Experimental study on the mechanism of cavitation-induced ventilation

In this study, the cavitating flow and cavitation-induced ventilation flow around a surface-piercing hydrofoil were investigated to gain in-depth understanding of the interaction mechanism between the vaporous cavity and free surface at low cavitation numbers. Experiments were conducted in a constrained-launching water tank to visualize the cavity using a high-speed camera. Unsteady cloud cavitation and cavitation-induced ventilation at atmospheric pressure were observed and analyzed while piercing the free surface. The flow regime map was summarized at a fixed aspect ratio of AR(h) = 1.5. Subsequently, a physical model was proposed to predict the maximum depression depth of the water surface (H) at the trailing edge of the hydrofoil. Both the physical model and experimental results reveal that the non-dimensional depth H/c has a linear relation to Fn2 c x Re-c x sin(2)alpha. Finally, a criterion for cavitation-induced ventilation based on the improved lifting-line theory and a physical model were proposed. A new relation H/L-c similar to alpha(0.5) was obtained, where L-c is the maximum cavity length. The results of this study can guide the design and application of hydrofoils for ventilation and cavitation processes

绕弹性水翼空化流动及其流激振动特性研究

该文采用基于径向基函数的流固耦合方法，对不同空化数下绕刚性水翼/弹性水翼的空泡形态及其流激振动特性进行了数值模拟。结果表明，绕弹性水翼空泡形态具有三个典型特征：(1)靠近翼尖前缘的附着空泡形态为三角形；(2)翼尖出现梢涡空化，梢涡空化随着空化数的降低而逐渐明显；(3)回射流作用下水翼存在云状空泡的周期性脱落。对不同空化数下弹性水翼的振动特性进行分析发现，随着空化数的下降，水翼振动的平均振幅及振幅波动值先增大再减小，平均振幅和振幅波动由大到小依次是：云空化>超空化>片空化

Experimental and numerical study on ventilated cavitation of high-speed projectile

In this study, ventilated cavitating flow characteristics around an axisymmetric projectile are investigated by combining experiments and numerical simulations. Experiments were carried out with a Split-Hopkinson pressure bar launch system and the pressure-equaling exhaust technology. Modular projectiles are designed to experimentally investigate the influence of head shape and ventilatory volume on flow characteristics. Large eddy simulation model is applied to obtain more flow field information. Compared with the conical head projectile, the hemispherical head projectile has a thinner attached cavity and more local detachment of the cavity. The statistical structure of the velocity and pressure fluctuations are analyzed by combining histograms and Q-Q diagrams. The results show that the pressure drag is dominant in the total drag and the periodic pulsation of the tail cavity and the stable vortex structure at the tail cause the variation of drag. The larger cavity volume changes the actual shape of the projectile, making the drag of the conical head projectile higher. The evolution characteristics of the cavitating flow field around the projectile with different ventilatory volumes are obtained, and the relationship between pressure fluctuation and chamber volume is derived. It is found that the reentrant jet causes a reverse flow at the nozzle, which leads to local pressure rise at the same interval. The above research work could contribute to the design and flow control of the ventilated cavity body

改进的物理融合神经网络在瑞利-泰勒不稳定性问题中的应用

基于相场法的物理融合神经网络PF-PINNs被成功用于两相流动的建模,为两相流动的高精度直接数值模拟提供了全新的技术手段.相场法作为一种新兴的界面捕捉方法,其引入确保了界面的质量守恒,显著提高了相界面的捕捉精度;但是相场法中高阶导数的存在也降低了神经网络的训练速度.为了提升计算训练过程的效率,本文在PF-PINNS框架下,参考深度混合残差方法 MIM,将化学能作为辅助变量以及神经网络的输出之一,并修改了物理约束项的形式,使辅助变量与相分数的关系式由硬约束转为了软约束.上述两点改进显著降低了自动微分过程中计算图的规模,节约了求导过程中的计算开销.同时,为了评估建立的PF-PINNS在雷诺数较高、计算量较大的场景中的建模能力,本文将瑞利-泰勒RT不稳定性问题作为验证算例.与高精度谱元法的定性与定量对比结果表明,改进PF-PINNs有能力捕捉到两相界面的强非线性演化过程,且计算精度接近传统算法,计算结果符合物理规律.改进前后的对比结果表明,深度混合残差方法能够显著降低PF-PINNS的训练用时.本文所述方法是进一步提升神经网络训练速度的重要参考资料,并为探索高精度智能建模方法提供了全新的见解

Study on the cavity dynamics of water entry for horizontal objects with different geometrical shapes

