Simultaneous study of pressure pulsation and structural fluctuations of a cavitated vortex core in the draft tube of a Francis turbine

Abstract

The paper represents a simultaneous structural dynamics analysis and static pressure pulsation of a cavitated vortex core in the draft tube of a Francis turbine. Comparison of the static pressure oscillation signal and the dynamics of the cavitated vortex core structure, which were quantified by the computer-aided visualisation led to the experimental modelling of the cavitated vortex core. Through the wavelet analysis significant correlation of the observed processes and their interaction was confirmed. The obtained results of the experiments enables us to create a numerical prediction of the cavitated vortex core dynamics based on the well-known Fanelli differential equation [1]. Measured pressure and structure fluctuations of the cavitated vortex core were introduced into the differential equation. Consequently, using the method of least squares, the characteristic coefficients of the model at different operating conditions in a Francis model turbine could be determined. Appropriate regression coefficient with its typically high values (r² > 0.7) confirms the correct choice of the applied type of differential equation and gives the opportunity to set up a phenomenological model that shows the interdependence between the complex cavitated vortex core dynamics and the performance characteristics of a turbine.

Keywords:

• Cavitation
• Francis turbine
• experimental modelling of cavitated vortex
• flow visualisation