Two-phase wake-mixing layer flow past a splitter plate: an experimental investigation

The wake-mixing layer flow developing past a splitter plate separating two parallel gas and liquid co-flowing currents is experimentally investigated in this work. Time-resolved particle image velocimetry (TR-PIV) measurements of the two-phase velocity field are simultaneously performed in gas and liquid streams, shedding light on both mean (time-averaged) and unsteady features of the flow configuration. A selected reference case is first analyzed, revealing the presence of a wake region within the flow field, right behind the splitter plate. By progressively moving downstream along the streamwise direction, a pure mixing layer region is retrieved. The effect of two governing flow parameters, namely the gas Reynolds number and the gas-liquid dynamic pressure ratio, is then investigated, focusing first on the mean flow topology. It is found that the streamwise extension of the wake is a monotonic decreasing function of the Reynolds number, and it vanishes for the highest value considered, the two-phase flow resulting in a pure mixing layer regime. The flow unsteady dynamics is then characterized by means of the spectral analysis of normal-to-flow velocity fluctuating quantities, performed in both gas and liquid flows. As major results, it is found that frequency spectra are characterized by a high frequency content in the low Reynolds configuration, the peak frequency depending on the streamwise location. On the other hand, by progressively increasing the Reynolds number the peak frequency shifts to lower values, and it becomes independent on the specific spatial location by increasing the dynamic pressure ratio. It is found that, at high Reynolds and dynamic pressure ratio values, velocity fluctuations are characterized by low frequency temporal oscillations synchronized over a large spatial extent of the flow field. The different regimes outlined by variation of the flow governing parameters are found to be consistent with convective/absolute instability behaviors highlighted by spatio-temporal linear stability analyses of the flow recently presented in literature.