脉动压力对空化的影响

为了消除空化试验的比尺效应,具体分析脉动压力对空化的影响,从泡动力学方程得出水中气核的发育时间和临界压力,得出气核的生长时间小于水流的负脉动周期;另由于水流脉动对气核的振荡作用,使气核提前膨胀,促使空化提前发生,计算空化数时需计入脉动压力项。根据N-S方程分析了脉动压力的模型律,结合原型测量结果,得出在强紊动区中脉动压力满足重力相似律,但在溢流坝面、陡槽底部等处脉动频率比尺不满足重力相似律,频率比尺约为1。在进行强紊动区的空化实验时,应注意水流脉动对空化的影响。

Effects of Fluctuation Pressure on Cavitation

The main objective of this research is to avoid the cavitation scale effect, the term of fluctuation pressure is analyzed in details. Based on the dynamic equation of bubble,development time of gas nucleus in water and the required critical pressure are gained. As development time of gas nucleus is less than negative fluctuation period. The fluctuation pressuer make the gas nucleus expend early and cut down the growth time of gas nucleus, and make cavitation arised early. So the fluctuation pressure term must be reckoned in while calculating cavitation number. Model similarity law is analyzed based on N-S equation. Combined with measured result of prototype, it is concluded that fluctuation pressure in strong fluctuation area satisfy with the gravity similarity law, but the tests reveal that fluctuation pressure in smooth boundary layer, such as on overall dam face or steep channel bottom, don't obey the gravity similarity, and the frequency scale ration is about 1. As a result, when conducting cavitation test in strong fluctuation area, it is necessary to pay attention to the effect of flow fluctuation on cavitation.