Environmental Engineering Reference
In-Depth Information
Turbulence Intensity
Turbulence intensity is a measure of the relative unsteadiness of the wind, defined as
follows:
s/ U = s 0 + s G
U
(6-7a)
s 0 = RMS ( u - U )
(6-7b)
where
σ /U = turbulence intensity
σ 0 = ambient turbulence (m/s)
σ G = turbulence generated by the rotor (m/s)
RMS ( ) = root-mean-square of ( ) during a given time interval (typically 6 to 12 min)
u = instantaneous free-stream wind speed (m/s)
U = steady (mean) free-stream wind speed during the time interval (m/s)
Ambient turbulence is normally about 0.12 U. Generated turbulence within the first few
diameters of distance downstream of a turbine is normally about 0.08 U .
Turbulence causes two opposing effects on the amount of wake interference. It tends
to increase entrainment of air from the free stream surrounding the wake, thus re-energizing
the wake and reducing the velocity deficit. At the same time turbulence increases the wake
diameter, which causes a given turbine to affect more downwind units. Thus, turbulence
spreads the energy loss over a wider area. As shown in Figure 6-15, array energy losses
generally decrease with increasing turbulence intensity, indicating that the positive re-
energizing effect is dominant. At the same time, however, fatigue loads increase with
increasing turbulence.
Atmospheric Stability
Recent research has shown that atmospheric stability (see Chapter 8) may be a major
parameter in the determination of wake size and structure [Kelley 1994]. Atmospheric
stability controls the size of eddies within the general wind flow, and therefore the rate of
entrainment of air from the free stream and diffusion of turbulence in the wake.
Effect of Turbine Configuration
The description presented so far of wake effects indicates that the interference of a
given wind turbine is essentially a consequence of its global or external fluid mechanics and
is thus not strongly dependent on the details of the turbine design. This can also be inferred
by noting that the principal wake effects continue to be experienced 10 or more diameters
downwind of a rotor. At this distance the rotor wake has started to develop the general
structure common to all deficit wakes. Although there do appear to be some small distinc-
tions ( e.g. , in the amount of generated turbulence ), we need not distinguish between
HAWTs and VAWTs. Instead, we can discuss wake interference as a general phenomenon.
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