Turbulent secondary flow in spanwise rotating ducts with increasing aspect ratio

Direct numerical simulations are conducted in this study to investigate fully developed turbulent flows in spanwise rotating ducts with different aspect ratios. The global friction Reynolds number Reτ is fixed at 180, while the global friction rotation number Roτ varies from 0 to 10. The duct aspect ratios (AR) considered include 0.5, 1, and 2. The objective of this study is to explore in depth the kinematic and dynamic characteristics of secondary flows under different rotation numbers and aspect ratios, as well as their impact on the flow physics of wall-bounded turbulence. The results show that as the rotation rate increases, the secondary flow structures in both square (AR = 1) and AR = 2 ducts follow an evolution path from corner vortices, through additional vortices, to large-scale circulations dominated by the Coriolis force, although differences exist in the generation, development, and decay processes of each structure. In contrast, the AR = 0.5 duct directly evolves from corner vortices to Coriolis-dominated circulations. Analysis of the secondary flow intensity indicates that the expansion of the spanwise scale allows the secondary flow to fully develop, and the secondary flow intensity in the duct is significantly enhanced. The study further confirms that the secondary flow in rotating ducts is mainly driven by the Ekman layer, and the Ekman layers in ducts with different aspect ratios exhibit similarities. Turbulence statistics and transport analysis further reveal that the square duct with a smaller AR shows higher sensitivity of mean velocity and turbulence fluctuations to changes in rotation number, with more pronounced suppression effects by rotation, while the secondary flow significantly enhances the local turbulence intensity and transport capacity.