Reduced Wind Velocity Research Articles

An integrated simulation-assessment study for optimizing wind barrier design

Wind barriers are artificial structures that are widely built to abate wind erosion by reducing wind velocity near surface, which requires optimal design in aeolian engineering. Previous studies have shown that numerical simulation is an effective method for optimal design of wind barriers. However, there still exist two challenging questions: 1) how to resolve fine-scale airflow fields around barriers? and 2) how to systematically evaluate the shelter efficiency? In the current study, we have conducted high-resolution 3D computational fluid dynamics (CFD) simulations for airflow passing through wind barriers then explored optimal designs. To validate the simulation results, we compared the simulated airflow results with those from wind-tunnel measurement. Moreover, we innovatively proposed a shelter index to evaluate the shelter efficiency, which has taken wind velocity reduction, economical cost and shelter degree into account. According to the calculated shelter index, wind barriers with porosity of 0.3–0.4 could provide the longest effective shelter distance, and a 2-row-a-belt scheme with inter-row spacing of 5–7h (h as the height of wind barriers) is the most effective. The optimal inter-belt spacing is suggested as 12–15h depending on local wind velocity. This study is intended to provide design references for constructing wind barriers in aeolian engineering.

LiDAR-Based Approach for Urban Ventilation Corridors Mapping

Cities, recognized as one of the roughest surfaces, significantly reduce wind velocity. This leads to a greater concentration of pollution and increased temperature in urban areas. A proper urban design may promote the air flow and mitigate these negative phenomena. This paper describes a novel approach to detect and analyze the potential urban ventilation corridors. The concept is based on the morphometric analyses, where roughness parameters (such as roughness length and displacement height), as well as porosity are investigated as opposed to previous studies. Commonly available light detection and ranging (LiDAR) data enable more detailed terrain study, as the topography and obstacle features (type, dimensions, and porosity) might be investigated. In comparison to the previous studies and long adapted practices, this approach takes advantage of all information incorporated in LiDAR data to deliver even more accurate results. The comparison of the detected corridors, with the officially designed corridors in our study area in Warsaw, suggests inaccuracies in their extent (only 40% of their areas might still be functioning) and calls for an evaluation. The results deliver areas that may be recognized as urban ventilation corridors and serve as air regeneration and ventilation system. They may also be used for developing new solutions for continuously evolving cities.

Numerical simulation of the effect of wind barrier openings on high-speed wind flow around a heliostat field

• A two-dimensional numerical simulation was conducted. • We investigated the effect of varying the wind barrier opening size on wind flow. • We analyzed the vertical profile of wind velocity and turbulence kinetic energy in the domain considered. • We proposed the recommended wind barrier opening size for reduction of wind velocity and turbulence kinetic energy. The application of heliostats for solar power tower plants has increased in recent years. Most of the solar power tower plants installed are on flat terrain, and they may be subjected to wind loads. In the present study, a two-dimensional numerical simulation of turbulent high-speed wind flow around a heliostat field was analyzed and the wind barrier effect on the flow was investigated with use of the computational fluid dynamics commercial code COMSOL ( k - ε turbulence model). The calculation was performed for different configurations of wind barriers with opening sizes of 0 m, 0.2 m, 0.5 m, and 1 m, respectively, and four heliostat orientations in each case: 0°, 30°, 60°, and 90°. The results show that the use of wind barriers reduces wind velocity in a solar power tower plant. We found that wind barriers with small openings (0.2 m and 0.5 m) are effective in reducing wind speed below 10 m from the ground and create, at the same time, less turbulence kinetic energy than the other configurations (0 m and 1 m).

Aerodynamic damping of inclined slender prisms

The aerodynamic damping of forward and backward inclined prisms has been identified through a series of forced vibration wind tunnel tests. The test results show that the amplitude of vibration and reduced wind velocity both have a significant effect on the aerodynamic damping of the inclined prisms, especially at reduced wind speeds where von Karman vortex lock-in occurs. The effect of forward and backward inclination on the characteristics of aerodynamic damping in inclined prisms is dissimilar to a vertical prism, and the characteristics have been discussed in terms of the unsteady aerodynamic force and Strouhal number in this study. This study not only advances our understanding on aerodynamic damping of inclined prisms, but also provides values that can be used to estimate aerodynamics and aeroelasticities of inclined prisms.

