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Journal of Fluids and Structures

Journal of Fluids and Structures 57 (2015) 206–218

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An experimental investigation on the characteristicsof fluid–structure interactions of a wind turbine model sitedin microburst-like winds

Yan Zhang, Partha P. Sarkar, Hui Hu n

Department of Aerospace Engineering, Iowa State University, Ames, IA 50011, United States

a r t i c l e i n f o

Article history:Received 6 December 2014Accepted 23 June 2015

Keywords:Wind turbine aeromechanicsMicroburst-like windFluid–structure interaction

x.doi.org/10.1016/j.jfluidstructs.2015.06.01646/& 2015 Elsevier Ltd. All rights reserved.

esponding author.ail address: huhui@iastate.edu (H. Hu).

a b s t r a c t

An experimental investigation is performed to assess the characteristics of the fluid–structure interactions and microburst-induced wind loads acting on a wind turbine modelsited in microburst-liked winds. The experiment study was conducted with a scaled windturbine model placed in microburst-like winds generated by using an impinging-jet-typedmicroburst simulator. In addition to quantifying complex flow features of microburst-likewinds, the resultant wind loads acting on the turbine model were measured by using ahigh-sensitive force–moment sensor as the turbine model was mounted at different radiallocations and with different orientation angles with respect to the oncoming microburst-like winds. The measurement results reveal clearly that, the microburst-induced windloads acting on the turbine model were distinctly different from those in a conventionalatmospheric boundary layer (ABL) wind. With the scales of the wind turbine model andthe microburst-like wind used in the present study, the dynamic wind loadings acting onthe turbine model were found to be significantly higher (i.e., up to 4 times higher for themean loads, and up to 10 times higher for the fluctuation amplitudes) than those with thesame turbine model sited in ABL winds. Both the mean values and fluctuation amplitudesof the microburst-induced wind loads were found to vary significantly with the changes ofthe mounted site of the turbine model, the operating status (i.e., with the turbine bladesstationary or freely rotating), and the orientation angle of the turbine model with respectto the oncoming microburst-like wind. The dynamic wind load measurements werecorrelated to the flow characteristics of the microburst-like winds to elucidate underlyingphysics. The findings of the present study are helpful to gain further insight into thepotential damage caused by the violent microbursts to wind turbines to ensure safer andmore efficient operation of the wind turbines in thunderstorm-prone areas.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Wind power market is growing rapidly in recent years in many countries around the world. With an average growth rateabout 30% during the past 10 years, the total installed wind energy capacity has reached 321 GW globally by the end of 2013(Wiser and Bolinger, 2014). As both the total number and the size of wind turbines increase, the structural integrity and

Fig. 1. Schematic of a microburst.

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operational safety of wind turbines are receiving more and more attentions. According to the information provided by 2014Caithness Windfarm Information Forum (2014) available at the website of http://www.caithnesswindfarms.co.uk/accidents.pdf, there were about 1500 wind turbine accidents and incidents in the UK alone in the past 5 years. Among all the turbinesafety issues, structural failure, typically caused by extreme winds in thunderstorms, has contributed about one fifth of thetotal accidents, and resulted in much more property losses than any other types of accidents.

Downburst, as a particular example of the extreme winds, could be a serious wind hazard to the structural safety of windturbines. Downbursts are quite common in many areas of the world. According to the 2011 Extreme Weather Sourcebook ofNational Center for Atmospheric Research (NCAR), approximately 5% of thunderstorms would produce downbursts that isone of the primary factors responsible for the estimated $1.4B of insured property loss each year in USA alone (data takenfrom 1950 to 1997). Fujita (1985) first classified the downbursts into microbursts and macrobursts, based on the horizontalextension of the divergent outburst flows. A microburst, as defined by Fujita (1985), is a strong downburst which producesan intense outburst of damaging wind with the radial extent being less than 4.0 km, or else is defined as a macroburst.Although a “microburst” has a smaller size than its counterpart, “macroburst”, it could produce a much stronger outflowwith the maximum wind speed up to 270 km/h.

The flow characteristics of a microburst are dramatically different from those of conventional atmospheric boundarylayer (ABL) winds and other wind hazards of wide concerns, e.g., tornadoes. As shown schematically in Fig. 1, a microburstcan produce an impinging-jet-like outflow profile diverging from its center with the maximum velocity occurring at analtitude of less than 50 m above ground (Hjelmfelt, 1988). Such extreme high wind speed and wind shear (i.e., velocitygradient) near the ground could produce a significantly greater damaging potential to built structures. Furthermore, amicroburst would also produce strong vertical velocity component in both the core region and the leading edge of theoutburst, which is very different from conventional ABL winds. Therefore, microburst-induced wind loading pattern is quitedifferent fromwhat is usually expected with conventional ABL winds. Due to the extreme damaging potential of microburststo built structures, a number of experimental and numerical simulation studies have been conducted in recent years toquantify the flow characteristics of the microburst-like winds and to assess the effects of the microburst-induced windloading on various built structures on the ground, such as transmission towers, grain bins, low-rise residential houses andhigh-rise buildings (Savory et al., 2001; Chay and Letchford, 2002; Sarkar et al., 2006; Sengupta and Sarkar, 2008; Zhanget al., 2013a,b, 2014a,b).

