“…Subsequently, many publications have been devoted to the study of the penalty method for the steady Stokes and NSEs, as well as for the unsteady NSEs, in continuous, semi-discrete and fully discrete cases, see, for example, [4,11,13,16,28] and reference, therein. Recent results on the penalty method can be categorised as follows: two-grid penalty method [3,15,2], iterative penalty method [7,14], methods based on different boundary conditions such as nonlinear slip boundary conditions [2,7,21], friction boundary conditions [22,24], slip boundary conditions [31], etc. Moreover, penalty method is used, very recently, for the stochastic 2-D incompressible NSEs [20], and for the incompressible NSEs with variable density [1].…”