Study On Aerodynamic Performance And Mechanism Of Trailing Edge Flap On Wind Turbine

With the rated power of wind turbines and the size of blades increasing, cost of wind electricity system increases a lot, so reducing the cost is an important issue to promote wind electricity development. The blade is a key component of wind turbines. Decreases of blade loads will result in decreases of loads of other related components, and then substantial savings may be realized to reduce the total cost. The trailing edge flap is considered to be one of the most effective active control methods. Steady and unsteady computational fluid dynamics methods are adopted in this paper to study the aerodynamic performances of a two dimensional wind turbine airfoil and a three dimensional wind turbine blade. Influences of trailing edge flaps on fluid structures and mechanism of trailing edge flaps are investigated. The flap could reduce the loads fluctuation resulting from wind variation, and then fatigue loads are reduced. The results may help to promote the engineering use of trailing edge flaps and provide a direction of adjustments of flap control methods and modifications of blade element momentum methods for study of trailing edge flaps. Four equations Transition SST model including a transition model is adopted to model the turbulence, and the dynamic mesh method is used to evaluate the grids as trailing edge flaps deflect.First, the two dimensional S809airfoil and three dimensional NREL Phase VI blade are simulated to verify the validation of the model. It indicates that the Transition SST model is more accurate than fully turbulent models. For small angles of attack, compared with the experimental results, the predicted lift coefficients of S809airfoil are accurate to within3%and the drag coefficients to within10%, but the drag coefficients predicted by fully turbulent models are larger than measurements by as much as100%. For the NREL Phase VI rotor, the predicted low speed shaft torques are accurate to within4%, more accurate than fully turbulent models whose error is about24%under partial separation conditions. Moreover, the predicted locations of laminar separation bubbles and transition points are very close to experimental results.Next, related issues of the trailing edge flap on a two dimensional airfoil are investigated:1) By analyses of the stationary trailing edge flap, the optimal parameters of the flap are obtained and the capacity of the flap to change the aerodynamic parameters of the airfoil is investigated. The results show that, among the studied parameters, a continuously deforming elastic trailing edge flap with10%chord length,10°flap angle and a shape function of one order power function obtains better results which mean large enough lift coefficients variations and fairly low drag coefficients and hinge moment coefficients variations as well as a stable flow field. When the angles of attack are not too big, the angles of trailing edge flap should be2.44times those of angles of attack to obtain the same variations of lift coefficients.2) Reduction of fatigue loads is achieved by oscillation of the trailing edge flap, so analyses of the dynamic properties of the trailing edge flap will help modify the control strategies according to properties of inflows and the trailing edge flap. Under constant inflows, influences of certain parameters, such as angular frequencies, amplitudes of deflection angles, lengths of the flaps, angles of attack, mean angles of deflection angles, et al., on the oscillating trailing edge flaps are investigated. When no obvious separations are observed, the lift coefficients lag the deflection angles and the phase differences will influence the effect of the trailing edge flap. The phase difference first increases with the angular frequency, and then decreases. Moreover, the phase difference decreases when the amplitude of deflection angles, length of the flap and angle of attack increase. If the trailing edge flap does not locate at its maximum or minimum angles, the lift coefficient varies linearly with the deflection angle. When the angular frequency decreases, as well as amplitude of deflection angles and length of flap increase, the absolute value of the slop increases, which indicates that the capacity of the flap alternating lift coefficients increases.During simulations of the oscillating trailing edge flap under constant inflows, the unsteady level can be described by the reduced frequency which takes the length of the flap as the characteristic length. When the value of the flap reduced frequency is greater than or close to0.01, the flow field appears obvious unsteady characteristic and the wake vorticity appears the trend of bend and rolling up. When the reduced frequency is big enough, periodic vortex shedding shows up. If the reduced frequency is fairly small, the flow could be considered as quasi-steady, and the wake vorticity is similar to that of the steady condition.3) With the optimal parameters of the trailing edge flap adopted and the phase angle of the flap modified according to the dynamic analyses, the influence of the trailing edge flap on an airfoil under sinusoidally varying wind is investigated. As the effect of a10%chord length trailing edge flap movement by the same frequency and phase angle with the wind, the variation of the normal force coefficient is reduced by90%. With a proper phase difference added to the flap movement, the reduction rate increases to93%, and the normal forces vary more smoothly. The trailing edge flap influences pressure almost all over the airfoil surface rather than the finite chordwise region of the flap.Finally, based on results of two dimensional analyses, the aerodynamic properties of the trailing edge flap on a three dimensional wind turbine blade are investigated. When the wind varies by the sine law, under the impact of an oscillating trailing edge flap, the reduction rate of the root flap bending moments is about27%～38%according to different cases. With a proper phase difference added, the reduction rate increases, and the moment curve is smoother. The flap influences not only the sections with a flap mounted, but also other spanwise regions. When the flap deflects, the pressure near the surface of the deflecting direction increases, and the streamlines deflect towards both sides along spanwise direction from the center of the flap, especially near the junction. Opposite changes appear on the other side of the flap. Considering the continuity of the fluid, when the pressure changes near the trailing edge flap, similar changes appear around the nearby sections. The influence of the trailing edge flap is most obvious on the very section where the flap locates, and with the spanwise distance from the flap increasing, the impact decreases.