The Remote Sensing of Wind in Complex Terrain

Abstract

Remote sensing instruments have been found to have relatively large deviations when compared to cup anemometery in complex terrain. This thesis investigates experimentally and theoretically, these deviations for a monostatic sodar. In order to do this, a number of experimental campaigns are undertaken, including campaigns located on a flat coastal site and a complex ridge. These campaigns represent a significant increase in the amount of data available for wind energy remote sensing studies. Using a mix of experimental data, new methodologies, novel theoretical techniques and numerical modelling, the deviations between cup anemometry and sodar measurements in complex terrain are qualified and quantified. The experimental data are analysed using robust statistical techniques that hitherto have not been applied in studies of this type. The statistical methods used here are suggested as a standardised technique that could be applied to future investigations. The experimental data, combined with theoretical modelling in this study show that wind curvature over monostatic instruments constitutes the largest mean deviation between remote sensing and cup measurements; in this case sodar measurements are, on average 5.9% lower. Through the novel use of potential flow modelling it was found that this deviation could be reduced to 0.05%. Further, it is not always necessary to employ time-expensive CFD modelling; that potential flow models are applicable in most cases. Experimental data and stochastic turbulence models are used to explore the decorrolation of remote sensing and cup measurements due to the spatial coherence of the flow. The reductions in the correlation statistics were found to be linear with separation distance. A reduction in goodness of fit parameter in flat terrain of 0.0032 per 100 m of separation is found, compared to a reduction of 0.005 per 100 m in complex terrain. This provides important information for the analysis of inter-comparison experiments where instruments may not co-located. Increased measurement scatter was found in complex terrain compared to flat terrain. This was investigated and found to be weakly correlated to wind speed. This provides evidence that increased scatter in complex terrain is due to higher wind speeds, rather than increased turbulence production, as is commonly believed. A new method is developed for using a 5 beam sodar, whereby wind speed measurements are found in separate quadrants around the sodar. It is validated and used to investigate small-scale deviations in flow. A deviation of 0.15 m/s is found over the scanning volumes in complex terrain and the wind sheltering effect of a wind break is investigated. The techniques developed in this work represent a significant contribution to the existing understanding of remote sensing use in complex terrain. It is hoped that the methods developed here will improve the accuracy of future wind resource assessments.