射流对绕水翼云空化流动抑制机理研究

为理解绕水翼云空化流动的发展机理和探究水翼吸力面开孔射流的影响,采用密度 修正的RNG $k$-$\varepsilon $湍流模型和Schnerr-Sauer空化模型对原始NACA66(mod) 水翼和采用射流后的 水翼的云空化非定常过程进行模拟和对比分析;采用在水翼吸力面近壁区设立监测线的方法对近壁区的流场进行监测,得到 近壁区汽相体积分数、回射流速度、压力及压力梯度的时空分布云图;开展了云空化流场特性的涡动力学分析,进而分析水 翼云空化的发生机理和射流抑制空化的抑制机理. 结果表明:游离型空泡在下游溃灭时产生强烈的局部高压,其向上游传播 导致前缘空穴的一次回缩,而空穴的二次回缩受回射流的影响. 回射流的发展区域受限于较高的压力梯度,高的压力梯度一 直存在,但回射流在一个周期内的首次出现需要时间的积累. 在水翼吸力面射流使得射流孔附近压力升高,弥补了由于空化 和绕流造成的压降,压力梯度增大,抗逆压能力增强,对回射流起到阻挡作用;另一方面,射流使得回射流区域面积和回射 流的强度也有所减小,从而对云空化的发展起到抑制的效果. $Q$准则的涡结构云图相比于汽相体积分数云图能显示复杂的 流动结构,前缘附着型空穴和尾缘游离型空穴内存在旋涡,回射流对空穴存在剪切作用造成空穴脱落. 而射流对空穴和回射 流的剪切和阻挡使云空化发展得到抑制. 

MECHANISM INVESTIGATION OF WATER INJECTION ON SUPPRESSING HYDROFOIL CLOUD CAVITATION FLOW 1)

In order to understand the development mechanism of cloud cavitation and investigate the influence of jet flow on the suction surface of hydrofoil, the unsteady evolution process of cloud cavitation of the original NACA66(mod) hydrofoil and the hydrofoil with jet flow is simulated and compared by using density modified RNG $k$-$\varepsilon $ turbulence model and Schnerr-Sauer cavitation model. Monitoring lines of the hydrofoil suction surface are adopted and then spatiotemporal evolution of the vapor phase volume fraction, re-entrant jet flow, pressure and pressure gradient near the hydrofoil wall is obtained. The flow field characteristics of the cloud cavitation are analyzed by vortex dynamic. The suppression mechanism of active jet is analyzed. The results show that the first retraction of the attached cavity is caused by the strong local high pressure when the dissociated cavity collapse downstream, and the second retraction is affected by the re-entrant jet. The re-entrant jet zone is limited to higher pressure gradient. High pressure gradient always exists, but the initial occurrence of re-entrant jet in a time period needs the accumulation of time. Jet flow on the suction surface of hydrofoil can reduce cloud cavitation. The suppression mechanism is that injected jet makes the pressure rise near jet holes, making up for the pressure drop caused by cavitation and circumfluence, so as to increase the pressure gradient and enhance the anti-pressure ability. On the other hand, the re-entrant jet strength is decreased. The vortex structure contour of Q criterion shows complex flow structure compared to the vapor phase volume fraction contour. Vortex cores are existed in the front attached cavity and dissociated cavity, while shear may be present in re-entrant jet zone. The shear function of the injected jet suppresses the development of cavitation and re-entrant jet.