Ibrahim, Ahmed A.Elbaz, Ahmed M. R.Melani, Pier FrancescoMohamed, Omar S....
13页
查看更多>>摘要：Enhancing the performance of Darrieus Vertical Axis Wind Turbines (VAWTs) is the key to boost their possible commercialisation in the yet highly competitive wind energy market. This study evaluates an active boundary layer control technique to increase the aerodynamic efficiency of Darrieus VAWTs. A cavity is created on the airfoil's suction side, where suction is applied to create a trapped vortex. High-fidelity Computational Fluid Dynamics (CFD) simulations are used after validation against experimental results. The effect of suction momentum ratio C-mu on the performance of the turbine is investigated. The turbine with suction cavity airfoils (SC) shows a higher power coefficient than the one with baseline airfoil (BL) blades, especially at low tip-speed ratios. For the selected test case, the net maximum predicted power coefficient is 0.435 for the SC turbine at TSR = 2.3, 0.375 for the baseline at TSR = 2.6. At the lower tip-speed ratios, i.e. TSR = 2, the power coefficient of the SC turbine is almost double that of the BL turbine. The feasibility of the active control technique is finally discussed, demonstrating its potential as an effective solution for the performance augmentation of the Darrieus turbine.
查看更多>>摘要：This paper reports the statistics-based results of damping ratios in buildings with different global characteristics. Through a literature survey and data filtration, 1887 full-scale measured damping ratios under micro-amplitude from 1009 existing buildings were integrated into a single database. Using the correlation coefficient, multiple linear regression (MLR), and Kruskal-Wallis analysis of variance, the correlation between the damping ratio and the various factors in the database were assessed. It was quantitatively verified that building height is the optimal predictor for damping, and structural type (combination of construction material with lateral force-resisting system) is the pivotal classification criterion for the value setting of the damping ratio. Some multi-grained predictive models of the first mode damping ratios with the MLR formula as the primary result are proposed. However, the MLR inference also shows that the model deviation caused by different damping evaluation techniques is non-negligible, and it cannot be presently quantified and eliminated. This also results in high quality damping data lacking for certain structural types. Therefore, for three specific structural types, the copula function is applied to obtain the full probability model between the building height and damping ratio and to generate random samples containing real probability information.
Matejicka, LubomirKoss, Holger H.Georgakis, Christos T.
12页
查看更多>>摘要：Adverse weather conditions in the northern regions during winter might lead to accumulation of ice or snow on bridge cables and consequently trigger ice shedding. The falling ice poses a risk for the traffic and pedestrians below, which often results in bridge closures and/or insurance claims. A passive surface-modifying device in the form of a steel wire mesh tautened on the surface of a cable sheath was previously found to substantially reduce the risk associated with falling ice through improved ice retention. In this paper, the cross-flow aerodynamic behaviour of bridge cables fitted with steel wire meshes is investigated in detail. The wire mesh generally resulted in higher levels of aerodynamic drag when compared with a plain cable sheath, although the Reynolds number dependence was highly reduced. Several angles of attack were investigated and no instability issues with respect to the across-wind vibrations were observed for any of the tested wire mesh configurations based on the quasi-steady theory. Even though vortex shedding was still present in all cases, the intensity of the fluctuating lift forces was much lower than that of the plain cable. To investigate basic aspects of the flow structure produced by the wire mesh, two flow visualisation techniques, combined with streamwise wind speed measurements in the near wake, were further used and are discussed.
查看更多>>摘要：Studies on jet-in-crossflow have been extensively conducted in the last century. In our paper, we present the results obtained from the measurements of four jets-in-crossflow model developed at a wind tunnel testing facility. The interaction between four jets in a row and a boundary layer crossflow was visualized using a propylene glycol-based fog, and hot-wire anemometer measurements were conducted for quantitative analysis. Vertical profiles of the mean wind speed, relative wind speed, and turbulence intensity are presented for distances of 3.37 and 6.84 diameters from the central plane of the row. Such analysis was also performed for distances between the jets of 1.5, 2, and 3 diameters and velocity ratios of 1.6, 2, and 3.5. A comparison with the literature showed the model performance is comparable to currently reported wind barriers.
查看更多>>摘要：Shipboard helicopter operations are significantly impacted by the characteristics of the ship airwake which depend on environmental conditions in addition to the ship superstructure. In this paper, the effect of free stream turbulence, wind speed and ship motion on the airwake frequency spectra of two 1:50 scaled ships with stern flight-decks was investigated using wind tunnel testing. Results from a full-scale sea trial are also presented. Using both the wind tunnel and the sea trial data in ship motion conditions corresponding to medium sea states, results showed the ship motion energy was transmitted to the ship airwake at the motion frequencies. For single-frequency ship motions, the energy transfer occurred at the motion frequency only, whereas for more realistic at-sea ship motions with a range of frequency components the energy was transferred at a broadband of frequencies corresponding to the motion frequency range. The way in which realistic motion component effects combine at sea and the amount of energy added to the airwake in realistic conditions is presented. The impact of incoming atmospheric turbulence was also examined.
