Numerical Study Of The Effect Of Surface Grooves On The Aerodynamic Performance Improvement Of Small Wind Turbines

Traditional large wind turbines may bring potential impact on climate conditions and thus leads to the risk of global warming.Decentralized small wind turbines can be used as an alternative scheme to meet the increasing power demand without substantial side effects.However,the low Reynolds number condition of small wind turbines would lead to laminar separation,transition and reattachment in the boundary layer of wind turbine blades,and hence degrading their service performance.The characteristics of the boundary layer at low Reynolds number are sensitive to the environmental parameters,which increases the difficulty of its controlling.Compared with other turbulence generators,the indented groove has less drag loss while reducing the boundary layer separation.It is considered to be an effective way to improve the aerodynamic performance of small wind turbines.However,the design principles and control mechanisms of indented grooves are not fully understood.Therefore,exploring the groove characteristic parameters and reveal its control mechanism is important in engineering applications.The research results further enrich and develop the understanding of flow separation control under low Reynolds number,and provide theoretical guidance for the design of small wind turbines.The prediction capabilities of the Unsteady Reynolds Average Navier-Stokes(URANS)method coupled with four transition turbulence models are evaluated.The decay behavoir and sensitivity of the four models to the inlet boundary conditions is discussed.The application principle of those fluid dynamics models under working conditions of small wind turbines is recommended.The research results give a further evaluation of the model's predictive ability under the conditions of lower Reynolds number with thinner airfoil,which is not considered in the previous research.And thus,is fundamental to accurately predict the transition characteristics of the flow field at low Reynolds numbers.The effect of the dimensionless parameters,which are the depth ratio(h/?)and the aspect ratio(h/w)and the groove profile and position to eliminate the laminar separation bubbles and improve the aerodynamic performance of the airfoil are studied.The vorticity field,time-averaged streamline and lift-drag characteristics are analyzed to discuss the influence of different characteristic parameters of the groove.The results point out that the key influencing characteristic parameter is the depth ratio,and the boundary layer thickness defined in the groove depth ratio should be calculated at the entrance of the groove front edge.Under different Reynolds numbers and angles of attack,the effective depth ratio range is between h/?=1.0-1.5.The groove should be located in front of the separation bubble.By comparing the relationship between the stability characteristics of the vortex in the groove and the aerodynamic performance of the airfoil,the controlling mechanism of the groove is analyzed.It is believed that the "air bearing" effect of the vortex formed in the groove is the possible explanation of the better controlling effect than other methods.Based on the results of the numerical simulation,the effectiveness of the surface grooves in eliminating separation bubbles was verified through water tunnel experiments.The pitching motion of airfoil is simulated based on the dynamic mesh method and verified by the existing simulation and experimental results.The controlling effect of three groove with different characteristic parameters under dynamic conditions is discussed.Under different pitching amplitudes and reduce frequencies,the lift coefficient,drag coefficient,pitch moment coefficient and aerodynamic damping coefficient with and without grooves are analyzed.The results show that under dynamic conditions,the groove structure can still eliminate the separation bubbles,and reduce the aerodynamic hysteresis and vibration characteristics.The design criteria and control mechanism in the static state are still applicable in the dynamic state.Also,the position of the groove has a significant effect under dynamic conditions,and the position of the trailing edge of the groove at x=0.1c can provide a better control effect.Finally,the influence of the groove structure on the overall operating efficiency of the small wind turbine was evaluated by the Blade Element Method.Arranging the indented groove on the blade increases the maximum power coefficient by 2%,reduces the thrust and torque at the root of the blade,and improves the stability of the blade.