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Numerical Investigation of Three dimensional Turbulent Wind Flow around Two Square Buildings with Hip Roofs

Numerical Investigation of Three dimensional Turbulent Wind Flow around Two Square Buildings with Hip Roofs

Abstract This computational work aims at studying the three – dimensional turbulent wind flow around two square buildings with pyramid roofs of trivial architectural forms for an arbitrary geographical location. The overall investigation substantially reduces to the fundamental problem of an external three – dimensional turbulent flow field past a mounted obstacle of predefined shape. The novelty of this research is that the independency of the numerical solution for any possible distribution of mesh points was demonstrated in a theoretical manner without the necessity of changing the original grid with simultaneous repetition of the computational process.

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Periodic stability analysis of wind turbines operating in turbulent wind conditions

Periodic stability analysis of wind turbines operating in turbulent wind conditions

Although this approach attains the two goals outlined above, one of its limits is that it can not be used with mea- surements obtained on a real wind turbine operating in the field, since the effects of wind turbulence are not considered within the PARX model structure. To address this issue, the same approach was extended to account for the presence of turbulence (Bottasso et al., 2014). Using this new technique, one first identifies a periodic autoregressive moving average with exogenous input (PARMAX) model, whose stability is then analyzed according to Floquet. Bottasso et al. (2014) showed only one example related to the first blade edgewise mode of a wind turbine rotor. The goal of the present pa- per is to expand and formulate in detail the PARMAX-based method originally proposed by Bottasso et al. (2014). A sec- ond goal of this paper is to compare the PARMAX method with the periodic operational modal analysis (POMA) (see Allen et al., 2011a), which is taken here to represent the ac- cepted state of the art for the stability analysis of wind tur- bines operating in turbulent wind conditions.

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Buffet loading, dynamic response and aerodynamic control of a suspension bridge in a turbulent wind

Buffet loading, dynamic response and aerodynamic control of a suspension bridge in a turbulent wind

The paper has two primary objectives. The first is to conduct buffet re- sponse experiments on a bridge deck section in a turbulent air stream. Mea- sured and theoretically predicted responses will be compared and analysed. We will compare buffet admittances obtained from wind tunnel experiments with the admittances calculated from unsteady, three-dimensional, thin aero- foil theory. The bridge deck is mounted elastically in the Honda wind tunnel at Imperial College London with heave and pitch freedoms, while the incident stream turbulence is generated by a newly designed grid that provides large length-scale turbulence. We found that the measured results are about 1.7 times higher at the resonant peak for pitch motions than the theoretically pre- dicted values, with closer agreement obtained for heave motions. We suspect that some self-buffet occurred. The second aim of this paper is to conduct buffet control experiments using controllable leading- and trailing-edge flaps. These flaps were mounted on the deck model and suspended elastically in wind tunnel facilities. The control inputs are the leading- and/or trailing- edge flap angles. Two controllers were developed: the first senses the deck position, while the second produces flap angle deviations that are responsive to the vertical velocity of the flap hinge points. The wind tunnel tests show that the controllers can improve the critical wind speed for flutter, while simultaneously suppressing buffeting.

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Dynamic Analysis of Overhead Transmission Lines under Turbulent Wind Loading

Dynamic Analysis of Overhead Transmission Lines under Turbulent Wind Loading

Separate time histories of 300 seconds were generated for 19 points on each conductor. Distance between each point on the conductor is about 20 m. The mean wind speed at respective heights for generating the time history is from DIN-1055-4:2005-03 [14]. Wind turbulence is modeled using the weighted amplitude wave superposi- tion (WAWS) model based on Shinozuka and Jan [15]. Details of simulation can be found in the work done by Clobes [16]. Von Kármán power spectral density function was used to characterize the power distribution of the turbulence in longitudinal direction, Kaimal for lateral direction and Busch and Panofsky spectrum for vertical turbulence. The cross-correlation of two neighboring points decreases with increasing distance between them. This point has to be kept in mind as the loads generated can be up to 10 - 20 times higher if each element length is very large. At low frequencies, the eddies have a large integral length scale and take long time to cross the structure. In this case the distribution of load equally over the length of element is justified. However, for eddies corresponding to higher frequencies, that are smaller than the element length, it is incorrect to consider the wind load fully coherent along the length of the element. The high frequency eddies actually compensate each other from one point on element to other. If the load along the element is considered same it could result in overesti- mation of the forces [17]. While conducting this work, initially a model was made with element length of 20 m and it was found that the tension in the conductor was almost 65% - 70% higher than the tension due to design loads. Equation (1) [17] gives the ideal element length, below which the forces can be considered to be coherent.

