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Darrieus turbine is a type of vertical axis wind turbines that unlike it's simple structure, behavior analysis is a hard computational task. Because of the complex flows around the machines, aerodynamic optimization problem that still remains an open question. In this paper, a numerical algorithm based on the Double Multiple Stream tube model is used to calculate the effect of the parameters that influence the efficiency of the Darrieus turbine. This method is a semi-empirical method using lift and drag coefficients obtained from experimental data. The comparison between the results of the present study with the experimental measurements shows that although the developed algorithm gives acceptable results, but, for higher rotational speeds gets than nominal rotational velocity, the model accuracy gets lower. The aim of this paper is to find optimal conditions, parametrically analyze the effect of blade thickness, solidity, Reynolds number, pitch angle and aspect ratio on turbine efficiency and start. The results show that increasing thickness, Reynolds number and solidity cause an increase in the turbine self-start capability. On the other hand, increasing the solidity of the turbine will reduce working range, and increasing the aspect ratio will increase efficiency especially at the nominal rotational velocity. The results also show that the designed turbine having variable solidity, can have the benefits of both low and high solidity turbines simultaneously. But manufacturing variable thickness blades doesn't have proper justification. Limited increase in pitch angle can also have positive effect on efficiency.

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Unlike HAWT, Darrieus wind turbines is faced with the self-start problem and high fluctuations at output torque. Because of the need for techniques based on meshing and coupling Eulerian fluid equations and the Lagrangian equations for moving rigid body, the calculation of rigid body acceleration and fluctuations torque of VAWT is very complicated and it is a function of the moment of inertia of the turbine. In most studies, regardless of this effect, the angular velocity of turbine is assumed to be fixed. In this study, for calculating the turbine rotational speed and position, the sum of wind-driven aerodynamic forces and external forces caused by friction and generators are calculated and placed into Newton's second law to calculate the acceleration, and integrating it in time steps. The simulation is performed unsteady and dynamic mesh is used for moving the rotor. The results could check the interaction between wind and rigid blades on the in the process of increasing the rotational speed of turbine, and simulate the rotor from the moment of rest until the turbine reaches its final rotational speed. The causes of reduction in torque at low rotational speed is investigated and it has been shown that high dynamic stall and passing high exergy flow into the rotor without interaction with blades results the power reduction. Moment of inertia has significant impact on the frequency and amplitude of rotational velocity, fluctuations of output torque and output power, which is important in mechanical analysis of blades’ fatigue.