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The linear-time-invariance notion to the Koopman analysis is a recent advance in fluid mechanics [Li et al., "The linear-time-invariance notion to the Koopman analysis: The architecture, pedagogical rendering, and fluid–structure association," Phys. Fluids 34(12), 125136 (2022c) and Li et al., "The linear-time-invariance notion of the Koopman analysis—Part 2. Dynamic Koopman modes, physics interpretations and phenomenological analysis of the prism wake," J. Fluid Mech. 959, A15 (2023a)], targeting the long-standing issue of correlating nonlinear excitation and response phenomena in fluid–structure interactions (FSI), or, in the simplified case, flow over rigid obstacles. Continuing the serial research, this work presents a data-driven, Koopman-inspired methodology to decouple nonlinear FSI by establishing cause-and-effect correspondences between structure surface pressure and the flow field. Exploiting unique features of the Koopman operator, the new methodology renders dynamic visualizations of in-sync, fluid–structure-coupled Koopman modes possible, fostering phenomenological analysis and statistical quantifications of FSI energy transfers. Instantaneous contribution contours and densities offer new angles to evaluate pathways of energy amplification and diminution. The methodology enables better descriptions and interpretations of phenomena occurring in the flow and on the boundary (walls) of an FSI domain and readily applies to a broad spectrum of engineering problems given its data-driven nature. [ABSTRACT FROM AUTHOR]

Ideal tree exhibits fractal characteristics, where the branch angle plays a significant role in shaping the morphology of trees, thereby influencing their wind resistance capabilities. Nevertheless, investigation into the aerodynamic effects of branch angle on trees with leaves remains relatively scarce. By subjecting various tree morphologies to controlled wind conditions, this study scrutinizes the aerodynamic responses and resulting loads experienced by one-order sympodial trees with differing branch angle configurations. The results reveal that the tree experiences unstable oscillations induced by irregular leaf vibration with an increase in wind speed, resulting in a rise in drag coefficient. Meanwhile, despite a higher drag force observed in the tree with higher branch angle at wind speeds below 20 m/s, the tree exhibits superior reconfiguration capabilities, enabling it to withstand stronger winds effectively. Subsequently, a reconfiguration process for the one-order sympodial tree is proposed, exhibiting a wavy streamlining effect. Finally, it is found that the sympodial tree structure can be regarded as a high-frequency filter to dissipate high-frequency branch vibration energy. The findings from this research endeavor hold significant implications for enhancing our understanding of the aerodynamics of trees with different morphology and the cultivation and selection of urban trees. [ABSTRACT FROM AUTHOR]

This study uses two wind tunnel testing approaches: a sole high-frequency pressure scanning (HPS) approach, and a synchronized particle image velocimetry (PIV) and multipoint pressure scanning (SPMPS) approach, to investigate aerodynamic characteristics of a rectangular cylinder. HPS and SPMPS share identical experimental settings and device arrangements, while the SPMPS sampling frequency is much lower than HPS because the PIV system has a low sampling frequency in this study. SPMPS can simultaneously capture the flow field and surface pressure information. PIV measurement provides instantaneous flow field information, helping to analyze flow characteristics, and surface pressure taps offer both high- and low-frequency surface pressure information. It was found that for the rectangular cylinder, the zones of high turbulent kinetic energy and turbulent shear stress are associated with the lower negative pressure coefficient. In addition, diverse coherent structures in the instantaneous flow field resemble different critical points such as the saddle point, the repelling focus, and the attracting focus, and these coherent structures are associated with drastic changes in the pressure distribution or extreme pressure values. In particular, there is a visible flow reattachment, and the lift coefficient is more sensitive to the pressure distribution around the trailing edge of the rectangular cylinder. The convective velocity of wall pressure fluctuations is calculated based on the spatial temporal correlation of HPS information, and the convective velocity on the upper surface of the cylinder of SR = 3.25 is around 3.1 m / s providing evidence that Taylor's hypothesis breaks down for wall pressure fluctuations. [ABSTRACT FROM AUTHOR]

Insufficiently studied, the phenomenon of twisted wind is prevalent in mountainous regions. This study focuses on investigating the characteristics and aerodynamics of a single high-rise building subjected to a twisted wind field (TWF). By comparing the 0° conventional wind field (CWF) with two TWFs (TWF15 and TWF 30) with maximum twisted angle of 15° and 30°, both wind tunnel testing and computational fluid dynamics (CFD) are employed. A comprehensive analysis is conducted to examine the differences between TWFs and CWF in terms of wind profiles, turbulence, aerodynamic characteristics of single high-rise buildings, and wake vortex morphology. The results demonstrate notable disparities betweenTWFand CWF. Particularly, the twisted wind induces inflection in the wind profile, leading to significant acceleration or deceleration. Moreover, the twisted effect generates an asymmetric distribution of average building surface pressure, thereby introducing the potential for asymmetric or torsional loading extremes. Phenomenologically, the twisted effect weakens shear layer separation, creates squeezed separation bubbles, and promotes premature reattachment, resulting in distinct flow field morphologies compared to CWF. Furthermore, as the wind twisted angle increases, the energy concentration associated with Bernard-Karman shedding diminishes, thereby significantly altering the wake characteristics. [ABSTRACT FROM AUTHOR]

