The momentum model combines momentum theory with blade element theory [3]. It studies
the behavior of air flow on the blades and its forces. They can be further divided into single
stream tube, multiple stream tube, and double-multiple stream tube models. In the single
stream tube model first developed by Templin for VAWT [4], the turbine is placed inside a
single stream tube and blade revolution is translated in an actuator disk. The effects of the
stream tube outside are assumed negligible, and the wind speed in the upstream and
downstream sides of the turbine are assumed to be constant. This model suffers from
performance accuracy prediction, due to the many assumptions, and usually gives higher
prediction values.