说明：  随机方向移动模型描述为节点先在整个移动区域内随机选择一个位置作为初始位置,随机选择运动速度的大小和方向,一直运动到边界上的一个点
(The random direction moving model is described as follows: firstly, the node randomly selects a position in the whole moving area as the initial position, and then randomly selects the size and direction of the motion speed until it moves to a point on the boundary)