Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 113

Figure 25: Vertical axis wind turbines.

Double Actuator Disk

0.7

0.6

0.5

Betz-Limit
first disk
2nd disk
total cp

0.4

0.3

0.2

0.1

0

0 0.1 0.2 0.3 0.4 0.5

a

Figure 26 : c P (a) for a double actuator disk.

Figure 27 shows a comparison between various methods of calculating c P . The
fi rst method [18] uses a simple procedure for the induced axial velocity which then
is a function of circumferential angle ϕ . To simplify matters this is assumed to be

max

a ( ϕ ) = ls / 2 sin ( ϕ ). This method results in a c which is only slightly smaller

P

than Betz

c . Holme's [66] method uses a vortex approach to describe the fl ow fi eld.
Here the unsteadiness was avoided by distributing the bound circulation around a
whole circle. This then gives a steady but non-symmetric model which also makes it
possible to calculate transverse forces. Figure 27 shows, however, only minor changes
to the fi rst method, that of Wilson [18]. A simple blade element method was formu-