模拟2维沙堆效果，如果每个点大于高度4，则向前后左右分1个高度的沙子，本身减去4-Analog 2维沙stack effect, if greater than the height of each point of 4, then after moving around at a high level of sand, itself minus 4

时域有限差分方法，一维电磁波在色散介质中的传播和散射3D分析方法，fdtd-Finite difference time domain method, one-dimensional propagation and scattering of electromagnetic waves in dispersive media 3D analysis methods, fdtd