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Crosswind Stability Evaluation of High-Speed Train Using Different Wind Models

Crosswind Stability Evaluation of High-Speed Train Using Different Wind Models

A suitable construction of the wind model is widely considered critical for effectively assessing the overturn- ing risk of a railway vehicle exposed to crosswinds. The construction of the wind model has a number of levels of complexity, ranging from the simple assumption of a steady wind to a more complex gust of a specific form (such as the Chinese hat gust) and a complete stochastic wind simulation that produces turbulent fluctuations of the correct magnitude and scale. In most previous stud- ies, the crosswind stability analysis of railway vehicles was based on the steady wind hypothesis, and a num- ber of effective results have been obtained. Krajnović et  al. [12] used large eddy simulation to study the flow around a simplified train model exposed to crosswind; an overshoot of approximately 30% observed in the yaw- ing moment coefficients indicated the importance of performing dynamic tests for fulfilling safety standards. Giappino et al. [10] compared the crosswind behavior on a high speed train and that on a low speed train through two subsequent analyses: measurement of the aerody- namic coefficients through wind tunnel tests on scale models and evaluation of the rollover risks by applying the definition of CWC based on static equilibrium. The steady wind hypothesis is convenient for either of wind tunnel test or CFD simulation. Furthermore, natural winds were represented as a gust of a specific form. Car- rarini [13], Wetzel and Proppe [14] set up an artificial gust model, considered the most influential but uncer- tain parameters (gust factor, gust length, and aerody- namic coefficients) as stochastic variables, and proposed a reliability analysis method for the crosswind stability of railway vehicles. In EN 14067-6 [15], the construction of a Chinese hat gust wind model and the corresponding assessment methods for the railway vehicle are described. However, simulations of a steady wind and an ideal gust are generally substantially simpler than real-world cases. When a moving vehicle is subjected to crosswind, the aerodynamic loads acting on the vehicle depend on the mean value of the relative wind–vehicle velocity as well as on the statistical properties of the wind [16]. There- fore, certain researchers proposed an alternative proce- dure, i.e., the turbulent wind model; it has spectral and correlation statistics similar to those of natural wind. The stochastic approach is used for studying realistic

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Measurement of turbulent water vapor fluxes using a lightweight unmanned aerial vehicle system

Measurement of turbulent water vapor fluxes using a lightweight unmanned aerial vehicle system

Payloads for lightweight UAVs have been developed pre- viously by C 4 to measure aerosol, radiation, cloud, and me- teorological properties. These measurements, when cou- pled with the UAV’s versatility, have allowed investigation of the atmospheric heating rates of black carbon using stacked UAVs (Ramanathan et al., 2007), developed links between cloud microphysics and albedo (Roberts et al., 2008), and established insights into the long range transport of aerosols and their influence on solar absorption (Ramana et al., 2010). Ideally, for direct comparison with surface tower fluxes, flying at low altitude over long flight legs over a uniformly homogeneous surface is desirable. The low altitude reduces vertical divergence, long legs enable capture of the low flux- contributing eddy frequencies, and the surface homogeneity simplifies horizontal flux interpretation. If these conditions are met, aircraft flux systems will sample a turbulent wind field broadly equivalent to that advected past a tower, but on a much shorter averaging time (in the form of straight and level horizontal runs) due to the rapid motion of the aircraft through the assumed “frozen” turbulent wind field (Taylor, 1938). Airborne systems therefore require higher frequency response instrumentation than their stationary counterparts, in order to capture the smallest eddies contributing to the flux. In reality, such conditions are rare, and research is progressing to reconcile (λ)E horizontal flux variability with surface inhomogeneities (Kiemle et al., 2011; Samuelsson and Tjernstrom, 1999; Mahrt et al., 2001; Desjardins et al., 1992).

