Characterization of cavitating flow regimes in an internal sharp-edged orifice by means of Proper Orthogonal Decomposition

Cavitating flow regimes of water at ambient temperature inside a sharp-edged orifice have been characterized by means of a snapshot POD combined with high-speed visualizations. The density related gray level of each experimental image was processed at each pixel point. An estimation of the energy contribution in image reconstruction of each POD eigenvalue was made. Coherent flow structures related to the most significant eigenfaces were detected. Finally an FFT analysis of the temporal eigenfunctions led to the identification of the main modal dynamic properties. It was found that the flow structure of the first POD mode was defined by a single symmetric cavity. This cavity is related to the extension of the cavitating cloud. Conversely polarities and asymmetries in flow structures of higher modes reveal the establishment of advecting vortices and swirling motions. The frequency analysis of the first four POD modes has been used for the characterization of the different cavitation regimes. As the cavitation number decreases, the jet cavitation is characterized by a rapid increase of the frequencies of modes 3 and 4, up to 1000 Hz. Instead, developed cavitation and supercavitation are characterized by a growth in frequency of the second mode up to 300 Hz, as well as they exhibit the highest FFT magnitudes of the first mode.