On the number of subproblem iterations per coupling step in partitioned fluid-structure interaction simulations

TL;DR

This paper investigates how the number of internal subproblem iterations affects the computational cost of partitioned fluid-structure interaction simulations, proposing an optimal iteration count and a new residual-based convergence criterion.

Contribution

It highlights the importance of internal subproblem iterations, introduces a systematic approach to determine their optimal number, and proposes a residual-based convergence criterion for coupled systems.

Findings

Internal iterations significantly impact computational cost.

Limiting subproblem iterations can reduce overall run time.

A residual-based convergence criterion is effective without extra tolerances.

Abstract

In literature, the cost of a partitioned fluid-structure interaction scheme is typically assessed by the number of coupling iterations required per time step, while ignoring the internal iterations within the nonlinear subproblems. In this work, we demonstrate that these internal iterations have a significant influence on the computational cost of the coupled simulation. Particular attention is paid to how limiting the number of iterations within each solver call can shorten the overall run time, as it avoids polishing the subproblem solution using unconverged coupling data. Based on systematic parameter studies, we investigate the optimal number of subproblem iterations per coupling step. Lastly, this work proposes a new convergence criterion for coupled systems that is based on the residuals of the subproblems and therefore does not require any additional convergence tolerance for the coupling loop.