Theoretical considerations in an approximate method for estimating the blade performance of a Darrieus-type cross-flow water turbine

Fluid forces acting on a Darrieus blade were examined with an inviscid flow analysis. Calculated results showed that the fluid forces generated by velocity distribution along a Darrieus blade surface were much larger than the other additional forces by a circular motion of a blade. And then it was clarified that a flow around a blade in a circular motion could be transformed approximately to a flow around a blade in a rectilinear motion by use of equivalent circlar camber and equivalent angle of attack, with the condition that an angle between the resultant force and camber line of the blade was equal mutually in the two flow fields. As the consequence, an approximate method based on fluid dynamical correspondence between a blade in a circular motion and a still one in a uniform flow was also proposed for predicting Darrieus blade performance with airfoil data obtained by a blade at rest in a uniform flow.