Reduced Wind Velocity Research Articles

Prediction and visualization of 3D wake field of a rectangular high-rise building in tropical island cities based on UAV measurements

It is especially crucial to utilize measured data as samples to increase the accuracy of wind field prediction. However, scarce wake field measured data of high-rise buildings leads to the deviation between prediction results and the actual wake distribution. In this study, a six-rotor unmanned aerial vehicle wind measurement system (UAVWMS) equipped with an anemometer is used to measure the wake field of a rectangular high-rise building in a tropical island city, and the characteristics of fluctuating wind velocity spectra and distribution patterns of the wind velocity and turbulence intensity in the wake region are analyzed; the Kriging method is adopted to predict and visualize the three-dimensional (3D) wake field based on the measured data from UAVWMS. The results show that the exponential law is better than the logarithmic law in fitting the incoming wind velocity profile in the tropical island city. The measured points in the wake region, located near the central axis of the building, exhibit a pronounced reduction in wind velocity and an increase in turbulence intensity compared to those in the incoming region. Compared to the incoming wind velocity spectra and empirical spectra, the peak frequencies of wake wind velocity spectra shift to the high-frequency band. In addition, the wake wind velocity spectra are lower in the low-frequency band and higher in the high-frequency band. When designing a measured scheme based on UAVWMS, it's crucial to place measured points surrounding the target spatial wake field as much as possible. It helps enhance the accuracy of predicting wind parameters within the 3D spatial wake field. The research results provide a theoretical reference for the measurement, prediction and visualization of the wake field of high-rise buildings in dense urban buildings.

Analysis of Wind Power Fluctuation in Wind Turbine Wakes Using Scale-Adaptive Large Eddy Simulation

In large wind farms, the interaction of atmospheric turbulence and wind turbine wakes leads to complex vortex dynamics and energy dissipation, resulting in reduced wind velocity and subsequent loss of wind power. This study investigates the influence of vortex stretching on wind power fluctuations within wind turbine wakes using scale-adaptive large eddy simulation. The proper orthogonal decomposition method was employed to extract the most energetic contributions to the wind power spectra. Vertical profiles of mean wind speed, Reynolds stresses, and dispersive stresses were analyzed to assess energy dissipation rates. Our simulation results showed excellent agreement when compared with wind tunnel data and more advanced numerical models, such as the actuator line model and the actuator line model with hub and tower effects. This highlights the important role of coherent and energetic flow components in the spectral behavior of wind farms. The findings indicate a persistent energy cascading length scale in the wake of wind turbines, emphasizing the vertical transport of energy to turbine blades. These results complement existing literature and provide new insights into the dynamics of wind turbine wakes and their impact on wind farm performance.

Aerodynamic interaction between galloping instability and vortices in corner-cut rectangular cylinders

Studies on bluff-body aerodynamics have emphasized that galloping instability is strongly associated with the Kármán vortex. This study discusses the aerodynamic interactions between the galloping instability and Kármán and motion-induced vortices, analyzes the effects of these vortices on vortex-induced vibration and galloping, and investigates the stabilizing effects of various corner cuts on a rectangular cylinder. Wind tunnel tests were performed on a rectangular cylinder with a side ratio of 1.5 under a smooth flow for seven different corner shapes. The rectangular cylinder with cut corners significantly reduced both the aerodynamic force coefficients and the Kármán vortex shedding intensity. Furthermore, spring-supported free vibration tests indicated that the onset reduced wind velocities were high in the response amplitudes of all corner-cut sections that were analyzed despite the significantly low onset reduced wind velocities of the Kármán vortex-induced vibration, which were denoted as the reciprocal of the Strouhal number and closely associated with the onset of galloping. This was attributed to the motion-induced vortex dominating the vibration when the Kármán vortex shedding intensity was reduced. Therefore, this study clarified one of the factors that affected the onset-reduced wind velocity of galloping.

