File Name: analysis of flow through propellers and windmills .zip
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However, experimental techniques capable of quantifying or even qualitatively visualizing the large-scale turbulent flow structures around full-scale turbines do not exist today. Here we use snowflakes from a winter snowstorm as flow tracers to obtain velocity fields downwind of a 2. The spatial and temporal resolutions of the measurements are sufficiently high to quantify the evolution of blade-generated coherent motions, such as the tip and trailing sheet vortices, identify their instability mechanisms and correlate them with turbine operation, control and performance.
Our experiment provides an unprecedented in situ characterization of flow structures around utility-scale turbines, and yields significant insights into the Reynolds number similarity issues presented in wind energy applications. Laboratory and numerical studies of near-surface phenomena occurring in the atmospheric boundary layer ABL are severely constrained by the inability to reproduce the complexities of real-life atmospheric flow conditions and by an inherently limited range of spatial scales leading to enormous differences in Reynolds number.
Our ability to gain such understanding is limited by the lack of experimental techniques able to quantify turbulent flows at the utility turbine scale. Current field measurement techniques provide only point velocity characterization sonic anemometers or velocity profiles LiDAR and Sodar at a coarse spatio-temporal resolution 7 , 8 , 9. Particle image velocimetery PIV , based on tracking the displacement of tracers in an illuminated flow field 10 , 11 , is the only measurement technique capable of obtaining planar velocity distributions with the spatio-temporal resolution required to study flow—structure interactions.
However, PIV has only been applied, so far, to small-scale turbine models, from 0. In addition, the in situ PIV implementation is severely constrained by a number of adverse environmental conditions in the field such as the background daylight and large dynamic range of wind velocity and direction. Consequently, PIV has yet to be applied to areas larger than a few square metres 18 , 19 , 20 , which is at least one order of magnitude smaller than what is required to make an impact in wind energy research.
In the experimental setup, a collimated 5-kW searchlight with specially designed reflecting optics is employed to generate a light sheet aligned with the mean wind direction for illuminating snow particles. The meteorological conditions contribute to optimal concentrations of snowflakes with good traceability and light-scattering capability, enabling flow visualization of coherent motions at elevations corresponding to the lower portion of the rotor behind the operating turbine.
By cross-correlating time-consecutive images, the wind velocity distribution in the illumination plane is obtained at a spatial resolution of 0. The resolution is sufficient for quantifying the spatio-temporal evolution of large vortical structures induced by the rotating blades in the turbine wake. Taking advantage of the fully instrumented 2. The advection patterns of snow particles within the light sheet elucidate the dynamics of unsteady vortical structures occurring in the turbine wake.
The hallmark vortical structures are the three-dimensional vortex tubes shed at the blade tips, which are referred to as the tip vortices. As the turbine rotates, the tip vortices describe helical trajectories evolving with the turbine wake and intersect the light sheet at multiple streamwise locations, where vortex cores create voids in the snow particle seeding due to the centrifugal effect on the inertially driven snow particles Figs 1 and 2.
Connected to, and positioned above each tip-vortex core, a vertical strip of low snowflake concentration is frequently observed Fig. This signature is the footprint of trailing vortex sheets generated along the entire span of each blade. Compared with the tip vortices, these structures are less energetic and persistent, and have been visualized in the past only at small laboratory scales For most of the time during the observation period, the tip-vortex structures are advected downstream steadily, projecting a periodic streak pattern or trail of equally spaced voids Fig.
The spacing between adjacent voids represents the helical pitch of tip-vortex spirals and, when decreased, tends to promote vortex interactions. Such interactions are observed only occasionally and involve two or more vortices with continual reduction in spacing, which then start to orbit around each other, ultimately merging into a new stronger vortex that rotates in the same direction with increased vortical circulation and size Fig.
More frequently, we observe events such as vortex collapse, visualized by a sudden homogenization of snow particle concentration in the collapsing vortex core and rapid vanishing of the characteristic void signature Fig. This phenomenon, manifesting a drastic reduction of vortex strength, is a result of the breakdown of the trailing vortex sheets, which are needed to sustain the tip vortices The unsteadiness of the trailing sheets can quickly trigger tip-vortex instability and cause vortex core collapse and, subsequently, void disappearance.
