Wind Yaw Research Articles

Modelling Yawed Wind Turbine Wakes: Extension of a Gaussian-Based Wake Model

Yaw-based wake steering control is a potential way to improve wind plant overall performance. For its engineering application, it is crucial to accurately predict the turbine wakes under various yawed conditions within a short time. In this work, a two-dimensional analytical model is proposed for far wake modeling under yawed conditions by taking the self-similarity assumption for the streamwise velocity deficit and skewness angle at hub height. The proposed model can be applied to predict the wake center trajectory, streamwise velocity, and transverse velocity in the far-wake region downstream of a yawed turbine. For validation purposes, predictions by the newly proposed model are compared to wind tunnel measurements and large-eddy simulation data. The results show that the proposed model has significantly high accuracy and outperforms other common wake models. More importantly, the equations of the new proposed model are simple, the wake growth rate is the only parameter to be specified, which makes the model easy to be used in practice.

Active flap control with the trailing edge flap hinge moment as a sensor: using it to estimate local blade inflow conditions and to reduce extreme blade loads and deflections

Abstract. Active trailing edge flaps are a promising technology that can potentially enable further increases in wind turbine sizes without the disproportionate increase in loads, thus reducing the cost of wind energy even further. Extreme loads and critical deflections of the blade are design-driving issues that can effectively be reduced by flaps. In this paper, we consider the flap hinge moment as a local input sensor for a simple flap controller that reduces extreme loads and critical deflections of the DTU 10 MW Reference Wind Turbine blade. We present a model to calculate the unsteady flap hinge moment that can be used in aeroelastic simulations in the time domain. This model is used to develop an observer that estimates the local angle of attack and relative wind velocity of a blade section based on local sensor information including the flap hinge moment of the blade section. For steady wind conditions that include yawed inflow and wind shear, the observer is able to estimate the local inflow conditions with errors in the mean angle of attack below 0.2∘ and mean relative wind speed errors below 0.4 %. For fully turbulent wind conditions, the observer is able to estimate the low-frequency content of the local angle of attack and relative velocity even when it is lacking information on the incoming turbulent wind. We include this observer as part of a simple flap controller to reduce extreme loads and critical deflections of the blade. The flap controller's performance is tested in load simulations of the reference turbine with active flaps according to the IEC 61400-1 power production with extreme turbulence group. We used the lifting line free vortex wake method to calculate the aerodynamic loads. Results show a reduction of the maximum out-of-plane and resulting blade root bending moments of 8 % and 7.6 %, respectively, when compared to a baseline case without flaps. The critical blade tip deflection is reduced by 7.1 %. Furthermore, a sector load analysis considering extreme loading in all load directions shows a reduction of the extreme resulting bending moment in an angular region covering 30∘ around the positive out-of-plane blade root bending moment. Further analysis reveals that a fast reaction time of the flap system proves to be critical for its performance. This is achieved with the use of local sensors as input for the flap controller. A larger reduction potential of the system is identified but not reached mainly because of a combination of challenging controller objectives and the simple controller architecture.

A level-set framework for the wind turbine wake analysis: from high-fidelity unsteady simulations to 1D momentum theory

In the context of yawed wind turbine wakes, the validity of the so-called 1D momentum theory is arguable. The 1D momentum theory emerges from an inviscid, steady and irrotational analysis of the streamtube surrounding a wind turbine while the wake aerodynamics under yaw is modified. In this work, the mean flow streamtube around a single DTU 10MW wind turbine, under yaw misalignement and turbulent inflow is investigated with Large-Eddy Simulation combined to the Actuator Line method. The simulations are performed on a highly resolved grid counting billions of elements. The streamtube construction from the mean flow is based on the transport of accurate conservative level set functions. Integration of local flow quantities within the streamtube presents similarities when the streamtube expansion occurs past the turbine until the velocity deficit starts to recover. Mean kinetic energy and momentum budget are then presented to explain the yawed wake recovery and deflection process. Background turbulence plays a key role in the recovery process while the deflection of the wake is impacted by pressure forces on the streamtube. From these budgets, four wake regions showing similar flow dynamics are defined and correlated to local flow structures.

