Wind Yaw Research Articles

Parametric dependencies of the yawed wind‐turbine wake development

AbstractYaw misalignment is currently being treated as one of the most promising methods for optimizing the power of wind farms. Therefore, detailed knowledge of the impact of yaw on the wake development is necessary for a range of operating conditions. This study numerically investigates the wake development behind a single yawed wind turbine operating at different tip‐speed ratios and yaw angles using the actuator‐line method in the spectral‐element code Nek5000. It is shown that depending on the tip‐speed ratio, the blade loading varies along the azimuth, resulting in a wake that is asymmetric in both the horizontal and vertical directions. Large tip‐speed ratios as well as large yaw angles are shown to decrease the vertical asymmetry of the yaw‐induced counter‐rotating vortex pair. Both parameters have the effect that they increase the spanwise force induced by yaw relative to the wake rotation. However, while the strength of the counter‐rotating vortex pair in the far wake increases with yaw angle, it is shown to decrease with the tip‐speed ratio. The vertical shift in the wake center is found to be highly dependent on the yaw angle and the tip‐speed ratio. These detailed insights into the yawed wake are important when optimizing potential downstream turbines.

A point vortex transportation model for yawed wind turbine wakes

In this study, stereo particle imaging velocimetry measurements are performed at multiple streamwise locations behind a yawed wind turbine to reveal the formation mechanisms of the counter-rotating vortex pair (CVP), and a point vortex transportation (PVT) model is proposed to reproduce the top–down asymmetric kidney-shaped wake (also referred to as a curled wake). Results indicate that the CVP formed behind a yawed wind turbine originates from the complex interactions between the hub vortex and the streamwise components of the blade tip vortices, which is fundamentally different from the case of a yawed drag disk where the hub vortex is absent. Specifically, when the yaw angle exceeds a critical value, a small part of the streamwise vorticity shed from the rotor disk edge switches its sign from negative to positive and subsequently merges with the concentrated hub vortex under mutual induction, creating a patch of positive vorticity; meanwhile, the remaining streamwise vorticity distributed along the rotor edge curls and evolves into another patch of negative vorticity. These two patches of streamwise vorticity essentially constitute the CVP. Based on the physics learnt from the experiments, the non-uniform cross-stream velocity fields are first reconstructed by a cloud of point vortices distributed along the rotor edge and a hub vortex located in the rotor centre, and subsequently used to numerically solve a simplified transportation–diffusion equation of the wake velocity deficit, which altogether constitute the PVT model. This physics-based reduced-order model is the first model capable of accurately reproducing the wake deformation behind a yawed wind turbine.

A momentum-conserving wake superposition method for wind farm power prediction

Analytical wind turbine wake models and wake superposition methods are prevailing tools widely adopted by the wind energy community to predict the power production of wind farms. However, none of the existing wake superposition methods conserve the streamwise momentum. In this study, a novel wake superposition method capable of conserving the total momentum deficit in the streamwise direction is derived theoretically, and its performance is validated with both particle imaging velocimetry measurements and large-eddy simulation results. Detailed inter-method comparisons show that the novel wake superposition method outperforms all the existing methods by delivering an accurate prediction of the power production and the centreline wake velocity deficit, with a typical error of less than 5 % (excluding the near-wake region). Additionally, the momentum-conserving wake superposition method is extended to combine the transverse velocities induced by yawed wind turbines, and the secondary wake steering effect crucial to the power optimization in active wake control is well reproduced.

Using computational fluid dynamics and field experiments to improve vehicle-based wind measurements for environmental monitoring

