Unsteady flow in a channel obstructed by a square rod (crisscross motion of vortex)

Abstract

A two-dimensional numerical computation was made for unsteady laminar flow in a channel obstructed with a square rod placed perpendicular to the flow in the channel center plane. The computation was made for several cases of different rod sizes, different Reynolds numbers, and different patterns of inlet flow velocity profiles. The vortex shedding frequency and the drag coefficient of the rod are discussed first. Then the behavior of the vortex street formed downstream of the rod is discussed in particular. The present computation reveals that the vortex street shows a different pattern of motion from its counterpart formed behind a rod placed in a uniform flow. In several cases of obstructed channel flow, it is found that vortices that are shed from the upper half of the rod cross the channel center plane toward the bottom channel wall as they move downstream, and those that are shed from the lower half of the rod move toward the upper half of the flow space. Flow visualization was made for the flow in a channel obstructed by a square rod and confirmed the appearance of this interesting crisscross motion of the vortices. The blockage ratio is indicated to be a major factor governing the conditions for the appearance of the crisscross motion of the vortex. Misalignment of the rod position leading to asymmetry of flow is found to cause no serious suppression of the crisscross motion of the vortex.