查看更多>>摘要：The turbulent flow past a simplified Intercity-Express 3 high-speed train at Re-H = 6 x 10(4) is investigated by a combination of wind tunnel experiments and numerical simulations using the large-eddy simulation (LES), the improved delayed detached eddy simulation (IDDES) and the unsteady Reynolds-averaged Navier-Stokes (URANS) simulation. This work aims to compare the predictive capabilities of LES, IDDES and URANS for the flow over a streamlined high-speed train. Numerical simulations are compared to experimental data for validation. Results show that the well-resolved LES is more accurate among the numerical methods used. Compared to the well-resolved LES, IDDES and URANS using the coarser mesh can produce similar mean flow, although IDDES and URANS are found to be slightly inaccurate for the coherent wake structures near the wall. However, for the near-wall flow instability concerning wake dynamics, Reynolds stresses, turbulence kinetic energy and the fluctuation of pressure, IDDES is found to be inapplicable. Overall, this study suggests that the well-resolved LES is appropriate to the flow of a streamlined high-speed train. Moreover, IDDES and URANS are proved to apply to the mean field of the studied flow.
查看更多>>摘要：The light-weight and high-speed of modern high-speed trains make the safety and comfort of trains running on bridges prominent in windy environments. The installation of wind barriers can significantly improve the running performance of high-speed trains by reducing the action of wind on the train body. By using the developed moving model test rig in wind tunnel, this paper analyzed the dynamic interaction of wind barrier on the aerodynamic characteristics of the moving train. The windproof performance of wind barrier with different air porosity is detailed discussed. Influences of train speed, wind velocity and lane position on train's aerodynamic coefficients are quantified according to the test results. The result reveals the difference of train's aerodynamic coefficients between static and moving model tests and found static model test may overestimate the reduction rates of wind barrier on train's aerodynamic coefficients. The decreased train speed and increased wind velocity increase the change values of side and overturning coefficients caused by the wind barrier, while decrease the change of lift coefficient. The wind barrier introduces the opposite law of changes in trains' aerodynamic coefficients with it running at the different lanes, leading to different windproof effect of wind barrier on train's aerodynamic characteristics.
查看更多>>摘要：Flow-induced vibration (FIV) of a heat exchanger tube may present both negative effects (e.g., causing fatigue damage of the tube) and positive effects (e.g., enhancing heat transfer efficiency). As a typical passive control approach, fins attached to the tube (or circular cylinder) present significant effects on its FIVs. This study carried out systematic wind tunnel tests to investigate the FIVs in the transverse direction (along with the drag forces) of a circular cylinder with various radial and longitudinal fins. For the radial group, 3 cases with spare, moderate, and dense fins were tested. For the longitudinal group, 24 cases with various fin numbers and arrangements were tested. For the longitudinal fin arrangement parallel to the flow direction (also known as a circular cylinder with single or dual splitter plates), various fin lengths were also tested. The experimental results showed that a radial finned cylinder always presents larger FIVs than a bare circular cylinder. The longitudinal fins may significantly increase or decrease the FIVs depending on the number and arrangement of attached fins. For example, a single splitter plate (placed either upstream or downstream of the circular cylinder) often increases the FIVs, while dual splitter plates with large lengths can effectively suppress the FIVs. The experimental results provide valuable guidance for selecting appropriate fin configurations to passively control the FIVs to a certain level that can enhance the heat transfer efficiency without introducing unacceptable fatigue damage.
查看更多>>摘要：This paper reports the effect of high reduced frequency on the aerodynamics of wind turbine blade under a deep dynamic stall at Reynolds number 135,000, of which the cross section is NACA0012 aerofoil with a constant chord length. Large-eddy simulations (LES) at reduced frequencies 0.1 and 0.15 were validated against reference data in the literature. Our LES data suggest that the lift, drag and moment coefficients are evidently dependent on the pitching frequency. The lift coefficient at the reduced frequency 0.4 increases up to 22% during the upstroke, and 64% during the downstroke compared to at the reduced frequency 0.2. The peak drag coefficient decreases up to 26% at the reduced frequency 0.4 compared to at the reduced frequency 0.2. The phase angle of dynamic stall shifts towards the downstroke regime as the reduced frequency increases. Pitching motion at the high reduced frequency (e.g. 0.4) significantly enhances the suppression of leading edge vortex during the upstroke, and delays the reattachment of the boundary layer until a very low angle of attack in the downstroke. This study can be beneficial for improvement in the parameterisation of the operational blade element method (BEM) of wind turbine blade design.
查看更多>>摘要：The scale effect on aerodynamics determines, to some extent, the development trend and limitation of large vertical axis wind turbines (VAWTs). Among many factors, Reynolds number is an important one affected by such effects and is therefore often used as a criterion to distinguish between small-scale and large-scale wind turbines. In this paper, a dimensionless analysis was first conducted, and the results showed that scaling the wind turbine as a whole only led to a change in Reynolds number, while other key factors remained constant. Based on such understanding, a univariate analysis was then carried out to investigate the effect of Reynolds number variation due to scaling effects on the aerodynamic performance of VAWTs. In the study, a series of high-resolution numerical simulations were performed on a simplified single-bladed VAWT model. By directly scaling the model size, the operating Reynolds number of the wind turbine was made to vary in the range of 1 x 10(4) to 5 x 10(6) at a specific solidity and reduced frequency. The analysis shows that over the range of Reynolds number investigated, as the size of the wind turbine model increases, the flow separation nearby the blade decreases, the strength of the dynamic stall vortex becomes weaker, and the power coefficient of wind energy extraction generally tends to increase, thus indicating that large-scale wind turbine is generally beneficial aerodynamically. Specifically, when VAWTs operate in a Reynolds number ranging from 5 x 10(4) to 5x10(5), the power coefficient increases significantly as the size of wind turbine increases, while when operating in the range of 5 x 10(5) ~ 5 x 10(6), the power coefficient still tends to increase, but in a decreasing increment ratio.
NSTL
Elsevier