Occurrence mechanism and oscillation characteristics of pulsation phenomenon arising in cavitation surge in cascade

In this study, numerical analysis of cavitating three-blade flat-plate cascade is performed with paying attention to cavitation surge which is a type of cavitation instabilities. A numerical method employing "locally homogeneous gas-liquid two-phase compressible medium model" is applied to solve the unsteady cavitating flowfield. From results of numerical analysis, few typical tendency of cavitation surge which is well known in experiments was reproduced numericaly, that frequency of cavitation surge decreases with lowering σ, and that cavitation surge is easy to occur at low flow rate condition. Additionaly, mechanism of pulsation phenomenon of cavitation surge in fiat-plate cascade was clarified, that collapse pressure of cloud cavity in downstream of cascade propagates to upstream on contact with re-entrant jet which is flowing backward inside a sheet cavity and causes oscillation in upstream pressure, also that negative angle of attack and choking of cascade throat are considered to cause of oscillation in flow rate. About dynamic transfer function of cavitation, certain degree of possibility was confirmed in quasi-steady mass flow gain factor M_quasi and cavitation compliance K_quasi for prediction of occurrence limit of cavitation instabilities. Additionally, for prediction of frequency of cavitation surge, it was confirmed that local cavitation compliance K_local is more available than quasi-steady K_quasi.