气液耦合系统中固有频率的实验研究

流体砰击现象广泛存在于海洋环境、航空航天等自然界与工程中.流体砰击大尺度结构过程中,自由液面破碎时会包裹气体进入流场,气液混合易导致局部砰击荷载增大,引起结构破坏的危险.砰击过程中,气室压力对自由液面固有模态的影响尚未有系统的研究报道.该文采用物理模型实验方法在二维储舱内设计并开展一系列实验,系统研究了两种不同的气室压力对耦合系统的固有频率和阻尼的影响.实验中采用高速摄影机记录了自由液面振荡过程,通过自主研制的图像处理软件提取自由液面波高.结果表明:在低气室压力下,晃荡能量主要集中于一阶固有频率;在高气室压力下,晃荡能量主要集中于二阶固有频率.随着气室压强的增大,影响液体晃荡的主要固有频率提高,而对应的阻尼比却随之降低.因此,气体可压缩性是研究流体晃荡的一个重要因素.

Experimental Investigation of Natural Frequencies of Gas-Liquid Coupled Systems in Tanks

The wave impact phenomenon widely exists in nature, ocean and aerospace engineering. When the wave impacts on the large-scale structure, the violent free surface may break and the wave tip entraps the air. As a consequence, wave impact with gas-entrapment may cause localized and large load, which may lead to structural failure. During the slamming process, the influence of gas on the natural modes of the free surface has not been systematically reported. A series of experiments were designed and conducted to study the influences of 2 different ullage space pressures on the natural frequencies and damping ratios of gas-liquid coupled systems. High-speed cameras were employed to record the free-surface vibration. Furthermore, the surface wave height was extracted with a self-made image-processing software. The results show that, the sloshing energy mainly concentrates on the lowest natural frequency of the free surface under a low ullage space pressure; while the sloshing energy mainly concentrates on the 2nd natural frequency of the free surface under a high ullage space pressure. As the ullage space pressure of the sloshing tank increases, the dominant natural frequency of the free surface will increase, while the corresponding damping ratio will decrease. Therefore, the gas compressibility is an important factor for the sloshing issue in the tank.