Abstract
Many problems of interest, particularly in the nuclear engineering field, involve coupling between the thermal and mechanical response of an engineered system. The strength of the two-way feedback between the thermal and mechanical solution fields can vary significantly depending on the problem. Contact problems exhibit a particularly high degree of two-way feedback between those fields. This paper describes and demonstrates the application of a flexible simulation environment that permits the solution of coupled physics problems using either a tightly coupled approach or a loosely coupled approach. In the tight coupling approach, Newton iterations include the coupling effects between all physics, while in the loosely coupled approach, the individual physics models are solved independently, and fixed-point iterations are performed until the coupled system is converged. These approaches are applied to simple demonstration problems and to realistic nuclear engineering applications. The demonstration problems consist of single and multi-domain thermomechanics with and without thermal and mechanical contact. Simulations of a reactor pressure vessel under pressurized thermal shock conditions and a simulation of light water reactor fuel are also presented. Problems that include thermal and mechanical contact, such as the contact between the fuel and cladding in the fuel simulation, exhibit much stronger two-way feedback between the thermal and mechanical solutions, and as a result, are better solved using a tight coupling strategy.
Highlights
The processes involved in capturing energy from nuclear reactions and converting that to usable form can involve extreme thermal environments
The individual physics models are solved using preconditioned Jacobian-free Newton Krylov (JFNK). This technique has been used in the work presented here to explore the relative performance of tight and loose coupling on thermomechanics problems
It shows the nonlinear iterations taken in the tight coupling approach, the number of fixed-point iterations in the loose coupling approach, and the total number of nonlinear iterations taken by the individual physics solutions in the loose coupling approach, accumulated for all fixed-point iterations in each step
Summary
The processes involved in capturing energy from nuclear reactions and converting that to usable form can involve extreme thermal environments. Because the gap conductance has a strong effect on the thermal response of the fuel system, the ability to efficiently and robustly solve the strongly coupled thermal and mechanical equations in the presence of evolving contact conditions is critical for a successful fuel performance modeling code. This paper describes the solution environment used to enable tightly and loosely coupled simulations of thermomechanical problems, provides a review of the equations governing thermal and mechanical response, and demonstrates the performance of loose and tight coupling strategies on simple thermomechanical problems with varying degrees of feedback between the two systems Following these simple demonstrations, the performance of these solution strategies is demonstrated on real-world nuclear engineering problems, first on a simulation of reactor pressure vessel response during pressurized thermal shock conditions and on a fuel performance simulation. This work extends similar studies presented by the authors in Novascone et al (2013) and Novascone et al (2013)
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