1. Introduction
Taiwan, benefitted by its unique geographical location, has rich wind resources particularly in the region of the Taiwan Strait. This geographical advantage clearly favors large-scale wind farm development, but the frequent incidence of typhoons in Taiwan is a real threat to wind farm safety, having already caused significant damage to installed wind turbines in recent years. For example, Typhoon Soudelor in 2015 collapsed six wind turbines and seriously damaged the blades of a seventh wind turbine in the Taichung wind farm. The maximum wind speed at a nearby meteorological mast was reported as 62.6 m/s by Liu and Chen [
1]. This event indicates that a wind turbine may not intactly survive the extreme winds of a typhoon if this scenario is not well-defined. Although both steady and turbulent wind conditions are considered in the Extreme Wind Speed Model (EWM) of the International Electrotechnical Commission (IEC) 61400-1 [
2] standard, the corresponding wind characteristics and induced aerodynamic loads on wind turbines under such extreme conditions still require further investigations and inspired this study to address the dynamic effect of gusts during extreme typhoons.
In order to define the proper design requirements for wind turbines operating in typhoon-prone areas, several studies have, in recent decades, investigated the characteristic wind conditions of typhoons, such as wind profile, turbulence intensity, and gust factor, based on measured data. Ishizaki [
3] proposed relationships between turbulence intensity, mean wind speed, ground height, and gust factor from a statistical analysis of typhoon measurements. Cao et al. [
4] analyzed the wind conditions of Typhoon Maemi through the wind speed samples measured by nine vane-type and seven sonic-type anemometers at a height of 15 m. They found that the turbulence intensity decreases with increasing wind speed and remains almost constant at high wind speeds, with a reported gust factor of 1.6. Clausen et al. [
5] proposed a method to characterize tropical cyclones and to derive the structural design wind speed at a given site based on the existing and publicly available cyclone data. Garciano and Koike [
6] employed an extreme wind speed delivered by generalized extreme value distribution to estimate the buckling strength by a buckling capacity model recommended by the ISO. They further assessed the probability of buckling failure of a wind turbine considering extreme wind speed distributions from both typhoon-prone and non-typhoon-prone areas, as well as the buckling resistance of a tower as a function of wind speed. They proposed a reference wind speed for wind turbines in typhoon-prone areas based on the annual extreme wind speeds measured at fifty weather stations around the Philippines. Their reference wind speed is about 16% higher than that suggested for wind turbines of Class I.
Several studies have focused on the damage inflicted to wind turbines by typhoon-induced wind loads, with a specific focus on the fluid-structure interactions. Ishihara et al. [
7] analyzed the damage to the wind turbines on Miyakojima Island after the onslaught of typhoon Maemi, where the estimated maximum gusts were over 70 m/s. They employed a finite element method (FEM) to forecast the displacement at the tower top together with the bending moment at the turbine foundation. Uchida et al. [
8] investigated the cause of blade damage to wind turbines in southern Honshu when Typhoon Melor struck Japan in 2009. The WRF-ARW meteorological model was employed to predict the mesoscale flow behavior, followed by a large eddy simulation (LES) model, i.e., RIAM-COMPACT, to describe the near-field flow features, and, finally, a Reynolds-Averaged Navier–Stokes (RANS) model was coupled with an FEM model to determine the flow field around the blades and the resulting stress on the blades. In a recent study [
9], the aerodynamic loads on a 5-MW wind turbine during an extreme gust, modelled in accordance with the IEC 61400-1 standard, were assessed via an unsteady RANS method, and the increase of blade loading was quantified and verified. The dynamic response of this 5-MW wind turbine on a floating platform in irregular seas was investigated via an aero-servo-elastic modeling approach [
10]. From the perspective of dynamic analysis, Amaechi et al. [
11] analyzed submarine hoses attached to a mooring buoy and suggested a dynamic amplification factor (DAF) of 2 on the environmental loads. Haddadin et al. [
12] defined the DAF of a lattice structure as the ratio between the peak total response and the peak quasi-static response, which is the definition adopted in the present study for the aerodynamic response of a wind turbine.
Unfortunately, no local wind characteristics during typhoons in Taiwan are explicitly disclosed in the aforementioned references. For this reason, the present study first applied a generalized extreme value analysis to local wind measurements during typhoons to propose an extreme wind speed and a gust model for the Taiwan region.
