On the Improvement of Starting Torque of Darrieus WindTurbine with Trapped Vortex Airfoil P. Mohan Kumar 1,2 M.Mohan Ram Surya 1 N.Srikanth 1 pmkumar@e.ntu.edu.sg mohanram001@e.ntu.edu.sg nsrikanth@ntu.edu.sg 1 Energy Research Institute at NTU, Nanyang Technological University,Singapore. 2 School of Science and Technology, Singapore University of Social Sciences. Abstract with high power coefficient, yet they are poor performers in low wind speed regime. As a potential solution to improve the low wind speed performance, a notched airfoil (KF-N- 21) was incorporated into straight bladed Darrieus rotor and experimentally compared with conventional NACA0021 rotor from the earlier study. The experimental results displays the superior performance of KF-N-21 in the low Reynolds number without degrading the performance at the high Reynolds number .This paper presents the computational validation of previous experimental investigation and to numerically explore the two dimensional unsteady flow field over the notched airfoil in comparioson with NACA 0021 .A detailed flow characteristics are presented in terms of velocity and pressure distribution by capturing the vortex bubble formation .The simulation results shows a good agreement with the experimental data endorsing the performance of KF-N-21 airfoil for a wide range of Reynolds number. Index Terms—Darrieus Wind Turbine,2D VAWT CFD,Kline Airfoil,Vortex,Starting Torque,Experiment„Numerical study I. I NTRODUCTION Wind power plays a major role in renewable energy generation, experiencing a tremendous growth in last two decades. Among vertical axis wind turbines, Darrieus type wind turbines are potential wind energy harvesting devices for challenging environments due to its distinguished advantages over Horizontal axis wind turbines (HAWT)(1). The Darrieus turbines are lift based and can achieve higher power coefficient among the vertical axis turbines. But they are not a preferred choice for low wind speed regions (2) due to poor self- starting capability (3) and inability to sustain its acceleration. The possible causes are low startup torque generation and high incidence angle respectively .From the experience of tailoring the airfoil profile for ease of manufacturing (4),au- thors proposed to incorporate a notch in the airfoil similar to Kline -Fogelman (KF) airfoil (5) .A detailed experimental investigation was carried out with three bladed Darrieus rotor, with KF-N-21 and NACA 0021 airfoil (6) .Both the models are tested in the wind tunnel for a range of tip speed ratio to obtain the C p -lambda in order to validate the proposed solution. The experimental results conclude that the KF-N-21 airfoil outperforms NACA 0021 for the Re range of. The objective of the current numerical study is to explore in detail over the complex vortex formation and associated flow characteristics. The study will provide an insight for further optimization of KF-N-21 airfoil geometry to increase the annual energy output of the turbine. II. CFD DOMAIN CHARACTERSTICS A 2D numerical simulation was carried out on a 3-straight bladed Darrieus turbine with KF-N-21 airfoil and NACA 0021 airfoil. The results are compared in terms of torque coefficient C t and power coefficient C p .The support arms are eliminated in the experimental model, instead the blades are supported by the end plates on the top and the bottom. The diameter of the end plates extends up to the rotor diameter with negligible drag. The current blade support with end plates eliminates the blade tip vortices. Another vital advantage is the elimination of parasitic drag (7),which is substantial for the model of this size and it is the primary reason for the discrepancy between the numerical predictions and the experimental value. With these enhancements,2D simulation will be appropriate for the current study. The rotor under investigation and the computational domain dimensions are shown in the Fig.1(a) and Fig.1(b). A. Description of the numerical flow field The computational study was carried out by using the commercial software -Ansys Fluent.The front side of the domain is assigned to inlet boundary condition with the wind speed varying from 4 m/s to 9 m/s. The rear side of the domain was assigned to outlet boundary condition of atmospheric pressure. Rest of the sides of the domain are considered as walls. A structured mesh has been generated in the vicinity of the blade, with y+ < 1 and edge sizing of 0.1 mm to capture the vortex bubble formation. A gradual cell growth is achieved from the blade boundary to adopt to the unstructured triangular elements of domain .Mesh independent study is performed to determine the optimum number of cells. The cell count is varied from 0.3 million to 0.6 million and the influence on the performance coefficients is less than 1.1 %. Hence an optimum cell counts of 0.5 ± 0.05 million was adopted for the entire study. Second order spatial discretization was employed to improve the numerical accuracy of the sliding mesh technique.Five complete revolutions are computed with constant time-step. The performance coefficients are computed for every degree of revolution, averaging the values for the last three revolutions, until a stabilized flow field is established. — The Darrieus wind turbines are simple in design