Experimental study on the effect of canyon cross wind yaw angle on airflow and flame characteristics in a tunnel

The effect of canyon cross wind yaw angle on airflow and flame characteristics of ethanol pool fires in a tunnel was investigated. The fire source location and canyon cross wind speed were studied as factors. The wind yaw angle was ranged in 45°–135° with an interval of 15°. The results showed that the flow field in the tunnel presented three different conditions with the boundaries of wind yaw angles 60° and 90°. The mass burning rate and flame tilt characteristics were affected by the flow field near the fire source. The mass burning rate decreased with the increasing wind yaw angle when the wind yaw angle was in 45°–90°, but remained steady when the angle was in 90°–135°. Additionally, a normalized mass burning rate model considering normalized distance to the upstream portal and the wind yaw angle was developed. The flame close to the upstream portal wiggled, rotated and oscillated irregularly due to the influence of turbulence induced by the external canyon cross wind. This made mean flame length and flame tilt angle in turbulence zone different from those in other zones. A prediction correlation between the flame tilt angle and the canyon cross wind speed and yaw angle was established.

A Wind Tunnel Study of the Windsor Body with a Streamlined Tail

<div class="section abstract"><div class="htmlview paragraph">The effects of adding a streamlined tail to a simple vehicle shape, represented by the Windsor Body has been investigated in a small scale wind tunnel experiment. The extended tail has a constant width, with a flat lower surface and a constant upper surface taper angle. The tail is truncated in steps to understand the trends in the principal aerodynamic characteristics. The slant surface and the base have been pressure tapped to indicate the contribution to drag and lift from these surfaces. The bodies have been tested over a range of yaw angles and wind tunnel airspeeds. The effects of adding wheels, albeit in a fixed ground experiment, has also been studied. The experimental data for the basic wheel-less body in a squareback configuration and with tapered tails of different length at zero yaw has been compared with an earlier CFD simulation of the same configurations. While it is recognized that the longer models have impractical shapes, the systematic development provides a useful insight into the development of low drag bodies.</div></div>

Influence of wind yaw angle and pylon interference on the buffeting responses of cable-stayed bridges during construction

The present study focuses on the effects of wind yaw angle and pylon interference on the buffeting responses of bridge decks during construction. Two typical cable-stayed bridges with rhombus pylons (RPs) and single-column pylons (SPs) in the maximum double cantilever state were selected as research objectives. The influence of the wind yaw angle and pylon interference on the lateral, vertical and torsional buffeting displacements were systematically analyzed through full bridge model aeroelastic wind tunnel tests. The research results show that the maximum buffeting responses appear when the wind yaw angle is in the range of to The effects of the wind yaw angle on the buffeting responses become more obvious in cases of high mean wind speeds. The pylon interference effects on the lateral buffeting displacements are more significant than those on the vertical and torsional buffeting displacements. The strong pylon interference effects may lead to non-conservative buffeting response estimation results using traditional buffeting analysis methods. This study demonstrates that it is of paramount importance to take into consideration the combined effects of the wind yaw angle and pylon interference in the design of long-span cable-stayed bridges during their construction stages.

Wind tunnel and numerical study of a floating offshore wind turbine based on the cyclic pitch control

Offshore wind energy is one of the most important sources of renewable energy to make sure supplying enough electricity for the world in the future. The floating offshore wind turbine (FOWT) operates under many influences such as both the aerodynamic force, the hydrodynamic force and coupled motion between the two. Therefore, in this paper about the effect of collective pitch control and cyclic pitch control on power and loads of FOWT are discussed under the uniform wind velocity of 10 m/s with wind tunnel and numerical investigation. FAST tool (Fatigue, Aerodynamics, Structures, and Turbulence) is used to simulate FOWT under the same wind conditions as wind tunnel experiments. A two-bladed downwind turbine model is used for this experiment to estimate the fluctuations of the power coefficient, thrust coefficient and moments. The pitch moment and the yaw moment are measured at the top of the tower and the fluctuation of loads is compared. The phase angle of the pitch angle is controlled by three actuators through a swash plate equipment. From the simulated results, the effect of the pitch angle change on the power coefficient is higher than the yaw angle change. For the yawed wind condition, the load of the wind turbine at the phase angle of ξ = 60° was equal to the optimum operating point. Thence, the load of the wind turbine was decreased when the cyclic pitch control method was applied.

