Static Loading

Which is also known as the long-term load or Dead load. Static

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

loading is also constant. The deflection induced by static loading is

proportional to the stiffness.

Cyclic Loading

There are two types of Cyclic loading;

1)

Qausi-static loading is time independent, the

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

the structure due to this kind of loading is proportional to the loading.

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.

Stochastic Loading

This type of loading can be seen as loading in a random and

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

Loading

Aerodynamic loading is simply forces that result from the

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