Improved expression for across-wind aerodynamic damping ratios of super high-rise buildings

To study across-wind aerodynamic damping ratios of super high-rise buildings, wind tunnel tests of multi-degree-of-freedom (MDOF) aero-elastic models with various aspect ratios and mass-damping parameters were carried out. The relationship between aero-damping ratio and reduced wind velocity (Vr), wind field roughness, aspect ratio, structural mass, structural damping as well as across-wind displacement was investigated systematically. It was found that the positive aerodynamic damping ratio is negatively correlated with the structural damping ratio and structural mass at Vr lower than critical reduced velocity of resonance (Vc). However, when Vr is close to or slightly bigger than Vc, the absolute value of negative aerodynamic damping ratio is positively correlated with structural damping but negatively correlated with structural mass. Therefore, the empirical function based on aerodynamic damping ratio alone cannot be reliably established, because the aerodynamic damping ratio alone is insufficient to assess aerodynamic effects. Thus, these considerations lead to a proposal of the concept of reduced damping ratio. This concept allows an improved empirical function of aerodynamic damping ratio to be established. Effects of the aspect ratio, reduced wind velocity, wind field terrain, structural damping and mass density were all considered in this empirical function.

Shelter effect efficacy of sand fences: A comparison of systems in a wind tunnel

Abstract The Lanzhou-Xinjiang High-speed Railway runs through an expansive wind area in the Gobi Desert and blown-sand disasters are a critical issue affecting its operation. To strengthen the blown-sand disaster shelter systems along the railway, the shelter effects of punching plate and wire mesh fences with approximately equal porosity (48%) were simulated in a wind tunnel. The experimental results showed that the wind velocity was reduced to a higher extent by the punching plate fence than by the wire mesh fence. When a single row of sand fencing was used, the wind velocity reduction coefficient (Rcz) values downwind of the punching plate fence and wire mesh fence reached 71.77% and 39.37%, respectively. When double rows of sand fencing were used, the Rcz values downwind of the punching plate and wire mesh fences were approximately 87.48% and 60.81%, respectively. For the flow field structure on the leeward side of the fencing, the deceleration zone behind the punching plate fence was more pronounced than that behind the wire mesh fence. The vortex zone was not obvious and the reverse flow disappeared for both types of fences, which indicates that the turbulent intensity was small. The sand-trapping efficiency of the wire mesh fence was close to that of punching plate fence. When a single row of sand fencing was set up, the total mass flux density decreased, on average, by 65.85% downwind of the wire mesh fence, and 75.06% downwind of the punching plate fence; when double rows of sand fencing were present, the total mass flux density decreased, on average, by 84.53% downwind of the wire mesh fence and 84.51% downwind of the punching plate fence. In addition, the wind-proof efficiency and the sand-proof efficiency of the punching plate fence and the wire mesh fence decreased with increasing wind velocities. Consequently, punching plate and wire mesh fences may effectively control the sand hazard in the expansive wind area of the Gobi Desert.

環境工学分野における随伴変数法の適用に関する研究 （第2報）風速低減を目的とした樹木配置及び排熱に効果的な開口位置の最適決定法の提案

環境工学分野における随伴変数法の適用に関する研究 （第2報）風速低減を目的とした樹木配置及び排熱に効果的な開口位置の最適決定法の提案

Wind tunnel test on the effect of metal net fences on sand flux in a Gobi Desert, China

The Lanzhou-Xinjiang High-speed Railway runs through an expansive windy area in a Gobi Desert, and sand-blocking fences were built to protect the railway from destruction by wind-blown sand. However, the shielding effect of the sand-blocking fence is below the expectation. In this study, effects of metal net fences with porosities of 0.5 and 0.7 were tested in a wind tunnel to determine the effectiveness of the employed two kinds of fences in reducing wind velocity and restraining wind-blown sand. Specifically, the horizontal wind velocities and sediment flux densities above the gravel surface were measured under different free-stream wind velocities for the following conditions: no fence at all, single fence with a porosity of 0.5, single fence with a porosity of 0.7, double fences with a porosity of 0.5, and double fences with a porosity of 0.7. Experimental results showed that the horizontal wind velocity was more significantly decreased by the fence with a porosity of 0.5, especially for the double fences. The horizontal wind velocity decreased approximately 65% at a distance of 3.25 m (i.e., 13H, where H denotes the fence height) downwind the double fences, and no reverse flow or vortex was observed on the leeward side. The sediment flux density decreased exponentially with height above the gravel surface downwind in all tested fences. The reduction percentage of total sediment flux density was higher for the fence with a porosity of 0.5 than for the fence with a porosity of 0.7, especially for the double fences. Furthermore, the decreasing percentage of total sediment flux density decreased with increasing free-stream wind velocity. The results suggest that compared with metal net fence with a porosity of 0.7, the metal net fence with a porosity of 0.5 is more effective for controlling wind-blown sand in the expansive windy area where the Lanzhou-Xinjiang High-speed Railway runs through.