In fact, as observed in the experiment, the inception of vortex collapse resides at the transition from stable to unstable evolution of the trailing sheets Fig. Note that Fig. The insert panels a — g show snapshots of tip vortices behind the turbine at different times in the time sequence. Red and grey shaded areas mark the occurrence of merging and collapsing vortices, respectively.
Overall, from Fig. Some physical explanations are provided here to understand the trends described above. As illustrated in Fig. Combined with the shifting of the vortex position due to the unsteady incoming wind, this perturbation on the vortex strength can lead to a rapid destabilization of wake vortical system and consequently to the grouping and merging events, as also evidenced in the numerical simulation by Delbende and Rossi These exceptions point to the conclusion that vortex interaction events may also be triggered by other factors that can influence the turbine wake and have not been considered in the present study.
Such factors could include, for example, unsteadiness presented in the turbine operation and the unsteadiness and inhomogeneity of the near-blade turbulent flow induced by aeroelastic effects. In Fig. The resulting velocity distribution reveals the dominant wake-flow features 2 , 12 , such as the travelling vortex cores, the momentum entrainment between vortices highlighted in Fig.
To illustrate the features of the velocity profile presented in Fig. In the close-up view of the instantaneous flow field Fig. These patterns confirm that the voids are indeed a strong signature of high-vorticity regions. The black dashed line is a canonical log-layer profile that fits the SLPIV mean velocity profile in the range of 0. A Galilean translated vector field u The velocity vectors are skipped in a and c in both x and z directions for clarity.
This apparent low variance of the tip-vortex circulation in our imaging domain suggests that the roll-up process of trailing vortices is likely to be completed upstream of our measurement location. The tip-vortex circulation from SLPIV measurements can be compared with the averaged bound circulation on one blade estimated from the turbine power.
The detailed analysis is provided in the Supplementary Note 2. The incomplete capture of blade-bound circulation could be contributed by multiple sources such as the assumptions made during our calculations, the uncertainty involved in our measured circulation details provided in Supplementary Note 1 , the effect of blade geometry, and the effect of atmospheric turbulence on tip-vortex evolution. It is noteworthy that the high Reynolds number atmospheric turbulent flows and the large scale of the turbine do not seem to strongly affect the circulation capture by tip vortices on average, but may contribute significantly to the unsteadiness presented in the tip-vortex circulation as shown in the following section.
The circulation measurements, coupled to unsteady forces imposed on the blades, suggest that unsteadiness of the incoming flow is amplified through the interaction with the turbine, leading to a variety of coherent motions and interacting modes in the turbine wake Fig.
The reduced variability in the power output is primarily attributed to the spatial averaging effect in the conversion of the wind to a lift force, heterogeneity in the wind spanning the large rotor area and the large rotational inertia of the turbine rotor and drive train. The apparent strategy employed by the turbine controller to rapidly adjust the pitch in response to the approach wind velocity to maintain the generator speed dampens the unsteady fluctuations of the lift and thrust, and ultimately leads to a reduction of the power fluctuations.
Another key manifestation of flow—structure interaction is described here in terms of unsteady structural loading, as quantified by 20 strain gauges installed on the turbine foundation Supplementary Methods. Our study presents the first measurements of unsteady flow structures behind a utility-scale turbine. The measurement technique, although constrained by specific weather conditions, allows us to make a number of novel contributions. First, SLPIV measurements within the turbine wake provide information of unsteady vertical structures, which can be correlated with turbine operations.
Although the grouping of tip vortices has been observed in a number of prior studies 23 , 28 , it was reported to appear much farther downstream of the turbine when compared with that in our study. By correlating the visualized vortex structures with the synchronized turbine performance information, our experiment has shown some connection between the vortex interactions and subtle changes in the turbine operation.
The early occurrence of such interactions, compared with the observations from laboratory experiments, is likely to be related to the significant unsteadiness that is uniquely presented in the turbulence of the ABL coupled with the subsequent response of the utility-scale turbine operation. As discussed in a number of recent studies 23 , 24 , 29 , the unsteadiness related to, for example, large perturbations due to incoming flow and rotor geometry, including inhomogeneities in the blade construction, can cause variation in initial vortex position and strength, and lead to a rapid destabilization of the vortex system and vortex interactions.
