LAUSR

Experiments were conducted to investigate the cavitation behavior of Helmholtz self-sustained oscillation jets. In this paper, highspeed photographic technology was used to capture the flow details of cavitation with various nozzle structures and operating pressures from 1.0 MPa to 3.0 MPa. Furthermore, an in-house code based on a matrix approach that processes gray values with various statistical methods was used to quantitatively evaluate the cavitation jet of waters generated by Helmholtz nozzles. The mean values of the normalized gray levels of the images were used as a metric to measure the lengths of the cavitation clouds. When the original data were smoothed by a 15th lowess filter to exclude the noise of the images, the periods of cavitation cloud shedding were obtained accordingly, and the average period had a magnitude of 10-4. All of the smoothed results of the original data captured a two-peak shape that comprises a large peak and a mild hump. Image processing results showed that the Helmholtz nozzle produced greater cavitation intensity than a traditional conical nozzle. The shear-layer instability waves disappeared within 10d1 downstream from the injector exit. The cavitation clouds propagated a distance of approximately 1–10d1 at various pressure ratios, and the clouds were continuous within this distance. The geometry of the nozzle strongly affected the length and shedding period of the cavitation cloud. As described in the discussion section of this paper, when the cavity length was doubled, the length of the cavitation cloud decreased approximately 26.5 %; however, the period of the cavitation cloud shedding increased approximately 10 %. The longest cavitation cloud appeared when the ratio of the cavity diameter and the upper nozzle diameter was eight. Excessively small or large diameters were not conducive to the development of cavitation, whereas the effect of the pressure ratio on the cavitation cloud length was positive. When other conditions remained unchanged, the lump shedding period decreased as the pressure ratio was increased to 25. When the pressure ratio exceeded 25, the cavitation cloud shedding period remained constant.