Adaptive time multiscale algorithms for fluid–structure interaction with impacts - Application to a row of PWR fuel assemblies under seismic loading

Abstract

The present article is dedicated to numerical methods for the simulation of the response of PWR fuel assemblies under external mechanical loading such as earthquakes. It proposes new algorithms to manage impact sequences within the time multiscale resolution of a strongly coupled fluid–structure partitioned problem. Preserving the computational efficiency imposes an adaptive strategy to adjust the time scale to force the solution of the costly pressure problem for the fluid only when it is necessary to account for the brutal variations in the structure acceleration resulting from significant impacts. The adaptation criteria must be built upon a thorough monitoring and characterization of all impacts, including the duration of the flight sequence before it occurs and the relative velocity between impact entities. Finally, solutions to mitigate the consequences of missed and unresolved impacts in time are provided to avoid spurious amplification of the variation of the acceleration under impact in the fluid velocity field through the time integration scheme. This leads to two classes of adaptive time multiscale algorithms, extensively tested and qualified through two chosen cases of growing complexity. The paper is completed by a full experimental validation of the proposed fluid–structure framework with contact and impacts, using the results available for a row of six fuel scale assembly immersed in water and submitted to a seismic excitation on a shaking table.