Effects of the direction of rotation axis on turbulent flows in rectangular ducts.

This article investigates the effects of the rotation axis orientation on turbulent flows in rotating rectangular ducts using direct numerical simulation (DNS). The rotation axis lies within the duct’s cross section, and its angle β relative to the bottom edge is varied from 0° to 90°, with simulations conducted for different friction Reynolds numbers (Reτ = 300, 454, 900), rotation numbers (Roτ = 2, 4, 8), and cross-sectional aspect ratios (ar = 1.0 and 2.0). Results indicate that increasing β to 45° significantly suppresses or even completely laminarizes turbulence, especially at lower Reynolds numbers or higher rotation numbers, while enhancing secondary flow; in square ducts, bulk velocity remains nearly constant, whereas it decreases monotonically in rectangular ducts with ar = 2. The study attributes turbulence suppression to the formation of an Ekman layer above the pressure wall, where a strong wall-normal variation in mean flow direction tilts low-speed streaks and disrupts near-wall turbulence regeneration. Additionally, periodic flow structures emerge in regions of severe turbulence suppression, and the complex interplay between rotation angle and aspect ratio warrants further research.