Generally, wind turbines are designed to operate in conventional ABL winds. The wake characteristics and resultant windloads acting on horizontal-axis wind turbines sited in conventional ABL winds have been studied extensively over the pastyears (Vermeer et al., 2003; Cal et al., 2010; Chamorro and Porte-Agel, 2010, 2011; Lebron et al., 2012; Yang et al., 2012; Huet al., 2012; Zhang et al., 2012; Tian et al., 2014; Jeong et al., 2014). A number of numerical simulations have also beenconducted by coupling stochastic or CFD turbulence models with aeroelastic models (e.g., FAST Jonkman and Buhl, 2005) toinvestigate the wind turbine loads subject to turbulent atmospheric boundary layer winds (Moriarty et al., 2004; Lee et al.,2011). Although extreme situations, such as Extreme Coherent Gust with Direction Change (ECD) and Extreme DirectionChange (EDC), have already been considered in the IEC standards for wind turbine design (International Standard, 2005),such standards are not applicable for non-conventional ABL wind conditions. While microburst-like winds have been shownto generate significantly different fluid–structure interaction characteristics and wind loading effects on both low-rise andhigh-rise structures (Savory et al., 2001; Chay and Letchford, 2002; Sengupta and Sarkar, 2008; Zhang et al., 2014a,b), onlyfew analytical studies can be found in literature to investigate microburst-induced wind loads acting on wind turbines. Forexample, Nguyen et al. (2011) studied the wind loads acting on a wind turbine sited in a simulated translational microburstwind by using an analytical model as suggested by Chay et al. (2006). They found that the simulated microburst wouldimpose 86% higher out-of-plane bending moment on turbine blades than a typical EDC load case defined in IEC standards(International Standard, 2005), and 20% higher bending moment than that of an ECD load case when there was no yawcontrol applied. Kwon et al. (2012) introduced a concept of gust loading factors into the analysis of the wind loads acting on

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wind turbines sited in gust-front winds, and suggested that the gust-front winds would induce approximately twice higherof the static loads on the wind turbine tower system than those in conventional ABL winds defined by ASCE-7 standards.However, these analytical studies were based on idealized mathematical models without taking the complicated flowfeatures and turbulence nature of microburst winds into account. As revealed in the Zhang et al. (2013a,b), microbursts areactually very complex vortex flows with intense downdrafts and violent outburst winds. While the intense downdraftdominates the flow feature in the core region of a microburst, the characteristics of the divergent outburst flow resemblesthose of a wall jet well with the maximum velocity occurring very close to the ground, i.e., lower than the hub-height of alarge-sized wind turbine. Furthermore, the much higher turbulence intensity levels in violent microburst winds may resultin much more significant peak loads acting on the wind turbine, in comparison with those in conventional ABL winds, whichare extremely dangerous to the structural integrity of the wind turbine. Therefore, it is necessary and highly desirable toimprove our understanding about the characteristics of the fluid–structure interaction between wind turbines and violentmicrobursts and the resultant wind loads acting on the wind turbines in order to improve our understanding about thepotential damage caused by violent microbursts to wind turbines.

In the present study, a comprehensive experimental investigation was conducted to assess the characteristics of thefluid–structure interaction and the microburst-induced wind loads acting on wind turbines. The experimental study wasperformed with a scaled horizontal axis wind turbine (HAWT) model placed in a microburst-like wind generated by usingan impinging-jet-typed microburst simulator available at Iowa State University. With the turbine model being mounted atdifferent radial locations away from the core center of the microburst-like wind and at different orientation angles withrespect to the oncoming outburst wind, the microburst-induced wind loads (i.e., both the aerodynamic forces andcorresponding moments) acting on the wind turbine model were measured quantitatively by using a high-sensitive load cellmounted under the tower base of the turbine model. During the experiments, the rotor blades of the turbine model was setin either free rotating or stationary to simulate the scenario of the wind turbine in either normal operation or shutdownstatus during a microburst event. The present study simulates a wide range of the possible situations when a microburst innature occurs nearby a wind turbine. To the best knowledge of the authors, this is the first effort of its nature. It is hoped thatthe findings derived from the present study would be helpful to improve our understanding about the potential damagescaused by violent microbursts to ensure safer and more robust operation of the wind turbines sited in thunderstorm-prone areas.

2. Experimental setup and wind turbine model

While microbursts in nature are transient phenomena with a life time of about 10 min, a steady impinging-jet flow wasfound to resemble the major features of a microburst at its maximum strength reasonably well (Hjelmfelt, 1988). Suchsimilarity has been proven by many researchers who successfully used impinging-jet-models to produce outburst flowprofiles to simulate microburst-like winds (Holmes and Oliver, 2000; Choi, 2004; Chay et al., 2006; Mason et al., 2005).Moreover, the flow characteristics of a microburst wind represented by a steady impinging jet flow would also be the mostcritical scenario to study the microburst-induced wind loading effects on built structures. It should be noted that, dynamicsimilarity is one of the greatest challenges to conduct laboratory experiments to simulate meteorological phenomena suchas microbursts. It will be very difficult, if not impossible, to match the Reynolds numbers of the microbursts in nature withthose of the impinging jet flows generated in the laboratories due to the significant scale difference of the two comparedcases. However, it has been found that, although the dynamic similarity is difficult to match, the measurement resultsobtained from laboratory experiments are still useful to reveal the flow characteristics of microburst-like winds and topredict the winds loads acting on test models induced by microburst-like wind as long as the Reynolds number of thelaboratory experiments is high enough. Therefore, impinging-jet-typed microburst simulators have been widely adopted togenerate microburst-like winds in laboratories to produce outburst flow velocity profiles resembling those of microburstwinds (Wood et al., 2001; Chay et al., 2006; Das et al., 2010; Zhang et al., 2013a,b).

