Saddle-point problems give rise to indefinite linear systems that are challenging to solve via
iterative methods. This paper surveys two recent techniques for solving such problems
arising in computational fluid dynamics. The systems are indefinite due to linear constraints
imposed on the fluid velocity. The first approach, known as the multilevel algorithm, employs
a hierarchical technique to compute the constrained linear space for the unknowns, followed
by the iterative solution of a positive definite reduced problem. The second approach …