Abstract. Vehicle-based measurements of wind speed and direction are presently used for a range of applications, including gas plume detection. Many applications use mobile wind measurements without knowledge of the limitations and accuracy of the mobile measurement system. Our research objective for this field-simulation study was to understand how anemometer placement and the vehicle's external airflow field affect measurement accuracy of vehicle-mounted anemometers. Computational fluid dynamic (CFD) simulations were generated in ANSYS Fluent to model the external flow field of a research truck under varying vehicle speed and wind yaw angle. The CFD simulations provided a quantitative description of fluid flow surrounding the vehicle and demonstrated that the change in wind speed magnitude from the inlet increased as the wind yaw angle between the inlet and the vehicle's longitudinal axis increased. The CFD results were used to develop empirical speed correction factors at specified yaw angles and to derive an aerodynamics-based correction function calibrated for wind yaw angle and anemometer placement. For comparison with CFD, we designed field tests on a square, 12.8 km route in flat, treeless terrain with stationary sonic anemometers positioned at each corner. The route was driven in replicate under varying wind conditions and vehicle speeds. The vehicle-based anemometer measurements were corrected to remove the vehicle speed and course vector. From the field trials, we observed that vehicle-based wind speed measurements differed in average magnitude in each of the upwind, downwind, and crosswind directions. The difference from stationary anemometers increased as the yaw angle between the wind direction and the truck's longitudinal axis increased, confirming the vehicle's impact on the surrounding flow field and validating the trends in CFD. To further explore the accuracy of CFD, we applied the function derived from the simulations to the field data and again compared these with stationary measurements. From this study, we were able to make recommendations for anemometer placement, demonstrate the importance of applying aerodynamics-based correction factors to vehicle-based wind measurements, and identify ways to improve the empirical aerodynamic-based correction factors.

The curled wake model: equivalence of shed vorticity models

In this work, we explore the spanwise velocities in the wake of a yawed wind turbine. In the curled wake model, these motions are induced by a collection of vortices shed from the rotor plane. The direction of the vorticity generated by yaw is aligned with the main flow (streamwise) direction. The streamwise vorticity induces velocities in the spanwise directions. These are the motions responsible for creating the curled wake mechanism. In this work, we explore a more accurate formulation for this mechanism, using a vortex cylinder. Under certain assumptions, the new and original curled wake models yield the same mathematical formulation. Also, both models predict an elliptic distribution of vortex strength, where the main difference is the location of the vortices.

Large-Eddy Simulation of Yawed Wind-Turbine Wakes: Comparisons with Wind Tunnel Measurements and Analytical Wake Models

In this study, we validated a wind-turbine parameterisation for large-eddy simulation (LES) of yawed wind-turbine wakes. The presented parameterisation is modified from the rotational actuator disk model (ADMR), which takes account of both thrust and tangential forces induced by a wind turbine based on the blade-element theory. LES results using the yawed ADMR were validated with wind-tunnel measurements of the wakes behind a stand-alone miniature wind turbine model with different yaw angles. Comparisons were also made with the predictions of analytical wake models. In general, LES results using the yawed ADMR are in good agreement with both wind-tunnel measurements and analytical wake models regarding wake deflections and spanwise profiles of the mean velocity deficit and the turbulence intensity. Moreover, the power output of the yawed wind turbine is directly computed from the tangential forces resolved by the yawed ADMR, in contrast with the indirect power estimation used in the standard actuator disk model. We found significant improvement in the power prediction from LES using the yawed ADMR over the simulations using the standard actuator disk without rotation, suggesting a good potential of the yawed ADMR to be applied in LES studies of active yaw control in wind farms.

Numerical studies on aerodynamics of high-speed railway train subjected to strong crosswind

Under the action of strong crosswind, the aerodynamic behavior of a rail vehicle at high speed will be changed significantly, which could directly affect the safe operation of the vehicle. With the help of the shape of train used in China, the aerodynamic characteristics of trains with scale of 1:1 is investigated using computational fluid dynamics numerical simulation method, which consists of the variation of aerodynamics force and moment with wind yaw angle, wind speed, train speed, and nose shape. After an initial validation against Baker’s results from wind tunnel test, the numerical model is then used to investigate the aerodynamic characteristics of the trains. The numerical results indicate that lift coefficient of the M train is slightly higher than TMC1 and TMC2 trains. Regardless of aerodynamics force coefficients, TMC1 reaches the maximum at a yaw angle of 75°. Aerodynamics force coefficient increases with both wind speed and train speed, but the change of which is not linear. Comparing aerodynamic force with different geometric dimensions of train nose, it is shown that height–width ratio is insensitive to side force and rolling moment, but sensitive to lift force from the yaw angle 0°–90°. The side force coefficient, as we most concern, is less than other results, when the length–width ratio is 1 and height–width is 0.87.

