Research on multiphase flows has been strongly improved over the last decades. Because of their large fields of interests and applications for chemical, hydraulic, coastal and environmental engineers and researchers, these flows have been strongly investigated. Although they are some promising and powerful numerical models and new computing tools, computations can not always solve all actual practical problems (weather forecast, wave breaking on sandy beach…). The recent and significant developments of experimental techniques such as Particle Imagery Velocimetry (PIV) and conductivity or optical probes have particularly led scientists to physical modeling that provide series of data used to calibrate numerical models. Flows with time and length scales that were not achievable in the past are now studied leading to a better description of physical mechanisms involved in mixing, diffusion and turbulence. Nevertheless, turbulence is still not well understood, particularly in two-phase flows. In the present chapter, we focus on a classical multiphase flow, the hydraulic jump. It occurs in bedrock rivers, downstream of spillways, weirs and dams, and in industrial plants. It characterizes the transition from a supercritical open-channel flow (low-depth and high velocity) to a subcritical motion (deep flow and low velocities). Experimentally, this two-phase flow can be easily studied. Furthermore, it involves fundamental physical processes such as air/water mixing and the interaction between turbulence and free surface. This flow contributes to some dissipation of the flow kinetic energy downstream of the impingement point, in a relatively short distance making it useful to minimize flood damages. It is also associated with an increase of turbulence levels and the development of large eddies with implications in terms of scour, erosion and sediment transport. These are some of the reasons that make studies on this flow particularly relevant. Although numerical and analytical studies exist, experimental investigations are still considered as the best way to improve our knowledge. After a brief description of the hydraulic jumps, the first part of this chapter aims to review some historical developments with special regards to the experimental techniques and physical modeling (similitude). In the second part, we describe and discuss the basic properties of the two-phase flow including void fraction, bubble frequency, bubble velocity and bubble size. The free surface and turbulence properties are presented as well. In the last part, we develop some conclusions, perspectives and further measurements that should be undertaken in the future
