Evolution of instability modes and spreading characteristics of a planar jet flow under long-wave excitations

Abstract

Switching phenomenon and parametric resonance have been identified in near field flows subjected to high-frequency controlled perturbations. This study reports the correlation between switching phenomenon, parametric resonance, and spreading characteristics caused by the instability modes after the end of the potential core of a planar jet (far field). Two periodically oscillating strips were used to control perturbations in free shear layers and three different long-wave excitations (i.e., symmetric, antisymmetric, and one-sided) were imposed at the excitation frequency (fe). The excitation frequency was fixed relative to the preferred mode frequency (fp) at fe = fp/4. To investigate the evolution of coherent structures, frequency spectral analysis and phase correlation were applied to hot-wire anemometer measurements. Under symmetric excitation, the instability mode of vortical structures is observed twice at mode switching, retaining the in-phase pattern. Jet puffing occurs when the cross correlation R12(0) is positive at the far field. These features cause vortex shedding to remain symmetric and suppresses the flapping motion of the jet. However, out-of-phase behavior reoccurs under antisymmetric and one-sided excitations and the cross correlation R12(0) becomes a negative after the end of the potential core. As a result, flapping motion is enhanced and the jet spreads wider after the end of the potential core.