A wall-bounded turbulent mixing layer flow over an open step: part 2. Unsteady characteristics

The unsteady characteristics of the wall-bounded turbulent mixing layer flow over an open step were investigated. Simultaneous measurements of wall pressure fluctuations and velocity fluctuations were accomplished using a microphone array and X-type gold-plated hotwire, respectively. To elucidate the scale-variant flow structure superimposed on the wall-bounded turbulent mixing layer flow, a multi-resolution analysis based on the maximum overlap discrete wavelet transform (MODWT) was performed. The large-scale vortical structure shedding at the frequency St = 0.075 was extracted using the autocorrelation of the continuous wavelet transformed wall pressure. The convective features of the vortices at St = 0.076 and the higher harmonic frequencies St = 0.15 and 0.29 were delineated in terms of the conditionally averaged velocity field and Reynolds stress field, the scale-variant cross-correlation of the wall pressure field, and the cross-correlation between the conditionally averaged wall pressure and the velocity field. The flow rate of fluid entrained through the open step was the highest when the large-scale vortical structure came into contact with the wall at x/H = 15.0.