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In this study, the interaction between a spherical solid particle descending in a fine-bubble plume and the fine-bubbles around the particle is investigated experimentally. The fine-bubbles, which are formed by water electrolysis and have an average diameter of 0.037 mm, rise via buoyancy to form bubble plumes. The average bubble velocity in the conduit is 0.05 mm/s. A spherical solid particle with a diameter of 11.1 mm and a density of 1130 kg/m3 is dropped into the fully-developed bubble plume. The particle falls in a meandering motion in the fine-bubble plume. In this process, the terminal velocity of the particles falling in the fine-bubble plume is almost identical to that of a particle falling in the quiescent water. Additionally, after the fine-bubbles have been separated from the particle surface, they flow into the wake of the particle, forming a stagnant region. The particle wake diameter increases threefold, as compared with its value in the downstream direction of the fine-bubble plume. [ABSTRACT FROM AUTHOR]

This paper provides a review of the general experimental methodology of snow-powered flow visualization and super-large-scale particle image velocimetry (SLPIV), the corresponding field deployments and major scientific findings from our work on a 2.5 MW utility-scale wind turbine at the Eolos field station. The field measurements were conducted to investigate the incoming flow in the induction zone and the near-wake flows from different perspectives. It has been shown that these snow-powered measurements can provide sufficient spatiotemporal resolution and fields of view to characterize both qualitatively and quantitatively the incoming flow, all the major coherent structures generated by the turbine (e.g., blade, nacelle and tower vortices, etc.) as well as the development and interaction of these structures in the near wake. Our work has further revealed several interesting behaviors of near-wake flows (e.g., wake contraction, dynamic wake modulation, meandering and deflection of the nacelle wake, etc.), and their connections with constantly-changing inflows and turbine operation, which are uniquely associated with utility-scale turbines. These findings have demonstrated that the near wake flows, though highly complex, can be predicted with substantial statistical confidence using supervisory control and data acquisition (SCADA) and structural response information readily available from current utility-scale turbines. Such knowledge can be potentially incorporated into wake development models and turbine controllers for wind farm optimization in the future. [ABSTRACT FROM AUTHOR]

Abstract: The wake of a circular cylinder coated with porous media was experimentally investigated by hot-wire anemometry, particle image velocimetry (PIV) and smoke-wire visualization. Mean velocity and Reynolds stresses in the streamwise and lateral are eliminated obviously for porous layer coating, especially in the centerline. The vortex formation length is elongated, and the width of the wake region is expanded by coating porous media. The Strouhal number of vortex shedding is 0.162 for the porous cylinder, which is smaller than 0.182 for the bare one due to the widened wake. Additionally, porous layer significantly regulates the flow field, which can be detected distinctly by smoke-wire visualization and transient PIV data. The proper orthogonal decomposition (POD) was further used to analyze PIV data with a view to extract dominant coherent structures in the wake. Moreover, a phase-averaged low-order model was applied to provide a comprehensive description of the dynamics of the first four modes. The first two POD modes, showing the signature of asymmetrical vortex shedding and predominating in the wake of the bare cylinder, are weakened dramatically for porous layer coating and present abrupt degradation in the midline. Meanwhile, mode 3 and mode 4, representing the symmetrical vortex shedding, occupy higher energy for porous cylinder. The study reveals that the porous layer coating delays the roll-up of shear layer and weakens the interaction between them, and thus, the vortex region is shifted further downstream.Graphical abstract: [ABSTRACT FROM AUTHOR]

