LAUSR

Abstract This experimental study presents some aspects of the flapping dynamics and flow characteristics in the wake of small flags ( M * = 0.42 ) inserted in rectangular channels of various aspect (0.2 to 1.0) and confinement (1.0 to 5.0) ratios. The experiments were performed in the range of up to U * = 48 or R e d h = 44 , 000 . Results revealed that flag start-up characteristics were highly dependent on initial conditions. The hysteresis loop of the critical velocity ranged from 15% to 48% and was partially explained by the residual alternating pressure difference between the two sides of the flag. The flag flapping dynamics was largely affected by the confining walls. The flapping amplitude was constrained by the channel width and the oscillation modes shifted from simple shapes in low velocity, high confinement ratios to complex shapes in high velocity, low confinement ratios. The dimensionless oscillation frequency St initially decreased at low U* and Redh, and then converged to constant values in high U* and Redh. In addition, the presence of the flag substantially increased the flow unsteadiness and turbulence levels inside the channel, especially in the near wake. The measurements reveal a possible mechanism of vortex formation due to the flag’s oscillation. The shed vortices further interact with each other and the channel walls, then eventually decay. Confinement has a profound impact on wake behaviour. The size of the channel, which constrained the flag amplitude, largely dictated the structure of the vortex cores in the near wake and their eventual decay. The turbulence levels (I) inside the channel also depended on confinement: wider channels have higher I levels due to larger vortices produced which are convected farther before decaying. Narrow channels limit flag oscillation amplitude, leading to smaller vortex structures and their rapid decay.