Experiments on the low-frequency oscillation of a separated shear layer

An experimental investigation has been carried out to study the effect of the lowfrequency oscillation of a separated shear layer. A separated shear layer was generated on a flat plate placed horizontally in a low-speed wind tunnel using a contoured wall at the top of the tunnel test section. Two different contoured walls were used to impose a low and high level of adverse pressure gradient in the flow. The time-resolved particle image velocimetry measurements were carried out to study the unsteady characteristics of the separated shear layer. The measured data reveals that the vortex shedding associated with the separating shear layer is regular for the low adverse pressure gradient case, whereas it is found to be irregular or intermittent for the high adverse pressure gradient. We find that the intermittent nature of the vortex shedding for the high adverse pressure gradient case is due to a low-frequency oscillation of the shear layer and the associated movement of the points of inflection in the velocity profiles. The short time-averaged velocity profiles in the intermittent vortex-shedding process are also found to follow the embedded shear layer scaling proposed by Schatzman and Thomas [J. Fluid Mech. 815, 592 (2017)]. We study the effect of this low-frequency oscillation on the stability characteristics of the separated shear layer and the vortex-shedding process. Based on the analyses, a nondimensional parameter (8*rms/8*) is proposed to quantify the interaction level of the low-frequency oscillation on the vortex shedding. We find that the interaction of the low-frequency oscillation on the vortex shedding vanishes as 8*rms/8 -> 0. Further, it shows that when the numerical value of this parameter approaches 0.23, the interaction is found to be intensified, leading to the separated shear layer either from a non-vortex-shedding state to vortex-shedding state or a vortex-shedding state to a non-vortex-shedding state.