36 research outputs found
A review of turbulence measurements using ground-based wind lidars
A review of turbulence measurements using ground-based wind lidars is carried
out. Works performed in the last 30 yr, i.e., from 1972–2012 are analyzed.
More than 80% of the work has been carried out in the last 15 yr,
i.e., from 1997–2012. New algorithms to process the raw lidar data were
pioneered in the first 15 yr, i.e., from 1972–1997, when standard
techniques could not be used to measure turbulence. Obtaining unfiltered
turbulence statistics from the large probe volume of the lidars has been and
still remains the most challenging aspect. Until now, most of the processing
algorithms that have been developed have shown that by combining an isotropic
turbulence model with raw lidar measurements, we can obtain unfiltered
statistics. We believe that an anisotropic turbulence model will provide a
more realistic measure of turbulence statistics. Future development in
algorithms will depend on whether the unfiltered statistics can be obtained
without the aid of any turbulence model. With the tremendous growth of the
wind energy sector, we expect that lidars will be used for turbulence
measurements much more than ever before
A six-beam method to measure turbulence statistics using ground-based wind lidars
A so-called six-beam method is proposed to measure atmospheric turbulence
using a ground-based wind lidar. This method requires measurement of the
radial velocity variances at five equally spaced azimuth angles on the base
of a scanning cone and one measurement at the centre of the scanning circle,
i.e.using a vertical beam at the same height. The scanning configuration is
optimized to minimize the sum of the random errors in the measurement of the
second-order moments of the components (u,v, w) of the wind field. We
present this method as an alternative to the so-called velocity azimuth
display (VAD) method that is routinely used in commercial wind lidars, and
which usually results in significant averaging effects of measured
turbulence. In the VAD method, the high frequency radial velocity
measurements are used instead of their variances. The measurements are
performed using a pulsed lidar (WindScanner), and the derived turbulence
statistics (using both methods) such as the u and v variances are
compared with those obtained from a reference cup anemometer and a wind vane
at 89 m height under different atmospheric stabilities. The
measurements show that in comparison to the reference cup anemometer,
depending on the atmospheric stability and the wind field component, the
six-beam method measures between 85 and 101% of the reference
turbulence, whereas the VAD method measures between 66 and 87% of the
reference turbulence
A six-beam method to measure turbulence statistics using ground-based wind lidars
A so-called six-beam method is proposed to measure atmospheric turbulence
using a ground-based wind lidar. This method requires measurement of the
radial velocity variances at five equally spaced azimuth angles on the base
of a scanning cone and one measurement at the centre of the scanning circle,
i.e.using a vertical beam at the same height. The scanning configuration is
optimized to minimize the sum of the random errors in the measurement of the
second-order moments of the components (<I>u,v, w</I>) of the wind field. We
present this method as an alternative to the so-called velocity azimuth
display (VAD) method that is routinely used in commercial wind lidars, and
which usually results in significant averaging effects of measured
turbulence. In the VAD method, the high frequency radial velocity
measurements are used instead of their variances. The measurements are
performed using a pulsed lidar (WindScanner), and the derived turbulence
statistics (using both methods) such as the <I>u</I> and <I>v</I> variances are
compared with those obtained from a reference cup anemometer and a wind vane
at 89 m height under different atmospheric stabilities. The
measurements show that in comparison to the reference cup anemometer,
depending on the atmospheric stability and the wind field component, the
six-beam method measures between 85 and 101% of the reference
turbulence, whereas the VAD method measures between 66 and 87% of the
reference turbulence