Chaotic mixing in a curved-square duct flow at very low Reynolds numbers

Chaotic mixing in a curved-square duct flow is studied experimentally and numerically. Two walls of the channel (inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction toward the exit of the channel. There are two parameters dominating the flow, the Dean number De (α the pressure gradient or the Reynolds number) and the Taylor number Tr (α the angular velocity of the wall rotation). It is found that excellent mixing is achieved at very low Dean numbers (low Reynolds numbers) if De ≤ 0:1Tr when the main axial flow and the wall rotation are in the same direction (Tr > 0), and good mixing is observed for De ≤ 0:1jTrj when the main flow and the rotation are in the opposite direction (Tr < 0), because Lagrangian chaos occurs for these cases. The flow is studied both experimentally and numerically, and the two results agree with each other very well. The mechanism of chaotic mixing is analyzed by observing the entanglement process of path lines.