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Design, Fabrication and Characterization of Low Speed Open-jet Wind Tunnel

Design, Fabrication and Characterization of Low Speed Open-jet Wind Tunnel

Abstract— A new low-speed open-jet wind tunnel has been designed and constructed at the University of Leeds. A series of Computational Fluid Dynamics (CFD) and experimental evaluations were conducted to determine the flow quality and to verify the wind tunnel suitability for aerodynamic studies. Two sets of results are presented in the current paper. Initially, mean velocity and turbulent intensity measurements in an empty test section using a Pitot-static tube and hot wire anemometer (HWA) were introduced. These results show that flow quality was significantly affected by boundary layer controllers (honeycomb and mesh screens) in the settling chamber and wide angle diffuser. Investigations were also conducted to evaluate the effectiveness of using an array of synthetic jet actuators (SJAs) for flow control in a wake behind a convex "hump" model (section of circular cylinder). These additional tests were conducted to validate the suitability of the wind tunnel for aerodynamics research.

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On the impact of non-Gaussian wind statistics on wind turbines – an experimental approach

On the impact of non-Gaussian wind statistics on wind turbines – an experimental approach

The experiments were conducted in a wind tunnel of the Uni- versity of Oldenburg in open jet configuration. The outlet of 0.8 m × 1 m (height × width) was equipped with an active grid for turbulence generation with a similar design as de- scribed by Weitemeyer et al. (2013). The grid is made of nine vertical and seven horizontal axes with square metal plates attached. To allow an individual motion of the axes and thus flow manipulation, 16 stepper motors were used. However, throughout the experiments, all axes were excited simultaneously. We define a flap angle α, whereas α = 0 ◦ is in alignment with the main flow direction (open) and ± 90 ◦ corresponds to maximum blockage. At α = 0 ◦ , the block- age of the grid is approximately 6 %, considering the cross- sectional area of the grid in relation to the wind tunnel outlet. The excitation protocols of the motors were designed so that two different flow situations with the same mean wind velocities and comparable turbulence intensities were real- ized. At the same time, they strongly differ in their distribu- tions of increments: one flow (A) being Gaussian distributed, the other one (B) being highly intermittent on a broad range of timescales, which shows a distinctly heavy-tailed distribu- tion of velocity increments. The resulting time series are dis- cussed in Sect. 4.1. The excitation protocol resulting in the intermittent flow featured an active flow modulation, where α was changed appropriately at a maximal rate of 50 Hz. For the Gaussian flow, the axes were not moved dynamically so that α ˙ = 0 ◦ .

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A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16 (S) during the dry season

A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16 (S) during the dry season

On high-altitude Andean tropical glaciers during the dry season, turbulent latent heat fluxes (LE) are generally an important sink of energy since they mainly consist of sub- limation, which is favoured by the low atmospheric pres- sure. The magnitude of H can change significantly between night and day due to changes in thermal stratification (e.g. Wagnon et al., 2003; Sicart et al., 2008). Synoptic forcing generally remains weak, and thermally driven winds domi- nate wind circulation (Litt et al., 2015). At night and during the morning, a marked temperature inversion at low height above the surface (2–3 m) favours the development of kata- batic flows (Fedorovitch and Shapiro, 2009). For weak syn- optic forcing, a wind-speed maximum is frequently observed at low height. Strong synoptic forcing causes strong winds at the glacier surface, while complex outer-layer interactions with the surface layer occur (Litt et al., 2015). During after- noons of the dry season, anabatic valley winds are frequently observed (Fedorovitch and Shapiro, 2009), while the tem- perature inversion is less marked. Net turbulent surface heat flux (H + LE) and associated errors may strongly depend on these wind-regime characteristics. A comprehensive review and quantification of sources of uncertainties, and compari- son of these errors with net turbulent fluxes under different wind regimes, is required to improve uncertainty assessment in energy balance studies.

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Combined vertical-velocity observations with Doppler lidar, cloud radar and wind profiler

Combined vertical-velocity observations with Doppler lidar, cloud radar and wind profiler

All three velocity-measuring instruments are most sensi- tive to particles or structures which are similar in size to the operating wavelength λ of the instrument (see Table 1). The Doppler lidar (λ = 1.5 µm) is most sensitive to aerosol particles (100 nm to 10 µm in diameter). The cloud radar (λ = 8.5 mm) is capable of sensing cloud droplets (10 to 100 µm). Additionally it shows large signals for hydromete- ors like drizzle droplets, rain droplets or ice crystals (100 µm to 10 mm). The wind profiler detects echoes from refrac- tive index fluctuations originating from turbulent eddies in the atmosphere with sizes of half its wavelength of λ = 0.62 m and Rayleigh scattering from particles. In this work, the combined operation of this unique set of instruments is demonstrated in order to study the vertical motions in and around layered clouds. The combination of wind profiler and cloud radar has been done before (Tridon et al., 2013). Pro- tat and Williams (2011) also showed the principle of com- bined vertical-velocity observations, but with a combination of 50 MHz wind-profiler and 3 GHz radar. It is the focus of this paper to show that an additional Doppler lidar can de- liver complementary information about the vertical veloc- ity of small cloud droplets and their fast-changing turbu- lent motion. For that purpose, first tentative efforts of the MOL/TROPOS measurement campaign COLRAWI (Com- bined Observations with Lidar, Radar and Wind Profiler) are presented here. It is shown that turbulent air motion, large- scale waves and the vertical movement of falling ice and water particles can be measured at once. In this way a co- herent picture of different kinds of vertical motions in the atmosphere can be drawn and the unbiased fall velocity of particles can be measured.

