Page 21 - Wind Energy Handbook

LIST OF SYMBOLS xxi

streamwise velocity at the rotor disc
U d
U w streamwise velocity in the far wake
U e1 extreme 3 s gust wind speed with 1 year return period
U e50 extreme 3 s gust wind speed with 50 year return period
U 0 turbine upper cut-out speed
U r turbine rated wind speed, defined as the wind speed at which the
turbine’s rated power is reached
U ref reference wind speed defined as 10 min mean wind speed at hub
height with 50 year return period
U 1 strain energy of plate flexure
U 2 in-plane strain energy
v fluctuating component of wind speed in the y-direction; induced
velocity in y-direction; in-plane plate deflection in y-direction
V airspeed of an autogyro; longitudinal air velocity at rotor disc,
U 1 (1 a) (Section 7.1.9); voltage (shown in bold when complex)
V(t) instantaneous lateral wind speed
VA electrical volt-amperes
fibre volume fraction in composite material
V f
blade tip speed
V t
w fluctuating component of wind speed in the z-direction; induced
velocity in z-direction; out-of-plane plate deflection
W wind velocity relative to a point on rotating blade; electrical power
loss
x downwind co-ordinate – fixed and rotating axis systems; downwind
displacement
x(t) stochastic component of a variable
x n length of near wake region
x 1 first-mode component of steady tip displacement
X electrical inductive reactance
y lateral co-ordinate with respect to vertical axis (starboard positive) –
fixed-axis system
y lateral co-ordinate with respect to blade axis – rotating-axis system
y lateral displacement
z vertical co-ordinate (upwards positive) – fixed-axis system
z radial co-ordinate along blade axis – rotating-axis system
Z section modulus
Z electrical impedance (bold indicates a complex quantity)
ground roughness length
z 0
number of teeth on pinion gear
z 1
z(t) periodic component of a variable

Greek
Æ angle of attack – i.e., angle between air flow incident on the blade and
the blade chord line; wind-shear power law exponent
â inclination of local blade chord to rotor plane (i.e., blade twist plus
pitch angle, if any)