A free-vortex analysis of a Darrieus rotor blade in nonsteady motion has been developed. The method uses the circle theorem to map a moving airfoil into the circle plane. The wake is modeled using point vortices. Nascent vortex strength and position are determined from the Kutta condition so that the nascent vortex has the same strength as a vortex sheet of uniform strength. The force on the airfoil is determined by two methods, integration of the pressure over the plate and from the impulse of the wake vortices. Both methods yield the same numerical results. A comparison with an analytical solution for a plunging airfoil gives excellent agreement. Results are shown for a one-bladed Darrieus Rotor at a tip speed ratio of three and two chord sizes. The numerical results indicate that the forces and moment on a Darrieus Rotor blade may be adequately approximated by quasisteady relationships although accurate determination of the local velocity and circulation are still required.

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