| Wind turbines are usually operated in highly unstable flow environments.The existence of turbulence could make the flow state around the rotating blade more complicated,which seriously affects aerodynamic performance prediction.Meanwhile,in order to further improve the applicability and conversion efficiency of wind turbines and reduce the cost of power generation,additional components can be added to change the flow around the rotating blade during the optimization design of the wind turbine.This technology has become the hotspot of current wind power research.However,most existing studies on such flow control devices are based on two-dimensional and static states.There is still a lack of evaluation and prediction research on aerodynamic efficiency changes of flow control devices considering unsteady flows such as turbulent inflow and oscillation.Application on rotary wind turbines has not been sufficiently verified.Therefore,this paper focuses on studying and analyzing the Gurney flap,using wind tunnel tests as the primary research method.Gurney flaps are applied in the wind turbine airfoil and rotating blade.The aerodynamic characteristics of the Gurney flap on the wind turbine airfoil and the rotating wind turbine with different turbulence intensities are studied,and the effects of the Gurney flap on static,dynamic airfoils and a three-dimensional rotating state are discussed.Combined with numerical simulations,the flow characteristics,the pressure distributions of the rotating blade and the mechanism of power enhancement of a wind turbine installed with Gurney flaps are studied,providing a reference for the aerodynamic analysis and prediction of a wind turbine with additional components.The main achievements are as follows:The aerodynamic characteristics of a static wind turbine airfoil with and without a Gurney flap under different turbulent conditions are studied by wind tunnel tests.The surface pressure and aerodynamic force coefficients(lift,drag and pitching moment)are compared and discussed.In order to quantify the aerodynamic efficiency of Gurney flaps,the benefit margin is introduced.Hence,the effects of geometric parameters and turbulence intensities on the aerodynamic characteristics of the static airfoil are ascertained.The results illustrate that flap height is critical for airfoil aerodynamic characteristics.The pressure difference at the trailing edge and aerodynamic efficiency caused by the Gurney flap increase with the turbulence intensity increase at 10.5%,but the efficiency of the Gurney flap decreases at a highly high turbulence intensity of 19.0%.Based on the experimental data,a semi-empirical formula is established to predict the lift increase of Gurney flaps on wind turbine airfoils.According to the time and frequency domain analysis of the near-wake flow field with and without a Gurney flap,adding the Gurney flap can lead the pulsation energy to migrate to the pressure surface at pre-stall conditions.In contrast,the Gurney flap can reduce the vortex shedding during boundary layer separation at the post-stall conditions.A dynamic airfoil aerodynamic test rig is designed and set up to expound the airfoil with Gurney flaps in the cases of high and low turbulent intensity,respectively.The results revealed that with the increase of turbulence intensity to 10.18%,the energy of the boundary layer increases,which could withstand a larger adverse pressure gradient,thus delaying stall separation.The difference in the depth stall angle becomes smaller,enabling the Gurney flap shows a better lifting ability during a whole pitching period,but the difference between the two maximum lifts becomes smaller.The aerodynamic control effect demonstrated that the Gurney flap has more minor lift changes and a positive aerodynamic damping coefficient under turbulent conditions,making the flapped airfoil more stable than the baseline airfoil.But the stability maintenance of lift coefficients in the stall range becomes worse.The flow field results indicate that the power spectrum density of the wake is mainly dominated by the periodic pitch motion and is affected by the vortex motion of different scales in the flow field.Turbulence makes the differences in flow distributions between the baseline airfoil and the flapped airfoil smaller.The existing L-B dynamic stall model is modified,the vortex motions during the upstroke and before the reattachment stage are mainly considered,and the test data comparison verifies the accuracy of the modified aerodynamic prediction model.According to the aerodynamic changes of the Gurney flap in the pitching period,the aerodynamic prediction module is added to this model.The variations caused by circulation and vortex motion are introduced to make the prediction model more suitable for a wind turbine airfoil with a Gurney flap.The time-averaged output power,load and power pulsation characteristics of a wind turbine with and without Gurney flaps are collected through a measuring test bench of the small-scale wind turbine.The time-averaged output power results revealed that the enhancement effect of the Gurney flap in the three-dimensional rotation is correlated with the aerodynamic variation of the corresponding airfoil equipped with a Gurney flap.The Gurney flap has no apparent power enhancement effect at a low tip ratio for the small-scale wind turbine used in the test.In contrast,Gurney flaps allow the rotating blades to operate better,resulting in a higher power coefficient and a more excellent optimum tip ratio at a high tip ratio condition.Meanwhile,the relationships between the instantaneous power pulsation of the wind turbine and the velocity pulsation of inflow are explored by the frequency domain method.Coupling behaviour existed between wind turbine output power and incoming flow.The output power spectral density function verified that the function attenuated exponentially by-11/3 in the inertia sub-region of the frequency domain.The addition of Gurney flaps caused the pulsation energy to appear at lower frequencies,with significant pulsation peaks in the original inertia sub-region.Research on thrust characteristics shows that the evolution of turbulence intensity has little influence on the time-averaged wind thrust coefficient but significantly increases the pulsation peak.The Gurney flap could make the thrust coefficient shift upward in the whole blade tip ratio range,and more attention should be paid to the instantaneous thrust coefficient pulsation peak caused by high turbulent inflow.Besides,a numerical simulation method is used to study the flow control mechanism of the Gurney flap installed in the rotating blades of a wind turbine,and the aerodynamic changes of the flow field and surface pressure of rotating blades with Gurney flaps are analyzed.A hot-wire anemometer is used to measure the instantaneous velocities at the horizontal plane of the hub height and the vertical plane behind the hub.The evolution law of the wake flow field of the wind turbine with and without Gurney flaps is analyzed to explore the differences in the wake characteristics.The flow field test results indicated that the additional velocity deficit caused by Gurney flaps installed in the middle of the blade is mainly in the rear of the flap installation position and the larger relative radius of the blade near the wake and its transition region.Gurney flaps also increase the value of turbulence intensity in the wake.The frequency-domain analysis shows that the differences in energy distribution caused by the Gurney flap are mainly reflected in the near wake region.After Gurney flaps are installed in the middle part of the blade,the near-wake power spectral density corresponding to the position behind the position of the Gurney flap and the position with a larger relative radius of the blade appeared to a significant peak value.Moreover,the vortex shedding frequency value is close to the rotation frequency of the wind turbine. |
PDF Full Text Request