中文说明：该程序为MATLAB编程计算二维D2Q9模型的管流程序，可以计算相应的速度分布和密度分布图。以及相应的平衡分布函数等
English Description：
The program is a matlab program for calculating the pipe flow of two-dimensional D2Q9 model, which can calculate the corresponding velocity distribution and density distribution. And the corresponding equilibrium distribution function