A numerical simulation method in time domain to study wind-induced excitation of traffic signal structures and its mitigation

Traffic signal structures, commonly used to control traffic, have exhibited fatigue failure in their connections due to wind-induced vibration. This study focuses on developing a generalized method to predict the coupled across- and along-wind responses of a traffic signal light structure in normal or yawed wind by combining the aerodynamic properties of such a multi-component structure with a numerical simulation technique to generate wind loads and resulting responses in the time domain. The methodology and data presented here can be applied to any signal structure to explore its dynamic behavior, estimate its fatigue life and devise mitigation means such as dampers. The tip response of the mast arm and the maximum stress at its joint were calculated for a given signal light structure and validated with field measurements to demonstrate the methodology for a range of wind speeds and yaw angles. Results indicate that yawed wind below a speed of 9 m/s from the backside of the signal light and vortex shedding of the mast arm are the primary reasons of large-amplitude vibration, and the maximum stress exceeds the endurance limit that can cause fatigue failure over time. A modified signal light design to mitigate this problem is proposed.

Mitigation mechanism of longitudinal ribs on rain-wind induced vibrations of stay cables

For cable stayed bridges rain-wind induced vibrations of stay cables are probably one of the most widespread phenomenon. Aerodynamic countermeasures have been implemented to tackle such vibrations, but there is still not sufficient insight on the inherent mitigation mechanisms. To this goal, a numerical model, based on lubrication theory, was employed in order to study the coupled cable vibration response, aerodynamic forces, and formation and oscillation of rivulets for stay cables equipped with longitudinal ribs. Coupled equations governing the synchronous cable motion and water film evolution were established in order to understand the effects of several key parameters associated with the vibration mitigation performance of the ribs. Such parameters include the cable inclination angle, the wind yaw angle, the number and the height of the ribs. Computed results were successfully validated against experimental data. For the various studied cases, it was apparent that the ribs did not stop the formation of rivulets, but they could affect both their position and oscillation ranges. Through such a control action they could further affect the oscillation range and frequency content of aerodynamic forces, mitigating or not cable vibrations.

The effect of wake position and yaw misalignment on power loss in wind turbines

For a single wind turbine, the efficiency of extracting energy from the wind depends on the ability to align the wind turbine with the dominating wind direction. Considering average power production, yaw misalignment is relevant when the wind turbine operates with maximum power coefficient. On the other hand, the power production is less sensitive to yaw misalignment in high wind speeds, where the available energy in the wind field is higher than the maximum wind turbine capacity. In a wind farm, the interaction between nearby wind turbines alters the flow, and the power production is reduced. The present study investigates how yaw misalignment affects the power production in these wake situations compared to yaw misalignment effects for a wind turbine in the free-stream. Two generic cases are presented in this paper, offshore and forest, where the atmospheric conditions alter the morphology of the wake and, therefore, the power output of a yawed wind turbine operating in wake conditions. The results show that, for a conventional downstream spacing further than 3 rotor diameters, yaw misalignment results in larger power loss in wake situations than in free-stream. In wake situations, the presented results also show that the spatial distribution of the deficit influences the relative power loss when the wind turbine is operating in yawed conditions.

A novel wake model for yawed wind turbines

One of the current major challenges in wind energy is to maximize energy production of wind farms. One approach in this effort is through control of wind turbine wake interactions, since undesirable wake interactions can introduce additional mechanical stresses on turbines, leading to early failures and reduce overall energy production of wind farms. To develop control strategies that can minimize wake interactions, it is essential to simulate wake behaviors accurately and quickly. In this work, a fast and accurate turbine wake model capable of modeling turbine wakes under yaw is presented. This model builds upon the work of existing wake models and is capable of producing results comparable to that of conventional full CFD simulations using a fraction of the computational cost. The accuracy and speed of the proposed model allows for the development of real-time turbine control strategies to maximize power output. The results of the proposed model are validated with previous numerical and experimental data.

Wake model for horizontal-axis wind and hydrokinetic turbines in yawed conditions

Predicting the spatial evolution of horizontal-axis turbine wakes is a key factor to enhance the performance of wind and hydrokinetic power plants. The yaw angle misalignment is not only important to account for the uncertainty of the wind direction, but also a control strategy to improve the energy production averaged over the turbine array. In this paper, a new wake model, based on the skew-normal distribution and conservation of mass and momentum, is proposed to predict the turbine wake velocity distribution for given yaw angles. The new model is experimentally validated through wake measurements of yawed miniature wind and hydrokinetic turbines, in wind tunnel and open channel flows, respectively. Predictive capabilities extend from the spatial distribution of the maximum velocity deficit at hub height, to the spanwise asymmetry of the velocity distribution about the wake center.