In the present study, an impinging-jet-based microburst simulator hosted in the Department of Aerospace Engineering ofIowa State University (ISU) is used to generate microburst-like winds, as shown in Fig. 2. A schematic of this simulator withdetailed dimensions can be found in Zhang et al. (2013a,b). A downdraft flow is generated through an axial fan driven by astep motor. The exhaust nozzle diameter (D) of the ISU microburst simulator is 610 mm (i.e., D¼610 mm). The distancebetween the nozzle exit and the ground plane (H) is adjustable up to 2.3 times the nozzle diameter. Honeycomb and screenstructures are placed upstream of the nozzle exit in order to produce a uniform jet flow exhausted from the ISU microburstsimulator. During the experiments, a three-component cobra anemometer probe (Turbulent Flow Instrumentation Pvt.,Ltd.), which is capable of simultaneously measuring all three components of the wind velocity vector, was used to quantifythe flow characteristics of the jet flow at the points of interest. It was found that the jet flow exhausted from ISU microburstsimulator was quite uniform across the nozzle exit, and the turbulence level of the core jet flow was found to be within 2.0%.For the measurement results given in the present study, the ground floor was fixed at 2D below the ISU microburstsimulator (i.e., H/D¼2.0). The flow velocity at the nozzle exit of the ISU microburst simulator was set to 13.0 m/s (i.e.,Ujet¼13 m/s), which corresponds to a Reynolds number of 5.2�105 based on the nozzle diameter, D, of the ISU microburstsimulator. Further information about the design, construction, and performance of the ISU microburst simulator as well asquantitative comparisons of the microburst-like winds generated by using the ISU microburst simulator with themicrobursts occurring in nature can be found in Zhang et al. (2013a,b).

Y. Zhang et al. / Journal of Fluids and Structures 57 (2015) 206–218 209

Fig. 3 shows the schematic of the triple-blade horizontal axis wind turbine (HAWT) model used in the present study,which has a hub height of 184 mm and rotor diameter of 254 mm. With the geometric scale of approximately 1:500, theturbine model would represent a 2 MW utility scale wind turbine with a rotor diameter of about 130 m and hub-height of

Fig. 2. ISU microburst simulator used in the present study.

Fig. 3. Schematic of the wind turbine model and the coordinate system used in the present study. (a) Schematic of the wind turbine model (b) Thecoordinate system and the orientation angle of the turbine model.

Table 1The primary design parameters of the wind turbine model used in the present study.

Parameter Rrotor Hhub drod dnaccele α a a1 a2

Dimension (mm) 127 225 18 28 61 100 15 70

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90 m in the field. As a result, the present study would simulate the scenario of a 2 MW utility scale wind turbine interactwith a microburst of 300 m in diameter in the field. The rotor blades of the turbine model are MA0530TE blades (WindsorPropeller Inc.), which are twisted blades with the pitch angle ranging from 201 at the root to 101 at the tip of the blades. Theblades have a chord length of 12 mm at tip, 19 mm in the middle, and 16 mm at root. The airfoil cross-section of blades has aconcave pressure surface and is well adapted for low Reynolds number applications. Since the blades were originallydesigned for propeller applications, they were mounted reversely with the pressure side of the blades facing the oncomingairflow during the experiments to improve their aerodynamic performance when used as wind turbine rotor blades. Theprimary design parameters of the wind turbine model are listed in Table 1. It should be noted that, the same turbine modelwas used by Hu et al. (2012) to study the characteristics of the dynamic wind loads acting on wind turbine sited in aconventional ABL wind.

In the present study, the microburst-induced wind loads acting on the turbine model were measured by using a high-sensitive force–moment sensor (JR3, model 30E12A-I40). The JR3 load cell is capable of measuring forces in three directionsand the moment (torque) about each axis. The measurement uncertainty of the JR3 load cell is 70.25% of the full range(40 N). The wind loading measurements were performed with the turbine model mounted at different radial locations (r/D¼0.0 to 2.0, with a spacing of 0.5, where r is radial distance from the imping jet center) and with different orientationangles (OA) with respect to the oncoming outburst flow direction (i.e., OA¼0 to 180 deg, with a spacing of 22.5 deg), asshown in Fig. 3. For each test run, the wind load data were taken for 30 s with a sampling frequency of 1000 Hz. Both themean values and the standard deviations of the instantaneous wind loads were analyzed in the present study to quantifythe characteristics of the microburst-induced wind loads acting on the wind turbine model. Since the radial location of r/D¼1.0 is a critical location where maximum wind speed typically occurs in the microburst-like wind as reported in Zhanget al. (2013a,b), the radial component of the flow velocity at the turbine hub height, Uhub, at the radial location of r/D¼1.0was used as the reference velocity (i.e., UhubE3.5 m/s) in the present study to normalize the measured force and momentdata for all the tested cases. It should also be noted that, when the turbine model was mounted at the radial location of r/D¼1.0 in the microburst-like wind, the tip-speed-ratio of the rotor model turbine was found to be λ¼Ω Rrotor=Uhub ¼ 4:1,where Ω is the angular speed of rotation measured using a laser tachometer. It is in the working range of a typical large-scalewind turbine on a modern wind farm i.e., λE4.0–8.0, as described in Burton et al. (2001).

