial vessel model

Blood flow in an artery is closely related to atherosclerosis progression. Hemodynamic environments influence platelet activation, aggregation, and rupture of atherosclerotic plaque. The existence of swirling flow components in an artery is frequently observed under in vivo conditions. However, the fluid-dynamic roles of spiral flow are not fully understood to date. In this study, the spiral blood flow effect in an axisymmetric stenosis model was experimentally investigated using particle image velocimetry velocity field measurement technique and streakline flow visualization. Spiral inserts with two different helical pitches (10D and 10/3D) were installed upstream of the stenosis to induce swirling flows. Results show that the spiral flow significantly reduces the length of recirculation flow and provokes early breakout of turbulent transition, but variation of swirling intensity does not induce significant changes of turbulence intensity. The present results about the spiral flow effects through the stenosis will contribute in achieving better understanding of the hemodynamic characteristics of atherosclerosis and in discovering better diagnosis procedures and clinical treatments.

Effects of secondary variables on infiltration rate of open refrigerated vertical display cases with single-band air curtain

Abstract Recently the authors of this work reported the effects of several primary variables that can affect the performance of a circulated air curtain in protecting open refrigerated vertical display cases against infiltration of outside warm air. Those variables were either associated with the dynamics of the air curtain flow or were geometrical variables. There were a few other variables that were thought to have lesser impact on infiltration or relatively much harder to manage and vary; therefore, they were categorized as, so-called, secondary variables and decided to be studied separately in a complimentary investigation. These variables were not included in the permutations of the primary variables as they would drastically increase the number of experiments in the published primary -variables study. Due to using a few simplified assumptions or ideal conditions in the primary study, the secondary variables’ effects were incorporated as several correction functions to modify the infiltration rate obtained from the primary study. The four secondary variables studied here are turbulence intensity of the air curtain jet at its discharge nozzle, average percentage of the space between the shelves that was filled with food products, difference between the temperatures of ambient air and the jet at the discharge nozzle, as well as the relative humidity between the aforementioned locations. For the ranges of the variables commonly used in typical medium-temperature display cases, it was found that the temperature and relative humidity changes are of little or no importance, while the turbulence intensity changes infiltration rate almost linearly and the food level varies it in a nonlinear manner.

Experimental study on airflow fluctuation characteristic of an underfloor air supply terminal unit

In order to investigate dynamic characteristic of underfloor air supply terminal unit, an IFV900A hot-wire anemometer was used to measure the corresponding velocity field. Turbulence intensity and power spectrum density exponent of air velocity signal were analyzed. The result showed that the outlet velocity distribution of underfloor air supply terminal unit was uniform. With increment of height, the velocity distribution trends to be uniform. Two velocity attenuation regions appear during airflow development. Turbulence intensity changes obviously with height. It is lower than that of mechanical wind. Turbulence intensity goes up with the increment of jetting distance. Power spectrum density exponent trends to the value of natural wind with increase of jetting distance and decrease of wind velocity. The exponent value approaches to the value of typical natural wind for the air velocity is 0.5 m/s under high supply air rate. With airflow diffusion, the fluctuation characteristic of airflow varies obviously with the jetting direction. The fluctuation characteristic of airflow changes to that of natural wind with the increase of height which can improve comfort of indoor environment.

Particle modulations to turbulence in two-phase round jets

The particle modulations to turbulence in round jets were experimentally studied by means of two-phase velocity measurements with Phase Doppler Anemometer (PDA). Laden with very large particles, no significant attenuations of turbulence intensities were measured in the far-fields, due to small two-phase slip velocities and particle Reynolds number. The gas-phase turbulence is enhanced by particles in the near-fields, but it is significantly attenuated by the small particles in the far-fields. The smaller particles have a more profound effect on the attenuation of turbulence intensities. The enhancements or attenuations of turbulence intensities in the far-fields depends on the energy production, transport and dissipation mechanisms between the two phases, which are determined by the particle properties and two-phase velocity slips. The non-dimensional parameter CTI is introduced to represent the change of turbulence intensity.

Effects of nonlocal turbulence on the mass transfer of dissolved species to reef corals

We examined the importance of nonlocal turbulence to the mass transfer of dissolved species from both cylinders and models of reef coral. Solid gypsum cylinders and gypsum‐coated model coral were dissolved in an outdoor flume at mean flow speeds of 4.5 and 10 cm s−1. Three model corals were used: two branched forms of the genera Acropora and Pocillopora, and one lobate form of the genus Platygyra. Turbulence intensities (Tu) were measured as the ratio of the standard deviation of the time‐variant flow to the mean. Tu of between 5% and 55% were created independently of the mean flow by pumping water through two vertical arrays of jets. Mass transfer coefficients increased by 10‐70% with turbulence intensity for all forms at both mean flow speeds considered (p < 0.05); however, mass transfer coefficients for the branched coral were not as sensitive to changes in turbulence intensity as for the lobate coral. Results for the cylinders were consistent with the engineering literature and indicate that the effects of turbulence intensity on mass transfer become weaker when branch size or flow speed (or both) become very small. Turbulence intensities measured around experimental coral communities and above naturally occurring reef communities typically vary between 15% and 35%. Assuming an average turbulence intensity of around 25%, then ignoring the effects of nonlocal turbulence altogether would result in uncertainties in mass transfer coefficients of at most ±10% for the lobate coral form and ±5% for the branched coral forms. Measuring external turbulence in bulk flows is not necessary for most evaluations of biogeochemical rates in benthic reef systems.

Flow visualization and laser measurement on particle modulation to gas-phase turbulence

Particle modulation to turbulence is investigated experimentally by means of PDPA, PIV and flow visualization for a gas-particle two-phase jet flow. Large particles can enhance the small-scale vortex, so that gas-phase turbulence intensity is increased, while small particles may delay the rolling up of the gas vortex, so that gas-phase turbulence intensity is attenuated. The critical particle size range for such different effects is between 150 μm and 200 μm, corresponding to the Stokes number is between 88 and 157 under the present flow conditions. The PIV results show small particles can retain the gas-phase vortex structure, while large particles can break large vortex structure. The particle Stokes number is not the only judgment standard whether particles enhance or attenuate gas-phase turbulence. The CTI (Change of Turbulence Intensity) number can mark off particle modulation on turbulence in two-phase flow, but more studies are needed.