Luminescence from hydrodynamic cavitation
The majority of the research on cavitation luminescence has focused on the sonoluminescence or chemiluminescence generated by cavitation induced through ultrasound, with a lesser body of work on the luminescence induced by laser- or spark induced cavitation. In such circumstances, the cavitation is generated in liquids where, on the broad scale, there is usually assumed to be no net liquid flow (although of course there are small-scale flows as a result of the cavitation itself, through radiation forces, streaming, microstreaming and turbulence). Little attention has been paid to the luminescence that accompanies (undesirable) cavitation in pumps and turbines or in marine propellers. In the present study, the sonoluminescence specific to air/water vapour bubbles, collapsing within a cavitation tunnel, is addressed. The particular case of leading edge cavitation over a two-dimensional hydrofoil is considered in detail. Hence, strong instabilities develop, causing the attached cavity to shed large clouds of micro bubbles. The spatial and temporal properties of the emitted luminescence were studied using an intensified charge coupled device video camera and a photomultiplier (PM). The light emission was found to extend downstream from the region of cavity closure, to the region where the travelling vortices collapse. Examination of the PM signal on short time scales showed that the emitted luminescence consisted of relatively intense flashes of short duration (as with other forms of luminescence). Individual flashes were often found to be clustered in time. Over longer time scales, clear evidence of periodicity was found in the PM signals. Further analysis showed that bursts of light were being emitted at the Strouhal frequency (for the shedding of transcient cavities).