As shown schematically in Fig. 3, CFr is defined as the coefficient of the radial component of the aerodynamic force actingon the turbine model (i.e., it is also usually referred as the thrust coefficient for the case with a wind turbine sited in aconventional ABL wind); CFt is the coefficient of the tangential component of the aerodynamic force with respect to thedirection of the outburst flow; CFz refers the coefficient of the vertical component of the aerodynamic force. CMr, CMt, CMz

are the corresponding moment coefficients about each axis with direction obeying the right-hand rule. The coefficients ofthe Force and moment were calculated by using following equations:

CFi ¼Fi

12ρU

2hubπR

2rotor

ð1Þ

CMi ¼Mi

12ρU

2hubπR

2rotorHhub

ð2Þ

where Fi and Mi refer to the components of the aerodynamic force and moment in radial, tangential and vertical directions;ρ is density of the air; Rrotor is the radius of the turbine blades; Hhub is the hub height of the wind turbine model (i.e.,Hhub¼184 mm for the present study).

It should be noted that, while the flow velocity at the turbine hub height would change greatly as the turbine model wasmounted at different radial locations in the microburst-like wind, the force and moment coefficients were defined by usingthe same reference velocity of Uhub (i.e., the velocity at turbine hub height with the turbine model mounted at the radiallocation of r/D¼1.0) in the present study. Therefore, the force and moment coefficients reported in the present study can becompared directly for the measurement data obtained at different test conditions.

It is well known that wind turbines will cease power generation and even shut down at high wind speeds. The windspeed at which shut down occurs is called the cut-out speed, which is usually at 25 m/s (i.e., Ucut-out¼25 m/s) for modernutility scale wind turbines. Having a cut-out speed is a safety feature which protects the wind turbines from damage. Inpractice, wind turbine shut down may occur in one of several ways. While an automatic brake may be activated by a windspeed sensor for some wind turbines, twist or “pitch” the rotor blades are also used to spill the wind for the shutdown ofwind turbines. Still others use “spoilers,” drag flaps mounted on the blades or the hubs, which are automatically activated byhigh rotor rpm’s, or mechanically activated by spring loaded devices which turn the wind turbines sideways to theoncoming wind stream.

Fig. 4. PIV measurement results of the microburst-like wind and the scale of the wind turbine model used in the present study.

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It should be noted that the wind speed detected by the anemometers located at the top of the turbine nacelle is widelyused for wind turbine control to determine whether the turbine should be shut down or not. Since a severe microburst innature could induce a strong outflow with the maximum wind speed up to 270 km/h, i.e., 170 mph (Fujita, 1985), windturbines would be shut down during a severe microburst event (i.e., the turbine blades would be kept stationary during asevere microburst event) if the wind speed detected by the anemometer probes at the turbine hub height is greater than thecut-out speed of the wind turbines. Since a microburst would produce an impinging-jet-like outflow profile with themaximum velocity occurring at an altitude of less than 50 m above ground (Hjelmfelt, 1988), it is also possible that the windspeed detected by the anemometer probes located at the turbine hub height (e.g., �90 m for modern utility-scale windturbines) may still be smaller than the cut-out speed when the wind turbine is sited in the outburst region of the microburst(e.g., at downstream location of r/D41.0 as shown in Fig. 4). As a result, the wind turbine would be in free rotation in themicroburst-like wind. With this in mind, two different cases, i.e., the rotor blades of the turbine model were set in either freerotation or stationary, were considered in the present study, and the measurement data for the two cases were comparedquantitatively. For the case with the turbine blades freely rotating, the rotor blades of the wind turbine model would rotatefreely driven by the oncoming microburst-like wind. For the case with the turbine blades stationary, the rotor blades werepitched to minimize the aerodynamic forces acting on the rotor blades and tapped firmly to ensure that the rotor bladeswould not rotate during the experiments. For the case with the blades stationary, the three rotor blades of the wind turbinemodel were fixed in such a way that one rotor blade is in horizontal, and the other two blades having phase angles of 601and �601 related to the horizontal plane, respectively.

3. Measurement results and discussions

3.1. Flow characteristics of the microburst-like wind

As aforementioned, microbursts are very complex flows with intense downdrafts and violent outburst winds, which havedistinctly different flow features in comparison with conventional ABL winds. Fig. 4 gives the PIV measurement resultreported in Zhang et al. (2013b) to visualize the unique flow features of the microburst-like wind generated by using the ISUmicroburst simulator. The cartoon of a wind turbine was also added into the figure in order to provide a visual comparisonof the scale of the turbine model with that of the microburst-like wind. As visualized clearly in Fig. 4, the intense downdraftwould cause airflow to stagnate at the center of the microburst-like wind as it impinges onto the ground, forming a dome ofhigh static pressure near the stagnation point as reported in Zhang et al. (2013b). After impingement, the streamlines of thediverging airflow were found to be curved, and the airflow would transition from downdraft to outburst flow in the regionof 0.5rr/Dr1.0. At the further downstream locations of r/DZ1.0, the radial component (Vr) of the flow velocity was foundto become dominant, and the vertical velocity component (Vz) becomes almost negligible. The profile of the radial flowvelocity in the outburst region resembles that of a wall jet flow, with the maximum velocity occurring at an elevation veryclose to the ground (i.e., z/Dr0.15), i.e., at a height lower than the hub-height of the wind turbine model as shown clearly inFig. 4. Further detailed discussions about the flow characteristics of the microburst-liked wind based on PIV measurementscan be found in Zhang et al. (2013a,b).