The characteristics of vortex interaction observed in our study would imply a fast dissipation of wake structures and significantly different wake growth for a utility-scale turbine.
All these issues pose important questions that will form the basis of future investigations, which will integrate the SLPIV technique with high-fidelity numerical simulations. Second, the current SLPIV measurement technique quantifies the instantaneous full-scale velocity field and tip-vortex circulation. Based on a general principle of PIV, our experiment is able to quantify the instantaneous turbulent flow fields at scales sufficiently large to make an impact on studying the aerodynamics around utility-scale turbines.
The time-averaged flow field presents general features of near-wake flows. The circulation of neighbouring tip-vortex cores computed from instantaneous velocity fields shows considerably larger differences compared with the corresponding turbine power fluctuations.
Although the current flow quantification is limited up to lower portion of the rotor wake, the whole-field flow characterization around the turbine can be achieved through some hardware improvements, which include employing more powerful search lights and cameras with higher sensitivity.
Other sophisticated data sets are envisioned in the near future by simultaneously measuring spatially resolved incoming and wake flows, as well as turbine performance and blade structural dynamics to achieve direct in situ characterization of flow—structure interactions. This capability provides us an opportunity to investigate the organization of turbulent structures in high Reynolds number boundary layer flows, including problems such as the vertical extent and scaling of ramp-like structures and inner—outer layer interactions, forest and urban canopies focusing on shear penetration and dispersion processes, wake of buildings, bridges or other civil infrastructures for wind engineering applications.
In addition, owing to the unique tracers used in our measurements, our technique can be naturally adapted to study a number of snow-related subjects for the cold climate regions, such as snow drift driven by large-scale turbulence, spatial variation of snowflake settling velocity, concentration and size distributions. The field station consists of a heavily instrumented 2. The met tower Supplementary Fig. Natural snowfall provides particle seeding in a large sampling region for an extended period of time in an environmentally benign and non-intrusive fashion details provided in Supplementary Methods.
In addition, snowflakes have strong light-scattering capability especially side scattering owing to their multi-facet crystalline structure, which lowers the illumination power required for particle imaging. Artificially generated snow was previously used for PIV measurement A major concern of seeding with snow particles is the limited traceability due to their inertial and gravitational effects associated with their relatively large density as compared with that of the air.
The detailed discussion of the traceability of snowflakes in the presented experiment is provided in the Traceability section in the Methods and Supplementary Methods. The sheet expansion angle is controlled by adjusting the mirror curvature. Supplementary Figure 6 illustrates a test of the light sheet generation at the Eolos site. The entire light system including a 6-kW generator is affixed to a trailer, providing good mobility for aligning the light sheet in the predominant wind direction as required for planar PIV measurements.
As the imaging device is tilted at an angle with respect to the ground as illustrated in Fig. To calibrate our current setup, we first ensure that the tripod-mounted camera is levelled about its optical axis, which is aligned perpendicular to the light sheet. Next, we measure the tilt angle of the camera and its horizontal distance from the light sheet. Based on this information and the focal length of the imaging lens, we can correct the distortion resulting from the tilt imaging and determine the physical scale and location of the sampling area.
However, the wind direction Supplementary Fig. In this period, the bulk Richardson number, estimated from met tower measurements, is 0. Supplementary Fig. The imaging device was then arranged perpendicular to the light sheet on the southern side.
The selected interrogation window size also ensured sufficient particle density for valid PIV vector calculations Supplementary Fig. The resulting spatial resolution dictated primarily by the interrogation window size was 0. Five of these eleven data sets Supplementary Table 1 were synchronized with the turbine operation.
The convection velocity for each individual vortex within the five data sets was calculated and used to determine the exact time at which each tip vortex was shed at the blade tip.
Blade Element Propeller Theory. A relatively simple method of predicting the performance of a propeller as well as fans or windmills is the use of Blade Element Theory. In this method the propeller is divided into a number of independent sections along the length. At each section a force balance is applied involving 2D section lift and drag with the thrust and torque produced by the section. At the same time a balance of axial and angular momentum is applied.