Wind-Induced Response Assessment of CAARC Building Based on LBM and FSI Simulation

It is very important for the wind-resistant design of high-rise buildings to assess wind-induced vibrations efficiently. The Lattice Boltzmann Method-based Large Eddy Simulation and Fluid–Structure Interaction techniques are used to identify the surface wind pressure and wind-induced dynamic response of a CAARC standard high-rise building. Compared with wind tunnel tests, a detailed analysis of the accuracy of simulated wind pressures and base moments of the CAARC model are discussed under multiple wind direction angles. The differences between one-way and two-way Fluid–Structure Interaction simulations are compared under two different reduced wind velocities. The research results show that the simulated mean surface wind pressures of building under seven wind direction conditions have an error within 15% compared to probe measurements, and the average and root mean square base bending moments agree well with the wind tunnel tests. The top transverse wind-induced vibrations of the buildings are significantly larger when the reduced wind velocity reaches 4.6, indicating that aerodynamic damping effects on structural responses should not be overlooked. The research findings of this article provide valuable technical references for the application of LBM methods in the wind load effect assessments of high-rise buildings.

Investigation of the intrinsic characteristics of two lock-in regions in a 10:1 π-shaped deck

The π-shaped deck, as a typical blunt body, is prone to vortex-induced vibration due to the surrounding flow. However, the intrinsic characteristics of sectional vibration are frequently disregarded, leading to a lack of a comprehensive understanding of the lock-in region characteristics. Therefore, a deep analysis of the spectrum characteristics was carried out using a combination of simultaneous vibration–pressure testing and computational fluid dynamics (CFD) to gain a deeper understanding of this phenomenon. In this study, the differences between the two lock-in regions were analyzed, and it was observed that the predominant frequency of vibration increased with an increase in reduced wind velocity in a certain range. Furthermore, it was found that, in the lock-in region, when the maximum amplitude occurred, the vibration predominant frequency was greater than the natural frequency. The Strouhal number (St number) of each lock-in region generally exhibited a step-like downward trend as the frequency ratio increased. Moreover, the first lock-in region was associated with the Karman vortex street in the wake region, as well as with the slow-moving vortex on the upper surface and the two vortices on the lower surface. The long tail vortex shedding in the wake region and the single vortex on the lower surface were the key reasons for the larger amplitude of the second lock-in region when compared to the first lock-in region.

Effects of Wind Barrier Porosity and Inclination on Wind Speed Reduction

Wind barriers play a vital role in protecting saplings until maturity when planted as vegetative windbreak forests. Most previous studies have focused on the porosity of wind barriers, but no studies have simultaneously examined the effects of the porosity and inclination, despite the potential of the inclination to decrease wind speed. We tested three wind barrier cases in wind tunnel experiments: (1) Case A (porosity of 0% with inclinations (90°, 80°, and 70°)), (2) Case B (porosity of 25% with inclinations (90°, 80°, and 70°)), and (3) Case C (porosity of 50% with inclinations (90°, 80°, and 70°)). The vertical and horizontal wind velocities were measured at three vertical and seven horizontal points behind the barriers. The results demonstrated a statistically significant decrease in the correlation between the distance and mean wind velocity for all cases, with up to a six-fold wind protection effect. The wind barrier with 0% porosity and a 90° inclination provided the highest degree of wind protection. However, the wind protection range was limited downwind, and recirculation of wind flow could occur in the leeward direction, potentially damaging saplings. A wind barrier with 50% porosity and 70° inclination sufficiently decreased the wind velocity and prevented recirculation of wind flow, demonstrating that both porosity and inclination considerably impacted the wind protection effect by reducing wind velocity. Our findings offer novel insights into the influence of wind barriers with varying porosities and inclinations and can provide valuable guidance for constructing efficient windbreak forests.

Earthen mounds (heuweltjies) of South Africa and their termite occupants: applicability of concepts of the extended phenotype, ecosystem engineering and niche construction.

