A Review of Sodar Accuracy

What accuracy and reliability can today be expected from SODAR wind measurements? Is there traceable evidence for performance? Environmental factors, turbulent fluctuations and non-uniform terrain all affect the wind speed uncertainty. So site-to-site variations for SODAR-mast comparisons can be large. On a uniform terrain site, differences between a SODAR and a mastmounted cup anemometer will arise due to turbulent fluctuations and wind components being measured in different spaces, as well as to variable background noise. We develop theories for turbulence-related random fluctuations due to finite sampling rates and to sampling from spatially distributed volumes. Effects can be minimized by selecting the environment and selectively filtering the data for periods of low fluctuations. But there is still real difficulty in answering the question: How good is a SODAR? Most field use, away from an idealized test environment, appears to produce SODAR-mast rms differences greater than the 0.1 m s-1 or less typically quoted by SODAR manufacturers. However, in these real environments it is likely that much of the difference arises from the mast sensors and the SODAR actually measuring in different spaces. We show some field results which reinforce this view. Both the turbulencerelated random fluctuations and systematic errors in complex terrain (where systematic wind shears arise) can potentially be removed by use of a vertical column geometry. Field results from a new bistatic receiver shed some light on the differences between such ‘common volume’ sampling and the usual monostatic sampling

Feasibility study for estimating the offshore shear layer from on shore measurements

This paper presents an open source computational fluid dynamics (CFD) study of air flow over a complex terrain. The open source C++ toolbox OpenFOAM has been used for the CFD analysis and the terrain considered is a scale model of Berlengas Island, which lies close to the Portuguese coast. In order to validate the CFD model, experimental work has been carried out in an open-section wind tunnel using hot-wire anemometry to measure the wind profiles above the island. In the majority of cases, the OpenFOAM CFD solutions show very good agreement with the experimental wind profile data, confirming that open source CFD solutions are possible for environmental flows over complex terrain. Such an analysis demonstrates the feasibility of estimating offshore boundary layer effects from onshore measurements

EU-Norsewind : Investigation of flow distortion effects on offshore instrumentation

In 2008 the EC programme NORSEWInD kicked off with a mission to deliver high quality offshore wind speed data for the wind industry. The aim of the project is to deliver offshore wind speed data to the wind industry by measuring offshore wind speed data from remote sensing instruments (LiDAR) on off shore platforms. This work reports on the techniques used to assesses the interference effects of the various mounting platforms on the measured wind speed data

Measurement and simulation of the flow field around a triangular lattice meteorological mast

The international standard IEC 61400-12-1 “Wind turbines – Part 12-1: Power performance measurements of electricity producing wind turbines” aims to provide a uniform methodology that will ensure consistency, accuracy and reproducibility in the measurement and analysis of power performance by wind turbines [1]. Annex G of this standard provides a methodology for the appropriate arrangement of instruments on the meteorological mast to ensure accurate measurement. For cup anemometers it provides recommendations about their location relative to the mast so that the effect of mast and boom interference on their output may be minimised. These recommendations are given for both tubular masts and lattice masts. This paper compares the flow distortion predicted by the IEC standard and the results of a 3D Computational Fluid Dynamics (CFD) simulation of a triangular lattice mast. Based on the results of wind tunnel and CFD simulation it was found that the flow distortion surrounding the lattice mast was over predicted by the method suggested in appendix G of IEC61400-12-1. Using the CFD data it was possible to determine, for a range of flow directions and mast heights, the distance from the mast that anemometers would need to be in order to be outside the flow distortion field