Dynamics of cavity structures and wall-pressure fluctuations associated with shedding mechanism in unsteady sheet/cloud cavitating flows

• Published in: Flow (Cambridge, England) Vol. 3
• Main Authors: Wang, Changchang, Zhang, Mindi
• Format: Journal Article
• Language: English
• Published: Cambridge Cambridge University Press 01.01.2023
• Subjects: Cavitation; Cavitation number; Clouds; Content analysis; Dynamic structural analysis; Dynamics; Flow control; Flow velocity; Fluid flow; Holes; Investigations; mPOD; Physics; Pressure; Pressure distribution; Pressure measurement; Probability density functions; Propagation; Proper Orthogonal Decomposition; Re-entrant jet; Reynolds number; Shedding; Shock waves; Shockwave; Statistical analysis; Wall-pressure fluctuations
• DOI: 10.1017/flo.2023.2
• ISSN: 2633-4259

The physics and mechanism of sheet/cloud cavitation in a convergent–divergent channel are investigated using synchronized dynamic surface pressure measurement and high-speed imaging in a water tunnel to probe the cavity shedding mechanism. Experiments are conducted at a fixed Reynolds number of Re  = 7.8 × 10 5 for different values of the cavitation number σ between 1.20 and 0.65, ranging from intermittent inception cavitation, sheet cavitation to quasi-periodic cloud cavitation. Two distinct cloud cavitation regimes, i.e. the re-entrant jet and shockwave shedding mechanism, are observed, accompanied by complex flow phenomenon and dynamics, and are examined in detail. An increase in pressure fluctuation intensity at the numbers 3 and 4 transducer locations are captured during the transition from re-entrant jet to shockwave shedding mechanism. The spectral content analysis shows that, in cloud cavitation, several frequency peaks are identified with the dominant frequency caused by the large-scale cavity shedding process and the secondary frequency related to re-entrant jet/shockwave dynamics. Statistical analysis based on defined grey level profiles reveals that, in cloud cavitation, the double-peak behaviours of the probability density functions with negative skewness values are found to be owing to the interactions of the re-entrant jet/shockwave with cavities in the region of 0.25 ~ 0.65 mean cavity length ( L c ). In addition, multi-scale proper orthogonal decomposition analysis with an emphasis on the flow structures in the region of 0.25 ~ 0.65 L c reveals that, under the shockwave shedding mechanism, both the re-entrant jet and shockwave are captured and their interactions are responsible for the dynamics and statistics of cloud shedding process.