Page 217 - Fluid Power Engineering

188 Chapter Nine

Probability distribution of wind speed at the hub is assumed to be
a Rayleigh distribution:

p (V hub ) = 1 − e −π(V hub /2 V ave ) 2 (9-2)

Wind profile as a function of height is assumed to follow the power
law with shear of γ = 0.2.
V (z) = V hub (z/z hub ) γ (9-3)

The turbulence standard deviation under normal conditions is as-
sumed to be:

σ 1 = I ref (0.75 V hub + 5.6) (9-4)

Extreme Wind Speed Model (EWM)
For computing design loads under steady extreme wind conditions
the following parameters are used or 50-year and one-year extreme
wind speed:

V e50 (z) = 1.4 V ref (z/z hub ) 0.11 (9-5)
V e1 (z) = 0.8 V e50 (z) (9-6)

For computation of steady extreme loads, a misalignment of ±15 is
◦
assumed.
For turbulent extreme wind the following wind conditions are
assumed:

V 50 (z) = V ref (z/z hub ) 0.11 (9-7)
V 1 (z) = 0.8 V e50 (z) (9-8)
(9-9)
σ 1 = 0.11 V hub
Similar to the normal wind profile model above, IEC 61400-1 defines
wind conditions for all other cases EWM, NTM, ETM, EDC, ECD,
EWS, and EOG. The wind conditions are used to compute the loads
on various components. The safety factors for ultimate strength and
fatigue are then used to size the components. Computations of design
loads has evolved to use of software programs that model aeroelastic-
ity of turbines subject to the wind regimes defined in the design cases.
Software programs in this domain include FLEX 5 (developed by Stig
8
Øye, Danish Technical University), HAWC2 (developed by Risoe,
9
Denmark), Bladed (developed by Garrad Hassan, UK) and others. In
these programs, the blades, nacelle, and tower are discretized to create