We analyze in this paper the convergence properties of the parareal algorithm for the symmetric positive definite problem u' + Au = g. The coarse propagator G is fixed to the backward-Euler method and three time i...
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We analyze in this paper the convergence properties of the parareal algorithm for the symmetric positive definite problem u' + Au = g. The coarse propagator G is fixed to the backward-Euler method and three time integrators are chosen for the F-propagator: the trapezoidal rule, the third-order diagonal implicit Runge-Kutta (RK) (dirk) method, and the fourth-order Gauss RK method. It is well known that the Parareal-Euler algorithm using the backward-Euler method for F and G converges rapidly, but less is known when one uses for F the trapezoidal rule, or the fourth-order Gauss RK method, especially when the mesh ratio J (= Delta T/Delta t) is small. We show that for a specified lambda(max)(the maximal eigenvalue of A or its upper bound), there exists some critical J(cri) such that the parareal solvers derived from these three choices of F converge as fast as Parareal-Euler, provided J >= Jcri. Precisely, for F the trapezoidal rule and the fourth-order Gauss RK method, Jcri depends on Delta T, Delta t, and lambda(max) and we present concise formulas to calculate Jcri, while for F the third-order dirk method, J(cri) = 4, independently of these parameters. Numerical examples with applications in fractional PDEs and uncertainty quantification are presented to support the theoretical predictions.
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