Abstract: A passive flow control technique, which is to drill a horizontal hole going from the front surface to the rear surface or an inclined hole going from the front surface to the top surface inside a circular cylinder of an aspect ratio H/D = 1, is proposed to control the rear recirculation region and the vortices near the free end surface. Here, both the diameter D and height H of cylinder are 70 mm. PIV measurement was performed with Reynolds number of 8570 in a water tunnel. Furthermore, in order to consider the effect of the hole position of the front surface, the cylinder models having different hole height of front surface from wall were tested. It is found that the rear recirculation zones of the horizontal hole cylinders are smaller than that of the standard cylinder. The Reynolds shear stresses and the turbulent kinetic energy are evidently reduced by the flow issued from the horizontal hole. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone are broken down into several small-scale vortices with decreasing the height of the horizontal hole. Although the rear separation zone of the inclined hole cylinder increases, the recirculation region near the free end surface decreases. The areas of large Reynolds shear stress and high turbulent kinetic energy increase in the rear recirculation zone. However, with increasing the height of the inclined hole, the Reynolds shear stress decreases.Graphical Abstract: [ABSTRACT FROM AUTHOR]

The wake flow originated by three-dimensional bluff bodies having the different aspect ratio is experimentally studied in this paper. The turbulent wakes generated by asymmetric- and semi-cylinders were first measured by the high-speed PIV. The time-averaged and instantaneous flow structures were analyzed in various planes. Then, the orthogonal wavelet multi-resolution technique was used to analyze the multi-scale flow structures. The instantaneous vorticity and Reynolds shear stresses of various frequencies were evaluated and compared among the different aspect ratio and body shape. It was found that the asymmetric-cylinder produces the aerodynamic downforce and its separation region is smaller than that of semi-cylinder. The large-scale structure makes the most contribution to vorticities, and the intermediate-scale structure dominates the flow structures near the tip of body. The large-scale structure makes the most contribution to Reynolds shear stress except the tip of body, accounting for about 55.9-79.2%. The maximum value of large-scale Reynolds shear stress appears at the end of separation region, and the small-scale Reynolds shear stress exhibits the maximum value in the shear layer of the body surface. Graphical Abstract: [ABSTRACT FROM AUTHOR]

To reveal the possibly existed flow phenomena that buried in a two-dimensional symmetric triangle wake, one-dimensional orthogonal wavelet transform and POD are combined to decompose the fluctuating velocity field into different wavelet components and modes. The features of reconstructed flow fields are analyzed in terms of fluctuating energy, time frequency distribution, space correlation and Reynolds shear stress. It is found that the first two wavelet components and POD modes can give representations to the most energetic large-scale structures, contributing about 77 and 73 % to the total fluctuating energy, respectively. Comparing with the first two wavelet components, the first two POD modes are more appropriate to represent the Karman-like vortical structures. The time-frequency and length scale characteristics of wavelet components suggest that frequency behavior can reflect the spatial-related length scale and the wavelet transform can be used to extract turbulent structures of different scales. Similar to the energy distribution, the most significant contributions to the Reynolds shear stresses comes from the large-scale structures that composed of first two wavelet components and POD modes, accounting for 88 and 80 % of the measured maximum Reynolds shear stress, respectively. Due to different classification criterion of wavelet and POD analyses, the combining use of these two methods tends to be more effective for analyzing multi-scale turbulent structures. Graphical abstract: [ABSTRACT FROM AUTHOR]

The three-dimensional dune wake flow is evaluated by large eddy simulation at a Reynolds number of 5,534, and validated by time-averaged results of particle image velocimetry measurement at a central streamwise plane. To reveal multi-scale characteristics of vortical structure, the instantaneous velocity, vorticity, and pressure were decomposed into the large-, intermediate- and small-scale components by three-dimensional wavelet multi-resolution technique, the scale of each wavelet component is quantified by two-point autocorrelation function with the central scale of 43, 20 and 9.6 mm, respectively. It is found that large-scale structure determines the formation of separation bubble and makes the most significance to the vorticity. Some intermediate-scale and small-scale streamwise vortices cause the upwelling of vortical structures, and they tend to be more active at the downstream. By visualization of pressure distribution, we can find that pressure distribution is mainly characterized by large-scale structure, and the distribution of small-scale structure can be interfered as a reason why large-scale vortical structure breaks into small vortices at the near outlet region. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]