Unsteady pressure measurements on an oscillating slender prism using a forced vibration technique

The unsteady aerodynamic forces acting on a slender prism were investigated using a forced vibration technique. The prism was driven to oscillate by an actuator and the unsteady distributed pressures, under different wind velocities and oscillation amplitudes, were measured. The measurement was calibrated with respect to driving oscillation, aerodynamic force coefficient, as well as motion-induced force coefficient. Then, the generalized and local aerodynamic force coefficients and the motion-induced force coefficients of the prism, which are functions of reduced wind velocity and oscillation amplitude, were analyzed. It shows that the effects of structural motion on the coefficients are significant in the crosswind direction while the effects are slight in the along-wind direction. Furthermore, in the crosswind direction, the coefficients tend to increase with oscillation amplitudes at low wind speeds while they are at a quasi-steady state at high wind speeds. These characteristics were analyzed from the perspectives of generalized and pointwise spectra, force-response coherences and Strouhal numbers of the prism. The study advances the understanding of the effect of structural motion on three-dimensional prisms, which can be utilized to improve response predictions of the prisms.

Flow field control to mitigate airborne sea salt adhesion on bridge girders

In order to realise effective maintenance and enhanced durability of structures, it is important to also reduce corrosion of bridges by airborne sea salt. The objective of this study is to reduce airborne sea salt adhesion amount on steel girder bridges by employing aerodynamic countermeasures. The study bridge is a typical metropolitan highway bridge with 8 I-shaped steel girders located in Japan. Aerodynamic countermeasure devices are employed to change the flow field around the bridge structure in an attempt to reduce wind velocity normal to the bridge girders. Devices existing on urban bridges such as noise barriers, median barriers, and facilities for passage of drainage pipes and electric cables, modelled as horizontal plates, are modified and investigated for their ability to reduce airborne sea salt adhesion amount. As additional devices, vertical plates are installed to change the flow separation and their applicability is also studied. Computational fluid dynamics is employed for flow field simulations and airborne sea salt adhesion amount is estimated by the improved concentration flux method. Findings indicate that horizontal plates and vertical plates significantly reduce airborne sea salt adhesion amount. Noise barriers and median barriers can also reduce airborne sea salt adhesion amount.

Protection efficiency assessment and quality of coastal shelterbelt for Dongshan Island at the coastal section scale

The coastal shelter forest in China is under threat of destruction and degradation because of the impact of human activities. Protection efficiency assessment of the coastal shelterbelt is an important component of shelter-forest remediation planning and sustainable management. In this study, a protection efficiency index (PEI) model was established using the projection pursuit method to assess the protective quality of the coastal shelter forest at the coastal section scale of Dongshan Island, China. Three criteria were used, including forest stand structure, forest belt structure, and windbreak effect; each criterion further comprised multiple factors. Based on survey data of 31 plots in the coastal shelter forest of Dongshan Island, we calculated PEI values using a projection of a pursuit model. The result showed 64.5 % of the PEIs fell at or below the middle level, which can indicate the status of the coastal shelterbelt is unsatisfactory. To further explore whether the different bays and land use types create significant differences in PEIs and evaluation indices, we used an ANOVA to test the influence of various bays and forms of land use on coastal shelterbelts. The results showed that PEI and most of the indices differed significantly by bay; mean tree height, mean DBH, mean crown width, stand density, vegetation coverage, and wind velocity reduction differed significantly by land use. Therefore, relevant measures for different locations, bays and surrounding land use can be proposed to improve the existing conditions of the coastal shelterbelt. The results of this study provide a theoretical and technical framework for future changes and sustainable management of coastal shelterbelt on Dongshan Island.

A method to estimate the structural parameters of windbreaks using remote sensing

Windbreaks are a major component of agroforestry practices and play an important role in agroforestry ecosystems. They can reduce wind velocity and protect shelter crops from wind damage and soil from wind erosion. Porosity is one of the most important structural parameters that affect wind speed and is widely used in the study of wind protection provided by windbreaks. In this paper, a method to estimate porosity using high-resolution satellite imagery is represented. Porosity was difficult to estimate through the direct use of remote sensing data due to the poor relationship with vegetation indices. Thus, two intermediate variables, that is, CL 2 × LAI and CL × LAI × W, which were highly related to porosity, were selected. Leaf area index (LAI) and average tree crown length (CL) were estimated using vegetation indices, and W, which refers to the width of a windbreak, was identified using object-based image analysis. Porosity was estimated using a statistical relationship between porosity and intermediate variables. The average prediction accuracy of the estimated porosity value was 76.104 %. Based on the estimated porosity value, the windbreaks were grouped into three types, and their efficiency of wind protection was evaluated. The evaluation result indicated that the windbreaks have a very good protective structure in the study area and they can effectively shelter crops from wind damage and erosion. This study can provide a useful guide for studying the wind protection provided by windbreaks on spatial and temporal scales using remote sensing.

