The Standard and Counter-Rotating VAWT Performances with LES

Abstract

Traditionally, the wind turbine performance is defined in terms of power extraction performance (expressed non-dimensionally as power coefficient, C_P, with its maximum value $C_{{P_{B} }} = 16/27$) while the turbine ability to start is normally ignored. Nevertheless, if a turbine cannot accelerate through start-up, its power extraction performance is severely limited, especially at low wind speeds. The criterion of starting behavior at relatively low Reynolds numbers, appropriate for the urban application, therefore offers another expectation to improve the overall performance concerning the period that the turbine needs to start might be achieved which might lead to a significant increase in energy turn-out. The work will focus upon vertical-axis machines of Darrieus type using an H-rotor in which the blades are straight and parallel to their axis of rotation. For the small such turbines, i.e. in low Reynolds number flows, some researchers have stated that the Darrieus-type turbine is inherently not self-starting. The concept of a vertical-axis counter-rotating rotor is used to overcome the starting drawback of small Vertical Axis Wind Turbines (hereafter VAWT). For this purpose, we attempt to simulate the flow around a Counter-Rotating VAWT (CR-VAWT) with Large Eddy Simulation (LES) and both starting behavior and power performance is outlined by comparing with an equivalent conventional turbine.