Wind loads on transmission tower bodies under skew winds with both yaw and tilt angles

Transmission tower networks play an important role in the infrastructure system of many countries around the world. Many studies have been directed towards the evaluation of aerodynamic coefficients of the vertical tower body sections of transmission towers. Most of these studies obtain drag coefficients of tower sections from wind tunnel tests with yawed wind. However, strong winds like hurricane, downburst, tornado etc., which are the leading causes of transmission tower failures, often attack the towers with both yaw and tilt angles. Transmission towers constructed on the top or slope of mountains may also suffer the action of skew winds with both yaw and tilt angles. This paper addresses wind loads of transmission towers under winds with both yaw and tilt angles. By performing wind tunnel tests with multi-balance synchronous force measurement, the wind loads acting on a lattice tubular steel transmission tower under skew winds were measured, and the drag coefficients under 19 yaw angles and 13 tilt angles were obtained. Besides providing a more accurate modified formula for the skewed wind load factor, concepts of tilted and combined wind load factors are proposed with recommended formulas. By comparing the experimental and standard-calculated values of skewed wind load factor under a tilt angle of 0°, it is found that the standards underestimate the skewed wind load factor by at least about 10%, whereas the error of the proposed formula is less than 4% under the design key angles. In this study, the values of tilted wind factor under a yaw angle of 0° and combined wind load factor are recommended as 0.3 and 0.2, respectively. When the recommended values are used in calculation, the deviations between calculated drag coefficients and wind tunnel test results are less than 5%.

Performance of buoyant shell horizontal axis wind turbine under fluctuating yaw angles

The paper presents the effect of fluctuating yaw angle and wind speed on the performance of a horizontal axis airborne wind turbine of three different shell shapes for the high altitude operational conditions. For this purpose, a numerical analysis has been performed by considering the ranges of yaw angle and wind speed of 0°−20° and 10m/s−25m/s, respectively. The turbine shell shapes are based on the three aerofoil profiles of NACA-5415, NACA-9415 and NACA-5425 to investigates the influence of the aerofoil camber and thickness. The turbine has been modelled using a NREL Phase IV rotor with the fixed rotor radius of 2m for all shell configurations. A 3-D numerical analysis has been conducted by implementing k−ω SST turbulence model to solve the Reynolds-averaged Navier-Stokes equations. Numerical results demonstrated that the increment in both the tip speed ratio and yaw angle augmented the rotor thrust coefficient. At the high yaw angles, the buoyant shell is prone to encounter instability due to unbalancing of the aerodynamic forces subjected to the shell body. Among all the studied shell configurations, the NACA-9415 aerofoil based shell displayed the relatively higher turbine power coefficient as well as the stable operation.

Blind test comparison on the wake behind a yawed wind turbine

Abstract. This article summarizes the results of the “Blind test 5” workshop, which was held in Visby, Sweden, in May 2017. This study compares the numerical predictions of the wake flow behind a model wind turbine operated in yaw to experimental wind tunnel results. Prior to the workshop, research groups were invited to predict the turbine performance and wake flow properties using computational fluid dynamics (CFD) methods. For this purpose, the power, thrust, and yaw moments for a 30∘ yawed model turbine, as well as the wake's mean and turbulent streamwise and vertical flow components, were measured in the wind tunnel at the Norwegian University of Science and Technology (NTNU). In order to increase the complexity, a non-yawed downstream turbine was added in a second test case, while a third test case challenged the modelers with a new rotor and turbine geometry. Four participants submitted predictions using different flow solvers, three of which were based on large eddy simulations (LES) while another one used an improved delayed detached eddy simulation (IDDES) model. The performance of a single yawed turbine was fairly well predicted by all simulations, both in the first and third test cases. The scatter in the downstream turbine performance predictions in the second test case, however, was found to be significantly larger. The complex asymmetric shape of the mean streamwise and vertical velocities was generally well predicted by all the simulations for all test cases. The largest improvement with respect to previous blind tests is the good prediction of the levels of TKE in the wake, even for the complex case of yaw misalignment. These very promising results confirm the mature development stage of LES/DES simulations for wind turbine wake modeling, while competitive advantages might be obtained by faster computational methods.

