Research on Aerodynamic Performance of an Wind Turbine Airfoil With Leading Edge Ice

The performance of wind turbine was influenced by the environment. Among them, airfoil with leading edge ice has a great effect on the changes of aerodynamic performance. This study calculated the performance of an wind tubine airfoil at two iced shape model by CFD simulation using LES. LES in various models has been developed to simulate turbulent flows, especially to separated flows. In this investigation, 2D LES has been used to simulate flow past a wind turbine airfoil with leading edge ice which is a classical separated flow. The results show that flow structure is more complex with abundant whirlpools signifying violent turbulence when airfoil with ice and leads to poorer performance of wind turbine.


INTRODUCTION
For the last decade the predominant configuration of modern wind turbines has been the three-bladed upwind pitch-controlled one.The researchers are often focous on the tubine power output.In all influence factors, the aerodynamic performance of blade is the dominant and complex one (Hansen, 2008).The response of the aerodynamic load depends on whether the boundary layer is attached or partly separated (Spera, 2009).Ice accretion can be a problem during certain weather which should be avoided due to the possible risk of decreased lift and increased drag on the blade.Some research shows that ice adhering to airfoil can cause severe problems, increased loads on the structural parts as well as altered aerodynamic behavior and changed the flow field structure increases the flow separations (Dalili et al., 2009).This is of course a subject of concern in regions with cold climate where the weather conditions make certain structures prone to ice accretion.
Accurate prediction of flow separation from airfoils is still a challenge for computational fluid dynamics today.As flow separation on airfoils is an undesirable phenomenon due to the fact that it is associated with lift breakdown and limits the usable range of the angle of attack.The reliable prediction of such flow configurations is great technical interest (Chao, 2006).As to the numerical studies, direct numerical simulations have good results for turbulent flows.But it needs more computational resources than LES for flow separation, transition, tip vortex and flow control (Tetsuro et al., 2007).Furthermore, LES has greater predictive potential than Reynolds-Averaged Navier-Stokes (RANS) methods.
In what follows, we study three airfoil models using 2DLES.One of the three model is standard airfoil.Another two models are airfoils with leading edge ice which summary frome from the literature (Hochart et al., 2008).The aim of this study is to provide an accurate simulation of the flow past a wind turbine airfoil in deep fall using a large eddy simulation in two dimensions and get detailed database to calculate the wind tuebine loads.

2D-LES governing equations:
The governing equations for LES of a neutrally stratified incompressible atmospheric boundary layer are the filtered Navier-Stokes equations: where turbulent stresses are defined as ߬ = ‫ݑ‬ ప ‫ݑ‬ ఫ തതതതത, ݂ is the Coriolis parameter, ݁ is the permutation tensor, ‫ݑ‬ is the geostrophic wind, ‫ݑ‬ ത and ܲ ത velocity and filtered non-hydrostatic component of pressure, respectively (Kim et al., 2006).In all simulations, we use periodic lateral boundary conditions and impose a stress-free condition on the upper boundary.At the lower boundary, the vertical component of velocity is set to zero and horizontal components of the turbulent stresses are defined based on the mean logarithmic wind profile assumption as follows 2DLES.
In two-dimensional incompressible turbulent flow, f is divided into two categories: where ݂ ̅ is large scale components, ݂ components: where, ∆ i= h i , h i is grid length in x equation and Navier-Stokes equations are filtered with Eq. ( 5), the continuity equation and equations are filtered:

Numerical model and boundary condition:
Numerical model: The standard airfoil in this DU21 which is a member of the airfoil family DU, where 21 mean that the thickness of the airfoil is 21% chord.It has good aerodynamic performance and lift drag ratio, mainly be used in the tail of wind turbine blade.Its airfoil-data coefficients w graphically in NREL technical report (Timmer and Van Rooij, 2003). (5) x i , the continuity Stokes equations are filtered with and Navier-Stokes ( 6)

Numerical model and boundary condition:
The standard airfoil in this study is DU21 which is a member of the airfoil family DU, where 21 mean that the thickness of the airfoil is 21% chord.It has good aerodynamic performance and lift-todrag ratio, mainly be used in the tail of wind turbine data coefficients were illustrated graphically in NREL technical report (Timmer and Van Figure 1 and 2 are the Grids of accreting rime and accreting glaze.Accreting rime and accreting glaze are the main forms of leading edge ice accretion.As can be seen from the picture, there are big differences between them at shape.For the purpose of improving the precision, the grid pattern was improved and locally refined.

