An unconventional tuft visualization method, along with an image processing technique adapted to the flow conditions, is proposed and implemented on a reduced-scale model of a Francis-type reversible pump-turbine in three different turbine stages such as turbine mode, runaway mode, and turbine break mode. The main goal of this technique is to visualize the complex flow developed during a rotating stall phenomenon. Fluorescent monofilament wires along with high-speed image processing and pressure sensors were installed in the narrow and vaneless gap between the impeller blades and guide vanes. Pressure fluctuations were analyzed along with tuft visualization to describe the flow with and without a rotating stall. The implemented tuft visualization method gives an adjusted qualitative description of the undergoing phenomena, making different flow behaviors such as backflow, high turbulences, recirculation, etc., evident. © 2012 by Begell House, Inc.