No Arabic abstract
A multiphysics finite volume method (FVM) solver, coupling neutronics and shock physics, is under development at Politecnico di Milano for the analysis of shock imploding fissile materials [1]. The proposed solver can be a useful tool to make preliminary safety assessment of subcritical plutonium experiments [2] and, more in general, to perform criticality safety evaluations in case of strongly energetic events (such as chemical explosions) involving fissile materials [3]. To this aim, a multi-group SP3 neutron transport model is coupled with a hydrodynamic shock physics model [4], suitable to describe the propagation of strong shockwaves in solid materials. The shock physics module implements a dynamic mesh to reproduce material deformations and its governing equations are written in an Arbitrary Lagrangian Eulerian (ALE) formulation to preserve the mesh quality in case of large distortions.
We present a shock capturing method for large-eddy simulation of turbulent flows. The proposed method relies on physical mechanisms to resolve and smooth sharp unresolved flow features that may otherwise lead to numerical instability, such as shock waves and under-resolved thermal and shear layers. To that end, we devise various sensors to detect when and where the shear viscosity, bulk viscosity and thermal conductivity of the fluid do not suffice to stabilize the numerical solution. In such cases, the fluid viscosities are selectively increased to ensure the cell Peclet number is of order one so that these flow features can be well represented with the grid resolution. Although the shock capturing method is devised in the context of discontinuous Galerkin methods, it can be used with other discretization schemes. The performance of the method is illustrated through numerical simulation of external and internal flows in transonic, supersonic, and hypersonic regimes. For the problems considered, the shock capturing method performs robustly, provides sharp shock profiles, and has a small impact on the resolved turbulent structures. These three features are critical to enable robust and accurate large-eddy simulations of shock flows.
The Hierarchical Schur Complement method (HSC), and the HSC-extension, have significantly accelerated the evaluation of the retarded Greens function, particularly the lesser Greens function, for two-dimensional nanoscale devices. In this work, the HSC-extension is applied to determine the solution of non-equilibrium Greens functions (NEGF) on three-dimensional nanoscale devices. The operation count for the HSC-extension is analyzed for a cuboid device. When a cubic device is discretized with $N times N times N$ grid points, the state-of-the-art Recursive Green Function (RGF) algorithm takes $mathcal{O}(N^7)$ operations, whereas the HSC-extension only requires $mathcal{O}(N^6)$ operations. %Realistic operation counts also depend on the system dimensions in $xyz$-directions and the form of contact self-energy matrix. Operation counts and runtimes are also studied for three-dimensional nanoscale devices of practical interest: a graphene-boron- nitride-graphene multilayer system, a silicon nanowire, and a DNA molecule. The numerical experiments indicate that the cost for the HSC-extension is proportional to the solution of one linear system (or one LU-factorization) and that the runtime speed-ups over RGF exceed three orders of magnitude when simulating realistic devices, such as a graphene-boron nitride-graphene multilayer system with 40,000 atoms.
The production, application, and/or measurement of polarised X-/gamma rays are key to the fields of synchrotron science and X-/gamma-ray astronomy. The design, development and optimisation of experimental equipment utilised in these fields typically relies on the use of Monte Carlo radiation transport modelling toolkits such as Geant4. In this work the Geant4 G4LowEPPhysics electromagnetic physics constructor has been reconfigured to offer a best set of electromagnetic physics models for studies exploring the transport of low energy polarised X-/gamma rays. An overview of the physics models implemented in G4LowEPPhysics, and its experimental validation against Compton X-ray polarimetry measurements of the BL38B1 beamline at the SPring-8 synchrotron (Sayo, Japan) is reported. G4LowEPPhysics is shown to be able to reproduce the experimental results obtained at the BL38B1 beamline (SPring-8) to within a level of accuracy on the same order as Geant4s X-/gamma ray interaction cross-sectional data uncertainty (approximately $pm$ 5 %).
High energy physics has a constant demand for random number generators (RNGs) with high statistical quality. In this paper, we present ROOTs implementation of the RANLUX++ generator. We discuss the choice of relying only on standard C++ for portability reasons. Building on an initial implementation, we describe a set of optimizations to increase generator speed. This allows to reach performance very close to the original assembler version. We test our implementation on an Apple M1 and Nvidia GPUs to demonstrate the advantages of portable code.
We present a parareal in time algorithm for the simulation of neutron diffusion transient model. The method is made efficient by means of a coarse solver defined with large time steps and steady control rods model. Using finite element for the space discretization, our implementation provides a good scalability of the algorithm. Numerical results show the efficiency of the parareal method on large light water reactor transient model corresponding to the Langenbuch-Maurer-Werner (LMW) benchmark [1].