Statistical and Spectral Analysis of the BulbT Conical Diffuser Flow

The three dimensional unsteady swirling flow inside the draft tube of a model bulb turbine is investigated using timeresolved stereoscopic particle image velocimetry. The flow is analysed around the best efficiency points for twoNSUB11/SUB values. At NSUB11/SUB = 170, the draft tube flow dynamics are linked to a known efficency drop of the turbine while at NSUB11/SUB = 130, this phenomenon does not occur. This paper presents the experimental setup and provides a complete overview of the measurement campaign based on statistical and spectral analyses of the velocity fields and data from pressure sensors distributed across the draft tube. After the efficiency drop, the intermittency and skewness of the draft tube pressure recovery are found to be reflected onto the velocity fields. For both runner rotation speeds, spectral analyses reveal the presence of a coherent and non-axisymmetric structure that rotates at the same frequency as the runner.