The distribution of turbulence intensity was also found to vary significantly in the microburst-like wind, which has greatimpacts on the dynamic wind loadings on the high-rise built structures as suggested by Zhang et al. (2014b). Morespecifically, in the core region of the microburst-like wind, the turbulence intensity is very small (�2.0%). The turbulenceintensity was found to increase rapidly as the radial distance from the center increases and become much greater in thetransition region and outburst region in comparison with than those in the core center. It should also be noted that, asreported in Hu et al. (2012), the turbulence intensity in a conventional ABL wind would decrease gradually with theincreasing elevation level away from the ground. However, the region with higher turbulence intensity in the microburst-like wind was found to concentrate at a higher elevation (i.e., near the turbine hub height), which is quite far away from theground plane.

In summary, the measurement results described above reveal clearly that the microburst-like wind is a complexturbulent flow with significantly different flow characteristics (i.e., different magnitude and distinct distribution pattern in

Fig. 5. Force and moment coefficients vs. the radial location of the wind turbine model. (a) Force coefficients (b) Moment coefficients.

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both the mean velocity and turbulence intensity) from those of a conventional ABL wind. As a result, the characteristics ofthe fluid–structure interaction between wind turbines and microburst-like winds are expected to become much morecomplicated, in comparison with those in conventional ABL winds.

3.2. Characteristics of the microburst-induced wind loads acting on the wind turbine model

3.2.1. Mean values of the microburst-induced wind loads acting on the turbine modelAs described above, the dynamic wind loads acting on the turbine model were measured by using a JR3 load cell with a

sampling frequency of 1000 Hz for 30 s for each tested cases. Both the mean values and standard deviations of theinstantaneous wind loads were analyzed in the present study to reveal the characteristics of the microburst-induced windloads. Fig. 5 shows the mean (i.e., time-averaged) values of measured aerodynamic forces and moments acting on the windturbine model as the turbine model was mounted at the different radial locations in the microburst-like wind. For themeasurement data given in Fig. 5 the turbine blades were set either stationary or freely rotating with the orientation angleof the turbine model being 0 deg (i.e., OA¼0 deg) with respect to the oncoming microburst-like wind, i.e., the turbine rotordisk is normal to the radial component of the microburst-like wind as shown in Fig. 3. It can be seen that, corresponding tothe complexity of the flow features of the microburst-like wind, the microburst-induced wind loadings acting on the windturbine model were found to vary significantly as a function of the mounted location of the turbine model. In general, theradial-component of the aerodynamic force Fr and the bending moment Mt were found to be the most prominent among allthe force and moment components for all the tested cases, except for the case with the turbine model sited at the center ofthe microburst-like wind. When turbine model sited at the center of the microburst-like wind (i.e., r/D¼0.0), corresponding

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to the intense downdraft in the core region of the microburst-like wind, while the vertical component of the aerodynamicforce was found to be the most significant component as expected, all the other force components are negligibly small. Asthe turbine model is moved away from the core center of the microburst-like wind (i.e., r/D increases), the radial-component of the aerodynamic force (Fr) and the bending moment (Mt) were found to increase rapidly. Both Fr and Mt werefound to reach their maximum values as the turbine model was mounted at the radial location of r/D¼0.5, while themaximum radial velocity was found to be reached at the radial location of r/D¼1.0. This is because the depth of the outburstflow at the radial location of r/D¼0.5 is much greater than those at the radial location of r/D¼1.0 and other furtherdownstream locations. As shown in Fig. 4, the “submerged” area of the turbine model in the high-speed outburst flowwould be much larger when mounted at the radial location of r/D¼0.5, thereby, the turbine model would experience muchlarger resultant wind loads, in comparison with those at r/D¼1.0 and the further downstream locations. Due to thesymmetric nature of the turbine model at OA¼0 deg in respect to the oncoming microburst-like wind, the mean lateralforce, Ft, and corresponding radial moment, Mr, were found to be always very small, which are negligible.

It should also be noted the CFr values of the wind turbine model sited in the microburst-like wind (CFrE0.45–1.2 for theradial location of r/DE0.5–2.0) was found to be much higher than the value reported in Hu et al. (2012) by using the sameturbine model sited in a conventional ABL wind (i.e., CFrE0.3 in a conventional ABL wind). It suggests that, with the sameoncoming flow velocity at the turbine hub height, the absolute values of the mean wind loads would become much greater(i.e., up to 4 times greater) when the turbine is sited in a microburst-like wind, in comparison with those in a conventionalABL wind. Furthermore, the turbine model was also found to experience a considerably large downward force Fz as it ismounted near the core region of the microburst-like wind (i.e., at r/DE0–0.5). Such substantial downward force induced bythe intense downdraft of the microburst-like winds could be very hazardous to the turbine structure integrity since windturbines are designed by assuming only to operate in conventional ABL winds without considering the downward forceinduced by the microburst-like wind.

