Rotor Heights Research Articles

Statistical characterization of marine wind structure over wind-turbine-relevant heights in tropical cyclones

Tropical cyclones (or typhoons, hurricanes) may influence the power production and structural integrity of offshore wind turbines. However, understanding of tropical cyclone wind structure over turbine-relevant heights remains limited. Based on observations from a wind lidar and a meteorological mast at a planned offshore wind farm site, this paper investigates the marine wind characteristics, e.g., wind speed shear, wind direction veer, and turbulence intensity, over turbine-relevant heights in tropical cyclones. Statistical distributions of the marine wind characteristics are analyzed, and their variations with upstream fetch conditions and hub-height wind speeds are examined. Some noteworthy features of the marine wind structure in tropical cyclones are observed, such as the median wind veer rate of 0.018°/m and veer angle of 2.2° across the turbine rotor heights, different wind shear coefficients and wind veer rates for upper and lower rotors, and levelling off or decrease of wind shear coefficient and turbulence intensity with increasing wind speed at hub-height wind speed over 25 m/s. The outcome of this study is expected to provide useful information for design and operation of offshore wind turbines, marine platforms, and other ocean structures in tropical cyclone-prone regions.

A critical insight into the most effective CFD settings to model atmospheric stability in wind energy applications

An accurate reconstruction of the Atmospheric Boundary Layer (ABL) is key for the estimation of energy production and loads in modern wind turbines since, at current rotor heights, the vertical structure of the ABL is heavily influenced by thermal stratification effects. The wind power community usually accounts for these effects by semi-empirical modifications to the neutral ABL profile obtained using linear solvers such as WAsP; this approach, however, may not be suitable for sites with complex terrain, time-dependent thermal effects, or dense canopies. In these applications, methods with higher fidelity, such as Computational Fluid Dynamics (CFD) approaches based on steady or unsteady Reynolds Averaged Navier-Stokes (RANS) equations are needed. While CFD is recognized as providing more accurate results for these realistic cases, its use is still not a common best practice. In the current study, the WindModeller CFD tool by Ansys was employed to assess the effect of various inlet boundary conditions on the predicted wind speed profiles in unstable and stable atmospheric conditions. The test case is a wind farm in North Dakota, USA, characterized by a simple terrain but strong atmospheric stability effects and temperature fluctuations. The comparison of CFD results with experimental measurements and WAsP-CFD simulations reveals a high level of agreement between CFD and experimental data in stable conditions. In the case of unstable conditions, despite the good representation of the wind field, a high sensitivity to inlet boundary conditions is highlighted.

Effect of Eddy Current Loss on the Performance of Wind Turbine Induction Generator

Eddy current losses in wind power generation systems are dissipated as heat, which has a direct impact on the generator's efficiency. Due to the induction generator's poor heat dissipation from the rotor, eddy-current losses may generate substantial heating. A precise combination of the rotor architecture and the lamination dimensions should be employed when developing a wind power induction generator. In addition to material selection, lamination dimensions have a critical role in reducing core losses. The paper shows how to determine the generated power, losses, total energy efficiency, and capacity factor of a wind turbine generation system using a simple and accurate mathematical approach. The proposed work adds to the body of knowledge on eddy current losses by examining different steel lamination thicknesses and rotor heights for a wind power induction generator with a slip range of -0.05 to +0.25 and a B of 1.5 T and a frequency of 50 Hz. This work proposes an analytical tool for estimating the thickness and height of laminations (d and H). The optimal choice of d and H gives excellent efficiency and ensures that the thermal and slip are less than the maximum allowable limits, according to the results. The proposed method was used to examine a 2.2 MW, 50 Hz, 4-pole wind turbine induction generator. When d=0.1 mm and H=0.2 mm, the generator operates with a wide range of slip and is much below the thermal stress limit, resulting in a 96 percent efficiency.

Aerodynamic Interactions of Counter-Rotating Coaxial Rotors Hovering in Ground Effect

With the renewed interest in counter-rotating coaxial rotor (CCR) vehicles, a comprehensive understanding of its performance under out-of-ground effect and in-ground effect (IGE) operating conditions is imperative. The current work reports on an experimental study of a modular three-bladed CCR system during IGE hovering conditions. Thrust and torque produced by the upper and lower rotors were individually measured for two different axial spacings between the rotors with varying rotor heights from the ground. Both fixed-collective (untrimmed) and constant thrust, torque-matched (trimmed) experiments were performed to understand the interdependent effects of rotor-to-rotor interactions and ground effect on the performance of the CCR and its individual rotors. The CCR performance increased monotonically with decreasing rotor height with performance qualitatively similar for the two rotor spacing cases. At the same time, individual rotor performance variation with rotor height showed nonmonotonic behavior due to complementary or opposing effects of the rotor-to-rotor interactions to the ground effect. In general, the lower rotor in CCR experienced the most relative improvement in performance during IGE operations and was also observed to degrade the upper rotor performance.

