Vertical axis wind turbine induced velocity vector theory

Abstract

Double multiple stream tube (DMST) models of the vertical axis wind turbine (Vawt) take its interference as unidirectional flow against the wind. However, in the standard blade-element momentum (BEM) theory of the horizontal axis wind turbine (Hawt), the interference velocity is proportional to the lift vector normal to the apparent wind. A high design tip speed ratio brings the apparent wind to tangential, making the BEM interference flow radial across the Vawt blades. The lateral drift induced by the Vawt-bound vorticity and the mean reaction velocity to the torque are not upwind at all. A new vector BEM formulation includes such crosswind Vawt interference. For the two Vawt passes through the wind, it predicts a lower and narrower operating peak of tangent and ‘passive pitch’ Vawts than the DMST or Hawts. However, it gives the pitch cam shape for broadening the operating peak and for benignly avoiding high downwind drag and reverse wake flow. The Prandtl tip correction is included and compared to isolated wing aspect ratio factors.