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Details of Grant 

EPSRC Reference: EP/M025039/1
Title: Breakdown of helical vortices in wind farms
Principal Investigator: Mao, Dr X
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Department: Engineering and Computing Sciences
Organisation: Durham, University of
Scheme: First Grant - Revised 2009
Starts: 01 September 2015 Ends: 11 September 2016 Value (£): 98,410
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Panel History:
Panel DatePanel NameOutcome
22 Apr 2015 Engineering Prioritisation Panel Meeting 22nd April 2015 Announced
Summary on Grant Application Form
The project investigates developments of helical vortices shed from tips of wind turbines and ubiquitously observed in offshore wind farms. The instabilities and breakdown of these vortices are critical to the development of the wake flow and consequently impose significant impacts on downstream wind turbines in terms of fatigue loading, lifetime and acoustic noise. Better understanding of the dynamics of helical vortices will lead to more advanced methods in wind turbine design and wind farm layout, more accurate predictions of wind turbine lifetime and energy generation of wind farms, and ultimately reduced price of wind energy. A better understanding of the dynamics of helical vortices will also be more generally useful, e.g. in aerodynamic performance of helicopter rotors or turbine impellers, erosion induced by tip-vortex cavitation in ship propellers, and breakdown of straight vortices.

At present there are no general numerical tools to calculate the global stability of helical vortices and the breakdown of these vortices driven by instabilities. Most of the existing work on helical vortices concerns experimental measurements of vortex breakdown or analytical stability analyses using simplified models, e.g. inviscid and thin helical vortex filaments perturbed by either short or long waves. As a result, the path to understanding the role of instabilities in breakdown is blocked, and so also is the capability to design wind turbines and farms taking into account the effect of wake flow on performances.

General numerical studies of global stabilities of helical vortices require a novel algorithm originating from classical stability theories, which have been used on straight vortices in both Cartesian and cylindrical frames. Such an algorithm to study helical vortices will be implemented in the Frenet frame, taking advantage of the homogeneity of the helical vortices in the vortex axis direction and therefore enabling the integration of existing stability theories across scales (from short-wave to long-wave) into one framework. Both theoretical models of helical vortices and experimentally measured mean flow in the wake of a turbine will be used as the base flow in stability studies. The results will be cross-validated against field measurements and theoretically derived instabilities.

Direct numerical simulations (DNS) of helical vortices initially perturbed by the most energetic modes will be conducted in the Frenet frame, which enables DNS at much larger Reynolds numbers than other frames. The simulations will reveal the role of various linear modes in the breakdown of helical vortices. The distribution of wind in the wake extracted from the simulation can be modeled as a more accurate alternative of the models used in current industry applications, e.g. the PARK model. This new model will be broadly useful in the design and operation of large-scale offshore wind farms, which require fast & precise predictions of the wake flow.
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