Optimal ${L^2}$ error estimates for 2D/3D incompressible Cahn--Hilliard--magnetohydrodynamic equations

Abstract: This paper focuses on an optimal error analysis of a fully discrete finite element scheme for the Cahn--Hilliard--magnetohydrodynamic (CH-MHD) system. The method use the standard inf-sup stable Taylor--Hood/MINI elements to solve the Navier--Stokes equations, Lagrange elements to solve the phase field, and particularly, the N\'ed\'elec elements for solving the magnetic induction field. Suffering from the strong coupling and high nonlinearity, the previous works just provide suboptimal error estimates for phase field and velocity field in $L^{2}/\L^2$-norm under the same order elements, and the suboptimal error estimates for magnetic induction field in $\H(\rm curl)$-norm. To this end, we utilize the Ritz, Stokes, and Maxwell quasi-projections to eliminate the low-order pollution of the phase field and magnetic induction field. In addition to the optimal $\L^2$-norm error estimates, we present the optimal convergence rates for magnetic induction field in $\H(\rm curl)$-norm and for velocity field in $\H^1$-norm. Moreover, the unconditional energy stability and mass conservation of the proposed scheme are preserved. Numerical examples are illustrated to validate the theoretical analysis and show the performance of the proposed scheme.