“…The weak Galerkin (WG) finite element method is a numerical technique for solving partial differential equations. Since it has been proposed by Wang [28], the WG method has been applied successfully to the discretization of several classes of partial differential equations and variational inequalities, e.g., second-order elliptic problems [2-5, 8, 9, 11, 16, 19, 29, 31], the Stokes equations [25,30,33,34], the Biharmonic equations [18,21,27,35], the Maxwell equations [22], the Oseen equations [15], the Helmholtz equations [20,23], the multi-term time-fractional diffusion equations [36], the parabolic integro-differential equations [37], the interface problems [17,24], the natural convection problems [6], and the elliptic variational inequality [26].…”