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

Abstract

The effects of the direction of rotation axis on flows in rectangular ducts are studied using direct numerical simulation. The rotation axis lies in the cross section of the duct. The angle of the rotation axis relative to the bottom edge of the cross section is altered from 0° to 90°. A series of cases are considered, including three Reynolds numbers Reτ = 300, 454, and 900, three rotation numbers Roτ = 2, 4, and 8, and two cross-sectional aspect ratios ar = 1.0 and 2.0. The results show that as the angle increases, the bulk velocity remains almost constant in the square duct while it decreases monotonically in the duct with ar = 2.0. When the angle increases from 0° to 45°, turbulence is significantly or even completely suppressed, while the secondary flow is gradually enhanced. Furthermore, with the same rotation number, turbulence is more strongly suppressed at a lower Reynolds number. As the angle further increases from 45° to 90° in the cases with ar = 2.0, the intensity of turbulence is recovered to some extent and the secondary flow gradually weakens. With the angle increasing from 0°, the Ekman layer is formed above the pressure wall and gradually strengthens, resulting in a drastic wall-normal variation of the mean flow direction and a tilting of the low-speed streaks near the wall, which may cause the weakening of the turbulence. In addition, in the flow fields where turbulence is severely suppressed, periodic structures are observed in the corner of the duct, which needs further study.