Boundary condition:
To capture the physics of vortex formation around the airfoil trailing edge and easy to comparative analysis the aerodynamic performance Re = 2×10 6 (U 0 = 3 0 m/s, C (Chord Length) = and attack angles from 0° to 90 simulated by the 2D-LES.In the simulation, field is calculated by using an C-H The inlet boundary is placed five times chord length (5C) upstream of the leading edge of the blade while the outlet boundary is placed ten times chord length (10C) downstream of the trailing edge of the blade (Yoshihide and Ted, 2009).

RESULTS AND DISCUSSION
The aerodynamic characteristics of airfoil mainly caused by the change of flow field.Figure 3  The streamlines show the developme vortices at leading end trailing edge.Focusing on the trailing edge separation this happens sooner and more pronounced for ice B when comparing the individual angle of attack plots with the ones for the clean airfoil and ice case A. That means at some attack of angle, the main forms of leading edge ice accretion.As can be seen from the picture, there are big differences between them at shape.For the purpose of improving the the grid pattern was improved and locally To capture the physics of vortex airfoil trailing edge and easy to the aerodynamic performance, (Chord Length) = 100 cm) to 90° were numerically In the simulation, the flow H type grid system.The inlet boundary is placed five times chord length ) upstream of the leading edge of the blade while en times chord length ) downstream of the trailing edge of the blade

RESULTS AND DISCUSSION
The aerodynamic characteristics of airfoil are mainly caused by the change of flow field.Figure 3  The horn-like protrusion shape severely affects the flow and thus the aerodynamic properties of the airfoil.It is obvious that the case A lift coefficient for all tubine work angles of attack (0°-30°) is lower than the clean airfoil due to the early leading edge separation and the more severe trailing edge vortices which developed at lower angles of attack than for the clean airfoil.The maximum CL of 1.05 is reached already at 11°.
It is seen that the lift coefficient is generally lower than both the clean airfoil and ice case A. C L increases with angle of attack until 8° where after there is a decrease followed by another increase.The course of the curve with the lower angle of attack for maximum lift and the value for maximum lift corresponds well with the flow development described and compared to the clean airfoil and ice A above.

CONCLUSION
Two-Dimensional (2D) Large-Eddy Simulation (LES) and a hybrid RANS/LES were used to predict the flow past a wind turbine airfoil (DU21) at Re = 2×10 6 and angle of attack from 0° to 90° aiming at comparative analysis of their features and performance.In order to check their feasibility and get the aerodynamic performance of an airfoil with leading edge ice.
The lift coefficient of case A and B is generally lower than for the clean airfoil at most work angles of attack.Drag is increased compared to the clean airfoil due to the more severe separation resulting in increased pressure drag.
Ice case A and B changed the flow field structure and ailfoil aerodynamic performance.For wind turbine, the phenomenon cut down the power and added loads of blade and wind turbine foundation.In some places, we can't altered the climate, but in order to ensure the safe operation of wind turbine, we must consider this factor in the wind turbine blade design.
Figure 1 and 2 are the Grids of accreting rime and accreting glaze.Accreting rime and accreting ܲ ത are the filtered hydrostatic component of ., 2006).In all simulations, we use periodic lateral boundary free condition on the upper boundary.At the lower boundary, the vertical component of velocity is set to zero and horizontal the turbulent stresses are defined based on the mean logarithmic wind profile assumption as dimensional incompressible turbulent flow, (3)

Fig. 1 :
Fig. 1: Grid of accreting rime, case A are Comparison of the flow field structure for three (a) Is the streamlines of clean airfoil at attack of angle 15°, (b) is streamlines of case A, (c) is case B

Fig. 3 :Fig. 4 :
Fig. 3: Comparison of the flow field structure for three case at 15° coefficient of case B is more low and case A and case B have in deep stall.As seen from Fig. 4a, at 45° angle of attack, clean airfoil trailing-edge vortex separation occurs, resulting in movement of the flow field near the airfoil stall.Full separation come up with the increase of initial attack