Stability behavior of the transmission line system under incremental dynamic wind load

Wind load is the principal cause for a large number of the collapse of transmission lines around the world. The transmission line is traditionally designed for wind load according to a linear equivalent method, in which dynamic effects of wind are not appropriately included. Therefore, in the present study, incremental dynamic analysis is utilized to investigate the stability behavior of a 400 kV transmission line under wind load. In that case, the effects of vibration of cables and aerodynamic damping of cables were considered on the stability behavior of the transmission line. Superposition of the harmonic waves method was used to calculate the wind load. The corresponding wind speed to the beginning of the transmission line collapse was determined by incremental dynamic analysis. Also, the effect of the yawed wind was studied to determine the critical attack angle by the incremental dynamic method. The results show the collapse mechanisms of the transmission line and the maximum supportable wind speed, which is predicted 6m/s less than the design wind speed of the studied transmission line. Based on the numerical modeling results, a retrofitting method has been proposed to prevent failure of the tower members under design wind speed.

XE112-2000 Wind Turbine Yaw Strategy With Adaptive Yaw Speed Using DEL Look-Up Table

With the increasing capacity of wind turbines, the importance of the yaw system has gradually become more prominent. The supervisory control and data acquisition (SCADA) system of XEMC Windpower Co., Ltd. is used in this paper to analyze the yaw data of the No. 5 wind turbine of Baozhong Mountain Wind Farm in Hunan Province. In order to evaluate the power generation and the damage equivalent load (DEL) of the yaw bearing during the yaw process, an economic model predictive control (EMPC) yaw strategy based on light detection and ranging (LIDAR) was proposed. EMPC takes the yaw error and wind speed as the disturbance of the cost function (CF) at the same time by establishing the yaw bearing DEL look-up table in advance. Discrete adaptive yaw speed control set is proposed to adapt to different wind conditions sets. Finally, the effectiveness of EMPC is verified using the model of XE112-2000 wind turbine in simulation software Bladed.

A field study on the aerodynamics of freight trains

A novel full-scale field test was undertaken to assess the aerodynamic performance of shipping containers loaded on inter-modal freight trains. The aerodynamic performance of an instrumented 48 ​ft container, located 185 ​m downstream of the locomotive, is assessed in the context of surface pressure, weather station, and GPS data sets. Previous studies on the aerodynamics of trains have been largely limited to low-resolution, reduced-order and scaled; field, numerical and wind-tunnel studies; respectively. The objective here was to determine the capacity of this novel field-based method to assess the aerodynamic performance of full-scale train containers for a large range of operating conditions. For low wind conditions, where the yaw angle is predicted to be low, measured surface pressure distributions on the front and base of the container are similar to that of past work however the magnitude of the drag coefficient was much lower, by up to 65%. This suggests that previous studies are yet to fully detail the drag profile of containers located at large downstream distance. Observed asymmetry of the pressure distribution on the front of the container are generally consistent with wind conditions measured at nearby weather stations and can be used as a proxy to determine the wind yaw angle at the train.

Experimental study on high-mode vortex-induced vibration of stay cable and its aerodynamic countermeasures

In recent years, high-mode vortex-induced vibration (VIV) of stay cables has been observed in some cable-stayed bridges. It can cause undue stress and fatigue in the anchorages at the deck and/or pylons and in the cables themselves. In this study, experimental investigations were performed to characterise and control the high-mode VIV of stay cables by using horizontal- and inclined-rigid-stay-cable models. First, wind tunnel tests using the horizontal-rigid-dimpled-stay-cable model were performed to investigate the VIV characteristics of dimpled stay cables and effective VIV suppression measures. Wind tunnel tests were then conducted on an inclined rigid dimpled cylinder with and without helical fillets to investigate further the VIV characteristics of dimpled stay cables and effective suppression measures. Finally, stay-cable field measurements were made for comparison with the experimental results of the stay-cable model. The results show that stay-cable VIV can occur in cases of low damping, and the VIV of the stay cable can be effectively suppressed by increasing the damping ratio of the stay cable. Moreover, helical fillets with a diameter of d = 2.0 or 3.5 mm, which are usually used to control rain–wind induced vibration of stay cables in cable-stayed bridges, cannot effectively suppress the high-mode VIV responses of stay cables. The inclined-dimpled-stay-cable model with a damping ratio of 0.2 % at inclination angles of 25° and 40° exhibits significant VIV for the cases of wind yaw angles of 0°, 15°, -15°, and -30°. However, helical fillets with d = 10 mm (d = 0.07D, where D is the diameter of the stay cable) and pitch of 12D can effectively suppress the VIV responses of the stay cables.

