Do ambient shear and thermal stratification impact wind turbine tip-vortex breakdown?

Abstract

Modern wind turbines experience uneven inflow conditions across the rotor, due to the ambient flow’s shear and thermal stratification. Such conditions alter the shape and length of turbine wakes and thus impact the loads and power generation of downstream turbines. To this end, understanding the spatial evolution of the individual wakes under different atmospheric conditions is key to controlling and optimising turbine arrays. With this numerical study we aim to obtain a better understanding of the fundamental physics governing the near-wake dynamics of wind turbines under shear and thermal stability, by examining their tip-vortex breakup mechanisms. Our approach considers scale-resolving simulations of a single turbine wake under a linear shear profile as well as the application of harmonic tip perturbations to trigger flow instabilities. For the subsequent analysis we use the proper orthogonal decomposition (POD) method to extract coherent structures from the flow, and we also calculate mean kinetic energy fluxes to quantify each coherent structure’s contribution to wake recovery. The wake’s helical spiral is found to hinder wake recovery for all studied ambient flow conditions, whereas the mutual inductance instability has positive MKE flux leading to an enhanced wake recovery. Finally, the ambient shear has the largest impact on the local MKE flux with respect to downstream location by changing the shape of the curve and location of extrema, whereas thermal stratification has only a minimal impact on the magnitude of the near-wake local MKE flux distribution.

Large-Eddy Simulations of wind turbine wakes in sheared inflows

“…As can be expected, higher shear causes the helical tip vortices to convect more quickly at the top than at the bottom resulting in a tilting of this flow structure. A proper orthogonal decomposition (POD) analysis, as performed by Hodgkin et al [10] for a wind turbine in shear and explained in [22], would show that the tip vortices can be identified as the dominant helical mode with an associated frequency and wavelength. In Fig.…”

“…The near-wake structure can be classified in three regions: linear, transition and turbulent based on the tip vortex behavior described above and as proposed in [10]. Note that, in our work, the classification is based only on visual inspection over a range of iso-surfaces.…”

“…Note that, in our work, the classification is based only on visual inspection over a range of iso-surfaces. A more robust way would have been to perform a suitable decomposition technique like POD as explained in [22] and used in [10]. Note also that we focus on the tip vortices as the largest flow structures in the wake that are presumably providing the bulk of the mixing and, hence, likely contribute strongest to the wake recovery.…”

“…Wind turbine wakes comprise of a large number of temporal and spatial structures that are created in the region of velocity-deficit downstream of the wind turbine, generated by the blade rotation [9]. Hodgkin et al [10] showed that shear has a significant impact on the evolution of the tip vortices and, therefore, on the shape of the wake, whereas thermal stratification only affects the near-wake region. In the literature, it was also shown that the presence of wind shear can produce a highly complex wake structure downstream of the turbine rotor that exhibits significant asymmetries, streamwise vorticity generation, non-periodicity, and nonuniformity [11].…”

Exploring the impact of different inflow conditions on wind turbine wakes using Large-Eddy Simulations

“…The presence of high wind shear has a twofold impact on wake evolution and turbine performance, affecting wake size, shape, and recovery, and impacting loads and power output [8]. Hodgkin et al [9] demonstrated that shear significantly impacts the evolution of tip vortices and, consequently, the shape of the wake. Furthermore, wind shear and turbulence can lead to the formation of highly complex wake structures downstream of the turbine rotor, characterized by significant asymmetries, streamwise vorticity generation, nonperiodicity, and non-uniformity [10].…”