Study Of Dynamic Stall Model Of Rough Surface Airfoil

In wind turbines,the blades are often eroded by pollutants such as insect debris and leaves,resulting in a rough surface that significantly affects the aerodynamic characteristics of the airfoil.However,currently,there is a lack of mathematical models that can accurately predict the dynamic stall performance of rough airfoils.To address this issue,this paper develops a dynamic stall model adapted to rough airfoils based on the Beddoes-Leishman(B-L)dynamic stall model.The model considers the delay effect of effective angle of attack caused by the roughness effect during the dynamic pitching motion of the airfoil.The impact of the roughness model on the calculation of blade operational loads is investigated based on this model.The main research content and conclusions are presented as follows:(1)The original B-L model has been modified to effectively improve the prediction accuracy of the dynamic stall model.To address the difference between the normal and tangential separation points that was not considered in the original B-L model,this study introduced Liu’s separation point prediction model based on the B-L model and developed a new dynamic stall model.The modified model was validated using the dynamic experiment results of the S809 airfoil under various mean angles of attack,amplitude of attack,and reduced frequencies.The results show that the modified model can match well with the experimental values and effectively improve the prediction accuracy of the original model.(2)Developed a dynamic stall model for rough airfoils.Compared to smooth airfoils,the surface roughness of airfoils reduces the sensitivity of the flow to the incoming wind,leading to earlier entry into stall and turbulence states.The dynamic stall model of smooth is no longer applicable to the actual operation of rough airfoils.To address this issue,an attenuation coefficient is introduced in the dynamic pitching motion to modify the effective angle of attack formula.The accuracy of the model is verified using rough S809,S814.S815,and S825 airfoils with different thicknesses.The study found that when the attenuation coefficient βis set to 0.9,which delays the effective angle of attack of the rough airfoil by about 10%compared to the smooth airfoil,the accuracy of the model’s predictions for dynamic aerodynamic characteristics is significantly improved.The results match well with experimental values.(3)The study examines the impact of the accuracy of the dynamic stall model on the operational loads calculation of the wind turbine.Based on the bladeelement momentum theory(BEM),the NERL 5MW wind turbine was studied using both smooth and rough airfoil dynamic stall models to evaluate operational loads.It was found that the smooth airfoil dynamic stall model significantly underestimated operational loads at the blade root(r<30%)with a high underestimation rate of up to 5%at wind speeds of 8,10,12,14,and 16m/s;However,at the blade tip(r>50%),the model was overestimated by about 1%to 2%.The impact on blade design cannot be ignored.The research presented in this paper has important theoretical significance and engineering application value for the assessment of wind turbine loads and blade design.