Angelou, NikolasDellwik, EbbaBekkers, Casper C. A.
11页
查看更多>>摘要：Trees are important natural wind engineering elements in both the urban and the agricultural contexts, but their aerodynamic description has been limited by its focus on young, flexible trees. Here, we provide a reformulation of the classical drag equation, which is also suitable for mature and wind-adapted trees. The new formulation is based on results from a full-scale experiment with focus on a solitary oak tree, for which we determined all terms in the drag equation experimentally. We also present a new photographical method for the accurate quantification of the tree's frontal area under highly variable outdoor lighting conditions. We used a database of images from a surveillance camera, from which a high-quality subset was automatically selected with machine learning algorithms. Compared to previous work on younger and smaller trees, the mature tree has a lower absolute value of the Vogel exponent, which indicates a relatively low degree of reconfiguration. The presented results underline the high efficiency of mature trees in reducing the momentum of the wind. The results can be used to quantify the effect of similar trees in wind simulations, and the new method for determining the frontal area can be applied in other tree measurement campaigns.
查看更多>>摘要：In this study, a limiter for turbulent concentration diffusivity, accounting for travel time, was introduced into the Reynolds-averaged Navier-Stokes equations (RANS) and the Eulerian dispersion model. Using conventional modeling of turbulent diffusivity with the simplified gradient diffusion hypothesis and a proposed method with a limiter, pollutant dispersion from an elevated point source in a turbulent boundary layer was predicted and the results validated by using data from an existing wind tunnel experiment. By using conventional modeling, it was difficult to give a good prediction of the entire concentration field by only adjusting the turbulent Schmidt number. In contrast, the proposed method controlled the turbulent diffusivity near the source, and in the downwind region, by adjusting the combination of model parameters in the limiter. As a result, the dispersion characteristics (concentration profile and plume half-width) near the source and in the downwind region were simultaneously well predicted. Finally, a parametric study was conducted on the mean concentration, and the optimal combination of model parameters in the limiter showed improved results of validation metrics ( FB = 0.04, MG = 1.09, NMSE = 0.03, VG = 1.14) than the conventional modeling ( FB = -0.11, MG = 0.75, NMSE = 0.11, VG = 1.62).
查看更多>>摘要：The wind-induced vibration of long-span bridges mainly manifests as flutter, galloping, vortex-induced vibrations (VIVs), buffeting, etc. Among these, VIVs cause vertical or torsional single-mode vibrations of the main girder; these affect the comfort of drivers or pedestrians on the bridge when vortex-induced resonance occurs. In this study, field measurements, numerical simulations, and wind tunnel tests were used to systematically investigate triggering mechanism during the sudden first and secondary VIV phenomena of Humen Bridge, along with the structural mode and damping characteristics and VIV aerodynamic mitigation effects, etc. The results show that the sudden first VIV was caused by the temporary installation of water-filled barriers, as these changed the aerodynamic configuration of the box girder; the decisive removal of these water-filled barriers can effectively suppress the VIV amplitude to some extent. The succedent secondary VIV was caused by the reduction of the structural damping; therefore, aerodynamic control and damping control measures are proposed. The aerodynamic control measures include the implementation of suppressor plates on the top of handrails and removal of the maintenance rails under the girder, whereas the structural damping control measures involve the successive installation of tuned-mass damper (TMD) to compensate for the reduced damping owing to the former large-amplitude VIV performance. After verification based on reduced-scale model wind tunnel tests, the aerodynamic control measures and structural damping control measures are successfully adopted and applied to Humen Bridge. These measures are shown to effectively suppress VIVs, as confirmed via succedent long-term on the-spot observations.
查看更多>>摘要：Since the stream-wise and vertical turbulent fluctuations have an important influence on the movement of sand and dust particles, the accurate quantitative estimation of natural wind fluctuations has an important role in improving the forecast level of wind erosion and dust storm. The high Reynolds number turbulence characteristics of aeolian sand flow and the complexity of sand-laden flows are the main difficulties affecting the accuracy of prediction. In this study, the characteristics of the stream-wise and vertical wind velocity fluctuations in sandladen flows are studied by analyzing the real-time data of the wind velocity field in the atmospheric surface layer (ASL). Then, a simple empirical model representing the stream-wise and vertical wind velocity fluctuations is proposed. The model not only considers the effects of large-scale and small-scale fluctuations, but also has the advantage of predicting the stream-wise and vertical wind velocity fluctuations at any height. Verification shows the results predicted by the model are in good agreement with the observation results, and the predicted saltation sand flux is closer to the experimental results than that without considering the turbulence effect or the vertical wind velocity fluctuations. Moreover, it is a convenient model for application especially in predicting aeolian sand transport.
