颗粒气体团簇行为实验研究

颗粒体系由于非弹性碰撞和摩擦等内秉的能量耗散特性,由宏观粒子形成的颗粒气体体系经常会有局部凝聚现象,这是颗粒气体体系与分子气体体系的最大区别之一.理解和预测这一现象的发生将有助于人们对远离平衡态体系的复杂现象,如有序结构、斑图和团簇形成的认知.这种局部凝聚现象可以类比于分子气体中亚稳分解形成的液滴,将气液相分离用于解释和寻求局部凝聚现象的此模型得到了分子动力学模拟的校验.但是实验的校验却由于宏观粒子运动受重力作用的影响难以在实验室中实现.作为实践十号卫星的前期实验,本文利用国家微重力实验室落塔装置,以水平激振装有不同尺寸和数目的颗粒样品,在短时微重力条件下,成功观察到颗粒气体团簇的形成;并将实验结果与颗粒气体类范德瓦耳斯气体分子相分离模型对比,由形成团簇样品的颗粒数密度条件,来实验确定了所选颗粒的恢复系数,得到直径为0.5 mm的钛珠颗粒的恢复系数在0.6—0.8之间,直径为1 mm的钛珠颗粒的恢复系数约为0.8,直径为2.5 mm的钛珠颗粒的恢复系数应大于0.8.

Exp erimental study of clustering b ehaviors in granular gases

Granular materials are widely spread in nature and in industry. Owing to the inelastic collisions between particles and frictions among particles, granular systems are dissipative in nature. This intrinsic dissipative nature causes local clustering in granular gas systems. This is a unique phenomenon compared with the molecular gases. Understanding and predicting the condition and parameter values when this phenomenon happens will be helpful for us to gain knowledge of the conditions of clustering or pattern formations in non-equilibrium complex systems. The clustering phenomenon in granular gas is analyzed using phase-separation modeling of van der Waals-like molecules. The results from the model are verified by molecular dynamics numerical simulations. However, due to the influence of the gravity, experimental verification is difficult in laboratory. In this work, we perform an experiment in micro-gravity environment provided by the drop tower of National Microgravity Laboratory Chinese Academy of Science. In the experiment we for the first time observe the phase-separation clustering phenomenon. Comparing the observation condition with the model prediction, we are able to indirectly obtain the restitution coefficients of particles used in the experiment. A model calculation for the spinodal regime under experimental conditions is performed for possible particle restitution coefficients, and a comparison with the experimental observation allows us to justify the values of the restitution coefficients. It is found that the coefficient is larger for bigger particles. For d=2.5mm titanium particles, the restitution coefficient is higher than 0.8; for d=1mm titanium particles, the restitution coefficient is about 0.8, and for d=0.5mm titanium particles, the restitution coefficient is between 0.6 and 0.8. This useful result can be essential for comparing experimental observation with the theoretical and the numerical results, and is crucial to the success in the SJ-10 satellite experiments.