# Static in the wind is converted into useful mechanical

loading is constant as time goes on. The deflection that occurs due to static
proportional to the stiffness.

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1)
loading varies slowly, so inertial effects can be ignored. The deflection of

2)
Dynamic cycling, Dynamic loads can also produce
loads that can be seen as cyclic, where they repeat over and over again. A wind
turbine is subjected to dynamic loads, the wind, which can be predicted over a
course of time. Dynamic cyclic loads result in deflection that is related to
the dampening forces of the wind turbine. Particularly when the load
application frequency is close to the natural frequency of the structure.

unpredictable manner. It stems from wind turbulence ad is highly relevant to
the fatigue response of a wind turbine.

Loads derived from Cyclic and stochastic turbulence are the
ones to watch out for as they are usually the loads that cause structural
failures in wind turbines especially those due to fatigue.

Aerodynamic

wind.

The vast amount of kinetic energy in the wind is converted into useful
mechanical work in the rotor of a wind turbine.
The most important concept here is the conservation of momentum, the
more we conserve the greater useful energy we produce. The momentum exchange
occurs in the wind flow direction. The forces present in the rotor plane
produce useful power which is perpendicular to the stream of wind flowing
towards the wind turbine.

It is ideal that the oncoming wind stream is slowed down to
produce useful mechanical energy. In most cases the average wind turbine begins
producing useful energy at wind speeds of 3-4 m/s (8 mph) the following speed
is noted as ‘the cut in speed’. The maximum rated output power is achieved at
speeds of 15 m/s (30mph) as the electrical generator isn’t capable of higher
wind speeds. The turbines usually switch off when ‘the cut-out speed’ is
reached, which is around 25 m/s (50mph) to avoid storm damage 1.

Figure 1 2

The oncoming wind
stream can also impose thrust loads, these can be very problematic if they are
not accounted for during the design stage of the structure. The thrust is the axial force applied by the wind on the rotor of a wind turbine. Because all action
yields an opposite reaction, the thrust is
therefore also the axial force applied
by the wind turbine on
the wind. 3

The square of the wind speed (V) is proportional to the
axial thrust (T). The rate of change of axial wind thrust (T) on a turbine is
given by the following equation:

(1)

Where:

T          Axial wind thrust kN

V          Wind speed m/s

Air density kg/

Inflow angle

Lift coefficient

Drag coefficient

c          Constant