Lagrangian coherent structures and material transport in unsteady flow of vertical-axis turbine wakes

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.