查看更多>>摘要：The conventional target for self-excited galloping/flutter control of a civil structure often focuses on the critical wind speed. In the present work, a nonlinear control target is introduced, i.e., to ensure that the vibration amplitude is lower than a threshold value (pre-specified according to the expected structural performance) before a target wind speed. Unlike the conventional control target, the nonlinear one can take into account the underlying large-amplitude vibrations before the critical state and/or the structural safety redundancy after the critical state. To obtain the most economical TMD parameters that enable the nonlinear target, an optimization procedure involving nonlinear aeroelastic effect is developed for galloping control based on the quasi-steady aeroelastic force model, and for flutter control based on a nonlinear unsteady model. Three numerical examples involving the galloping/flutter control of different cross-sections are analyzed to demonstrate the different results designed by the conventional and nonlinear targets. It is demonstrated that the nonlinear target and optimization procedure can lead to more economical design results than the conventional ones in the galloping/ flutter control for a structure with relatively large post-critical safety redundancy, and they are more reliable than the conventional ones for a structure that may experience large-amplitude vibrations before the critical wind speed. These superiorities of the nonlinear control target and new optimization procedure suggest that they may be utilized in the TMD parameter optimization for galloping/flutter control of structures in a wide domain of engineering fields.
查看更多>>摘要：The swirl ratio is one of the key parameters for determining the tornado-like vortex structure. Various forms of definition of the swirl ratio have been employed in the physical and numerical modeling of tornado-like vortices, which makes the interpretation of a tornado-like vortex structure inconsistent. In this study, a series of large eddy simulations were performed to model stationary tornado-like vortices by considering two methods, with and without rotating downdraft at the inlet, to generate swirling flow. The evolution from single-vortex, to vortexbreakdown, to vortex-touchdown to multi-vortex flow configurations with increase in swirl ratio are presented, with analyses of various velocity and geometric ratios of simulated tornado-like vortices. Results show that the tornado-like vortex and its velocity and geometric ratios depend on whether or not there is a rotating downdraft when the swirl ratio is low, even when the dependence becomes weak at larger swirl ratios. A critical swirl ratio is discovered to be able to characterize the tornado's vortex structure, dimensionless velocity and geometric ratios. In addition, the various forms of definition of the swirl ratio correlate with each other in a linear or log-log-linear relationship, so they can be converted quantitatively to each other.
查看更多>>摘要：This paper describes the galloping occurrence conditions of four-bundled conductors and the galloping suppression effect of diagonally arranged loose spacers, which is a type of spacer applicable for four-bundled conductors with two rotational clamps placed at diagonal positions. Field observations were conducted at a full-scale test line in Japan where in-cloud icing frequently occurs. A total of 40 ice accretion events were observed across four winter seasons, and galloping data were obtained in a non-countermeasure phase set with normal spacers and a countermeasure phase set with loose spacers. The relationship between the total displacement amplitude and tension variation was determined to identify the conditions under which large galloping occurred based on tension data, in addition to the displacement data. Galloping was found to occur at wind speeds exceeding 9 m/s, and large galloping oscillations were observed when a certain relationship was satisfied between the wind speed and the amount of ice accretion. Furthermore, it was observed that the loose spacers reduced the galloping oscillation amplitude by approximately 40%. The rotational clamps operated normally under severe natural conditions, and the suppression effect was caused by the difference in the ice accretion characteristics of the rotatable subconductors owing to their rotation.
查看更多>>摘要：Crosswind stability is one of the most studied topics in the field of train aerodynamics. The interaction between a train and strong crosswinds can lead to the train overturning. One way to reduce the train overturning risk is to equip the railway line with windbreak barriers. In this work, the results obtained during wind tunnel tests on an ETR1000 high-speed train in the presence of different types of windbreaks are shown. The goal is to underline the effects of different flow parameters and geometric parameters of the infrastructure's models on the train's aerodynamic coefficients. Comparing the different test results made it possible to provide indications on which parameters are important to avoid experimental errors and reproduce as much as possible the real full-scale phenomena. Particular attention shall be paid to the dimensions of the splitter plate because a limited upwind width can lead to underestimation of aerodynamic coefficients by more than 30%. The length of the barriers in front of the train is fundamental and too short length can lead to coefficients overestimation of about 25%. Finally, coefficients measured with only the upwind windbreak row are lower (up to 50%) than those measured with a two rows configuration.
Wang, TianhangKwok, Kenny C. S.Yang, QingshanTian, Yuji...
15页
查看更多>>摘要：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.
查看更多>>摘要：This paper examines wind loads acting on multi-span roofs of low-rise buildings by performing wind tunnel tests with models whose parameters are roof shape (sawtooth, gable), roof slope (5 degrees, 10 degrees, 20 degrees, 30 degrees, 45 degrees) and the number of spans (1-5). Large suctions can occur at either a high corner/edge or a low corner of sawtooth roofs, depending on roof slope and the location of roof span. Low corners of multi-span gable roofs experience large negative pressures on edge roofs of low-pitch roof slopes and on middle roofs of steeper roof slopes. Relatively large positive pressures on both sawtooth and multi-span gable roofs can occur at roof troughs of low-pitch roof slopes and their magnitude becomes larger as roof slope increases. For steeper roof slopes, large positive pressures are also observed at roof ridges of both roof shapes although their magnitude and the location of the roof span depend on roof shape. Both the size and the magnitude of suctions on walls between spans of sawtooth roofs increase with roof slope. Comparisons with ASCE7-16 design values were performed and the results imply that ASCE7-16 underestimates both positive and negative wind loads on most roof zones of multi-span roofs.
NSTL
Elsevier