Blade element momentum theory is a theory that combines both blade element theory and momentum theory. It is used to calculate the local forces on a propeller or wind-turbine blade. Blade element theory is combined with momentum theory to alleviate some of the difficulties in calculating the induced velocities at the rotor. This article emphasizes application of BEM to ground-based wind turbines, but the principles apply as well to propellers. Whereas the streamtube area is reduced by a propeller, it is expanded by a wind turbine. For either application, a highly simplified but useful approximation is the Rankine—Froude "momentum" or "actuator disk" model , This article explains the application of the "Betz limit" to the efficiency of a ground-based wind turbine.
Wind energy, together with other renewable energy sources, are expected to grow substantially in the coming decades and play a key role in mitigating climate change and achieving energy sustainability. One of the main challenges in optimizing the design, operation, control, and grid integration of wind farms is the prediction of their performance, owing to the complex multiscale two-way interactions between wind farms and the turbulent atmospheric boundary layer ABL. From a fluid mechanical perspective, these interactions are complicated by the high Reynolds number of the ABL flow, its inherent unsteadiness due to the diurnal cycle and synoptic-forcing variability, the ubiquitous nature of thermal effects, and the heterogeneity of the terrain. Particularly important is the effect of ABL turbulence on wind-turbine wake flows and their superposition, as they are responsible for considerable turbine power losses and fatigue loads in wind farms. These flow interactions affect, in turn, the structure of the ABL and the turbulent fluxes of momentum and scalars.
tuator disk concept still constitutes the most important analysis tool for the simulation of the ﬂow through. propellers or turbine rotors.
Blade element momentum theory
Wind energy is a form of solar energy. Wind turbines convert the kinetic energy in the wind into mechanical power. A generator can convert mechanical power into electricity . Mechanical power can also be utilized directly for specific tasks such as pumping water. The US DOE developed a short wind power animation that provides an overview of how a wind turbine works and describes the wind resources in the United States.
ТРАНСТЕКСТ заклинило на восемнадцать часовМысль о компьютерном вирусе, проникшем в ТРАНСТЕКСТ и теперь свободно разгуливающем по подвалам АНБ, была непереносима. - Я обязан об этом доложить, - сказал он вслух. В подобной ситуации надо известить только одного человека - старшего администратора систем безопасности АНБ, одышливого, весящего четыреста фунтов компьютерного гуру, придумавшего систему фильтров Сквозь строй. В АНБ он получил кличку Джабба и приобрел репутацию полубога. Он бродил по коридорам шифровалки, тушил бесконечные виртуальные пожары и проклинал слабоумие нерадивых невежд.
Компьютер, который постоянно отслеживал работу ТРАНСТЕКСТА, оказался выключен, вокруг не было ни души. - Эй! - крикнул Чатрукьян. Ответа не последовало. В лаборатории царил образцовый порядок, словно здесь никто не появлялся уже много часов. Чатрукьяну было всего двадцать три года, и он относительно недавно начал работать в команде обеспечения безопасности, однако был хорошо подготовлен и отлично знал правила: в шифровалке постоянно дежурил кто-то из работников его службы… особенно по субботам, когда не было криптографов.
Она много читала о таких вирусах - смертоносных программах, в которые встроено излечение, секретный ключ, способный дезактивировать вирус. Танкадо и не думал уничтожать главный банк данных - он хотел только, чтобы мы обнародовали ТРАНСТЕКСТ. Тогда он дал бы нам ключ, чтобы мы могли уничтожить вирус. Сьюзан стало абсолютно очевидно, что план Танкадо ужасным образом рухнул. Он не собирался умирать.
Тогда бы время, необходимое для дешифровки, составило двадцать лет. Производственное управление АНБ под руководством заместителя оперативного директора коммандера Тревора Дж. Стратмора торжествовало победу. ТРАНСТЕКСТ себя оправдал.
- Клянусь, я сделаю. Этим я и занимался сегодня весь день - считывал тексты с его терминала, чтобы быть наготове, когда он сделает первый шаг, чтобы вмонтировать этот чертов черный ход. Вот почему я скачал на свой компьютер его электронную почту. Как доказательство, что он отслеживал все связанное с Цифровой крепостью. Я собирался передать всю эту информацию в прессу.