Aerodynamics and aeroelastics of wind turbines - Wind Power Generation and Wind Turbine Design - page 114

3-bladed H-Darrieus - Comparison of Methodes

0.6

Simple Actuator Disk
Linear Vortex Theory
Blade Element
CFD

0.5

0.4

0.3

0.2

0.1

0

0 0.5 1 1.5 2 2.5 3 3.5 4

Tip Speed Ratio

Figure 27: Comparison of different methods of describing a small VAWT.

lated by Stickland [74] in the 1970s. Here the the fl ow is divided into various
stream tubes and an internal computer code interation ensures the actual balance
of axial momentum. As a result of including drag the c P (TSR) curve now becomes
much narrower and the maximum also decreases to values common for HAWTs.
In addition CFD methods can be applied to VAWT. To avoid time-consuming
unsteady computations a special technique called interface averaging between
two computational volumes with different frames of references were used. One
volume in the vicinity of the rotor experiences inertial forces due to rotation. The
delicate point now is how to pass over to the second volume where no inertial
forces are applied. In summary a much smaller

max
c may be predicted by this
method. Meanwhile [71] 3D transient CFD simulation have been carried out.
Further research is obviously necessary to reduce the large differences obtained
by the various methods. It should be noted that manufacturers of small wind tur-
bines try to use the vertical axis principle again. Unfortunately no particular sys-
tem seem tobe able to compete against HAWT. This is mainly because most
systems do not possess a dynamically stable supporting structure.

P

7.2 Diffuser systems

Wind-concentrating systems such as shrouds, ring-wings and diffusers have been
being investigated for a period of about 50 years. A summary has been given by
van Bussel [20], together with a 1D momentum theory and a collection of experi-
mental data. Special emphasis has been given to a discussion of the infl uence of
the diffuser's exit pressure. It should be noted that most of the designs try to use a