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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Optimising measurement strategies to maximise project value: Is the industry making false economies at the expense of project value?' taking place on Tuesday, 11 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Christiane Montavon ANSYS UK Ltd, United Kingdom
Co-authors:

(1) ANSYS UK Ltd, Abingdon, United Kingdom (2) SSE Renewables, Bristol, United Kingdom

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Abstract

Sensitivity of wind flow to atmospheric stability for a complex site with multiple masts

Introduction

Site complexity, both in terms of terrain and forestry, determines the speed, turbulence intensity (TI) and shear exponent factor (SEF) of the wind flow that will be experienced by turbines located on the site. Atmospheric stability conditions, and in particular diurnal cycles affecting the site, play a significant role in modulating these parameters. Understanding how the stability conditions affect the wind flow is important for turbine suitability and energy yield assessments. When modelling is based on the assumption of a neutral atmosphere, these assessments may not capture key site-specific wind flow characteristics.

Approach

For a site in moderately complex terrain, with complex forestry, equipped with three masts, we investigate approaches to classify the prevailing surface stability conditions. Since the masts do not provide stability information, stability conditions are derived from mesoscale hindcast data, concurrent with the mast data. The sensitivity of the relative wind speed, TI and SEF to the stability classification is quantified using data gathered at the masts. A complementary CFD analysis is performed for a range of directions, upstream wind speeds and surface stability to help understand the trends observed in the data.

Main body of abstract

This investigation focuses on a site of moderate terrain complexity with complex forestry, including varying forestry heights, varying tree density and felled regions. The site is equipped with 3 masts, with anemometers measuring at approximately 35m, 50m, 65m and 81m. Concurrent wind data at the masts is available for a period of 13 months. Mesoscale model hindcast data covering the same period are also used to derive the variation in surface stability conditions. Mast to mast ratios of wind speed and TI, as well as the SEF at the individual masts, are binned by wind direction, producing mean and standard deviation of these parameters, overall and by surface stability.
A detailed CFD analysis of the site is carried out with ANSYS’ WindModeller software using the Katul forest canopy model. In the immediate surroundings of the mast and turbine locations, a highly accurate representation of the forestry height is derived from LIDAR flyover data. Beyond the extent of the LIDAR scan, the forestry height is derived from roughness data. The CFD analysis investigates the sensitivity of wind speed ratios, TI ratios, and SEF by direction to parameters such as the Froude number (associated with the stability conditions above the boundary layer), and the surface stability conditions. The resulting sensitivity is compared to that seen in the data for the derived stability classes.


Conclusion

Comparing the trends by direction between the observations and model results shows that the inclusion of stability effects in the simulations accounts for the variability seen in the observation of the SEF and TI by direction, variability not otherwise explained by purely neutral simulations. Since atmospheric stability can have a key influence on the flow distribution at complex sites, it is important to take this into account when conducting flow modelling for turbine suitability or energy yield assessments. In the absence of stability measurements on site, the model input characterising the stability conditions can be inferred from mesoscale hindcast data.


Learning objectives
This contribution distinguishes between the concepts of freestream stability (above the boundary layer) and surface stability (associated with heat fluxes at the ground). It quantifies how both aspects of stability conditions affect the TI, SEF and wind speed ratios and demonstrates that the variability observed in these parameters can be accounted for when including stability effects in CFD simulations.