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Winglets (WLs) have recently been used to improve the performance of horizontal axis wind turbine (HAWT). The WL geometry is a key parameter for diverging blade tip vortices away from turbine blades and reducing induced drag. The present study focuses on the effect of winglet height (H) and toe angle ( w ) on the turbine performance. The performance of a three-bladed rotor of 1 m diameter with SD8000 aerofoil is numerically investigated using ANSYS 17.2 CFD on a polyhedral mesh. The model is hence validated by comparing results for power coefficient (C pw ) with experimental values available in the literature. Four different values of H are considered while keeping w constant at 0 • . H of 0.8%R is proved to be the best height for performance enhancement. It increases C pw by 2.4% at tip speed ratio = 7. The toe angle effect is studied for upwind and downwind WLs. The results show that C pw increases as w increases up to w = +20 • at all values of . C pw increases by 6% at = 7. Downwind WL always reduces C pw . The present results are well explained by the resulting vectors map near the blade tip. Using WL with the optimum H and w , causes 6% increase in C pw as compared to rotor without WL. K E Y W O R D SCFD, toe angle, wind turbine, winglet, winglet height INTRODUCTIONGlobal warming and fossil fuel emissions are the main drive and motivation for finding alternative sources of energy. Wind energy is one of the most viable alternative energy sources. It is expected to support the global electricity by more than 20% by 2030. 1 Many researchers have studied the aerodynamics behavior of the flow around wind turbines in order to understand the wind kinetic energy extraction by rotor. The flow around wind turbine is very complicated due to turbulence generation and vortices. Experimental studies need sophisticated measurements techniques and equipments. Verified numerical modeling has been widely used in order to better understand the flow within the turbine and in its wake.Blade Element Momentum Theory (BEMT) and Computational Fluid Dynamics (CFD) are the most common approaches that are used to calculate the aerodynamic forces. 2 BEMT is the basic theory of wind turbine blade design by combination between momentum theory and blade geometry. It solves set of equations at each blade element by balancingThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

In this paper 3D simulations of four different horizontal axis wind turbine (HAWT) blade shapes with the same radius (0.65m) and airfoil profile (NACA4418) are presented. The first blade shape is optimal twist and tapered (OPT); this blade is designed using blade element momentum (BEM) theory. The second is un-tapered and optimal twist (UOT), this blade has the same twist distribution as the (OPT) but with a constant chord. The third is tapered un-twisted (TUT), this blade has the same chord variations as the OPT blade. The fourth is untapered un-twisted (UUT). The effect of nacelle, shaft and tower existence on the performance of the four designs has been investigated also in the present work.All simulations are performed by using shear stress transport (SST) k-ω turbulence model. The power coefficient of OPT blade reach to 0.317 at TSR = 5. Meanwhile, the maximum power coefficient (Cp=0.3348 at TSR=4) has been recorded in the UOT blade. The TUT and UUT blade recorded a lower power coefficient, this is due to their always operations in stall and turbulence conditions.

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