A numerical physio-chemical model of the NOx-O3 photochemical cycle in the near-wake region of an isolated residential/office building has been presented in this study. The investigation delves into the dispersion of reactive air pollutants through the lens of fluid phenomenology and its impact on chemical reactivity, formation, transport, deposition, and removal. Computational fluid dynamics (CFD) simulations were conducted for the ground-point-source (GES) and roof-point-source (RES) scenarios. Results show that the Damköhler number (Da), which quantifies pollutants' physio-chemical timescales, displays a strong inverse proportionality with the magnitude and spread of NO-increasing Da reduces human exposure to the toxic NO and NO2 substantially. When different wind directions were considered, the dispersion range of NO exhibited varying shrinking directions as Da increased. Furthermore, as Da increases, the concentration ratio Kno2/Knox which quantifies the production of NO2 resulting from NO depletion, forms sharp high-low gradients near emission sources. For GES, the dispersion pattern is governed by the fluid's phenomenological features. For RES, the intoxicated area emanates from the building's leading-edge, with the lack of shielding inhibiting pollutant interactions in the near-wake, resulting in scant physio-chemical coupling. The NO2/NOx distribution follows a self-similar, stratified pattern, exhibiting consistent layering gradients and attributing to the natural deposition of the already-reacted pollutants rather than in-situ reactions. In the end, building design guidelines have been proposed to reduce pedestrian and resident exposure to NOx-O3. [ABSTRACT FROM AUTHOR]

This study focuses on the aerodynamic nonlinearity and flow field phenomenology of structure-motion-induced dynamics in fluid–structure interactions (FSI), which is essential for response prediction. Through dynamic-meshing large-eddy simulations with near-wall resolution, the nonlinear aerodynamic damping in the still wind has been isolated by forced vibration, and its phenomenological characteristics and physical mechanisms have been analyzed. The results show that nonlinear aerodynamic damping can account for up to 30% of the total damping, which cannot be ignored in response prediction. The study also reveals that the three-dimensional vorticity dynamics vary nonlinearly with structure motion, leading to the hysteresis effect between aerodynamic forces and displacement. Furthermore, in-depth phenomenological analysis discloses eight types of coherent flow field substructures, including the Stick, Phone, Bowknot, Crutch, Droplet, Bat, Horn, and Flag substructures, which are solely induced by structural motion. Insights into these substructures' formation, evolvement, dissipation, and superposable magnitude have been disclosed. This research offers a new perspective on understanding the physical nature of aerodynamic damping in FSI, serving as a reference for various FSI applications, including bridges, high-building design, and other related fields. [ABSTRACT FROM AUTHOR]

Galloping is characterized by large and periodical oscillations which may lead to collapse of slender structures. This study is the first attempt of a comprehensive experimental and theoretical investigation of galloping of transversely inclined prisms. A modified quasi-steady model is proposed with a constant term to estimate the galloping of a transversely inclined prism, which is later experimentally investigated by conducting a static Synchronous Multi-Pressure Sensing System (SMPSS) test and an aeroelastic test in a boundary layer wind tunnel. The galloping responses of the prisms were measured in the aeroelastic test, while the aerodynamic force coefficients were determined from the SMPSS test. These experimental results were subsequently utilized to validate the quasi-steady model. Based on the proposed model, the galloping responses of the prisms were predicted and compared with the experimental results. The experimentally measured and theoretically predicted galloping responses are discussed with respect to aerodynamic damping ratios, onset galloping wind speeds, distributed pressure coefficients, point pressure spectra and vortex shedding frequencies. Interesting findings are summarized. [ABSTRACT FROM AUTHOR]

This study examines effects of the splitter plate placed in the near wake of a circular cylinder on the performance of a piezoelectric wind energy harvester through wind tunnel experiments. The kinetic energy of the harvester is gained by wind-induced vibrations of the circular cylinder. The splitter plate is attached to the leeward side of the cylinder. The ratio of the splitter plate length to the diameter of the circular cylinder (Lsp/D) ranges from 0.25 to 2.00. After attaching the splitter plate with an appropriate length, the harvester is able to sustain large amplitude vibrations beyond the wind speed range corresponding to vortex-induced vibrations. Thus, the upper bound of the wind speed range for the harvester to harness wind energy is eliminated, which significantly increases the efficiency of the harvester. Compared to the different lengths of the splitter plate, 0.65D has been found to be the optimal length for maximizing the harvested power. [ABSTRACT FROM AUTHOR]