Heuweltjies are earthen mounds found throughout the Succulent Karoo of South Africa and are inhabited by the termite Microhodotermes viator. Many have assumed that heuweltjies are constructed by the occupying termites. Consequently, heuweltjies have been used as an example of several important concepts in ecology and evolution: the extended phenotype, ecosystem engineering and niche construction. However, recent findings demonstrate that M. viator does not directly construct heuweltjies. Rather, termite colonies enrich the soil around their nests with plant nutrients, which promotes development of widely separated patches of denser vegetation. Eventual formation of heuweltjies represents a response of the physical environment to the windbreak effect of the denser vegetation patches (localized reduction of wind velocity and resultant deposition and accumulation of airborne sediment). Other structures constructed by the termites are justifiably regarded as extended phenotypes. Identification and investigation of a complex cascade of processes are required to more precisely assess the manner in which this termite species functions as an ecosystem engineer or niche constructor, thereby significantly influencing the availability of resources within local ecosystems. Environmental alterations that are either directly or indirectly generated by social animals that construct large, communal nests represent ecological processes that contribute significantly to local biodiversity. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

A transport model of convective flow on self-excited vibrating flat flexible roofs using LES

This study aims to investigate the underlying mechanisms for the self-excited vibration of a one-way long-span flat roof. The large eddy simulations (LES) on both rigid and forced vibrating roofs with different vibrating frequencies are performed to obtain the aerodynamic forces and flow field data simultaneously. Firstly, the LES simulations on the rigid and vibrating roofs are validated against the reference experiments. Subsequently, the energy evolutionary characteristics of the aerodynamic forces on vibrating roofs are elucidated by the working energy analysis. Moreover, the interaction between the flow field and the vibrating roof is also revealed by flow visualizations, cross-correlation analysis, and spectral analysis. The synchronization between the transporting of vortices above the roof and the roof vibrations is qualitatively revealed and quantitatively confirmed at the occurrence of the self-excited vibration. Furthermore, a transport model of the convective flow is proposed and validated for the interaction between the vortex transporting and roof vibration. Ultimately, a semi-empirical function is established to predict the critical wind velocity of long-span roofs by modeling the vortex convection velocity in relation to the reduced wind velocity. The outcomes of this study will contribute to a deeper understanding of the flow mechanisms involved in self-excited vibrations of long-span roofs, ultimately informing the wind-resistant design of such structures in practical engineering applications.

Droplet Penetration Model Based on Canopy Porosity for Spraying Applications

Analysing the penetration and droplet deposition characteristics in the canopy of fruit trees is critical for optimising the operational parameters of air-assisted spraying equipment, achieving precise application of chemicals, and improving the effectiveness of fruit tree pest and disease control. We used a mobile LIDAR system to detect the tree canopy characteristics and optical porosity and conduct wind tunnel experiments to investigate the interaction between the tree canopy, the airflow field, and the droplet penetration ratio in the canopy of fruit trees. The results show that the relative wind velocity decreases rapidly during canopy penetration, and that the minimum value occurs at the back of the canopy. The smaller the optical porosity, the greater the reduction in wind velocity is. The quadratic exponential regression model had the highest coefficient of determination (R2) (0.9672) and the lowest root mean square error (RMSE) (5.56%). This paper provides information on optimising the spraying parameters, improving the pesticide utilisation rate, and selecting the optimum spraying conditions and application parameters.

Investigation on modal coupling–aeroelastic effect of a super tall building by MDOF aeroelastic model test

The prototype buildings in existing wind tunnel tests on the aeroelastic effect of super tall building are mostly hypothetical and not consider the modal coupling. In this paper, a multi-degree-of-freedom aeroelastic model was designed and wind tunnel tests were performed to research the modal coupling–aeroelastic effect of a super tall building, which exhibited significant aeroelastic effects and modal coupling during typhoon. The characteristics of wind-induced response of the model during different wind speeds were discussed. Complex mode decoupling technique and Bayesian approach were used to identify and analyze the variation of the first two modal parameters of the model with reduced wind velocity. Finally, wind tunnel test results were compared with field measurements. Results show that compared with the narrow-side windward, the cross-wind vortex-induced resonance of the model is more obvious when the model is wide-side windward. The along-wind modal frequency and damping ratio show similar variation with reduced wind velocity as cross-wind modal frequency and damping ratio. Moreover, the along-wind mode shape approaches the cross-wind mode shape at the reduced wind velocity of resonance. The peak accelerations predicted from the aeroelastic model test are in good agreement with the field measurements.

