本文以MD方法模擬二維非彈性顆粒子系統的冷卻過程﹙freely cooling system﹚，探討不同彈性係數下簇集現象形成的情況，並觀查不同的邊界條件對此現象的影響。當系統處於齊次狀態﹙homogeneous state﹚時，系統能量衰減正比於t -2，此結果與Haff’s cooling law相符合，而系統在非齊次態時簇集現象﹙clustering﹚將會展現。在非齊次狀態下，能量衰減與彈性係數成並非單純的線性關係。這是因為空間分佈的不均勻造成速率較小的顆粒子有較高的碰撞機率，並使彈性系數小於臨界彈性系數的系統反而擁有較高的系統總能量。本文末並觀察具有切線方向磨擦之系統的冷卻過程。 We consider the dynamics of an ensemble of identical, inelastic, hard disks in a square domain, with three kinds of dierent boundary conditions, (i) double periodic bound- aries, (ii) a pair of smooth, elastic walls in the x-direction and periodic boundaries in the y-direction, (iii) four smooth and elastic walls. Starting with the almost elastic case, in which the coeÆcient of restitution is just a little less than 1, the homoge- neous regime resembles a classical non-dissipative gas and there is no large structure. When decreases, the system becomes inhomogeneous and spatial non-uniformity occurs. Clusters appear when is even smaller, large clusters of disks form, break, and reform. As time goes by, the cluster stays in a status of hydrodynamic shear state, or collapse. Inelastic collapse, which is a dynamic singularity of the binary collision model, is caused by the many-body dynamics. Numerical simulations show that the energy decay in the homogeneous regime is proportion to t