As shown in Fig. 5, the microburst-induced wind loads experienced by the turbine model were also found to be quitedifferent when the turbine blades were set stationary, in comparison with the freely rotating case. As reported in Tian et al.(2014), when sited in a conventional ABL wind, a wind turbine would experience much larger thrust and bending momentfor the case with its blades in rotation than those with the rotor blades stationary. However, interestingly, a very differentfashion was observed when the same turbine model was mounted in the microburst-like wind. As shown clearly in Fig. 5,when the turbine model was mounted at r/D¼0.5 (i.e., near the boundary of the core region of the microburst-like wind),the values of CFr and CMt for the case with the turbine blades stationary were found to be considerably larger than thosewith the turbine blades freely rotating. When the turbine model was moved further downstream into the outburst region(i.e., r/DE1.0), the values of CFr and CMt for the stationary case were still found to be slightly larger than those of therotating case. The differences between the two compared cases were found to decrease gradually as the distance away fromthe center of the microburst-like wind increases.

The distinct differences in the characteristics of the microburst-induced wind loads acting on the turbine model arebelieved to be closely related to the unique flow features of the microburst-like wind. When a wind turbine is sited in aconventional ABL wind, the resultant wind loads acting on the wind turbine are expected to be much higher at the upperhalf (i.e., the region above the turbine hub height) of the wind turbine due to the much higher wind speed at the higherelevations away from the ground. However, since the microburst-like wind would have a wall-jet-liked velocity profile inthe outburst flow with the maximumwind velocity located at a height much lower than the hub-height of the wind turbinemodel, only the lower half of the wind turbine would be submerged into the high-speed diverging airflow. As a result, thewind loads acting on the wind turbine would mainly come from the lower half of the turbine rotation disk (i.e., the regionbelow the turbine hub height). As illustrated schematically in Fig. 6(a), for the case with the rotor blades freely rotating,when the turbine blades rotate into the lower half of the rotation disk, they will be submerged into the high speed outburstwind near the ground. A substantially large aerodynamic force would be generated to drive the blades to rotatecontinuously, which results in a positive thrust force acting on the turbine blades. As the turbine blades rotate into theupper half of the rotation disk, the turbine blades would rotate with the same rotational speed as they were at the lower halfof the rotation disk due to the inertial effects. However, since the velocity of the oncoming airflow would decreasedramatically in vertical direction as shown in Fig. 4, the turbine blades would actually work as “propeller blades” to addenergy to the oncoming low-speed airflow, instead of extracting kinetic energy from the oncoming airflow, at the higherelevations. As revealed clearly in the schematic of the velocity vector triangles given in Fig. 6(b), the resultant wind speedmay cause a negative thrust force acting on the turbine blades, which would counterbalance the positive thrust and bendingmoment acting on the turbine blades as they were at the lower half of the rotation disk.

3.2.2. The variations of the microburst-induced wind loads as a function of the orientation angleSince the direction of the oncoming airflow in a real microburst event in nature could be arbitrary in respect to the rotor

disks of the wind turbines, a parametric study was conducted in the present study to assess the variations of the microburst-induced wind loads as a function of the orientation angle (OA) with respect to the oncoming microburst-like wind, andsome of the measurement results are given in Fig. 7. For the measurement data given in Fig. 7, the turbine model wasmounted at the radial location of r/D¼0.5, where the turbine model was found to experience the maximum radialaerodynamic force and bending moment. It can be seen that, for the case with the turbine blades stationary, both values ofCFr and CMt would decrease as the orientation angle increases, reaching their minimum values at OA¼90 deg, and then

Fig. 6. Schematic of the flow velocity vectors relative to the cross-section of the turbine blade.

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increasing again as the orientation angle increases from OA¼90 deg to OA¼180 deg. Such trend is found to be consistentwith the variation of the projected area of the turbine rotation disk along with the direction of the oncoming outburst flow.It can also be seen that, while CFr and CMt were found to reach their maximum values at OA¼0 deg and OA¼180 deg, i.e.,with the turbine rotation disk normal to the oncoming outburst wind, the tangential component of the aerodynamic force Ftand its corresponding moment, Mr, were found to be greatest when the turbine model is mounted with an approximateorientation angle of 112.5 deg with respect to the oncoming outburst wind (i.e., OAE112.5 deg). Since the turbine modelwas exposed to the same downwash flow when mounted at the radial location of r/D¼0.5, the vertical component of theaerodynamic force, Fz, and its corresponding moment, Mt, were found to be almost independent on the orientation angle.

For the case with the turbine blades freely rotating, since the rotor blades of the turbine model would not able to rotateas the turbine model was set at a relatively high orientation angle (i.e., for the cases OA445 deg) with respect to theoncoming outburst wind, the measurement data can be obtained only when the turbine model has a relatively smallorientation angle. As shown clearly in Fig. 7, the values of Fr and Mt for the case with the turbine blades freely rotating werefound to be consistently smaller than those with the turbine blades stationary at all the three tested orientation angles, i.e.,OA¼0.0, 22.5, and 45 deg. The differences between the two compared cases were found to become smaller and smaller asthe orientation angle increases. It should be noted that, a series of experiments were also conducted to investigate theeffects of the turbine orientation angle on the microburst-induced wind loads with the turbine model mounted at furtherdownstream locations (i.e., r/DZ1.0). Since the measurement results with the turbine model located at further downstreamlocations reveal a very similar trend as those given in Figs. 6 and 7, the measurement data are not shown here forconciseness.

