Mechanism of front stagnation point unsteadiness for flow around a square cylinder

Abstract

The unsteady characteristics and mechanisms of the front stagnation point oscillation for the flow around a square cylinder are investigated with the two-dimensional time-resolved particle image velocimetry technique in the range of 1420 ≤ Re ≤ 8600. The front stagnation point oscillates with the frequency equal to the wake vortex-shedding frequency. The Reynolds number has no effect on the amplitude and the dimensionless frequency of the front stagnation point. It is revealed that the front stagnation point moves from the centerline to one side when the wake vortex from the opposite side is forming and moving toward the centerline. This growing wake vortex stays attached to the rear surface of the square cylinder. After the wake vortex is detached from the rear surface, the front stagnation moves from the maximum displacement back toward the centerline. Furthermore, the quantitative correspondence between the location of front stagnation point and the wake pattern is revealed via time-lag correlation. The movement of the front stagnation point and the variation of typical proper orthogonal decomposition coefficient of the wake are nearly opposite in phase.