The rotating air-water ring, featuring a bubbly flow, in the vaneless gap of a reduced scale physical model of a pump-turbine operating in dewatered condition is investigated. High speed visualizations are performed together with pressure fluctuation measurements to estimate the bubbles size, the velocity and pressure fields of the two-phase flow as a function of the cooling water discharge and of the gauge pressure in the draft tube cone of the pump-turbine. An image processing method is developed to compute the bubbles size and to estimate both tangential and radial velocity components of the bubbles. The results show that the size of the bubbles in the rotating air-water ring increases by increasing the gauge pressure while the cooling discharge through the labyrinth seals has no influence. The pressure and the velocity fields in both radial and tangential components are estimated as a function of the gauge pressure. Pressure fluctuations and the corresponding frequency are explored and correlated to the interaction of the rotating air-water ring with the rotating parts and the stationary parts of the pump-turbine. The modal decomposition of the harmonic components of the pressure fluctuation is analytically derived to evidence the rotor-stator interaction in the vaneless gap causing pressure fluctuation.