查看更多>>摘要：In the present study, a comparison has been performed numerically to explore the impact of internal rotor type on power characteristics of a two-bladed hybrid vertical axis wind turbine (VAWT). For this purpose, the internal rotor of a hybrid wind turbine built with the conventional semicircle and Bach-type blades is considered under two free-wind speeds (U-infinity) of 5 and 10 m/s. Computations are conducted at three tip speed ratios (TSRs) of 1.5, 2.5, and 3.5. Effects of different relative positions between the external and internal rotors, called attachment angle, (phi) such as 0?, 45?, and 90? are studied in detail and the obtained results are validated against the available data for Darrieus and Savonius rotors, separately. The results illustrate that the internal rotor type, attachment angle, and TSR have considerable impacts on the hybrid rotor performance. Moreover, it is demonstrated that the Bach-type Savonius rotor has better performance than the conventional semicircle one in a two-bladed hybrid wind turbine regardless of phi, TSR, and U-infinity. The maximum improvement of 20% is reported for hybrid wind turbine equipped with Bach-type rotor under phi = 90?, TSR = 1.5, and U-infinity = 5 m/s.
查看更多>>摘要：Modern multi-megawatt wind turbines have long, slender and flexible blades, which experience significant vibration due to aerodynamic loads. These blades are generally made of composite layups with cambered airfoil sections, which induce elastic and dynamic coupling between bending and torsional modes. In addition to these phenomena, the aerodynamic pitching moment also arises due to the offset of the blade pitch axis from the aerodynamic centre. With this in mind, the present work aims to develop a computationally efficient complete multi-body dynamic aero-servo-elastic model of an onshore horizontal axis wind turbine, including bending-torsion coupling and aerodynamic pitching moment. 3D wind fields are generated using TurbSim, and the responses of the coupled system are solved under different operational scenarios, which signify the impact of the aforementioned coupling. It is found to be particularly detrimental at higher wind speeds than the rated value. But, proper design and implementation of structural layup can enhance the aerodynamic performance of the blade. Finally, the optimal design of composite fibre orientation of a benchmark turbine blade is investigated to show its beneficial effects in load/response mitigation.
查看更多>>摘要：Under weather conditions such as snow and icing in cold regions, snow and ice accumulation on bogies severely affect the running safety and increase the energy consumption of urban rail trains. To analyze the snow and ice accumulation process, experiments for real urban rail train non-powered bogie were conducted for the first time in the Central South University Icing and Snowing Wind Tunnel (CSU-ISWT). Two anti-snow structures (de-flectors and wheelset snow shields) with low energy consumption were designed and discussed based on comparative experiments. The following data were collected after each experiment: dynamic accumulation process, final distribution of snow and ice, and total accumulation mass on the bogie. The results showed that the distributions on the original and optimized bogies were similar. Much more snow and ice accumulated on the lower part of the bogie than on the upper part and covered more on the windward side of the frame than on the leeward side. In the experiments with anti-snow structures, the snow and ice mass accumulated on the bogies decreased by 53.35% and 52.64% in the deflectors and wheelset snow shields cases, respectively. The proposed structures were proven to significantly improve anti-snow performance on the bogies. Therefore, the presented initial research can remarkably strengthen the transportation capacity and safety of urban rail trains in winter.
查看更多>>摘要：Ultra-long stay cables characterized as high flexibility are prone to large-amplitude vibrations. Much attention has been paid to the vibrations of stay cables under normal wind conditions in the past decades, while the dynamic response under extreme winds is reported rarely. This study investigates the exceptional wind-induced vibration of the stay cables of a kilometer-level cable-stayed bridge based on the monitoring data. The mean wind velocity and extreme wind duration are firstly obtained using the wind records of the structural health monitoring system (SHMS). The effects of the turbulence intensity and mean wind velocity on the cable vibrations are then analyzed. To explore the motion trajectory features, the in-plane and out-of-plane dynamic displacements are estimated by an acceleration-based approach. Besides, the modal properties, including the frequencies and damping ratios, are identified. The result shows that the stay cables present multi-modal vibrations with frequencies higher than 3 Hz, which is different from rain-wind-induced vibration (RWIV). The out-of-plane vibration amplitude under the extreme wind is much larger than that of the in-plane due to the absence of dampers. Furthermore, the out-of-plane damper is suggested to mitigate the large-amplitude vibration of ultra long stay cables under exceptional winds.
