Y-shape outlet structures are used to release the turbine water into a reservoir. Classical design oftenuses a Y-shape over a certain length and with a specific opening angle. Depending on the geometry,especially in turbine mode, significant undesired flow separations may occur at the bifurcation.Investigations were performed with physical model tests using ultrasonic profilers at 1 and 2 MHzemission frequencies in order to assess an approach to control the design of an outlet structure bymeasuring the flow field and its fluctuations. The distribution and fluctuation of the flow velocities, bothat the exit section as well as along the tunnel axes for both outlet openings of the Y-shape geometryallow detecting potential flow detachment at the bifurcation.