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众所周知,用显式的差分格式对时间相关的N-S方程求数值解时,时间步长须受稳定性条件的限制,计算效率较低。特别是在紊流附面层中,近壁面的节点必须位于粘性底层内,网格间隔十分小,由此确定的计算步长亦十分小。因此,欲达到稳定状态的结果要耗费大量的机时。本计算在近壁面的法线方向上的细网格区采用了守恒的“delta”形式的隐式格式,而在其余地区及流向方向仍采用MacCormack的两步显式格式,从而提高了计算效率。为了减小非线性振荡,还采用了[2]所提出的开关函数,取得了很好的结果。
It is well-known that when using the explicit difference scheme to solve the time-dependent N-S equation in numerical solution, the time step must be limited by the stability conditions and the computational efficiency is low. Especially in turbulent overburden, the nodes near the wall must be within the adhesive bottom, the grid spacing is very small, and the calculated step size is also very small. As a result, a large amount of machine time is required to achieve a steady state result. The proposed algorithm uses a conservative “delta” implicit scheme for the fine grid in the normal direction of the near-wall surface, while MacCormack’s two-step explicit format is used for the rest of the region and the direction of flow to improve computational efficiency . In order to reduce the non-linear oscillation, the switch function proposed in [2] is also used, and good results have been obtained.