Abstract
In recent years, the H-rotor vertical-axis turbine has attracted considerable attention in the field of wind and tidal power generation. After a series of complex spatiotemporal evolutions, the vortex shed from turbine blades forms a turbulent wake with a multi-scale coherent structure. An analysis of the wake characteristics of twin turbines forms the basis of array optimisation. This study aimed to examine the instability characteristics of a twin-turbine wake with two rotational configurations. The dynamic evolution characteristics of coherent structures with different scales in the wake were analysed via wavelet analysis. The results show that an inverse energy cascade process occurs after the high-frequency small-scale coherent structures induced by rotation lose their coherence. This self-organising characteristic is more apparent in the quasi two-dimensional wake of a forward-moving counter-rotating turbine (Array 1) than in that of a backward-moving counter-rotating turbine (Array 2). With greater organisation and coherence, the wake of Array 1 exhibits low-frequency instability characteristics dominated by a large-scale coherent structure. In addition, the signals reconstructed using wavelet transform show that asymmetric modes exist between low-frequency large-scale coherent structures. The experimental results provide a new perspective on the instability mechanism of twin-turbine wakes, as well as important data for numerical modelling.
Highlights
Wind and tidal power generation technologies have advanced rapidly in recent years
Turbine wake is a type of unsteady turbulent flow
To reduce the calculation for wake simulation, Rajagopalan et al [3] used the CFD (Computational Fluid Dynamics) method to solve the generalised actuator disk rather than the dense blade grid and adopted the aerodynamic force acting on the wind turbine to the control equation
Summary
Wind and tidal power generation technologies have advanced rapidly in recent years. Turbine wake is a type of unsteady turbulent flow. The wake effect exerts an important influence on the power generation efficiency of a turbine, fatigue life of a rotor, and stability of a power grid. H-rotor turbine is the core equipment for vertical-axis wind power generation and tidal power generation [1,2]. The vertical axis turbine has a broad prospect in offshore wind power generation. The swept surface of a rotating vertical-axis H-rotor turbine (or horizontal axis turbine) blades is a cylinder (or disk). The turbine rotor can be described by a generalised actuator disk or actuator cylinder. To reduce the calculation for wake simulation, Rajagopalan et al [3] used the CFD (Computational Fluid Dynamics) method to solve the generalised actuator disk rather than the dense blade grid and adopted the aerodynamic force acting on the wind turbine to the control equation. Wu [4] first described the nonlinear actuator disk model of Energies 2020, 13, 4310; doi:10.3390/en13174310 www.mdpi.com/journal/energies
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