Figure 1 illustrates the framework of this paper. A measurement-based approach to describe the extreme wind conditions of typhoons is given in
Section 2. Then,
Section 3 uses this extreme condition to conduct steady and transient RANS simulations, and examines the aerodynamic loads on a parked wind turbine. Because of a lack of in-situ load measurements of a wind turbine in such an extreme typhoon in Taiwan, a benchmark study of the target wind turbine under its rated condition was performed to ensure the reliability of the RANS results. Finally,
Section 4 discusses the load amplification factors due to gust effects based on the comparison of the steady and unsteady simulation results. Note that the amplification induced by the turbulent wind in EWM is not performed in the present study, so only the partial dynamic behaviour of this wind turbine is resolved by the simulations.
4. Results and Discussion
To investigate the characteristics of the aerodynamic loads acting on the wind turbine, several cases with different wind conditions were simulated. The predicted cases were separated into two categories: in the first category, the flow field around the parked wind turbine was simulated with constant extreme wind speed. These simulations were conducted in steady mode since the rotor was not moving and a mean wake was of main interest. Flow separation may occur behind the tower, but it is expected to be insignificant, especially in high wind conditions. In the other category, the flow field around the parked target wind turbine was simulated with a gust in transient mode. To enhance the numerical stability of the transient simulations, a precursor calculation of constant wind speed was first conducted. Then, one gust period with a varying wind speed was simulated using the precursor flow field as an initial condition. Four blade configurations were studied in the steady simulation, while only one blade configuration was investigated in the gust simulation.
4.1. Steady Aerodynamic Loadings
The simulation results of steady extreme wind speed are summarized in
Table 5. The drag force
acting on the tower as well as rotor is insignificantly impacted by the blade configuration, except the case of 60°–180°–300° when the tower is directly shadowed by a blade. In this case the rotor shows a lower aerodynamic loading because of a small mean blade height implying a low incoming wind speed along with zero angle of attack, whereas the tower loading slightly grows due to a local flow acceleration due to the blockage effect of the blade. However, the total drag of the tower and rotor delivers a similar magnitude. The rotor torque
is relatively sensitive to the blade configuration and fluctuates in the range between 1.2 MN-m and 5.0 MN-m where the blade configuration of 60°–180°–300° clearly shows the smallest value benefitted from its advantage in mean blade height and effective angle of attack. The blade pitch moment
is approximately −0.167 MN-m in average, where the minus sign indicates aerodynamic loading tending to decrease the blade pitch angle, i.e., to lead to an unfavorable blade position to acquire higher blade pitch moment. In the range of
the blade pitch moment generally declines with the blade azimuthal angle, while the blade pitch moment grows with the blade azimuthal angle for
. As expected, the blade pitch moment at
is the smallest among all studied blade positions due to a small local wind speed as well as angle of attack. The wind turbine suffers from a higher overturning risk along the y axis than along the x axis because
is much higher than its counterpart
that constantly changes its direction in a rotor rotation period. Additionally,
varies little among different blade configurations and has a mean value of 32.3 MN-m. The overturning moment along the y axis is mainly contributed by the rotor and tower where they deliver a comparative importance in
. The twisting moment
is mostly governed by the rotor but its magnitude is about one-order of magnitude smaller than
and approximately two-order of magnitude less than
.
4.2. Dynamic Amplification
The gust simulation results for the blade configuration of 0°–120°–240° are shown in
Table 6, which lists the aerodynamic loading at the time of peak wind speed, i.e.,
= 3 s, as well as the time instance of the maximum loading during the unsteady gust. Wind profile is depicted in
Figure 15, which also shows the low speed wake behind the blades and tower. The pressure contour at
= 2 s is plotted on the right-hand side of
Figure 15 to illustrate a positive pressure gradient along the wind direction during the acceleration phase of the gust.
In order to investigate the correlation between the aerodynamic loads and wind speed in the gust cases, the amplification factor,
, is defined as follows [
12]:
where
denotes any physical quantity, and
is the maximum absolute value of the physical quantity
. Both the rotor and tower drags reach their peak values somewhat in advance of the 3-s peak.
Figure 16a compares the time history of different drag components in a gust period where top represents the contribution of rotor-nacelle assembly. The aerodynamic loading is found to principally mimic the wind speed variation. The tower drag force plays a major role in the total drag while the contribution of nacelle is relatively trivial. The rotor torque
behaves similar to the rotor drag force and
precedes the gust peak by 0.13 s. Different from the rotor torque, the peak blade pitch moments echo the arrival of the top wind speed.