Integrated wind farm layout and control optimization

Abstract. The objective of this paper is to investigate the joint optimization of wind farm layout and wind farm control in terms of power production. A successful fulfilment of this goal requires the following: (1) an accurate and fast flow model, (2) selection of the minimum set of design parameters that rules or governs the problem, and (3) selection of an optimization algorithm with good scaling properties. For control of the individual wind farm turbines with the aim of wind farm production optimization, the two most obvious strategies are wake steering based on active wind turbine yaw control and wind turbine derating. The present investigation is limited to wind turbine derating. A high-speed linearized computational fluid dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) solver models the flow field and the crucial wind turbine wake interactions inside the wind farm. The actuator disc method is used to model the wind turbines, and utilizing an aerodynamic model, the design space of the optimization problem is reduced to only three variables per turbine – two geometric and one carefully selected variable specifying the individual wind turbine derating setting for each mean wind speed and direction. The full design space is spanned by these (2N+NdNsN) parameters, where N is the number of wind farm turbines, Nd is the number of direction bins, and Ns is the number of mean wind speed bins. This design space is decomposed into two subsets, which in turn define a nested set of optimization problems to achieve a significantly faster optimization procedure compared to a direct optimization based on the full design space. Following a simplistic sanity check of the platform functionality regarding wind farm layout and control optimization, the capability of the developed optimization platform is demonstrated on a Swedish offshore wind farm. For this particular wind farm, the analysis demonstrates that the expected annual energy production can be increased by 4 % by integrating the wind farm control into the design of the wind farm layout, which is 1.2 % higher than what is achieved by optimizing the layout only.

Lidar measurements of yawed-wind-turbine wakes: characterization and validation of analytical models

Abstract. Wake measurements of a scanning Doppler lidar mounted on the nacelle of a full-scale wind turbine during a wake-steering experiment were used for the characterization of the wake flow, the evaluation of the wake-steering set-up, and the validation of analytical wake models. Inflow-scanning Doppler lidars, a meteorological mast, and the supervisory control and data acquisition (SCADA) system of the wind turbine complemented the set-up. Results from the wake-scanning Doppler lidar showed an increase in the wake deflection with the yaw angle and that the wake deflection was not in all cases beneficial for the power output of a downstream turbine due to a bias of the inflow wind direction perceived by the yawed wind turbine and the wake-steering design implemented. Both observations could be reproduced with an analytical model that was initialized with the inflow measurements. Error propagation from the inflow measurements that were used as model input and the power coefficient of a waked wind turbine contributed significantly to the model uncertainty. Lastly, the span-wise cross section of the wake was strongly affected by wind veer, masking the effects of the yawed wind turbine on the wake cross sections.

Generation and decay of counter-rotating vortices downstream of yawed wind turbines in the atmospheric boundary layer

Abstract

An adaptation of the super-Gaussian wake model for yawed wind turbines

The wake of a yawed wind turbine tends to be deflected and does not follow the incoming wind direction. This phenomenon, often referred to as “wake deflection”, has recently received a lot of attention from the wind energy research community: it is possible to dynamically optimize a wind farm power production by yawing some turbines to deflect their wakes, thus minimizing wake losses over the whole wind farm. In the present work, an analytical model for estimating the deflection of a yawed wind turbine wake is proposed. This new model is based on a super-Gaussian shape assumption for the velocity profile, enabling the estimation of the wake deflection in both near and far wake regions. First, the super-Gaussian model will be briefly introduced. Then, the new wake deflection model will be presented, as well as an updated version of the well known Jimenez model. This revised model allows a proper comparison with the proposed super-Gaussian model, since it is based on a similar definition of the wake width. Finally, results will be compared with experimental data from the literature.

Experimental load measurement on a yawed wind turbine and comparison to FAST

Current interest in unsteady aerodynamics of yawed turbines for wake redirection and cyclic load alleviation requires a clear understanding of turbine load fluctuation with azimuth position. To that end, an experimental 3.5 m diameter wind turbine rig was designed to measure rotor torque, flapwise/edgewise blade root bending moment, and normal force coefficient at r/R=0.66 and 0.82. The turbine was tested inside a large scale wind generation facility with a blockage ratio of 7%. Simultaneously and in time-resolved fashion, measurements were collected to produce phase-averaged performance parameters that are presented versus azimuth while the turbine is operating at different tip speed ratios and yaw angles. The NREL FAST code predictions were then compared to experimental data under the same conditions. It was found that in non-yawed conditions, FAST predictions were accurate but discrepancies start to emerge when the turbine is yawed and operating in dynamic stall conditions. In non-yawed cases, the discrepancy between experimental and FAST data is less than 7% when the flow is attached and increases to 22% when the flow is stalled. One of the main sources of discrepancy found is in the process of correcting the airfoil coefficients to account for 3D flow effects using AirfoilPrep.

