A complete characterization of two phase air-water flows, expressed in terms of flow fields of the two phases and of concentration and geometric features of the air bubbles, plays an important role in many engineering applications. In civil engineering, many hydraulic structures exhibit the entrainment of large amounts of air, therefore, their operation is highly dependent by the two phase flow patterns (Granata et al. 2011, 2015, Granata and de Marinis 2017). An experimental study on a horizontal air-water flow, generated by a submerged jet and laden with fluorescent tracers, has been performed at the Laboratorio di Ingegneria delle Acque, Cassino University, and at the Institute of Marine Engineering (CNR-INM) laboratory, Rome, Italy. The analysis of the air-water jet has been performed by means of the Particle Image Velocimetry (PIV) technique, combined with the Laser Induced Fluorescence (LIF). Two different regimes have been examined, the “two-way coupling” and the “four-way coupling”, according to the air-water interactions. For the simultaneous and separated analysis of the two phases, a discrimination algorithm, based on the geometric features and pixel intensity of the seeding particles and air bubbles, has been used (Di Nunno et al. 2018, 2019). Furthermore, the measurement of the bubble geometric features has been performed by means of a direct ellipse fitting algorithm. A statistical analysis has been conducted, which has highlighted the effect of the air flow rate increase on the two-phase flow pattern. In particular, an increase of turbulence and an upward orientation of the jet is observed, due to the interaction between the water and the air bubbles buoyancy forces. Furthermore, greater interactions between bubbles grouped in clusters occurs, which results in a lower variation of the air bubbles shape and orientation along their path. The aim of this study is to provide a better understanding of the bubble-water and bubble-bubble interactions and an evaluation of the effects of the air bubbles in the flow pattern of the jet.

Acknowledgements

Activities described here were partially conducted under Grant 2018 "MISALVA – Metodologie Innovative per la Sicurezza degli ALVei Arginati” by the Ministry of Ambiente, Tutela del Territorio e del Mare, Rome, Italy.

References

• Di Nunno, F., Alves F.P., de Marinis, G., Di Felice, F., Gargano, R., Granata, F. and Miozzi, M. (2018). “Experimental study of air-water two-phase jet: bubble size distribution and velocity measurements.” Journal of Physics: Conference series, 1110.
• Di Nunno, F., Alves F.P., de Marinis, G., Di Felice, F., Gargano, R., Granata, F. and Miozzi, M. (2019). “Two-phase PIV-LIF measurements in a submerged bubbly water jet.” Journal of Hydraulic Engineering, in press. Granata, F., de Marinis, G., Gargano, R. and Hager, W. (2011). “Hydraulics of Circular Drop Manholes.” Journal of Irrigation and Drainage Engineering, 137, 102-111.
• Granata, F., de Marinis, G. and Gargano, R. (2015). “Air-water flows in circular drop manholes.” Urban Water Journal, 12(6), 477-487.
• Granata, F. and de Marinis, G. (2017). “Machine learning methods for wastewater hydraulics.” Flow Measurement and Instrumentation, 57, 1-9.