The influence of geometrical shapes on cavity evolution, flow fields and vortex structures evolution during water entry processes are investigated by experimental and numerical methods in this paper. The cavity evolution for water entry of four horizontal objects with different geometrical shapes are captured by high-speed photography. The experimental results indicate that although the geometrical shape has influence on cavity shape and size, the pinch-off depths are very close for the open cavity cases. In the numerical study, a compressible two-phase flow solver combing with large eddy simulation (LES) and dynamic overset mesh based on OpenFOAM (R) platform is adopted to observe the cavity dynamics, flow fields and vortex structures information. Numerical results reveal that the cavity produced by the object with trapezoid section expand at a relatively higher speed in the radial direction to form a larger cavity due to the more kinetic energy transference to the fluid. The vorticity evolution shows that for the object with circle section, the vortex structures alternately shed at the cavity tail shortly after the cavity separates from the free surface. However, for the object with trapezoid section, the cavity is squashed distorted due to the hydrostatic pressure without alternating vortex shedding

Studies on two-bubble energy transfer model with radiant-receiver structure

A two-bubble model with radiate (alpha bubble)-receive (beta bubble) structure is constructed to study the energy transfer from one bubble to another. The influence of the non-dimensional distance d and the initial energy ratio psi on the energy transfer rate is investigated via numerical simulation. The relative received energy epsilon, relative jet energy J, and energy transfer rates eta are defined to quantify energy transfer. Results show that the energy transfer rate decreases with the increase of d and psi when the two bubbles' initial radius is identical. With the increase of d, the interactions between two bubbles are weakened, and the relative received energy satisfies the law of epsilon proportional to 1/d(2). With the increase of psi, the maximum inner pressure of the beta bubble increase first and then decreases, while the jet energy of bubble beta changes with the law of J proportional to psi. It is found that the energy storage capacity increases with the bubble radius by simulating different bubble radius ratios

Seal types of water-entry cavities generated by the impact of spheres with decreasing Bond number

The evolution of water-entry cavities gives rise to interesting dynamic phenomena that occur in nature and engineering fields. The evolution and seal types of the cavities remain unsettled at Bo less than 10(-2) since it is difficult to propel a small sphere to impact a water surface at high speed. In the present study, an experiment based on the laserdriven principle is conducted to accelerate a small sphere to hundreds m/s. The types of cavity seal are classified as quasi-static, shallow seal, deep seal, and surface seal within a Bo range of 1-7.57x10(-2). The transition mechanism from a shallow seal to a deep seal is investigated by analyzing the cross-sectional characteristics of cavity necks. With a further decrease in Bo (5.69x10(-2)-8.41x10(-3)), the shallow seal is changed directly to a surface seal since the pinch-off time of the cross section at the cavity neck decreases with Bo, and hence, the deep seal disappears. Finally, a parameter study is conducted by varying We and Bo in the air cavity. Two boundaries are obtained: We approximate to 64 for the transition between quasi-static and shallow seals and We approximate to 326 for the transition between shallow seals and surface seals

Monitoring long-term ecological impacts from release of Fukushima radiation water into ocean

4月13日，日本政府宣布其将福岛第一核电站的核废水排入太平洋的决定，尽管其一再强调经过处理和稀释后的核废水的辐射水平会低于饮用水设定的标准，但此举仍遭到当地渔民和附近沿海国家包括韩国、俄罗斯、中国和朝鲜的强烈反对，联合国和包括绿色和平组织等在内的非政府组织对日本这一决定表示深切担忧。国际原子能机构（IAEA）表示会对核废水的排放进行严格监督，保证其在安全水平之内。关于“福岛核辐射”的网络检索量在4月13日激增，此举引起国际上广泛关注。在此背景下，国际期刊Geography and Sustainability邀请欧洲科学院院士、发展中国家科学院院士、我校吕永龙教授研究团队撰写文章，科学分析了日本福岛核事故的生态影响及核废水排放入海会带来的潜在的生态影响，并提出了应采取的应对措施，呼吁国际科技界联合起来对此做出回应。该文的第一与通讯作者为吕永龙教授，共同作者包括苑晶晶博士以及杜荻、孙斌和易小洁三位博士生。参与单位包括厦门大学、中国科学院生态环境研究中心、中国科学院大学和中丹科教中心。【Abstract】After 10 years of the Fukushima Nuclear Accident, Japan decided to release the nuclear wastewater into the Pacific Ocean on 13 April 2021. It is apparent that Japan has chosen the most cost-efficient way to deal with the contaminated water, however, great opposition and concerns have been raised internationally due to the ecotoxicological features of radioactive materials and their harmful impacts on the environment. Here we analyze the ecological impacts caused by the nuclear accident and the potential impacts of releasing the nuclear wastewater into the ocean. Science-based solutions are proposed through a third-party evaluation and strict environmental assessment, multi-stakeholder public participation, integrated monitoring of the neighboring coastal countries, long-term international collaborative research, and setting up international convention for ecological compensation.This study was supported by the National Natural Science Foundation of China (Grant No.71761147001 and 42030707) and the International Partnership Program of the Chinese Academy of Sciences (Grant No.121311KYSB20190029). 该研究得到了国家自然科学基金（71761147001和42030707）和中国科学院国际大科学计划项目（121311KYSB20190029）的资助