Experimental Study Of Unsteady Characteristics Of Turbulent Mixing Layer Flow Over An Open Step And A Square-Edged Rib

The time-mean, spectral and unsteady characteristics of a wall-bounded turbulent mixing layer flow over an open step and a square-edged rib were investigated experimentally. Extensive spatio-temporal measurements of the wall pressure fluctuations and velocity fluctuations were performed using a microphone array and an X-type gold-plated hotwire probe, respectively. The author developed a systematic data processing method in order to elucidate turbulent characteristics based on auto-spectra, cross-correlation, coherence, wavenumber-frequency spectra, CWT (Continuous Wavelet Transform) and MRA(Multi-resolution Analysis) based on MODWT (Maximal Overlap Discrete Wavelet Transform).The turbulent flow over an open step was one of branches of turbulent flows over a backward-facing step. The dominant characteristics of turbulent flows over a backward-facing step were found to be amalgamation of the rolled-up vortices, shedding of the large-scale vortical structures, and flapping of the separation bubble. However, when the step was open and therefore able to entrain the ambient fluid, the resulting turbulent mixing flow exhibited distinctively different characteristics. Downstream of the step edge, the shear layer encompasses the low-speed entrained flow, which significantly altered the structure and turbulence properties of the wall-bounded turbulent mixing layer. The majority of previous studies in this area were mainly focused on time-mean characteristics such as entrained flow mass, which played an important role in increasing mixing and forming film-cooing. On the contrary, few studies were concerned on unsteady behaviors. In the present study, the unsteady characteristics of turbulent flow over an open step were elucidated using multi-field synchronized measurements and the evolution of vortices buried in turbulent mixing layer.A two-dimensional subsonic open-circuit wind tunnel was set up. A microphone array was employed to measure the wall pressure fluctuations, and an X-type gold-plated hotwire probe was used to survey the instantaneous velocity field of the turbulent mixing layer flow. Three configurations, with step heights of H /δ=0.67, 1.00 and 1.67 were chosen for comparison. The time-mean and spectral characteristics of turbulent mixing layer over an open step were disclosed in terms of the time-mean velocity profiles, Reynolds shear stresses, wall pressure spectrum, velocity spectrum, coherence and wavenumber-frequency spectrum of wall pressure fluctuations, and cross-correlations of wall pressure fluctuations and velocity fluctuations. The time-mean velocity distribution below the step edge ( ? 1≤y /H≤0) was found to be similar for the three configurations. The mixing layer was observed to grow rapidly with increasing the step height. Due to the influence of the downstream wall, the redeveloping shear layer was elevated when the step height was increased from H /δ=0.67 to H /δ=1.67. Inspection of the velocity fluctuations revealed a strong perturbation of the flow by the step of height H /δ=1.67, resulting in an intensification of the large-scale vortical structures. The streamwise distribution of wall pressure fluctuation coefficients indicated that the wall pressure fluctuations increased considerably with increasing step height. Coherence and wavenumber-frequency spectra of the wall pressure fluctuations showed weaker dispersion of the large-scale vortical structures at larger step heights. Close inspection of the cross-correlation of the streamwise velocity fluctuations and wall pressure fluctuations revealed that the large-scale vortical structure changed from backward to forward motion with increasing step height, causing the elevation of the redeveloping shear layer. Comparison of autospectra of velocity spectra and wall pressure fluctuations indicated that the contribution of local flow structures to the wall pressure fluctuations increases with increasing step height.In order to get insight into unsteady characteristics of the wall-bounded turbulent mixing layer flow over an open step, the wall pressure fluctuations and velocity fluctuations were simultaneously measured using a microphone array and X-type gold-plated hotwire, respectively. Inspection of the instantaneous wall pressure field and the autocorrelation of the continuous wavelet transformed wall pressure disclosed the deterministic presence of large-scale vortical structures shedding at the frequency St=0.075. A wealth of information on the unsteady features of the scale-variant vortices superimposed in the turbulent mixing layer flow was demonstrated by using multi-resolution analysis based on the MODWT. The convective features of vortices at St=0.075 and the higher harmonic frequencies St=0.15 and 0.29 were delineated by examining the conditionally averaged velocity field and Reynolds stress field, the conditonally averaged wall pressure field, and the cross-correlation between the conditionally averaged wall pressure at x/H=15.0 and the velocity field. The signatures of vortices at successively higher harmonic frequencies were clearly detected, even for St=2.34, in the mutli-resolution wall pressure field. In a straightfoward manner, the approximate entrainment rate of the fluid flow through the open step was obtained, and was found to be modulated by the presence of the large-scale vortical structures. The maximum entrainment flow rate through the open step was obtained when the large-scale vortical structure came into contact with the wall at x/H=15.0.