二维不可压缩Rayleigh-Bernard对流，交错网格，中心差分，矩形边界，上壁冷却，下壁加热，采用Boussinesq近似-Code is for 2D incompressible Rayleigh-Bernard convection, using staggered grids, middle difference, box boundary and Boussinesq approximation. The upper boundary is cooled while the lo