Actuator grid method for turbulence generation applied to yawed wind turbines

The aerodynamics of yawed wind turbine wakes remains a major investigation topic, especially for the use of yaw angle in control strategies. Large-Eddy Simulations are employed here to study the influence of yaw and inflow conditions on the prediction of the wind turbine wake structure. A single wind turbine setup with different yaw angles and three different inflow conditions is investigated and discussed with respect to experimental data. The wind turbine blades are modeled using the actuator line method (ALM) while tower and nacelle are represented with a body-fitted unstructured mesh. The high levels of upstream turbulence, experimentally generated by turbulence grids, are emulated here with oscillating ALM. Such approach demonstrates to be highly predictive on the turbulent flow characteristics compared to the emptied wind tunnel experimental data. Results with turbine show good agreement to the experiment data with only a slight overestimation on the magnitude of the wake deflection due to yaw. When compared to a deflection model, the confinement of the wind tunnel is highlighted. The radial and azimuthal time-averaged angle of attack exhibits a high probability of dynamic stall near the hub for a yawed turbine. These results show large discrepancies on the blade loading, highlighting the need for improved and specific models.

A comparative study of wake interactions between wind-aligned and yawed wind turbines using LES and actuator line models

An LES based OpenFOAM solver is coupled with the actuator line model for the simulation of wind farms and the prediction of power production. The tool developed is validated for wind aligned and yawed tandem wind turbines. Two NREL 5MW turbines in a tandem configuration are considered. The solution parameters are first studied in detail. The development of vortical wakes behind the upstream and the downstream turbines are simulated under uniform and atmospheric boundary layer inflow conditions, and the power productions of the turbines are predicted. The impact of a yawed upstream rotor on its wake deflection and the resulting power production of the tandem configuration are assessed. It is shown that the coupled solver developed is quite successful in capturing the wake deficits and wake deflections, and predicting the power production of tandem wind turbine configurations.

Buffeting Response of Cable-Stayed Bridge during Construction under Skew Winds and Pylon Interference

Buffeting is a kind of wind-induced vibration phenomenon of long-span cable-stayed bridges which is easy to occur. Balanced cantilever method is widely employed for the construction of cable-stayed bridge. The maximum double-cantilever state is the most dangerous state of wind resistance. Through the aeroelastic model wind tunnel experiment, the buffeting response of a cable-stayed bridge in construction under skew winds is studied in this paper. The variation law of buffeting response with the wind yaw angle is compared between the two cases with and without pylon. The experimental results show that for the same wind yaw angle the lateral and vertical buffeting responses of the main girder increase with the wind speed approximately in a quadratic curve, while the torsional buffeting response tends to increase linearly. At the same wind speed, the buffeting response of the main girder presents non-monotonic changes with increased wind yaw angle. Under skew winds, due to the disturbing effect of the pylon and the influence of the three-dimensional flow field, the buffeting response of the main girder will be affected by the aerodynamic interference of pylon. When the wind yaw angle is small, the interference effect is not obvious, with the increase of wind yaw angle and wind speed, the interference effect begins to appear. Generally speaking, the aerodynamic interference of the bridge pylon on the buffeting responses can be ignored.

Mitigating Large Vibrations of Stayed Cables in Wind and Rain Hazards

This paper presents an experimental investigation of stayed cable vibrations in dry-wind and rain-wind coupling hazards. To mitigate large vibrations of the cable, the use of spiral wires wrapped around the cable is proposed. By testing two cable models in a wind tunnel in dry and rain conditions for different yaw angles and wind speeds, the effectiveness of using the spiral wires to mitigate large vibrations is clarified. Finally, the paper provides a further understanding of the complex mechanism of wind-induced and rain-wind-induced vibrations. It is found that the low-frequency vortex flows in the wake play a significant role in the excitation of large responses of the cable in high wind speeds. The spiral wires dismiss these low-frequency flows and then reduce the large vibrations.