A shortcut for IMEX methods: integrate the residual explicitly

In numerical time-integration with implicit-explicit (IMEX) methods, a within-step adaptable decomposition called residual balanced decomposition is introduced. This decomposition allows any residual occurring in the implicit equation of the implicit-step to be moved into the explicit part of the decomposition. By balancing the residual, the accuracy of the local truncation error of the time-stepping method becomes independent from the accuracy by which the implicit equation is solved. In this way, the requirement of a small enough residual in an iterative solver is relieved in favor of overall computational efficiency. In order to balance the residual, the original IMEX decomposition is adjusted after the iterative solver has been stopped. For this to work, the traditional IMEX timestepping algorithm needs to be changed. We call this new method the shortcut-IMEX (SIMEX). SIMEX can gain computational efficiency by exploring the trade-off between the computational effort placed in solving the implicit equation and the size of the numerically stable time-step. Typically, increasing the number of solver iterations increases the largest stable step-size. Both multi-step and Runge-Kutta (RK) methods are suitable for use with SIMEX. Here, we explore the efficiency of SIMEX-RK methods in overcoming parabolic stiffness. Examples of applications to linear and nonlinear reaction-advection-diffusion equations are shown. In order to define a stability region for SIMEX, a region in the complex plane is depicted by applying SIMEX to a suitable PDE model containing diffusion and dispersion. A myriad of stability regions can be reached by changing the RK tableau and the number of solver iterations.