This study reports the first time-resolved particle image velocimetry characterization of a planar two-phase mixing layer flow, whose velocity field is measured simultaneously in gas and liquid streams. Two parallel air and water flows meet downstream of a splitter plate, giving rise to an initially spanwise invariant configuration. The aim is to elucidate further the mechanisms leading to the flow breakup in gas-assisted atomization. The complete experimental characterization of the velocity field represents a database that could be used in data-driven reduced-order models to investigate the global behaviour of the flow system. After the analysis of a selected reference case, a parametric study of the flow behaviour is performed by varying the liquid and gas Reynolds numbers, and as a consequence also the gas-to-liquid dynamic pressure ratio , shedding light on both time-averaged (mean) and unsteady velocity fields. In the reference case, it is shown that the mean flow exhibits a wake region just downstream of the splitter plate, followed by the development of a mixing layer. By increasing both and, the streamwise extent of the wake decreases and eventually vanishes, the flow resulting in a pure mixing layer regime. The spectral analysis of the normal-to-flow velocity fluctuations outlines different flow regimes by variation of the governing parameters, giving more insights into the global characteristics of the flow field. As a major result, it is found that at high and values, the velocity fluctuations are characterized by low-frequency temporal oscillations synchronized in several locations within the flow field, which suggest the presence of a global mode of instability. The proper orthogonal decomposition of velocity fluctuations, performed in both gas and liquid phases, reveals finally that the synchronized oscillations are associated with a low-frequency dominant flapping mode of the gas-liquid interface. Higher-order modes correspond to interfacial wave structures travelling with the so-called Dimotakis velocity. For lower gas Reynolds numbers, the leading modes describe higher frequency fingers shedding at the interface.

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 G direction, a pure mixing layer region is retrieved. The effect of two governing flow parameters, namely the gas Reynolds number '46 and the gasliquid dynamic pressure ratio ", is then investigated, focusing first on the mean flow topology. It is found that the streamwise extension of the wake GF is a monotonic decreasing function of '46, and it vanishes for the highest '46 value considered, the two-phase flow resulting in a pure mixing layer regime. The flow unsteady development is then characterized by means of the spectral analysis of normal-to-flow (i.e. along H direction) velocity fluctuating quantities E0 ¹C°, 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 '46 configuration, the peak frequency depending on the streamwise location G. On the other hand, by progressively increasing '46 the peak frequency shifts to lower values, and it becomes independent on the specific spatial location by increasing ". It is found that, at high '46 and " 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 spatiotemporal linear stability analyses of the flow recently presented in literature.