3.2.3. The standard deviations of the dynamic wind loads acting on the wind turbine modelAs revealed from the measurement results given above, microburst-like winds usually have much higher turbulence

intensity levels and more complicated distribution pattern, in comparison to conventional ABL winds. As a result, theresultant wind loads acting on wind turbines are expected to fluctuate much more significantly when sited in microburst-like winds. In the present study, the standard deviations of the instantaneous wind load acting the wind turbine model wasalso analyzed in order to assess the fluctuation extent of the dynamic wind loads induced by the violent microburst-likewinds. Table 2 summarizes the standard deviations (i.e., root-mean-square values) of the dominant components of thedynamic wind loads (i.e., σCFr and σCMt ) as the turbine model was mounted at different radial locations in the microburst-like wind. It should be noted that, with the same wind turbine model sited in a conventional ABL wind, as reported by Huet al. (2012), the standard deviations of the coefficients of the thrust forces and the corresponding bending moment wasfound to be about 0.11 and 0.15, respectively. However, as shown in Table 2, due to the more complicated distributionpattern and much higher turbulence levels in the microburst-liked winds, the standard deviations of the dynamic windloads acting on the turbine model (i.e., σCFr and σCMt ) were found to be significantly higher (i.e., �up to 10 times higher)than those reported in Hu et al. (2012) with the same turbine model sited in a conventional ABL wind.

Fig. 7. Force and moment coefficients vs. the orientation angle of the turbine model at r/D¼0.5.

Y. Zhang et al. / Journal of Fluids and Structures 57 (2015) 206–218 215

It can also be seen that, both σCFr and σCMt values were found to vary substantially as the mounted location of the turbinemodel changes in the microburst-like wind. For the case with the turbine blades stationary, both σCFr and σCMt were found toreach their peak values when the model turbine was mounted at the radial location of r/DE1.0, where the maximumwindspeed would occur as shown in Fig. 4. However, for the case with the turbine blades freely rotating, the maximum values forboth σCFr and σCMt were found to be reached at the radial location of r/D¼0.5, which is believed to be corresponding to thehighest rotation speed of the turbine blades at the radial location of at r/D¼0.5. Interestingly, while the turbine model wasfound to experience considerably larger mean wind loads for the case with the turbine blades stationary as shown in Fig. 5,the standard deviations of the dynamic wind loads acting on the turbine model were found to be always greater for the casewith the turbine blades freely rotating at all the tested radial locations. The larger fluctuation amplitudes of the dynamicwind loads acting on the wind turbine for the case with the turbine blades freely rotating are believed to be closely relatedto the shedding of the unsteady wake vortex structures from the rotating rotor blades, similar as those visualizedquantitatively in Yang et al. (2012). It should be noted that, the extreme high peak loads acting on wind turbines induced bymicroburst-like winds are very hazardous to the structure integrity of the wind turbines, which can cause permanentstructure failures of wind turbines when they are exposed in violent microburst-like winds.

Based on Fast Fourier transform (FFT) analysis of the instantaneous wind load measurement data obtained at a samplingrate of 1000 Hz, the power spectra of the dynamic wind loads acting on the turbine model can be determined. Fig. 8 gives anexample of the power spectra of the measurement data as the turbine model was mounted at the radial location of r/D¼0.50 with the turbine blades either freely rotating or stationary. Since very similar features were also seen in the power

Table 2The standard deviations of the dynamic wind loads acting on the wind turbine model sited in the microburst-like wind.

The location of the turbine model in the microburst-like wind

Turbine blades stationary Turbine blades freely rotating

Radial-component force,σCFr

Bending momentσCMt

Radial-component force,σCFr

Bending momentσCMt

r/DE0.0 0.76 0.58 0.80 0.67r/DE0.5 0.61 0.58 1.51 1.57r/DE1.0 0.78 0.64 1.36 1.29r/DE1.5 0.69 0.47 0.84 0.77r/DE2.0 0.63 0.50 0.98 0.95

Fig. 8. Power spectrum of the bending moment with the turbine model mounted at the radial location of r/D¼0.5 and 0 deg orientation angle.(a) Stationary, (b) free rotation.

Y. Zhang et al. / Journal of Fluids and Structures 57 (2015) 206–218216

spectra of the other components of the force and moment measurement data, only the power spectra of the bendingmoment (i.e., Mt) were presented here for conciseness.