Evaluating the improvements of thermal comfort by different natural elements within courtyards in Singapore

Heat mitigating natural elements such as trees, shrubs, grass and water have been broadly studied, while their effects on semi-open spaces have been almost ignored. This paper aims at measuring and comparing the heat mitigating effects of four natural elements in Singaporean courtyards. Four typical courtyard scenarios (each featuring a natural element) are simulated in Envi-met and a comprehensive analysis of air temperature, mean radiant temperature, wind velocity and relative humidity is done to clarify the respective advantages of each natural elements. Daily variation of climate indicators reveal that swimming pools and grass have similar effects on air temperature, mean radiant temperature, wind velocity, relative humidity, and on Physiological Equivalent Temperature, with their differences only being 0.1 °C, 0.3 °C, 0.1 m/s, 0.3%, and 0.3 °C, respectively. Compared with grass, trees do slightly reduce wind velocity by 0.1 m/s, still they significantly reduced the daily Physiological Equivalent Temperature by 2.1 °C. Fountains are better at reducing air temperature by 2.1 °C, compared with swimming pools, but because they also increase relative humidity by 12.8%, they end up performing not so differently from swimming ponds and grass in terms of Physiological Equivalent Temperature.

Characterizing the spatial variations of wind velocity and turbulence intensity around a single Tamarix tree

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

Comparison of near wind farm wake measurements from scanning lidar with engineering models

The presence of offshore wind farms causes downstream regions of reduced wind velocity, i.e. wind farm (cluster) wakes, which can affect the power of wind farms downstream. Engineering models are now being used to simulate the effects of these wakes, and an important requirement for model validation is a comparison with full-field measurements. Our objective in this paper is to parametrize and validate two engineering wake models with long-range lidar measurements. We use a long-range scanning Doppler lidar to scan the near wake region of a 400 MW offshore wind farm and compare the wind velocities in the wake to the outputs of two engineering models: FarmFlow and flappy. We adapt FarmFlow to solve the flow in highly unstable atmospheres by modifying the boundary conditions, which enables the comparison of velocity profiles behind the farm. The models perform qualitatively well in predicting the wake deficit and shape close to the farm and at lower heights. They predict higher wake losses within the farm when compared to production power data in a strongly unstable atmospheric case. However, the current analysis is limited due to the lack of inflow measurements for model initialization, compounded by limited data availability. We discuss the possibilities and limitations of long-range scanning lidar data for cluster wake model validation and the need for inflow measurements for model initialization. We conclude that with detailed inflow measurements, scanning long-range lidars could serve as a good tool for the validation of wind farm wake models.

Effects of end condition and aspect ratio on vortex-induced vibration of a 5:1 rectangular cylinder

A series of wind tunnel tests were carried out to study the end condition and aspect ratio effects of a sectional model on the vortex-induced vibration (VIV) of a 5:1 rectangular cylinder. Five aspect ratios varying from 9 to 32 as well as eight endplates with different sizes were examined in the tests. Static and elastically mounted sectional models were employed to measure the surface pressure and VIV response of the cylinder, respectively. Sectional fluctuating lift forces, shedding frequency, as well as their spanwise distribution and correlation on the sectional model were obtained and discussed. Results indicated that the aspect ratio had a significant effect on VIV amplitudes, and small aspect ratios tended to remarkably underestimate the VIV amplitudes. There existed an end interference region with a length of 6–8 D, where the sectional lift forces could be, on average, half the value for the non-interference region. Besides, the aspect ratio was positively related to the spanwise correlation of sectional lift forces. Both end condition and VIV had little effect on the length of the end interference region. In the VIV ‘lock-in’ range, with the increasing reduced wind velocity the fluctuating lift force first increased and reached a maximum value earlier than the VIV amplitude, and after that further increase of reduced wind velocity led to decreased fluctuating lift force (toward a value even smaller than that of the stational model). This complicated variation of sectional lift forces implied that vortex shedding was highly nonlinear and self-limiting. Finally, an empirical method based on the test results was explored to account for the influence of aspect ratio on VIV amplitude.