Validation of the real-time-response ProCap measurement system for full field wake scans behind a yawed model-scale wind turbine

For an accurate prediction of the complex flow conditions in wind farms, model scale wake flow measurements represent important references in well-defined boundary conditions. State-of-the art flow measurement techniques are often time-expensive and require elaborate post-processing to assess the data quality. In this wind tunnel study we demonstrate the advantages of the real-time-response system Probe Capture (ProCap) for measurements of a complex three-dimensional wind turbine wake flow. The complex wake flow behind a yawed model wind turbine is measured with both a Laser-Doppler Anemometer (LDA) and the ProCap system. Both the streamwise and vertical flow component show an accurate agreement with the LDA reference experiment for various measured downstream distances and turbine yaw angles. Areas of strong rotation in the wake flow are accurately resolved by the ProCap measurement system confirming its applicability for wind turbine wake measurement. The system appears to greatly facilitate and speed up larger wake surveys by a factor of 30 and thus has the potential to enhance the understanding of the complex flow topology in wind turbine wakes.

Wind tunnel experiments on wind turbine wakes in yaw: effects of inflow turbulence and shear

Abstract. The wake characteristics behind a yawed model wind turbine exposed to different customized inflow conditions are investigated. Laser Doppler anemometry is used to measure the wake flow in two planes at x∕D = 3 and x∕D = 6, while the turbine yaw angle is varied from γ=-30∘ to 0∘ to +30∘. The objective is to assess the influence of grid-generated inflow turbulence and shear on the mean and turbulent flow components. The wake flow is observed to be asymmetric with respect to negative and positive yaw angles. A counter-rotating vortex pair is detected creating a kidney-shaped velocity deficit for all inflow conditions. Exposing the rotor to non-uniform highly turbulent shear inflow changes the mean and turbulent wake characteristics only insignificantly. At low inflow turbulence the curled wake shape and wake center deflection are more pronounced than at high inflow turbulence. For a yawed turbine the rotor-generated turbulence profiles peak in regions of strong mean velocity gradients, while the levels of peak turbulence decrease at approximately the same rate as the rotor thrust.

Influence of ground roughness on the wake of a yawed wind turbine - a comparison of wind-tunnel measurements and model predictions

Wind-tunnel measurements in the wake of a three bladed horizontal axis wind turbine model (HAWT) operating in yawed conditions are presented. Measurements are made at a constant tip speed ratio within two neutrally-stratified turbulent boundary layers of different aerodynamic roughness length to investigate its influence on the wake trajectory. It is found that the wake is less deflected with increasing terrain roughness. The recorded flow field is then used to assess the predictive capability of two analytical wake models.

Inflow-based flap control on a 10MW-scale wind turbine using a spinner anemometer

In the paper, an individual flap control (IFC) algorithm based on wind speed measurements obtained with a spinner anemometer is presented. The controller uses flap actuators with the aim to remove any deterministic source of load variation on blades, associated with asymmetries of the inflow. Such load variations are concentrated on multiples of the rotational frequency (p multiples) and they are mainly due to wind yaw, inclination and atmospheric boundary layer (ABL) shear. The aim of the controller is to assist operation of the conventional feed-back individual pitch controller (IPC) and thereby reduce its control duty cycle. The performance of the proposed controller is assessed through aero-elastic simulations for the 10MW DTU Reference Wind Turbine. A subset of normal operation fatigue and ultimate Design Load Cases (DLCs) of the IEC has been simulated and load reduction capabilities of the control algorithm have been compared against those of the standard individual pitch control (IPC) loop.

Effects of central stabilizing barriers on flutter performances of a suspension bridge with a truss-stiffened deck under skew winds

To improve the flutter performance of a suspension bridge with a 1088-m-span truss-stiffened deck, the aerodynamic measures of upper and lower central stabilizing barriers were investigated at first via wind tunnel tests of sectional model under the normal wind condition. The yaw wind effect on the flutter performance of the bridge with the above aerodynamic measures was then examined via a series of wind tunnel tests of oblique sectional models. The test results show that the effect of the lower central stabilizing barrier on the flutter critical wind speed is remarkably different from that of the upper central stabilizing barrier for both the normal and skew wind cases. The inclination angle +3° is the most unfavorable inclination angle to the flutter performance of the truss-stiffened suspension bridge no matter whether the aerodynamic control measures are adopted or not. Furthermore, for most cases, the lowest flutter critical wind speed occurs when the incident wind deviates from the normal direction of the bridge span by a small yaw angle between 5° and 10°.