English

English

A Study on Aerodynamic Damping and Aeroelastic Instability of Helical-shaped Super Tall Building

In this paper, aeroelastic instability and aerodynamic damping ratio of a helical model which shows better aerodynamic behavior in both along-wind and crosswind responses on a super tall building was investigated by an aeroelastic model test, and the aerodynamic damping ratio was evaluated from the wind-induced responses of the model by using Random Decrement Technique. Aerodynamic damping ratios evaluated in this study were verified through comparison with previous results obtained by quasi-steady theory. As a result, the aeroelastic instability of the helical model in crosswind direction were not occurred for any conditions with increasing the reduced wind velocity while the square model generally encounters aeroinstability due to the vortex shedding. The aerodynamic damping in along-wind direction for the helical and the square model increased monotonically both with reduced wind velocity, i.e., there is no relation with modifications of building shapes. On the other hand, in crosswind direction, the characteristics of aerodynamic damping ratio with reduced wind velocity for helical model were quit different from those of the square model.

Effects of Exponentially Modified Sinusoidal Oscillation and Amplitude on Bridge Deck Flutter Derivatives

With the use of a computational fluid dynamics (CFD) numerical simulation, in this study an unusual exponentially modified sinusoidal (EMS) oscillation was forcibly imposed on deck section to simulate amplitude-varying self-excited forces. This approach facilitated the investigation of the influence of EMS transient oscillation and amplitude on self-excited forces and flutter derivatives. Study results show that for an ideal thin plate, the sensitivities of H2*, H4*, A2*, and A4* to oscillation decaying/diverging ratio (ξ) were higher than those of H1*, H3*, A1*, and A3*. With different amplitudes, ξ, and reduced wind velocities, the self-excited forces of one flat plate and two bluff bridge decks were numerically calculated and the flutter derivatives were extracted and compared with the theoretical and experimental results. If |ξ| < 0.1, its influence was almost negligible, by which the traditional sinusoidal oscillation technique was validated. The heaving and torsional amplitudes had different influences on each flutter derivative. The influences were also related to deck cross sections and reduced wind velocities. The proposed numerical simulation of EMS oscillation enables a better understanding of the influence of oscillation amplitude and ξ on bridge deck flutter derivatives.

Measurement of Wind Flow Behavior at the Leeward Side of Porous Fences Using Ultrasonic Anemometer Device

Efficiency of wind fences is evaluated by the reduction of wind velocity and the effective shelter area behind the fence. Fence porosity is the most influential structural parameters in the porous fence design, and has significant impact on the structure of wind velocity and turbulence leeward. In this work, three fences with porosity of 23%, 35% and 45% have been tested under 10m/s inlet velocity in an environmental wind tunnel. An ultrasonic anemometer was used to measure three dimensional velocity vectors and the fluctuations of velocity vectors with a designed spatial resolution. The obtained data were used to calculate velocity magnitudes, turbulence intensities and turbulent kinetic energies in the test domain. Porosity influencing the structure of turbulence intensity and effective fence zone were discussed in this paper. Neither the distribution of wind velocities nor the structure of turbulence alone can provide sufficient information to assess the performance of porous fences. Consideration of the targeted reduction rate of wind velocity needs to be taken for optimal fence design.

Effect of shelter porosity on downwind flow characteristics

Previous wind-tunnel studies were focused mainly on lonely standing windbreaks or wind fences with respect to their wind velocity reduction efficiency and effective shelter distance. In presented wind-tunnel study, we investigated the effects of a three different fence porosities (0.5, 0.25 and 0) embodied in a shelter-like building for coal convey by means of two-component Laser Doppler Anemometry (LDA). The turbulent flow characteristics behind the fences were compared with those performed without the fence. For characterization of the fence effectiveness we used following quantities: wind-speed and turbulence kinetic energy reduction, and time fractions of the turbulent coherent structures associated with the sediment transport (sweeps and outward interactions). Results from mentioned quantities revealed that for the case of embodied fence the shelter construction has significant impact on the flow characteristics behind. The fence of the 0.5 porosity has been indicated as the most shelter effective considering the studied quantities.

