Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 60

Figure 5: Iso-speed plot of local fl ow speed around a cylindrical mast, norma-
lised by free-fi eld wind speed (from left); analysis by two-dimensional
Navier-Stokes computations [3].

Figure 5 shows an example of fl ow distortion around a tubular mast. It can be seen
that there is a deceleration of the fl ow upwind to the mast, acceleration around it and
a wake behind it. The least disturbance can be seen to occur if facing the wind at 45°.
The fl ow distortion around a lattice mast is somewhat more complicated to
determine. Additionally to the orientation of the wind and the distance of the ane-
mometer to the centre of the mast it also depends on the solidity of the mast and
the drag. Figure 6 shows an example of fl ow distortion. Again a deceleration in
front of the mast can be observed while there is acceleration at the fl anks. Mini-
mum distortion is achieved when the anemometer is placed at an angle of 60°.
More details on how to determine the fl ow distortion can be found in [3] or [2].
In general, mounting of the anemometer at the same height as the top of the
mast should be avoided since the fl ow distortion around the top of the mast is
highly complex and cannot be corrected for.

2.5 Measurement period and averaging time

The energy yield is typically calculated referring to the annual mean wind speed
of the site. Unfortunately the annual average wind speed varies signifi cantly.
Depending on the local climate the annual averages of wind speed might vary
around

15% from one year to the next. To reduce the uncertainty of the inter-
annual variability it is strongly recommended to perform a long-term correction
of the measured data (see Section 3.1). On a monthly scale, the variations of the

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