Figure 16b displays the blade pitch moment of the blade configuration of 0°–120°–240°. The wind unsteadiness interestingly leads to a more uniform peak blade pitch moment among the three blades accompanied by a slight increase in the mean value. The overturning moment
and
show a leading phase of the gust peak while the phase of
lags the gust peak by 0.16 s. The phase inconsistency between the maximum aerodynamic loadings and the gust peak is a contribution of time terms in the momentum equations. In the unsteady flow the flow velocity as well as its time derivative contributes to the stagnation pressure whereas the contribution of time derivative vanishes in the steady case.
Figure 17 depicts the normalized profiles of wind velocity, the time derivative of wind velocity and the drag force experienced by the wind turbine in a gust period where the superscript * represents the physical quantity normalized by its maximum value in a gust period.
Figure 17 indicates that the wind velocity just experiences a sharp deceleration process as the flow velocity slowly reaches the gust peak. The combining effect of a gradually rising wind velocity and a steep velocity gradient in time clearly leads to a peak-load offset away from the time instance of gust peak, especially the aerodynamic load is governed by the stagnation pressure. For the aerodynamic load mainly stemming from viscous shear, such as the blade pitch moment and the twisting moment, this offset behavior is less unnoticeable.
Table 7 summarizes the amplification factor of aerodynamic loads given in
Table 5 and
Table 6. The ratio of the peak tower drag in gust to the steady tower drag is approximately proportional to the square of the gust factor,
, but the corresponding ratio for rotor drag varies quite linearly with the gust factor. This is explained by the blunt shape of the tower and the streamlined geometry of the rotor blade where the former is governed by stagnation pressure characterized by the square of flow velocity and the latter is dominated by viscous shear governed by the velocity gradient. The rotor torque
is mainly contributed by the lateral force exerted on the blades where pressure and shear components are both important in the head wind condition. This accounts for an amplification factor of
falling between
(shear dominant) and
(pressure dominant). The unsteady amplification of blade pitch moment is not very significant where only 10% increase is found for this blade configuration (0°–120°–240° or
-shaped). The gust exhibits a positive impact on
with a peak reduction by 64% in a
-shaped blade configuration. Contrary to
, the extreme moments along other two axes, i.e.,
and
, are governed by the shear effect where the corresponding amplification factor is close to the gust factor.
5. Conclusions
Taiwan, due to its geographical location, has an excellent wind conditions and high potential for wind power production as well as strong typhoons which have damaged several wind turbines in recent years. The aerodynamic loads during typhoons were investigated by using the extreme wind conditions prescribed in IEC 61400-1 together with a measurement-calibrated gust factor. The extreme wind conditions proposed in the present study were based on meteorological measurements from the Zhangbin area during typhoon invasions from 2007 to 2014. A statistical approach was employed to convert the raw data into a fitting formula to quantitatively describe the extreme wind conditions for typhoons. Our study shows that the 1-min average wind speed was 36 m/s, the maximum 3-s average wind speed was 45.36 m/s, and the maximum instantaneous wind speed was 64.85 m/s, where the maximum instantaneous wind speed in Taiwan was about 20 m/s higher than the values suggested in the IEC standards. Additionally, the gust factor for typhoons was estimated at 1.26 with a gust period of 6 s. This local gust factor was used to calibrate the EOG model constant, such that a transient gust time series was reconstructed for the unsteady simulation. The turbulence intensity during typhoons was found to be higher than those of turbulence types defined in the IEC standards at low wind speeds below 30 m/s, but it declines quickly with the growth of wind speed.
The aerodynamic loads acting on the NREL 5-MW wind turbine in a steady extreme wind condition as well as an unsteady extreme gust were studied by RANS simulations. The simulation results show that the extreme aerodynamic loads acting on the wind turbine are obviously underestimated when the extreme wind condition only adopts a constant wind speed. The amplification factor of aerodynamic loads is predicted as follows: 1.54 for the drag force, 1.4 for the rotor torque, 1.3 for the overturning moment along the y direction, 1.1 for the blade pitch moment, and the yawing moment by 1.35 times. The amplification factor is governed by the nature of aerodynamic loading. It is close to the square of the gust factor in a stagnation-pressure dominant case while it approximates to the gust factor when viscous shear plays a major role. The quantitative amplification factors may not be universal for different wind turbines in different areas because the present study is limited to the Taiwan local wind conditions and this specific wind turbine. With a concern of lacking validation under extreme typhoons in this study, more load measurements of in-situ wind turbines are expected in the future. Lastly, it is suggested to conduct future studies on the dynamic amplification of this reference turbine by adopting the proposed extreme turbulence intensity in EWM, such that the full aerodynamics can be resolved and compared.