Numerical simulation of wind veering effects on aeroelastic responses of thousand-meter-scale super high-rise buildings

The atmospheric boundary layer (ABL) consists of the atmospheric surface layer (ASL) and the Ekman layer. In the Ekman layer, which typically accounts for the upper 90% of the ABL, the wind veering is generally remarkable along the height. In the wind-resistant design of thousand-meter-scale super high-rise buildings, it is of vital importance to consider the wind veering effects. However, the relevant research on this issue has been rare so far. In this study, based on the authors’ previous study on wind loads acting on thousand-meter-scale super high-rise buildings immersed in the veering wind field, the structural aeroelastic responses of a thousand-meter-scale square-section building model with an aspect ratio of 9:1 under various reduced wind velocities without considering wind veering at 0° wind direction and considering wind veering at 75° wind direction are numerically simulated by utilizing the commercial software ANSYS Workbench 19.1. The wind veering effects on structural aeroelastic responses are comprehensively analyzed. Additionally, the structural dynamic responses with and without considering aeroelasticity are compared. The effects of aerodynamic damping on wind-induced vibration responses are further discussed, and the underlying mechanisms are illustrated in the light of instantaneous flow fields.

Experimental study on proximity interference induced vibration of two staggered square prisms in turbulent boundary layer flow

Slender structures placed close to each other could greatly magnify the dynamic response due to interference effects. This paper presents the results of experimental studies of the proximity interference induced vibration of two identical square prisms in a turbulent boundary layer to clarify its excitation mechanism. The upstream interfering prism was static, while the principal prism was dynamic and free to vibrate only in the crosswind direction. The divergent vibration behavior at the critical interfered location was examined in detail. Based on the time-resolved particle image velocimetry (TR-PIV) technique and unsteady pressure measurement, the vortex shedding process and corresponding pressure distribution of the dynamic principal prism were described by examining their correlation with the motion time histories. The biased gap flow between the two prisms was found to control the vibration of the dynamic principal prism. The interaction between the vortex shedding in the wake of the dynamic principal prism and the gap flow is dramatically intensified with increasing wind velocity. Such effects create a large fluctuating lift force on the dynamic principal prism at high reduced wind velocity, compared with measurement taken with two static prisms, which drives the prism to undergo a divergent crosswind response.

Wind farm cumulative induction zone effect and the impact on energy yield estimation

The cumulative induction zone effect arising from the operation of multiple wind turbine generators and their interactions within a wind farm is investigated within the framework of computational fluid dynamics. A computational fluid dynamic model is benchmarked and validated against measured lidar data from an offshore wind farm. A range of wind farm array configurations are investigated and the results are used to advise adjustments relevant to wind farm energy production assessments. Based upon the offshore wind farm array configurations investigated, the outcomes indicate that the cumulative induction zone effect results in a reduction in the induction zone wind velocity of approximately 0.3–1.6%, with a corresponding reduction in energy yield of 0.4–1.2% depending on the wind farm configuration, the inflow wind velocity and the atmospheric stability conditions. In the case of larger wind farms, investigations to date indicate that the cumulative induction zone effect is more pronounced with the appropriate energy adjustment factor predicted to be increased by a factor of up to two. Additionally, wind farm configurations and wind speed distributions differing from those considered within this work may result in cumulative induction zone effect values outside those stated.

