STABILIZED LEAPFROG BASED LOCAL TIME-STEPPING METHOD FOR THE WAVE EQUATION

Local time-stepping methods permit to overcome the severe stability constraint on explicit methods caused by local mesh refinement without sacrificing explicitness. Diaz and Grote [SIAM J. Sci. Comput. 31 (2009), pp. 1985-2014] proposed a leapfrog based explicit local time-stepping (LF-LTS) method for the time integration of second-order wave equations. Recently, optimal convergence rates were proved for a conforming FEM discretization, albeit under a CFL stability condition where the global time-step, Delta t, depends on the smallest elements in the mesh (see M. J. Grote, M. Mehlin, and S. A. Sauter [SIAM J. Numer. Anal. 56 (2018), pp. 994-1021]). In general one cannot improve upon that stability constraint, as the LF-LTS method may become unstable at certain discrete values of Delta t. To remove those critical values of Delta t, we apply a slight modification (as in recent work on LF-Chebyshev methods by Carle, Hochbruck, and Sturm [SIAM J. Numer. Anal. 58 (2020), pp. 2404-2433]) to the original LF-LTS method which nonetheless preserves its desirable properties: it is fully explicit, second-order accurate, satisfies a three-term (leapfrog like) recurrence relation, and conserves the energy. The new stabilized LF-LTS method also yields optimal convergence rates for a standard conforming FE discretization, yet under a CFL condition where Delta t no longer depends on the mesh size inside the locally refined region.