AbstractThe flow field through the vertical axis wind turbineis assumed to be two-dimensional in the mid-plane of the machine. The vorticity shed by the blades is concentrated into vortex blobs, i.e. circular regions with uniform distribution of vorticity. The blob radiiare assumed to grow with time starting from a zero value at the time of their generation, according to√40νt in whichνis kinematic viscosity. The bound vorticity of the blades isconcentrated in their aerodynamic center. The velocity field induced by the vortices is calculated by Biot-Savart integration. The lift-and drag coefficients of the blades are taken from static airfoil data for a profile Reynolds number calculated with the mean relative velocity during one revolution. The vorticity shed by a blade during a time step is taken equal and opposite to the change of circulation on the blade during the same time step in order to keep constant the total amount of vorticity in the flow field. The motionof the free vortices is calculated by Run ge-Kutta integration of four thorder based on the velocity vector in the center of the blobs.Results are presented shewing the vertex streets for one-and three bladed machines forone tip speed ratie. For a one bided machine, a power coefficient versus tip speed ratio characteristic is shown. Fera three-bladed machine, the agreement be t we en the calculated power coefficient obtained by free vortex analysis and obtained by mu ltiple stream tube theory is shown.