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Assessment of Dynamic Effect of Transmission Line Conductor Longitudinal Reaction Due to Downburst Loading

Assessment of Dynamic Effect of Transmission Line Conductor Longitudinal Reaction Due to Downburst Loading

Earlier research was directed to full-scale measurements trying to provide field data for the newly explored wind event. Metrological projects like the Northern Illinois Meteorological Research on Downbursts (NIMROD) and the Joint Airport Weather Studies (JAWS) were reported by Fujita (1985a), while the Federal Aviation Administration Lincoln Laboratory Operational Weather Studies (FLOWS) project has been reported by Wolfson et al. (1985). More recent field measurements have been reported by Orwig and Schroeder (2007) who presented the results obtained from a linear array of mobile towers for two captured events. They also presented their analyses for the results that were compared to synoptic wind data analysis. Choi (2004) has also reported measurements taken using one tower for more than 50 thunderstorm events. The author furtherly investigated the different variables affecting the velocity profiles of the measured events. In addition, a relatively large project, that took place between 2009 and 2012, has been reported by Solari et al. (2015a) as an extensive in-situ wind monitoring network. The Wind and Ports project relies on 22 ultrasonic anemometers to capture high-resolution thunderstorm records that are then processed to extract statistical properties of thunderstorm events. Gunter and Schroeder (2015) have also presented measurements collected using two mobile Doppler radars to provide enhanced understanding of the vertical profiles through analyzing three events.

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Investigation into the flow physics of large experimental offshore wind farms in a
        turbulent boundary layer wind tunnel, John James Turner

Investigation into the flow physics of large experimental offshore wind farms in a turbulent boundary layer wind tunnel, John James Turner

Corten et. al. aimed to get a basic understanding of the power production stabilization of wind farms with an 8 row and 3 column array of two bladed turbines. They also drew attention to the overestimate in efficiency of classical wind farm models that optimistically assumed a steady state too early in the farm [37]. Cal et al. examined the horizontally averaged structure of the flow and measured the Reynolds turbulent shear stresses to be of the same order as the power extracted by the wind turbines [25]. This leads us to the conclusion that the vertical kinetic energy is the driver of the power production stabilization in infinite wind farms. Chamorro and Porté-Agel took hotwire measurements in a miniature aligned wind farm and characterized the regions within and above the farm noting that large wind farms could be treated as a special case of surface roughness. They noted a strong enhancement of turbulence levels around top tip height and the growth of an internal wind turbine array boundary layer [31]. Chamorro and Sotiropoulos studied a staggered wind farm and showed that the momentum transfer is improved with larger spacings, however, a large distance is required for the adjustment of flow statistics above the farm [32]. Hamilton et al. performed a statistical study of the sweeps and ejections that contribute to the vertical kinetic energy flux from the flow above and argued that sweeping motions dominate the net entrainment of kinetic energy that can ultimately be used at the wind turbine location [60]. The last study listed in Table 2.2 (Study 11) is the current study [111]. The present work can be considered an extension of these studies. A photograph of one setup of the wind turbine array discussed in this thesis using a combination of 0.25 m diameter porous disks and model turbines can be found in Figure 2.6. This photograph is used here for a scale reference for this and future sections. This array is positioned to examine the worst case flow direction where the turbines are arranged in aligned rows and columns with a uniform incoming wind direction.

