Abstract
In this study, the unsteady phenomena of vertical-axis turbine wake flow were analyzed in two rotational speed conditions from a Lagrangian perspective. The wake flow field was obtained using particle image velocimetry. The spectral proper orthogonal decomposition method was used to decompose the wake modes with different frequencies. The Lagrangian coherent structure theory based on the finite-time Lyapunov exponent was used to study the material transport and mixing in the wake evolution process. The results show that the Lagrangian coherent structures can capture the boundary of the wake vortex. At a low rotation speed, a vorticity–flux window is found, through which the fluid is transported to the rotation-induced structures. At a high rotation speed, the increasing material transport between the separation bubbles and the incoming flow eventually leads to shedding of the large-scale vortex structures, accompanied by a decrease in the length of the separation zone. The material transport and mixing revealed in the unsteady flow of turbine wakes have significance in guiding the study of the flow control mechanism.
Highlights
The use of traditional fossil fuels has increased environmental and climatic problems
Baba-Ahmadi and Dong16 used the large eddy simulation (LES)/actuator line method (ALM) technique to study the wake of a tidal current turbine; the results showed that the change in turbulence intensity and turbulent kinetic energy had a significant influence on the wake characteristics
Turbine wake contains a wide range of turbulent scales
Summary
The use of traditional fossil fuels has increased environmental and climatic problems. To alleviate the energy crisis and protect the environment, researchers have proposed replacing traditional fossil energy with clean renewable energy and changing the energy utilization structure. As a clean renewable energy, oceanic energy has large reserves. It is difficult to develop because the environment is severe.. Marine renewable energy includes tidal current energy, wave energy, temperature difference energy, and salt difference energy.. The application prospects of tidal current energy are promising. Compared with other marine renewable energy sources, tidal current energy is regular, predictable, and environmentally friendly, with a high energy density, which is conducive to grid-connected power generation.
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