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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Peter Argyle Loughborough University, United Kingdom
Co-authors:
Peter Argyle (1) F P Simon Watson (1)
(1) Loughborough University, Loughborough, United Kingdom

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Currently working as a Research Associate at Loughborough University, Mr Argyle is building on the work in his doctoral thesis investigating offshore resource assessment and turbine wake interaction, especially the incorporation on atmospheric stability into CFD simulations. Having studied Meteorology to Masters Level at the Universities of Reading and Oklahoma and has since worked for companies specialising as utility curators and business intelligence consultancy.

Abstract

The effects of boundary layer height and ekman spiral on wakes within cfd simulations of offshore wind farms

Introduction

The lower background turbulence intensity found at offshore locations is known to cause wind turbine wakes to propagate further downstream than at sites on land for equivalent wind speeds. Thus wakes in offshore farms are highly significant and predicting their propagation and degradation is an important part of assessing a planned offshore farm’s potential yield. With modern farms extending over large areas, it is conceivable that variation in wind direction from the Coriolis Effect will alter how wakes interact. This study uses computer simulations to investigate its significance and possible links to the height of the atmospheric boundary layer.

Approach

A number of Computational Fluid Dynamics (CFD) simulations were run, validated against the Nysted offshore wind farm under different atmospheric conditions. Utilising actuator-disk theory within a transient version of the popular RANS k-ε turbulence model, features such as a Coriolis driven Ekman spiral below a stably stratified capping layer are investigated to assess their effects on turbine wake development, interaction and dissipation. Atmospheric stability is approximated by lowering the height of the capping layer to represent more stable conditions whilst simulations excluding the layer are more representative of unstable conditions.

Main body of abstract

Using the software package Windmodeller to drive the Ansys CFX CFD software, actuator disks representing the turbines at the Nysted Offshore wind farm are simulated individually and in groups to assess wake sensitivity to model features. Each model is compared against the standard RANS k-ε model and validation data from the Nysted farm. To account for variation in measured wind direction within the validation data, results from three simulations are combined for each multi-turbine case study. To maximise analysis of wake interactions, a narrow 5° wind direction bin is applied to the measured data, with simulations representing the standard down-the-line direction and ±2.5°. Simulated hub height wind speed is 8m/s, compared to measured values of 8±0.5m/s.
Unlike the basic RANS model, transient simulations require a trial and error process to obtain the required boundary conditions. This significantly increases the cost for each case study. However, models benefit from more realistically representing the atmospheric boundary layer. Whilst the Coriolis Effect works to ‘steer’ individual wakes away from downstream turbines, its significance depends on the inter-turbine spacing. If the turbines are close, each wake will still significantly affect the following machine’s production. By contrast, if the turbines are spaced far apart, the wind direction may vary significantly along the row, with wakes effecting turbines in neighbouring rows. A capping inversion restricts vertical turbulent mixing, thus lower inversions force wakes to expand horizontally, reducing the recovery from mixing with faster winds above and increasing horizontal wake interaction within and between turbine rows.

Conclusion

The behaviour of modelled turbine wakes are found to be significantly affected by both the Coriolis parameter and the height of a stably stratified capping layer. In general, the existence of the Coriolis parameter reduces modelled wake losses, particularly for turbines in the first half of the farm. By comparison, the inclusion of a stratified capping layer is shown to increases wake losses, as would be expected in more stable atmospheric stability conditions. Lowering the height at which the stratified layer begins further hinders vertical wake dissipation, thus prolonging their longevity and increasing their significance to offshore wind resource assessment.


Learning objectives
Running CFD simulations is a computationally demanding task and increasing the complexity of the RANS model, such as by making it transient to incorporate the Coriolis force and a stably capping layer. The objective of this work is to learn whether such additions to the well-known RANS k-ε turbulence model are significantly worth the extra cost to simulation time by returning more accurate predictions of wake interactions within a large offshore wind farm.