The operation of a reversible pump-turbine in condenser mode requires to dewater the impeller by closing the guide vanes and injecting compressed air in the draft tube cone to reduce the friction torque on the impeller blades. A water discharge is injected through the labyrinth seals for cooling purpose. The interaction of the water cooling discharge and the blades of the impeller causes the formation of an air water ring in the vaneless gap between the impeller blades and the closed guide vanes. Depending on the operating conditions, the hydrodynamic characteristics of the air-water ring in the vaneless gap change and affect the stability of the pressure in the machine and of the resisting mechanical torque transmitted through the coupling of the impeller and the shaft. This research aims to experimentally elucidate the hydrodynamic properties of the air-water ring as a function of the operating condition and to investigate the influence of this flow on the stability of both pressure and torque. High-speed visualizations are performed together with pressure fluctuations and torque measurements to establish a correlation between the flow characteristics and the pressure and torque swings. A cross-spectral analysis between the signals is performed as well as phase averaging to investigate the periodicity of the recorded instability. Based on the results achieved in this analysis, a multivariate adaptive regression spline surrogate model of the pressure coefficient at stable condition is built as a function of the relevant operating parameters. (C) 2018 Elsevier Ltd. All rights reserved.