再做Adina30个例子中的第18个时碰到下列问题,请指教

再做例18流构耦合时总是碰到下列提示:
Model may be unstable! Ratio 1.9389E+11 of largest to smallest diagonal
elements in factorized coefficient matrix is greater than 1.E+11. The
largest diagonal element 3.92409E+05 corresponds to
y-velocity at node (label) 548 and the smallest
diagonal element 2.02385E-06 corresponds to pressure
at node (label) 714.
请问模型不稳定是什么意思?
谢谢!