Abstract The effects of nano-alumina additives on the instability of round water jets were experimentally studied. The flow visualization inside and near the exit of two transparent nozzles was obtained by high-speed shadow imaging. A spectrum analysis method was applied to determine the surface wavelengths by performing fast Fourier transform on the image grayscale standard deviation data. According to the flow structure inside the sharp-edged nozzle, an integrated model for wavelength prediction was constructed based on the boundary layer instability theory. The effects of the nanoparticle additives on the jet instability were mainly caused by viscosity enhancement and cavitation promotion. The effects were divided into three categories according to the cavitation state in the nozzle. Firstly, for the no-cavitation flow, the nanoparticles exerted a frequency-reduction effect owing to the viscosity increase. Secondly, for the partial cavitation state, the nanoparticles had an increasing-frequency effect owing to the internal cavitation enhancement. Thirdly, for supercavitation or high-speed flow, the dominant frequency no longer changed, whereas the nanoparticles increased the jet instability owing to the enhancements in the external cavitation and aerodynamic force.

中文翻译：

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圆形纳米流体射流低速不稳定性实验研究

摘要 实验研究了纳米氧化铝添加剂对圆形水射流不稳定性的影响。通过高速阴影成像获得了两个透明喷嘴出口内部和附近的流动可视化。应用光谱分析方法通过对图像灰度标准偏差数据进行快速傅立叶变换来确定表面波长。根据锐边喷嘴内部的流动结构，基于边界层失稳理论构建了波长预测的集成模型。纳米颗粒添加剂对射流不稳定性的影响主要是由粘度增加和空化促进引起的。根据喷嘴中的空化状态，将影响分为三类。首先，对于无气穴流动，由于粘度增加，纳米颗粒发挥了降频作用。其次，对于部分空化状态，由于内部空化增强，纳米颗粒具有频率增加效应。第三，对于超空化或高速流动，主频率不再改变，而纳米粒子由于外空化和空气动力的增强而增加了射流的不稳定性。