We consider the combination of discontinuous Galerkin discretizations in space with various time integration methods for linear acoustic, elastic, and electro-magnetic wave equations. For the discontinuous Galerkin method we derive explicit formulas for the full upwind flux for heterogeneous materials by solving the Riemann problems for the corresponding first-order systems. In a framework of bounded semigroups we prove convergence of the spatial discretization.For the time integration we discuss advantages and disadvantages of explicit and implicit Runge-Kutta methods compared to polynomial and rational Krylov subspace methods for the approximation of the matrix exponential function. Finally, the efficiency of the different time integrators is illustrated by several examples in 2D and 3D for electro-magnetic and elastic waves.∂ t E(u(t)) = M∂ t u(t), u(t) 0,Ω = − Au(t), u(t) 0,Ω = 0, i.e., the energy is conserved E(u(t)) = E(u(0)) for all t ∈ [0, T ].We study three different applications fitting into this framework, namely acoustic, elastic, and electro-magnetic waves.
ApplicationsIn all applications, the operator A corresponds to a linear system of J first-order differential equations.Acoustic waves Acoustic waves in an isotropic medium with variable density ρ ∈ L ∞ (Ω) are described by the secondorder scalar equation for the potentialWe assume ρ(x) ≥ ρ 0 > 0 for a.a. x ∈ Ω. Introducing the pressure p = ∂ t ψ and the flux q = −∇ψ this corresponds to the first-order system ρ∂ t p + div q = 0 , ∂ t q + ∇p = 0 with J = D + 1 components. We define the operators M (q, p) = (q, ρp), A(q, p) = (∇p, div q)