空泡流非稳态现象的流动控制

FLOW CONTROL ON UNSTABLE CAVITATION PHENOMENA

处于跨临界阶段的空泡流必然导致强烈的周期性冲击和振动．空泡流的激振来源于空泡云的周期性大规模脱落．空泡云的形成和发展与流动的边界层效应有着强烈的相关性，且空泡末端的局部流动直接影响空泡流的整体稳定性．本试验在ＮＡＣＡ１６０１２水翼表面粘附一条展向１ｍｍ厚１０ｍｍ宽的挡流条，尝试以干扰水翼上表面局部流动的方法来影响整个空泡流的形态及其流动稳定性，最终在一定的空泡数范围内抑制了空泡流激振现象，并从试验研究的角度探索了空泡云脱落的机理．

Periodicity of cavitating flow in transcritical stage results in violent vibration, noise and impact,and also earlierises the fatigue and severes the cavitation erosion. The flow of cavity closure is always in the state of most instability and turbulence, especially it will generate vortical cavitating flow when in transitional status. The excitation of vibration is caused by the large-scale cloud cavitation shedding. The formation and development of cloud cavitation have strong corre- lation with boundary layer effect. Flow structure in the local region of cavity closure remarkably influences the general stability of cavitation flow. A trip bar, 1 mm in thickness and 10mm in breadth, was attached on the upper surface of a NACA16012 hydrofoil in the experiment, with which the local flow region could be modified with the changes of the whole cavitation patterns and flow instability. It is found out that, the efficiency of trip bar on the suppression of cavitation fluctuation is affected by the dimension and position of cavitation development.Better effects were acquired when cavity closure is upstream of the trip bar, and cavity is low in thickness dimension. As a result, the self-excited fluctuation of the hydrofoil cavitation flows was suppressed to some extent in a certain range of cavitation number for the trip bar in 35% chordwise position. However the trip bar in 75% chordwise position did not make favorable effect in this test. The generally accepted hypothesis of reentrant-jet is depicted to discuss the mechanism of cloud cavitation shed- ding. Furthermore, a new explanation based on the experimental results was proposed on trial to explore the real mechanism of the instability of transcritical cavitation flow.