Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 498

Turbulence Models Non-Uniform in Space and Steady in Time
The earliest models of wind inflow to a rotor were of this type and were developed
primarily to represent tower shadow (flow distortion to a downwind rotor caused by the
tower wake), not turbulence [Spera 1978]. However, the free-stream wind field itself can
be highly non-uniform, as illustrated in Figure 8-16. These variations have been measured
in the field by rotational sampling methods [ e.g. , Verholek 1978, Connell 1981], which will
be discussed later. Rotational sampling is a procedure that uses an array of anemometers
distributed along the hypothetical path of a section of a turbine blade. These anemometers
record the time history of wind speed that would be experienced by that blade section.
In order to use a non-uniform, steady wind model to represent rotationally-sampled
turbulence, the assumption is made that spatial variations in wind speed are quasi-static ,
changing slowly in comparison with the rotational period of the rotor. Any mathematical
function with periodic frequency might be used to convert the time series of wind speed at
each rotor station to ( y,z ) coordinates. The swept area of the rotor is then simply divided
into sectors, and a wind speed is specified for each sector. Presently, many turbulence
inflow models uses this procedure.
Figure 8-16. Schematic diagram of the non-uniform wind field that typically flows into
the swept area of a wind turbine rotor. [Sundar and Sullivan 1981]
Models Uniform in Space and Unsteady in Time
This type of model may also be called a planar gust front or a discrete gust model.
The gust profile (in the coordinates y and z ) is constant and envelopes the rotor uniformly,
but its level changes with time. However, a given turbulence eddy may appear uniform to
a small-scale rotor, but non-uniform to a larger one. This illustrates the need to model gust
amplitude , duration, number of occurrences, and dimensions together. Generally, the larger