螺槳葉尖渦中之微汽泡空化

Micro-Bubble Cavitations in Propeller Tip Vortices

某螺槳之一系列單獨試驗中發生一現象，就是在前進係數降低至某個門檻值之後，其葉尖渦空化似乎爆裂成一群微汽泡。為了釐清這種在葉尖渦中出現的微汽泡空化之來源，我們還進行了另外兩型螺槳的可視化實驗，並與上述的現象作比對。我們也以質點影像測速法（PIV）來測量葉尖渦的速度分佈。結合PIV和數值模擬的結果之分析顯示，當微汽泡空化清楚地出現在葉尖渦中時，葉尖渦中之最小壓力仍高過蒸汽壓。我們並使用一個簡單的尺度律來呈現，在這個案例中，片狀空化有效地減少螺槳葉尖之負載，也就是代表葉尖渦空化很不容易產生。所有這些研究結果都指向：微汽泡空化來自於螺槳吸入面上的片狀空化，而非葉尖渦空化之爆裂所造成。 A series of open-water tests for a certain propeller show that its tip vortex cavitation appears to be bursting into a cluster of micro bubbles when the advance coefficient of the tests is reduced below a threshold. To elucidate the origin of formations of micro-bubble cavitation in tip vortices, we perform visualization experiments with two other propeller geometries for comparisons. We also perform particle image velocimetry (PIV) experiments to measure the velocity distributions of tip vortices. Analyses combining results of PIV and numerical simulations indicate that the minimal pressure in the tip vortex is still above the vapor pressure when micro-bubble cavitation clearly appears. A simple scaling law shows that in our case the sheet cavitation effectively unloads the tip, i.e., tip vortex cavitation can't be easily generated. All these results suggest that it is not the tip vortex cavitations that burst to form micro bubbles but the sheet cavitations attaching on the suction sides of the propeller blades.