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Experimental and numerical analysis of a TLP floating offshore wind turbine

Experimental and numerical analysis of a TLP floating offshore wind turbine

In order to investigate the hydrodynamic performance of a TLP FOWT a test campaign was carried out by CEHINAV-UPM research group for Iberdrola and published by Rodriguez et al. (2014) . This concept consisted of a central cylindrical column with four square section horizontal pontoons at its base and each pontoon connected with two tendons to the sea bed. Regular, operational, survival, failure and transport tests performed were for a simulated 80m water depth. The paper presents the experimental setup, free decay tests, regular wave motion RAOs, irregular wave responses, tendon loads and accelerations. In order to include wind effect into the tests a calibrated turbine was used and controlled with the data measured through real time platform motion tracking. They also compared their results with available in-house numerical simulations and other results found in literature. Their experimental results indicated that the natural periods and damping values are similar to those published in the literature. The surge values were slightly smaller than reference values which was put down to the reduced water depth. All the RAOs were very small except surge which is typical for TLPs. Due to the coupling of surge and heave motions, the heave motion response contained components at twice the fundamental wave frequency. It is also reported that no slack in the tendons occurred during the testing period.

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WWER-type   NPP Spray Ponds Screen (J036)

WWER-type NPP Spray Ponds Screen (J036)

Forty cases of fluid flows above the spray ponds zone are simulated, based on analysis of the available input data and the site conditions. CFD simulation is done with the FASTEST/3D program package. The mathematical model is based on the Reynolds-averaged Navier-Stokes and continuity equations and k-ε turbulence model, describing the fluid motion in complex turbulent flow at stationary conditions. The model consists of a system of six nonlinear partial differential equations. The droplets are approximated as ideal spheres. Two types of simulations are carried out – a study on the wind velocity field in the vicinity of the spray ponds (FS-1) and a study on the distribution of the water droplets (FS-2).

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Sail or sink: novel behavioural adaptations on water in aerially dispersing species

Sail or sink: novel behavioural adaptations on water in aerially dispersing species

Movement across water surfaces taking advantage of wind currents has been reported in pioneering work on species that have a particularly close association with water, such as Dolomedes raft spiders [32], but it has not been documented in strictly terrestrial species such as those used in the current study which are highly dis- persive and known to use long-distance aerial dispersal throughout their life stages [24]. In our study we use la- boratory experiments and observations to test whether common ballooning linyphiid and tetragnathid spiders, which respectively represent ~ 11 % and 2 % of all spider species ([33] http://www.wsc.nmbe.ch/), have evolved strategies that may allow them to survive on water.

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Wind tunnel measurements of turbulent boundary layer flows over arrays of ribs and cubes

Wind tunnel measurements of turbulent boundary layer flows over arrays of ribs and cubes

trasted over different surface configurations. The effect of roughness elements on the roughness sublayer (RSL) was also investigated (Placidi and Ganapathisubramani 2015). Besides, turbulence structure was characterized by autocorrelation, quadrant analyses as well as spectra over cube-type arrays (Castro et al. 2006). These experimen- tal studies have enriched our understanding of turbulent flows over rough-wall TBLs. However, more wind tunnel results are needed to study the effect of surface configu- rations on the turbulence behavior and the associated street-level ventilation over urban areas.

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A Study on the Turbulent Characteristics within the Hurricane Boundary Layer

A Study on the Turbulent Characteristics within the Hurricane Boundary Layer

Although endeavours of taking direct measurements of turbulence variables within the HBL has been made as early as the 1970’s, as described by Moss (1978) and Sethraman (1979), few studies have been performed to derive a reliable model of turbulent mixing because of the rarity of measurements, which is due to the difficulty and danger of taking measurements at a sufficiently low altitude in the HBL. Until very recently, the invention of research aircraft at a sufficiently low altitude while maintaining safe and operational, like the ones used in the Coupled Boundary Layer Air-Sea Transfer Experiment, make it possible to take direct measurements of turbulence variables, like wind velocity variances and turbulent momentum fluxes, within the HBL. An other currently available technique of analyzing the HBL turbulence is remote sensing techniques. As described by Lorsolo et al. (2010), remote sensing techniques are able to reveal a crude structure of the HBL turbulent kinetic energy. In addition to studies based on direct observations over water, measurements taken over land are also used to analyze turbulence characteristics within the HBL, as in the study of Yu et al. (2008). Although research aircraft measurements provide, so far, the most reliable observations of the HBL turbulence, the scarcity of its measurement data hampers retrieving a comprehensive wind structure of the HBL. Limitations of the remote sensing observations and over land tower observations are crude resolutions and restrictions in measurement height respectively. Without a reliable direct observation, numerical models describing turbulent mixing in the standard ABL are used extensively to model the HBL turbulence for engineering applications.

