high enough (more than 2 millions based on inflow velocity and
rotor diameter) to ensure a fully turbulent flow. The nondimensional
torque coefficient of the turbine rotor is defined as:
CQ ¼ Q
1
2 rV2
refDR (4)
where the turbine diameter D and radius R are used as characteristic
lengths; the reference velocity Vref is an absolute velocity
which can be for instance the upstream farfield velocity magnitude
V0 in the case of a uniform upstream farfield flow. The turbine
power coefficient is defined as:
CP ¼ P
1
2 rV3
ref D (5)
Since P ¼ uQ, the power coefficient can also be expressed as:
CP ¼ CQ l (6)
where l denotes the tip speed ratio (TSR):
l ¼ u R
Vref
(7)
The VAWT