A unifying characteristic among bluff bodies is a similar far-wake structure independent of the shape of the body. While it is intuitive to consider the flow around an isolated turbine blade in terms of bluff body dynamics, the composite wake of multiple blades originating from a single turbine introduces significant complexity to this analogy. A VAWT proves an interesting example. During its rotation, a straight-bladed VAWT sweeps the surface of a right circular cylinder; thus it is reasonable to postulate that the wake dynamics of the two can be related. This rotation, however, is also one of the complicating factors in drawing an equivalence between the wake of a cylinder and that of a VAWT.
A prominent feature of bluff body flows is the periodic shedding of vorticity into the
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wake, which a number of investigators have explored, along with techniques to control it (see, e.g., the review by Choi et al., 2008). One approach relevant to VAWTs is the forced rotation of a cylinder (see, e.g., Mittal & Kumar, 2003), which can lead to vortex suppression when the tip-speed ratio (λ) is greater than unity; this phenomenon has also been observed in pairs of cylinders (Chanet al., 2011). A consequence of vortex suppression is reduced drag, which implies less axial momentum lost in the wake. VAWTs regularly operate at λ > 1, with some operating potentially as high as λ = 10 (Sutherland et al., 2012). Furthermore, since the power extracted by a wind turbine scales as the cube of the incoming wind speed, if the rotation of a VAWT causes similar modification of wake vorticity as it does for cylinders, even a modest effect on the mean flow recovery could have substantial implications for controlling power output within a wind farm.
The critical difference between VAWTs and cylinders is that a cylinder rotation must be externally forced, implicitly adding energy to the flow, while a wind turbine extracts energy from the incident flow. As was shown in Chapter 3, the wake dynamics of a VAWT are solely determined by the kinematics and aerodynamic properties of the turbine, irrespective of whether the flow or a motor drives the rotation; this is true provided that the net torque (i.e., time-averaged) due to drag, produced by the blades, does not exceed the net torque due to lift. However, it still remains an open question as to how the difference in energy exchange manifests itself in the wake of a rotating cylinder as compared to that of a VAWT, since their aerodynamic characteristics are significantly different. Additionally, a VAWT typically operates at Re ≈ 106, whereas rotating cylinder experiments where vorticity suppression has been observed were atRe≈102 (Kumar et al., 2011; Chan et al., 2011).
Most experimental investigations of the VAWT wake have focused on either the blade- scale or near-wake aerodynamics. As a VAWT rotates, each turbine blade is subject to dynamic stall as well as blade-wake interactions, both of which are functions of the rotation rate (Laneville & Vittecoq, 1986; Fujisawa & Shibuya, 2001; Ferreira et al., 2009; Edwards et al., 2015). Recent experiments by Dunne & McKeon (2015) demonstrated that the essential physics of the dynamic stall process on a VAWT airfoil can be captured using a low- order model identified by dynamic mode decomposition. In the near-wake of a VAWT, it has
been shown that the flow is characterized by an asymmetrical and three-dimensional mean velocity field, with the dynamics dominated by the vorticity shed from the blades (Battisti et al., 2011; Tescione et al., 2014; Bachant & Wosnik, 2015). Despite these advances, what appears to be lacking is a detailed study of the spatio-temporal evolution of the VAWT wake that extends to the far field.
Recent work by Rolin & Port´e-Agel (2015) has examined the far wake of a VAWT, up to approximately 7 rotor diameters downstream of the turbine. They noted that the effect of the boundary layer in the core of the wake was to re-energize the region with downward- entrained momentum. Such characterizations of the far wake are necessary in order to identify its dominant features, which has added significance when turbines are to be placed in close proximity to one another, as in a wind farm. For example, work by Iungo et al.
(2013) has shown the presence of a hub vortex instability in horizontal-axis wind turbine wakes, which the authors suggest could be excited by the vortex shedding from the rotor disc acting as a bluff body. They also noted that the characterization of the far wake is fundamental for wind farm design due to its practical implications, such as fatigue loads on downstream turbines.
The focus of this chapter is to present new experimental data that explores the spatio- temporal dynamics of the VAWT wake. Specifically, we examine how the dynamics change as the turbine geometry approaches that of a circular cylinder, either statically, by increas- ing the number of blades, or dynamically, by increasing the tip-speed ratio (λ). Particle image velocimetry is used to measure the velocity in the wake of three different laboratory- scale turbines: a 2-bladed, 3-bladed, and 5-bladed VAWT, as well as a circular cylinder of the same diameter and height as the turbines. The dynamic characteristics of the wake velocity are analyzed using spectral analysis and proper orthogonal decomposition. The time-averaged velocity is also examined and the wake recovery is compared with theoretical approximations for turbulent free-shear flows.
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