As shown in clearly in Fig. 8(a), a well-defined dominant peak (fvortex) can be identified in the power spectrum for thecase with the turbine blades stationary, which is believed to be closely related to the periodical shedding of the large-scaleprimary vortices in the microburst-like wind. As described in Zhang et al. (2013a), the large-scale primary vortices aregenerated due to Kelvin–Helmholtz instabilities at the strong shear layer between the high-speed outburst flow of themicroburst-like wind and the low-speed ambient flow. As shown in Fig. 8(b), in addition to the well-defined dominant peak(fvortex) corresponding to the periodic shedding of the large-scale primary vortices in the microburst-like wind, a series ofobvious peaks can also be identified in the power spectrum for the case with the turbine blades freely rotating. Thecorresponding frequencies of the series of the obvious peaks identified in the power spectrum were found to be correlatedvery well with the rotation frequency of the turbine blades f0 and its harmonic frequencies, i.e., nn f0. It indicates that theinstantaneous wind loads acting on the turbine model would be influenced not only by the shedding of the large-scaleprimary vortices in the microburst-like wind, but also by other factors such as wind shear, yaw, rotation-inducedunbalances, and the periodically-shedding of the tip and root vortices as visualized in Hu et al. (2012). Due to thesuperposition of the effects of the large-scale primary vortices and the unsteady wake vortex structures sheddingperiodically from the rotating turbine blades, the fluctuation amplitudes of the microburst-induced wind loads acting onthe turbine model would become much greater for the case with the turbine blades freely rotating, which is confirmedquantitatively by the measurement data given in Table 2.

It should be noted that, while the test conditions and controlling parameters of the present study with a microburstsimulator are intrinsically different from those of the experimental study described in Hu et al. (2012) with a conventionalABL wind tunnel, the values of the measured mean and fluctuating wind loads described above were obtained with the factsthat the same turbine model was used in the two studies and the coefficients of the wind loads are defined based on thesame reference velocity, i.e., the wind speed at the turbine hub height. However, since the microburst-induced wind loadsexperienced by a wind turbine may vary significantly as a function of the scale ratio between the wind turbine and themicroburst, the mounted site of the turbine in relation to the microburst center, the operating status (i.e., with the turbineblades stationary or freely rotating), the orientation angle of the turbine with respect to the oncoming microburst-like wind,

Y. Zhang et al. / Journal of Fluids and Structures 57 (2015) 206–218 217

extra caution is needed to directly compare the wind loads acting on wind turbines in microburst winds with those in ABLwinds. While it is almost impossible to simulate all the possible scenarios experienced by wind turbines operating inthunderstorm-prone areas, the findings of the present study are believed to very valuable to gain further insight into thepotential damage caused by the violent microbursts to wind turbines to ensure safer and more efficient operation of thewind turbines.

4. Conclusions

An experimental study was conducted to assess the characteristics of the fluid–structure interactions and resultantdynamic wind loads acting on a wind turbine model sited in a microburst-like wind. In addition to quantifying the uniqueflow features of the microburst-like wind, the microburst-induced wind loads acting on the wind turbine model wasmeasured quantitatively as the turbine model was mounted at different radial locations and orientation angles with respectto the oncoming microburst-like wind. The measurement results reveal clearly that, in comparison with those with thesame turbine model sited in a conventional atmospheric boundary layer (ABL) wind, the characteristics of the fluid–structure interactions between the wind turbine and the microburst-like winds become much more complicated, and theresultant wind loads acting on the turbine model become significantly higher for the case with the turbine model sited inthe microburst-liked wind. Some major findings of the present study are summarized as follows:

1)

Due to the violent and turbulent nature of the microburst-like winds, the microburst-induced wind loads acting on theturbine model were found to be up to 4 times higher for the mean values of the wind loads, and up to 10 times greater inthe term of the fluctuation amplitudes (i.e., with the scales of the wind turbine model and the microburst-like windsused in the present study), compared with those with the same turbine model sited in a conventional ABL wind.

2)

The radial component of the aerodynamic force (Fr) and bending moment (Mt) were found to be the most dominantcomponents among all the microburst-induced wind loads acting on the wind turbine model. Both the radial force andbending moment were found to reach their peak values as the turbine model was mounted at the radial location of r/D¼0.5, where the microburst-like wind would transition from intense downdraft to violent outburst flow.

3)

The intense downdraft near the core region of the microburst-like wind would result in significant downward wind loadsacting on the wind turbine model when sited near the core region of the microburst-like wind, which may pose a serioushazard to the turbine structure integrity.

4)

Unlike those in conventional ABL winds, the wind turbine model sited in microburst-liked wind was found to experiencelarger mean radial aerodynamic force (Fr) and bending moment (Mt) for the case with the turbine blades stationary, incomparison with the case with the turbine blades freely rotating. The difference between the two compared cases wasfound to decrease as the orientation angle increases.

5)

Interestingly, while the mean values of the microburst-induced wind loads acting on the wind turbine model were foundto be higher for the case with the turbine blades stationary, the fluctuation amplitudes of the dynamic wind loads actingon the wind turbine model were found to be much higher for the case with the turbine blades freely rotating due to thesuperposition of the effects of the large-scale primary vortices in the microburst-like wind and the unsteady wake vortexstructures shedding periodically from the rotating turbine blades.

6)

The microburst-induced wind loads was found to be a function of the orientation angle with respect to the oncomingmicroburst-like wind. The radial-component of the aerodynamic force (Fr) and bending moment (Mt) were found toreach their peak values at OA¼0 deg and OA¼180 deg, corresponding to the largest blockage area of the turbine rotationdisk along with the direction of the diverging microburst-like wind.

The findings derived from the present study are believed to be very helpful to gain further insight into the potentialdamage caused by the violent microbursts to wind turbines to ensure safer and more efficient operation of the windturbines in thunderstorm-prone areas.

Acknowledgments

The project is funded by National Science Foundation (NSF) under award numbers CMMI-1000198 and CBET-1133751.

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