超高压水射流喷头水动力特性研究

基于对超高压水射流喷头的外部参数定量化分析,给出关于射流核心参数的优选方法,旨在提高水射流效率。首先,根据超高压水射流除锈喷嘴的工作特点,考虑到水的压缩性和空化效应,建立单束定冲角、多束旋转喷头的三维数值模型,通过改变靶距、入射角度、转速等外部特征参数,实施了超高压水射流除锈喷头水动力性能模拟研究。然后,重点分析单束定冲角喷嘴靶距、入射角度对靶面剪切应力、打击压强分布的影响,以及射流等速核长度与最佳射流靶距的关系。发现当靶距等于喷嘴射流等速核长度时,壁面剪切应力达到最佳水平。此外,通过分析高速旋转射流卷吸效应、靶面水垫作用对靶面所受剪切应力、打击压强分布的影响,得到最佳转速范围和对应线速度。初步阐明了射流冲击剥离的机理、单束定冲角以及多束旋转射流的特征参数对射流效果的影响,可为超高压除锈喷头的设计、装配提供参考。     

基于ESGA遗传算法的水射流自驱旋转喷头优化设计

超高压水射流自驱旋转型喷头是目前广泛应用于船壁除锈的一种装置,其布局方式直接影响船壁除锈的效率和质量,目前喷头布局多依赖工程经验,缺少准确的理论分析和优化技术支持。针对水射流自驱旋转型喷头的布局优化问题,在传统遗传算法（genetic algorithm,GA）的基础上,提出一种基于“锦标赛选择”的精英策略遗传算法（elitist strategy genetic algorithm,ESGA）,该算法通过采用种群进化过程中精英个体直接保留到下一代的进化策略,从而有效提高算法的全局收敛能力和算法的鲁棒性。结合旋转喷头扫掠冲击性能和轨迹特征,以喷头移动路径垂直打击面上的能量分布均匀度为衡量标准,建立超高压水射流自驱旋转型喷头的螺旋扫掠冲击离散化时间优化模型,并分别利用两种遗传算法对其进行优化改进。对一字形水射流自驱旋转型喷头的布局优化研究发现,经ESGA算法优化的旋转喷头,其扫掠冲击能量分布均匀度较原喷头布局提升了47.2%,其收敛精度也高于GA算法。经对ESGA算法优化后的喷头实验验证发现,ESGA优化方案较原设计方案除锈效率提高了42.0%。改进的ESGA优化算法可行性强,能够在收敛...     

A new kind of hairpin-like vortical structure induced by cross-interaction of sinuous streaks in turbulent channel

This work is motivated by previous experimental and numerical studies which reveal that the hairpin vortex could be formed by the interaction between spanwise adjacent low-speed streaks. To prove that such an interaction mechanism is still applicable in the normal direction, two sinuous low-speed streaks with the same streamwise phase are set to be in the upper half and bottom half of a small size channel, respectively, and their evolution and interaction are investigated by direct numerical simulation. A new kind of hairpin-like vortical structure, distributed in the normal direction and straddled across both halves of the channel, is found during the cross-interaction process of the low-speed streaks. The influence of such a normal-distributed hairpin-like vortex(NHV) on the turbulent statistical regularity is also revealed. It is observed that the NHV can lead to a sudden surge of wall skin friction, but the value of the normal velocity as well as the streamwise and spanwise vorticity sharply decrease to zero in the center of the channel.