查看更多>>摘要：The conventional design of tunnels includes the low utilization rate of non-mechanical ventilation power, and is not suitable for the ventilation design of extra-long highway tunnels. The main purpose of this paper is to study the allowance of the non-mechanical wind pressure in each part of an extra-long highway tunnel with shafts, and to propose a tunnel ventilation method with a higher utilization rate of the non-mechanical wind pressure. Based on the ventilation network, the loop division method of a common shaft ventilation tunnel is first established, and the pressure modules in the tunnel are calculated. Then, each module is input into the ventilation network model, the non-mechanical wind pressure margin of each loop is calculated, and the distribution method of the required air volume is adjusted to ensure that the non-mechanical wind pressure margin is reasonable. Finally, according to the distribution results, the tunnel fan can be configured, and the shaft resistance can be adjusted if necessary to ensure the maximum utilization rate of non-mechanical ventilation power in the tunnel. According to the design results of the Funiushan tunnel in China, the energy-saving effect of this method is evident, which verifies its rationality.
查看更多>>摘要：Engineering atmospheric boundary layer (EABL) simulations performed using Computational Fluid Dynamics (CFD) can give key insights in successfully addressing a variety of topics in environmental and structural aerodynamics. To improve understanding of this complex topic, a computational evaluation of the shear stressdriven (SSD), pressure-driven (PD) and body-force-driven (BFD) EABL flows was performed using the opensource CFD code OpenFOAM (R). The EABL simulations were computationally evaluated by using the successor domain technique (SDT) and precursor domain technique (PDT). The boundary conditions applied in the CFD algorithm using OpenFOAM (R) to computationally model the SSD, PD, and BFD EABL flows were reported and discussed. A developed CFD approach may also be satisfactorily used in other relevant CFD codes, including but not limited to Ansys CFX (R), Ansys Fluent (R), STAR CCM+(R). The impact of the EABL models on surface pressure distribution on a low-rise cubic building, the total building drag force and the wind velocities in pedestrian-level areas was computationally evaluated by using major Reynolds-averaged Navier-Stokes (RANS) turbulence models, i.e., the standard k-epsilon, re-normalization group (RNG) k-epsilon, realizable k-epsilon, Wilcox's k-omega, and Menter's k-omega shear stress transport (SST) turbulence models. The obtained computational results indicate that the SSD and PD EABL flows may be successfully modeled in cases where additional modifications of the officially released CFD code are not required in regard to the PDT. The PD EABL flow may be successfully modeled by using all studied RANS turbulence models in combination with the PDT. The EABL flow throughout an empty computational domain is homogeneous even without using a body force because the employed CFD code is capable of creating the required pressure gradient that drives the flow along the computational domain. While the surface pressure estimation on the windward body surface and in the recirculation zones was generally proven to be an issue when using steady RANS two-equation turbulence models, surface pressures in the stagnation zone on the windward cubic building surface and pressures on the top and the side building surfaces in the present study agree well with the experiments when using the RNG k-epsilon and Menter's k-omega SST turbulence models. The pressure distribution on the building surfaces, the total building drag force and wind characteristics in the vicinity of a lift-up building are not considerably affected by the choice of the EABL model, while they are substantially affected by the type of RANS turbulence model. The best agreement with the experiments was achieved when using the RNG k-epsilon and Menter's k-omega SST turbulence models, so these models may be recommended for future applications.
