A Multi-Physics Adaptive Time Step Coupling Algorithm for Light-Water Reactor Core Transient and Accident Simulation

A new reactor core multi-physics system addresses the pellet-to-cladding heat transfer modeling to improve full-core operational transient and accident simulation used for assessment of reactor core nuclear safety. The rigorous modeling of the heat transfer phenomena involves strong interaction between neutron kinetics, thermal-hydraulics and nuclear fuel performance, as well as consideration of the pellet-to-cladding mechanical contact leading to dramatic increase in the gap thermal conductance coefficient. In contrast to core depletion where parameters smoothly depend on fuel burn-up, the core transient is driven by stiff equation associated with rapid variation in the solution and vulnerable to numerical instability for large time step sizes. Therefore, the coupling algorithm dedicated for multi-physics transient must implement adaptive time step and restart capability to achieve prescribed tolerance and to maintain stability of numerical simulation. This requirement is met in the MPCORE (Multi-Physics Core) multi-physics system employing external loose coupling approach to facilitate the coupling procedure due to little modification of constituent modules and due to high transparency of coupling interfaces. The paper investigates the coupling algorithm performance and evaluates the pellet-to-cladding heat transfer effect for the rod ejection accident of a light water reactor core benchmark.

MODELAGEM DA EMISSÃO DE LUZ BRANCA CONTÍNUA POR CONVERSÃO ASCENDENTE DE ENERGIA EM NANOMATERIAIS

In the last decade, the emission of continuous bright white light by nanomaterials of metal oxides has been observed when excited with lasers in the near infrared region. However, the origin of this phenomenon of emission of continuous white light by upconversion of energy is not yet established. Therefore, in this work, we aimed at developing a quantitative model based on the power balance equation to describe this process considering thermal emission as the main mechanism for white light generation. The model was implemented in the Rates program, in Fortran language, to solve differential equations using the 4th order Runge-Kutta method with an adaptive time step. The results show that the proposed model is adequate for the description of the most relevant features of this phenomenon, which allows its use in the design of new materials and to improve experimental conditions.

A Finite-Element Based Framework for Transient Rotor Dynamic Simulations

Transient simulations play a key role in the analysis and subsequent design of structural components with one or more rotating parts. A framework is proposed to this effect, centered around the finite-element solver OptiStruct, consisting of a time integration scheme built on the Newmark family with an appropriate adaptive time-step control. The process accounts for a computationally efficient handling of nonlinearities that might arise through bearings and casings. This solution is detailed starting from the governing equations for transient rotor dynamics to the nuances of the time marching scheme, and this process is applied to a test case from which conclusions are drawn that might be of interest to practicing engineers. These conclusions include insights into enforced motion, operation at or near critical speeds, rotor damping and contact. This work is aimed at producing a user-friendly and robust tool and process for the practicing engineer to perform complex rotor dynamic analysis.

Influence of Two External Excitations for Brake Stick-Slip Behavior Using New Numerical Calculation Method

In this paper, a dynamic brake model has been constructed by incorporating the brake rotor's speed and the brake normal force as excitation sources. By introducing two permissible errors (ε1,2), a novel computation algorithm is proposed to reduce the ill-conditioning, arising from the nonlinear friction. Its validation illustrates that the proposed method, using double-changed time-steps and smarter adaptive time-step reduced method, is more reliable than other integral equation solvers with a higher accuracy as well as less computation time. Moreover, the influences of external excitations on the dynamic characteristic of the brake system are also analyzed, and an estimation for the occurrence of unstable vibration is investigated. The results demonstrate the different contributions of the two external excitations on the dynamic characteristic. The brake system has more unstable vibration at a higher brake normal force and a lower brake rotor's speed with small fluctuation. Furthermore, the higher brake rotor's speed could generate more positive damping effect, which could reduce and suppress the occurrence of the sick-slip vibrations. In practice, these instabilities can be minimized by appropriate selection of the two external, which can be adjusted according to the advanced working requirements.

A general-purpose time-step criterion for simulations with gravity

ABSTRACT We describe a new adaptive time-step criterion for integrating gravitational motion, which uses the tidal tensor to estimate the local dynamical time-scale and scales the time-step proportionally. This provides a better candidate for a truly general-purpose gravitational time-step criterion than the usual prescription derived from the gravitational acceleration, which does not respect the equivalence principle, breaks down when $\boldsymbol {a}=0$, and does not obey the same dimensional scaling as the true time-scale of orbital motion. We implement the tidal time-step criterion in the simulation code gizmo, and examine controlled tests of collisionless galaxy and star cluster models, as well as galaxy merger simulations. The tidal criterion estimates the dynamical time faithfully, and generally provides a more efficient time-stepping scheme compared to an acceleration criterion. Specifically, the tidal criterion achieves order-of-magnitude smaller energy errors for the same number of force evaluations in potentials with inner profiles shallower than ρ ∝ r−1 (i.e. where $\boldsymbol {a}\rightarrow 0$), such as star clusters and cored galaxies. For a given problem these advantages must be weighed against the additional overhead of computing the tidal tensor on-the-fly, but in many cases this overhead is small.

A Fast Algorithm for Onboard Progressive Flooding Simulation

The need for decision support after a flooding casualty requires the development of fast and accurate progressive flooding simulation procedures. Here, a new quasi-static technique is presented, proposing a differential algebraic formulation capable to consider independently the flooding process in the internal rooms. The proposed method is efficient while simulating long flooding chains along rooms connected by similar size openings, a condition that likely occurs on large passenger ships. Moreover, the computational performances of the simulation procedure have been enhanced by adapting the time step to the progressive flooding pace. The adoption of an adaptive time step algorithm reduces significantly the calculation time. The novel procedure has been tested on the recommended benchmark cases for flooding simulations, highlighting the accuracy and flexibility of the proposed method.