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Forest impacts on snow accumulation and ablation across an elevation gradient in a temperate montane environment

Forest impacts on snow accumulation and ablation across an elevation gradient in a temperate montane environment

While few studies in maritime forested environments on the energy balance exist, there is evidence of longwave ra- diation as the dominating term during rain on snow (ROS) events within forests (Berris and Harr, 1987; Mazurkiewicz et al., 2008; Garvelmann et al., 2014). Berris and Harr (1987) showed that longwave radiation accounted for 38–88 % of all ROS event snowmelt. Garvelmann et al. (2014) found that in two ROS events longwave radiation accounted for 55.1 and 38.8 % of the net energy balance, although this may be bi- ased low due to the inability to accurately capture tree trunk temperature. Although Mazurkiewicz et al. (2008) did not differentiate between radiation terms, they found that net ra- diation was the largest contributor to melt. The highly nonlin- ear relationship between air temperature and incoming long- wave radiation formulation is apparent in the net radiation budget analysis. Infrequent cloud-free days and the warm, dense forests of the study area combine to emit a significant amount of longwave radiation to the snow surface (Berris and Harr, 1987; Sicart et al., 2004; Garvelmann et al., 2014). This leads to a positive net snow surface energy balance and mid- winter melt events, most pronounced at the warmer lower el- evation sites. With prolonged exposure to longwave radiation emitted by the canopy and the high efficiency of warm forest canopy interception capabilities, low elevation maritime sub- canopy snowpacks are relatively thin and do not persist long enough into the spring season to benefit from forest shading. This creates a radiative paradox where the longwave radia- tion emitted by dense and relatively warm forest cover ex- ceeds the resulting reduction in shortwave radiation due to forest shading (Sicart et al., 2004; Lawler and Link, 2011; Lundquist et al., 2013). The higher elevation sites experience colder air temperatures, higher wind speeds, and lower for- est density, which combine to decrease C IE and the impact of

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Magnetic reconnection and intermittent turbulence in the solar wind

Magnetic reconnection and intermittent turbulence in the solar wind

Analysis. — We use 3 s resolution magnetic field measure- ments from the Magnetic Field Investigation (MFI) [46] and proton moments from the 3D Plasma Analyzer (3DP) [47] onboard the Wind spacecraft. The data intervals used in this investigation are listed in Table I, and were originally selected randomly and then carefully scrutinized for reconnection exhausts. In the solar wind reconnection exhausts are identified as roughly Alfvénic-jetting plasma (based on the antiparallel field components) that are bounded on one side by correlated changes in the antiparallel components of V and B and by anticorrelated changes in those components on the other side. The list of identified reconnection exhausts that we use is assembled by application of these methods. In addition, current sheets are identified in a separate list as a reversal in at least one geocentric solar ecliptic (GSE) component of the magnetic field vector. While this method will miss current sheets where no field component actually reverses sign, it will not significantly affect our results since these are likely to be associated only with small fluctuations and not the most intermittent structures of interest. The numbers of reconnection exhausts (RE) and current sheets (CS) identified in this way are listed in Table I.

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Near-Wake Flow Dynamics of a Horizontal Axis Wind Turbine

Near-Wake Flow Dynamics of a Horizontal Axis Wind Turbine

The near wake region is characterized by a vortex structure resulting from the interaction between the upstream flow and the vorticity sheets generated at the blades and convected downstream. Schematics of this wake vortex topology are presented in Figure 1-2. In general, the trailing vorticity results from the variation of the circulation along a typical blade with finite span. The location at which the circulation is forced to zero indicates the termination of the vortex sheets, which occurs at the root and tip of each blade in case of wind turbine (Whale, 2000). The vortex sheets are trailing downstream in a helical pattern. These vortex sheets tend to roll up, shortly downstream of the rotor (see Figure 1-2 (a)). According to Figure 1-2 (b) and (c), two major vortex tubes can be observed originated from the blade: the vortex helix from the blade tips and from the roots with the helical motion in the reversed direction of the rotation of the rotor (Schepers et al., 2012). The inclination angle of the helical trajectories is a function of the tip speed ratio. For high tip speed ratios, the inclination angle of the vortex tube is small and the layer, encompassing the tip vortices, can be considered as an annular shear zone separating the flow within the wake from the ambient flow (Gomez et al., 2005; Vermeer et al., 2003; Grant and Parkin, 2000). Figure 1-2 (d) displays the formation of the annular shear zone downstream of the turbine, for the case of high tip speed ratio.

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