Table 2.

The value of $\parallel \nabla ·{\mathbf{u}}_h^n\parallel$ with different Rayleigh numbers $Ra$.

$\mathit{R}\mathit{a}$	10	100	150	180
Algorithm 2	1.82 × 10${}^{-8}$	2.65 × 10${}^{-10}$	2.02 × 10${}^{-11}$	4.96 × 10${}^{-12}$
Algorithm 1	3.50 × 10${}^{-18}$	/	/	/

The mark “/” means that the iterative step was larger than 600.