This study aims to expand the aeroelastic unstable range of a circular cylinder for improving the efficiency of a vortex-induced vibration (VIV)-based wind energy harvester. The kinetic energy of the harvester is provided by flow-induced vibration of a circular cylinder. Two small-diameter cylindrical rods were attached on two sides of the circular cylinder parallel to the cylinder axis and symmetrical to the stagnation line at a series of circumferential locations. This was inspired by rain-wind-induced vibrations of stay-cables of cable-stayed bridges. It was found that attaching two small-diameter cylindrical rods at the circumferential location θ=60° significantly expands the aeroelastic unstable range for the circular cylinder. The wind energy harvester with this configuration harnesses the wind energy beyond the VIV onset wind speed and is sustained over the range of wind speed. Therefore, this configuration possesses a dramatic superiority over a plain circular cylinder as the kinetic source of a wind energy harvester. [ABSTRACT FROM AUTHOR]

This letter presents the effects that fitting fins to various corners of a square-prism galloping-based piezoelectric energy harvester (PEH) has on its performance, based on results from a series of wind tunnel model tests. The results show that attaching fins to the leading edge significantly improves the efficiency of the harvester, achieving a maximum power 2.5 times that attained by a plain square prism PEH. Furthermore, a length that is 1/6 of the prism's cross-sectional width is found to be optimal for fins that are attached to the harvester. [ABSTRACT FROM AUTHOR]

Through comparing the mean wind profiles observed overland during the passages of four typhoons, and the gradient wind speeds calculated based on the sea level pressure data provided by a numerical model, the present paper discusses, (a) whether the gradient balance is a valid assumption to estimate the wind speed in the height range of 1250 m ~ 1750 m, which is defined as the upper-level mean wind speed, in a tropical cyclone over land, and (b) if the super-gradient feature is systematically observed below the height of 1500 m in the tropical cyclone wind field over land. It has been found that, (i) the gradient balance is a valid assumption to estimate the mean upper-level wind speed in tropical cyclones in the radial range from the radius to the maximum wind (RMW) to three times the RMW, (ii) the super-gradient flow dominates the wind field in the tropical cyclone boundary layer inside the RMW and is frequently observed in the radial range from the RMW to twice the RMW, (iii) the gradient wind speed calculated based on the post-landfall sea level pressure data underestimates the overall wind strength at an island site inside the RMW, and (iv) the unsynchronized decay of the pressure and wind fields in the tropical cyclone might be the reason for the underestimation. [ABSTRACT FROM AUTHOR]

To investigate the applicability and limitations of both physical (wind tunnel test) and numerical (weather prediction) simulations of the atmospheric boundary layer, field measurements from a Doppler SODAR and a wind-profiler were combined to serve as validation criteria when comparing the results from a numerical simulation conducted by the Weather Research and Forecast model (WRF) and from wind tunnel testing. The comparisons focused on the simulation of the typhoon boundary layer, and revealed that a major drawback of wind tunnel testing is the use of an unrealistic approaching wind profile. As a result, the wind tunnel test results should only be considered valid when the measured wind profile is influenced predominantly by the underlying terrain. Meanwhile, the relatively coarse resolution of the underlying terrain model used in the numerical weather prediction system may lead to an inaccurate mean wind speed profile at lower altitudes, especially when the winds are coming from the land fetch. [ABSTRACT FROM AUTHOR]

The turbulence intensity, defined as the standard deviation of the longitudinal wind speeds divided by the mean wind velocity, is one of the important turbulent characteristics in atmospheric boundary layers. In-situ measurements of turbulence intensities above heights of 10 m are, however, limited by the availability of suitable instrumentation. The present study discusses a method to estimate turbulence intensity from the turbulent kinetic Energy Dissipation Rate (EDR) through the use of a turbulent length scale model. Provided that the EDRs can be extracted directly from raw measurements taken by remote sensing techniques, the proposed method is applicable for estimating the turbulent velocity fluctuations where only remotely sensed EDRs are available. The validity of the proposed method was evaluated in two case studies. When the measurements taken by a Doppler sodar under various weather conditions were adopted to calculate both the longitudinal turbulent velocity fluctuations and to extract the EDRs, the comparisons between the observed and estimated turbulent velocity fluctuations quantitatively show the validity of the proposed method. It has been found that the proposed method is valid when (a) there are no topographic obstacles upstream, (b) the wind speed in a typhoon boundary layer is high enough to make the selected turbulent length scale model applicable and (c) the EDR values are large enough to ignore any noise contained in the EDR data. [ABSTRACT FROM AUTHOR]