Effect of aspect ratio on vertical-axis wind turbine wakes

Variability of the rotor aspect ratio is one of the inherent characteristics of vertical-axis wind turbines (VAWTs) which differentiates them especially from the more conventional horizontal-axis wind turbines. In this study, we intend to investigate the effect of rotor aspect ratio on VAWT wakes. In particular, we aim to find out whether a common behaviour exists in the mean flow field of such wakes. In order to do so, we first design and perform a set of numerical experiments (using our already validated large-eddy simulation framework) to obtain the mean flow field of the wakes of three VAWTs of different aspect ratio (2, 1 and ) and the same thrust coefficient (). After observing the obvious differences in these three wakes, by using the classical momentum integral and the concept of momentum diameter, we come up with an appropriate normalization length scale , where is the rotor diameter and is the rotor height. By normalizing the lengths (both streamwise and lateral) involved in the mean velocity profiles by , we obtain a remarkable collapse of the wake profiles for the three aspect ratios. As a corollary, cross-sections of wakes of turbines with different aspect ratios eventually converge to a circular shape - not an elliptical one, for example, as one might presume intuitively. This result influences the modelling of VAWT far wakes and, in turn, has implications on the optimal configuration of VAWT farms.