People’s Perceptions about the Socio-Economic and Environmental Impact of Coastal Green Belt in Bangladesh

Understanding the perceptions and attitudes of local people towards afforestation is crucial for successful afforestation. To better understand the people's perceptions about the coastal green belt, we surveyed the Satkhira and Bhola districts during January 2021. A questionnaire survey of 200 respondents was conducted by a purposive and random sampling technique to obtain quantitative data. On the other hand, two Focus Group Discussions (FGDs) were performed to obtain qualitative data based on the diversity of age, sex, education, and occupation variables. We used to evaluate the respondent’s perceptions about the coastal green belt by 5-point Likert scale data. The majority portion of the people in our study area was poor and their profession was fisherman and housewife among males and women respectively. The majority of them agreed that the green belt helped them in numerous ways, to rear their cattle and built a house near the green belt, by promoting the growth of crops, and further protect them from different natural calamities by reducing wind velocity. Shelterbelts greatly enhance tourism in that area, improved communication facilities, and increase their socio-economic condition and values. Shelterbelt has long-term benefits, peoples have positive attitudes towards shelterbelt and they seek training to maintain this shelterbelt smoothly. Respondents wanted fruit tree species as shelterbelt species near the embankment. This information can be used for policy formulation in terms of successful plantation by considering people’s attitudes, which may work in both mitigation and adaptation of climate change in the coastal remote areas of Bangladesh.

Wake-induced instabilities of parallel circular cylinders with tandem and staggered arrangements

When two circular cylinders are closely arranged in parallel, the downstream cylinder frequently exhibits wake-induced vibrations (WIVs). This study investigates the classifications of WIVs in various arrangements of cylinders by considering the generation mechanisms of WIVs through a series of wind tunnel tests. The generation mechanisms are defined through flutter and time-history analyses using the quasi-steady theory. Finally, this study suggests a classification map of the arrangements with four types of generation mechanisms: vertical flutter, anti-phase coupled flutter, in-phase coupled flutter (quasi-steady applicable), and in-phase coupled flutter (quasi-steady inapplicable). Vertical flutter occurs in closely staggered cylinders and tandem cylinders with a horizontal center distance measuring 2.5–9.5 times the cylinder diameter. A key parameter for distinguishing coupled flutter is the contribution of the aerodynamic coupling terms. Furthermore, for closely staggered cylinders, two types of vibrations occur, depending on the reduced wind velocity. The arrangement of the two cylinders is slightly changed because of the static displacement of the downstream cylinder depending on the wind velocity, resulting in a change in the generation mechanism. The results pertaining to the generation mechanisms effectively explain the WIV responses of the two cylinders in various arrangements.

Design, manufacture, and testing of an innovative ridging device for controlling of wind erosion

Wind erosion is a serious problem in the world, especially in Iran. Several studies have used windbreaks and barriers to reduce wind erosion and wind-induced dust emissions; however, there is no study on the development of the ridge with a device. In this study, a new machine was designed and manufactured to create a ridge by mixing soil with biopolymer. The device was developed in three sections (machine, mixing, and ridge maker sections). Mechanisms that control movements are chain drives, brakes, and clutches that enable the device to operate in different topological regions. As part of a test phase of the device, three levels of polymer suspension (10, 20, and 30%) were added to the soil and a ridge was made by the ridging device. These ridges were created in one-hectare flat agricultural land. The ridging device efficiency was tested in 48 h field measurements during September 2018, where three measurements, including penetration resistance, aggregate stability, and ridge height and width were carried out. The results showed 1.5 kg cm−2 penetration resistance in the ridge using 30% volumetric suspension. The highest ridge was 43 cm with 30% suspension. Aggregate stability values of the ridges at 30% and 20% volumetric suspensions were 0.88 and 0.78, respectively. Overall, the results showed that creating a ridge with an optimum height could reduce wind velocity, soil transportation, and limit saltation in the ridge windward and leeward zones.