Separation and Surface Nuclei Effects in a Cavitation Susceptibility Meter

This work is concerned with the operation of a Cavitation Susceptibility Meter ( CSM) based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. Both cavitation and velocity at the venturi throat, where cavitation occurs, are monitored optically by u a Laser Doppler Velocimeter. A computerized system is used for data acquisition and reduction. The throat pressure is determined indirectly from the upstream pressure and the local flow velocity. The results of the operation of the CSM are presented and critically discussed with reference to the occurrence of flow separation and surface nuclei effects, which represent the most stringent operational limitations of CSM's. Separation in the diffuser increases the minimum achievable throat pressure above the susceptibility of most cavitation nuclei commonly found in technical waters. At relatively high tensions surface nuclei can generate extensive sheet or spot cavitation even on optically finished glass surfaces, thereby preventing the measurement of free stream nuclei susceptibility. These phenomena are not easily overcome and bring therefore into question the utility of existing CSM designs for the measurement of free stream nuclei concentration and susceptibility at realistic values of the applied tension, as required for cavitation studies.