The importance of round-robin validation when assessing machine-learning-based vertical extrapolation of wind speeds

Abstract. The extrapolation of wind speeds measured at a meteorological mast to wind turbine rotor heights is a key component in a bankable wind farm energy assessment and a significant source of uncertainty. Industry-standard methods for extrapolation include the power-law and logarithmic profiles. The emergence of machine-learning applications in wind energy has led to several studies demonstrating substantial improvements in vertical extrapolation accuracy in machine-learning methods over these conventional power-law and logarithmic profile methods. In all cases, these studies assess relative model performance at a measurement site where, critically, the machine-learning algorithm requires knowledge of the rotor-height wind speeds in order to train the model. This prior knowledge provides fundamental advantages to the site-specific machine-learning model over the power-law and log profiles, which, by contrast, are not highly tuned to rotor-height measurements but rather can generalize to any site. Furthermore, there is no practical benefit in applying a machine-learning model at a site where winds at the heights relevant for wind energy production are known; rather, its performance at nearby locations (i.e., across a wind farm site) without rotor-height measurements is of most practical interest. To more fairly and practically compare machine-learning-based extrapolation to standard approaches, we implemented a round-robin extrapolation model comparison, in which a random-forest machine-learning model is trained and evaluated at different sites and then compared against the power-law and logarithmic profiles. We consider 20 months of lidar and sonic anemometer data collected at four sites between 50 and 100 km apart in the central United States. We find that the random forest outperforms the standard extrapolation approaches, especially when incorporating surface measurements as inputs to include the influence of atmospheric stability. When compared at a single site (the traditional comparison approach), the machine-learning improvement in mean absolute error was 28 % and 23 % over the power-law and logarithmic profiles, respectively. Using the round-robin approach proposed here, this improvement drops to 20 % and 14 %, respectively. These latter values better represent practical model performance, and we conclude that round-robin validation should be the standard for machine-learning-based wind speed extrapolation methods.

Long-Term Observations and High-Resolution Modeling of Midlatitude Nocturnal Boundary Layer Processes Connected to Low-Level Jets

AbstractLow-level-jet (LLJ) periods are investigated by exploiting a long-term record of ground-based remote sensing Doppler wind lidar measurements supported by tower observations and surface flux measurements at the Jülich Observatory for Cloud Evolution (JOYCE), a midlatitude site in western Germany. LLJs were found 13% of the time during continuous observations over more than 4 yr. The climatological behavior of the LLJs shows a prevailing nighttime appearance of the jets, with a median height of 375 m and a median wind speed of 8.8 m s−1 at the jet nose. Significant turbulence below the jet nose only occurs for high bulk wind shear, which is an important parameter for describing the turbulent characteristics of the jets. The numerous LLJs (16% of all jets) in the range of wind-turbine rotor heights below 200 m demonstrate the importance of LLJs and the associated intermittent turbulence for wind-energy applications. Also, a decrease in surface fluxes and an accumulation of carbon dioxide are observed if LLJs are present. A comprehensive analysis of an LLJ case shows the influence of the surrounding topography, dominated by an open pit mine and a 200-m-high hill, on the wind observed at JOYCE. High-resolution large-eddy simulations that complement the observations show that the spatial distribution of the wind field exhibits variations connected with the orographic flow depending on the wind direction, causing high variability in the long-term measurements of the vertical velocity.

Properties of the offshore low level jet and rotor layer wind shear as measured by scanning Doppler Lidar

AbstractThe atmospheric flow phenomenon known as the Low Level Jet (LLJ) is an important source of wind power production in the Great Plains. However, due to the lack of measurements with the precision and vertical resolution needed, particularly at rotor heights, it is not well‐characterized or understood in offshore regions being considered for wind‐farm development.The present paper describes the properties of LLJs and wind shear through the rotor layer of a hypothetical wind turbine, as measured from a ship‐borne Doppler lidar in the Gulf of Maine in July–August 2004.LLJs, frequently observed below 600 m, were mostly during nighttime and transitional periods, but they were also were seen during some daytime hours. The presence of a LLJ significantly modified wind profiles producing vertical wind speed shear. When the wind shear was strong, the estimates of wind power based upon wind speeds measured at hub‐height could have significant errors. Additionally, the inference of hub‐height winds from near‐surface measurements may introduce further error in the wind power estimate. The lidar dataset was used to investigate the uncertainty of the simplified power‐law relation that is often employed in engineering approaches for the extrapolation of surface winds to higher elevations. The results show diurnal and spatial variations of the shear exponent empirically found from surface and hub‐height measurements.Finally, the discrepancies between wind power estimates using lidar‐measured hub‐height winds and rotor equivalent winds are discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Bird movements at rotor heights measured continuously with vertical radar at a Dutch offshore wind farm

Assessing the impacts of avian collisions with wind turbines requires reliable estimates of avian flight intensities and altitudes, to enable accurate estimation of collision rates, avoidance rates and related effects on populations. At sea, obtaining such estimates visually is limited not only by weather conditions but, more importantly, because a high proportion of birds fly at night and at heights above the range of visual observation. We used vertical radar with automated bird‐tracking software to overcome these limitations and obtain data on the magnitude, timing and altitude of local bird movements and seasonal migration measured continuously at a Dutch offshore wind farm. An estimated 1.6 million radar echoes representing individual birds or flocks were recorded crossing the wind farm annually at altitudes between 25 and 115 m (the rotor‐swept zone). The majority of these fluxes consisted of gull species during the day and migrating passerines at night. We demonstrate daily, monthly and seasonal patterns in fluxes at rotor heights and the influence of wind direction on flight intensity. These data are among the first to show the magnitude and variation of low‐altitude flight activity across the North Sea, and are valuable for assessing the consequences of developments such as offshore wind farms for birds.