Analysis of vortex induced vibration frequency of super tall building based on wind tunnel tests of MDOF aero-elastic model

To study the vibration frequency of super high-rise buildings in the process of vortex induced vibration (VIV), wind tunnel tests of multi-degree-of-freedom (MDOF) aero-elastic models were carried out to measure the vibration frequency of the system directly. The effects of structural damping, wind field category, mass density, reduced wind velocity (Vr), as well as VIV displacement on the VIV frequency were investigated systematically. It was found that the frequency drift phenomenon cannot be ignored when the building is very high and flexible. When Vr is less than 8, the drift magnitude of the frequency is typically positive. When Vr is close to the critical wind velocity of resonance, the frequency drift magnitude becomes negative and reaches a minimum at the critical wind velocity. When Vr is larger than12, the frequency drift magnitude almost maintains a stable value that is slightly smaller than the fundamental frequency of the aero-elastic model. Furthermore, the vibration frequency does not lock in the vortex shedding frequency completely, and it can even be significantly modified by the vortex shedding frequency when the reduced wind velocity is close to 10.5.

A New Approach in Converting Desert into Pasture: Fengning County, Hebei Province, China

The thematic focus of this paper is to introduce a way to prevent desertification in the steppe climatic region by reducing wind velocity on ground surface and introducing a know-how of a specially designed drip-irrigation system that is sustainable in arid climate. The current desertification processes are rapidly expanding over the steppe climatic region worldwide, closely overlapping with Steppe Climatic Region (BS) of Koppen Climatic Classification. The basic thought in my study is to prevent desertification in the steppe climatic region by reducing wind velocity so that evaporation does not exceed precipitation. By applying Bernoulli’s equation for a one-kilometer-square pilot project in Fengning County, Hebei Province, China, it is estimated that four layers of tree windbreak in a square shape are sufficient to reduce wind velocity for the reforestation (Kim 2011). To avoid frequent failure of reforestation in the arid area owing to buildup of calcium layers and salt flat, a specially designed drip-irrigation system is introduced. Utilizing the favorable physical settings in the high altitude in arid area, a rotating solar power plant is planned to provide for pumping underground water for the drip irrigation. It is estimated that six kilowatts are sufficient power to support irrigation for a one-kilometer-square area, with a few dwelling units in the innermost 400-square-meter area. Once the Fengning pilot project is proven to be successful, it is hoped that the same system may be extended to the adjacent arid region and beyond.

여름철 공원 수림지와 잔디밭의 온열쾌적지수 개선 효과

The purpose of this study was to evaluate human thermal comfort in summer by the type of greenery in parks and to explore planning solutions to supply a comfortable thermal environment in parks. The research was conducted in three different land cover types: a park with multi-wide-canopied trees(WOODLAND), park with grass(LAWN) and park with pavement(PAV) as reference sites in Hamyang-Gun SangrimPark. Field measurements of air temperature, relative humidity and wind velocity, short-wave and long-wave radiation from six directions(east, west, north, south, upward and downward) were carried out in the summer of 2014(August 21-23 and 29-30). Mean Radiant Temperature() absorbed by a human-biometeorological reference person was estimated from integral radiation and the calculation of angular factors. The thermal comfort index  was calculated by Rayman software, ,   were calculated using the  Calculator and the Thermal Comfort Calculator of Richard DeDear. The results showed that the WOODLAND has the maximum cooling effect during daytime, reduced air temperatures/ by up to 5.9°C/35°C compared to PAV and lowered heat stress values despite increasing relative humidity values and decreasing wind velocity. While the LAWN had very slight cooling effects during daytime, reduced air temperatures/ by up to 0.9°C/3°C compared to PAV, the improvement effects of the thermal comfort index was very slight. However, during nighttime the microclimatic and radiant conditions of WOODLAND, LAWN, and PAV were similar owing to the absence of solar radiation, reduction of wind velocity and an increase in relative humidity. Because the shading and evapotranspiration effects of the WOODLAND were much greater than the evapotranspiration effects of the LAWN, it can be said that the solutions for supplying comfortable thermal environment in parks are to amplify the green volumes rather than green areas. This study was undertaken to evaluate the human thermal comfort in summer of WOODLAND/LAWN parks and to determine the improvement effects of thermal comfort index. These results can contribute to the provision better thermal comfort for park users during park planning.