查看更多>>摘要：This paper investigates transverse galloping of rectangular cylinders characterized by 0.7 < B/D < 2.8 (B: width; D: depth) in uniform flows at relatively high Reynolds numbers (>> 10(3)), in which the oscillation is self-excited and the phenomenon is triggered from rest. First, a better understanding of potential links between static and dynamic transverse galloping issues is acquired using a novel viewpoint that considers key characteristic points on the static force and dynamic response curves. Then, new perspectives on the transverse galloping generation mechanism are presented. Analysis of the present study indicates that the traditional and most widely known time-averaged flow pattern approach is not sufficient to explain possible transverse galloping physical mechanisms. A new time-varying flow pattern approach is needed and is proposed in this paper. This new approach shows that a quantitative change in the occurrence ratio of intermittent reattachment flow, which leads to a qualitative change in the time-varying flow pattern type, is the generation mechanism for certain important, interesting static and transverse galloping dynamic behaviors.
查看更多>>摘要：Owing to a capacity for high flexibility and low damping, long-span bridges are subjected to vortex-induced vibrations (VIVs) under operational conditions. Longterm monitoring data with machine-learning algorithms indicate the potential for automating the VIV assessment of long-span bridges. These methods require a significant amount of labeled data, whereas obtaining such data is normally not feasible owing to the limited availability of VIV datasets. This study leverages supervised learning techniques to develop an automatic classification method for VIVs. To address manual data labeling and develop an optimum model, a three stage strategy is presented: 1) Semi-supervised labeling, 2) deep neural network (DNN) training, and 3) identification of an optimum parameter range. First, semi supervised labeling is employed to automatically label the dataset into either VIV or non-VIV classes. Second, a DNN model is trained using the wind and vibrational features of labeled data. Finally, the optimum parameter range is determined by analyzing the peak factor distribution, confusion matrix, and corresponding velocity-amplitude curve of the classified test datasets. An application of the model to a long-span, cable-stayed bridge is illustrated to assess the classification performance based on actual monitoring data. The DNN with the suggested labeling process demonstrates consistent and accurate detection of VIVs.
查看更多>>摘要：The influence of various tree planting configurations on tracer gas dispersion in an urban street canyon is studied numerically, using OpenFOAM Large Eddy Simulation (LES). A recycling and rescaling turbulent inflow generation method is implemented and validated against a canonical backwards-facing step case. The street canyon simulations are based on the CODASC experiments, where the tracer gas was emitted from line sources at street level. The effect of tree stand density on the pollutant dispersion is studied for the first time using LES. Performance metric analysis of the CODASC simulations shows that the influence of tree crown porosity, and tree stand density, on the concentration field within the canyon can be captured by LES. The simulations show that tree crowns reduce the effectiveness of the canyon vortex at ventilating the street, and enhance the mean tracer gas concentration at pedestrian level on the leeward wall. A novel feature of the research arises from concentration probability density functions at pedestrian height in the canyon. Pedestrians can be exposed to pollutant concentrations almost three times the mean value at the leeward wall. The results have implications for urban greening strategies in city streets with heavy traffic.
查看更多>>摘要：By analysing a set of thunderstorm-induced events acquired on the Italian north-west coast, the separation of the relevant wind velocity signals into a time-varying mean and a residual fluctuation is considered. It is assumed that, within a structural design process, the former is used for a quasi-steady evaluation of the aerodynamic loads and the latter to estimate the dynamic response. A procedure based on the continuous wavelet transform is proposed to obtain the time-varying mean velocity and its outcomes are compared with those of different moving average techniques. The wavelet filtering appears to provide a time-varying mean that better describes the main time variations of the original velocity signals; a decrease of the correlation between mean and residual velocities is also found. The derivative of the time-varying mean wind speed leads to a time-varying mean acceleration, whose characteristics and possible role in a design process are discussed. An estimate of the acceleration-induced contribution to the aerodynamic loads on a slender body is also carried out through the Morison equation. The force term directly related to acceleration is shown to generally have a limited influence on the total design loads, except for bodies that are considerably elongated in the flow direction.
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Elsevier