Detailed Computational Investigation of a Hovering Microscale Rotor in Ground Effect

A compressible, structured, overset Reynolds-averaged Navier–Stokes-based solver is used to simulate a microhovering rotor in ground effect to demonstrate the capability of the code to provide accurate tip-vortex-flowfield predictions, and to provide a good understanding of the ground–wake interactions in conditions prevalent in helicopter brownout. The performance validation at different rotor heights above the ground shows good correlation with the experimental thrust and power measurements. A detailed comparison of the predicted tip-vortex flowfield shows good agreement with the vorticity contours and radial-velocity profiles obtained from the particle-image-velocimetry measurements during experiments. The examination of the in ground effect tip-vortex flowfield suggests high degree of instabilities close to the ground. The induced velocities arising from the proximity of the rotor tip vortices cause flow separation at the ground. An analysis of the eddy-viscosity contours near the ground indicates higher turbulence levels in the flowfield at smaller rotor heights above the ground. With a decreasing rotor height, the increased interactions between the tip vortices and the ground boundary layer result in the intermixing of eddy viscosity from the tip vortex to the ground boundary layer.

Aerodynamic and acoustic integral characteristics of porous rotors

This paper describes porous rotors manufactured from open cell aluminium foam. Rotor construction and theoretical description of fluid flow through rotating porous material are presented. Nine porous rotors made of materials with three different average pore sizes, with or without inducer, three rotor heights and two inlet diameters were selected and compared to a classical rotor with blades. Measurements involved two parts, measurement of pressure drop on non-rotating rotors while integral flow characteristics were measured on rotating rotors. Pressure drops for selected rotors agree well with theoretically calculated values based on Darcy-Forchheimer equation and Ergun equation. Measurements of integral aerodynamic and acoustic characteristics for selected rotors are presented. At low volume flow rates porous rotors have lower aerodynamic efficiency, comparable dimensionless pressure and lower noise compared to classical rotors with blades.

A numerical scheme for solving a periodically forced Reynolds equation

A modified Reynolds equation is used to model the air-film in a high-speed squeeze-film bearing. The axial position of the bearing stator is prescribed as a finite amplitude periodic oscillation. A numerical approach is considered for solving the uncoupled and coupled periodic problems associated with this model. The uncoupled problem requires the computation of the squeeze-film dynamics when the rotor is held at a fixed axial position and the coupled problem incorporates the additional air–rotor interaction since the rotor position is unknown and modelled as a spring-mass-damper system. The details of a Fourier spectral collocation scheme are provided for the reduction of the modified Reynolds equation to a system of non-linear, first-order ordinary differential equations in space. Using the Matlab boundary value problem solver bvp4c this system of equations is solved to give the periodic pressure distributions and rotor heights. The high degree of accuracy in the spectral collocation scheme is demonstrated through comparison with an appropriate analytical solution. Further analysis indicates that the direct periodic solver is at least 10 times faster than the equivalent Crank–Nicholson finite-difference scheme. For changing values of a selected physical parameter the method of arc-length continuation is employed to track branches of solutions computed using the spectral collocation scheme. A selection of results is presented to demonstrate the range of accessible solutions and the robust nature of the numerical scheme. Copyright © 2010 John Wiley & Sons, Ltd.

Acoustic Emissions From Active Cooling Solutions for Portable Devices

Due to ever increasing consumer demand, today's marketplace is full of portable computing devices such as notebook computers, gaming machines, personal digital assistants, and smart phones. In each of these technologies, processing power and functionality levels are continuing to increase, resulting in increased thermal management issues. The former of these technologies already use an active cooling solution with a profile of 10-15 mm while the latter two are quickly approaching the limits of passive cooling. Since a common characteristic in each of these technologies is the desire to reduce profile height, there exists a need to develop low profile cooling solutions that are capable of supporting this growing market. However, a prerequisite to developing such solutions is an accurate understanding of how the noise level characteristics of the driving fans scale with height. This paper experimentally addresses the acoustic scaling of low profile fans. The rotor heights range from 0.5 mm to 15 mm with diameters from 15 mm to 32 mm, giving an aspect ratio of height relative to diameter of 0.03-0.6. The results can be used as a foundation for the development of active cooling in mobile devices where both correlations for noise levels and basic design criteria are identified.