Large Eddy Simulation Of Airfoil For Wind Turbine

Under the background of increasing prominent globle energy crisis,wind energy as a kind of clean,environment-friendly energy has been coming up a rapid development.Wind turbine is a power machine that converts wind energy into mechanical energy.As one of the key components of the wind turbine,the wind turbine determines the ability of the wind turbine to capture wind energy.And airfoil is the basic element of rotor,it can be said that the aerodynamic performance of airfoil is a key parameter affecting the power of wind turbine.Therefore,research on the aerodynamic performance of wind turbine airfoils is very important.For this reason,this article focuses on the aerodynamic performance and flow characteristics of wind turbine-specific airfoils.Based on the N-S equations,the two-dimensional S809 airfoil is numerically simulated using RANS(Reynolds Average Navier-Stokes)and LES(Large Eddy Simulation)methods,respectively.The variation of the aerodynamic performance of the two-dimensional airfoil with the angle of attack is studied,and the simulation results based on the RANS are compared with the time-averaged results of LES.Furthermore,the three-dimensional S809 airfoil is modeled and numerical simulations are performed using the LES method.The aerodynamic performance and three-dimensional flow phenomena of the three-dimensional airfoil along with the angle of attack are studied.Finally,the flow field of the two-dimensional airfoil and the flow field of the three-dimensional airfoil are compared and analyzed,and the influence of the three-dimensional flow phenomenon on the flow characteristics of the airfoil is studied.The main conclusions of the research on wind turbine-specific airfoil S809 are as follows:(1)At small angles of attack,the large eddy simulation time-averaged results are basically similar to the steady flow field;when the attack angle is large,the time-averaged flow fields of the large eddy simulation and steady flow fields are different,and the larger the angle of attack is,the larger the difference is.The performance of the LES method is better than that of the RANS method in capturing the meso-structure of the flow,especially for the capture of tiny vortex structures.(2)For the two-dimensional S809 airfoil,the unsteady characteristics of the flow around the airfoil are studied by analyzing vorticity cloud diagram,pressure cloud diagram,pressure coefficientat different times during the flow around the airfoil at the same attack angle and flow velocity.The results show that at small angle of attack,the unsteady characteristic is not obvious,and a small number of small swirls are followed by the tail edge of the airfoil,and the unsteady characteristic is remarkable at the large angle of attack,and a large range of separation vortices are formed on the surface of the airfoil.Moreover,with the increase of the angle of attack,the unsteady characteristic is more obvious,and the separation vortex shows obvious periodic variation.The turbulent characteristics of the flow around the airfoil are analyzed under different attack angles and flow conditions.The results show that the turbulence intensity,turbulent kinetic energy and Reynolds stress are larger in the direction of the wing chord when the attack angle is small,and with the increase of attack angle,the turbulence intensity,turbulent kinetic energy and Reynolds stress of the airfoil are larger with the increase of the angle of attack.The value of the airfoil above the chord direction is larger,and its numerical value increases obviously.At the same time,with the development of airfoil wake,turbulence intensity,turbulent kinetic energy and Reynolds stress gradually decrease to 2.05°angle of attack,and resume to the state of incoming approximately 2.5 times the chord length from the airfoil trailing edge,and when the angle of attack increases,the recovery distance of wake increases to 20.16°angle of attack.At the back of the airfoil,about 15 times the chord strength they resume to the flow state.(3)For the three-dimensional S809 airfoil,the three-dimensional vortex structure of the airfoil is analyzed under the condition of different attack angles.The results show that when the attack angle is small,mainly manifested as turbulent vortex structures along the flow direction,and the turbulence vortex structure increases with the increase of the attack angle.The separation vortex is produced in the front edge of the three dimensional airfoil,and the phenomenon of 3D flow is obvious.The vortex of the tail edge of the airfoil alternately shedding forms a shape similar to the vortex street structure in the wake region of the airfoil.Under the condition of the same angle of attack,the pressure coefficients of three dimensional airfoils at different cross sections along the span direction are compared and analyzed.The results show that there are differences in the distribution of pressure coefficients along the different sections along the span direction,and the difference is small when the angle of attack is small,and the difference is small with the increase of the angle of attack.When the angle of attack increases,the difference becomes more and more obvious.It shows that the three-dimensional flow phenomenon of the three-dimensional airfoil is not obvious when the attack angle is small,and the three-dimensional flow phenomenon becomes more and more obvious with the increase of the angle of attack.Moreover,the shape and size of the vortex structure of the three dimensional tail edge shedding is different from that of the two dimensional wing tail edge shedding.(4)Under the condition of the same angle of attack,the distribution characteristics of the surface friction coefficient and the limit streamline of the three dimensional airfoil are studied.It is found that the surface friction coefficient of the front edge of the airfoil is larger,and this region exhibits the characteristics of attached flow with the increase of the angle of attack.When the area with larger distribution of friction coefficient becomes smaller,and the area with limited streamlines of airfoil surface becomes larger than 18.19°angle of attack,the friction coefficient is larger only in the smaller region of leading edge position and tail edge position,and the limit flow line of airfoil suction surface is basically in a disordered state.(5)Under the condition of the same angle of attack,the variation of Reynolds stress along y direction at different chord position and wake area of airfoil suction surface is analyzed.It is found that when the angle of attack is 2.05°,The Reynolds stress values in three directions at the selected four monitoring points are basically zero,showing laminar flow characteristics.With the increase of the angle of attack,the Reynolds stresses in the three directions in the middle position of the suction plane of the airfoil are increased.But its number is less than the Reynolds stress value of the monitoring point at the tail edge position,and the Reynolds stress value of the three directions near the wake monitoring point of airfoil is generally larger than the Reynolds stress value of the far wake monitoring point in three directions.