Hybrid smoothed particle hydrodynamics

We present a new algorithm for enforcing incompressibility for Smoothed Particle Hydrodynamics (SPH) by preserving uniform density across the domain. We propose a hybrid method that uses a Poisson solve on a coarse grid to enforce a divergence free velocity field, followed by a local density correction of the particles. This avoids typical grid artifacts and maintains the Lagrangian nature of SPH by directly transferring pressures onto particles. Our method can be easily integrated with existing SPH techniques such as the incompressible PCISPH method as well as weakly compressible SPH by adding an additional force term. We show that this hybrid method accelerates convergence towards uniform density and permits a significantly larger time step compared to earlier approaches while producing similar results. We demonstrate our approach in a variety of scenarios with significant pressure gradients such as splashing liquids

Field-Scale Testing and Numerical Investigation of Soil-Boulder Interaction under Vehicular Impact Using FEM and Coupled FEM-SPH Formulations

A computational approach that couples the Finite Element Method and the Smoothed Particle Hydrodynamics method may be advantageous for simulating the response of complex, physical systems involving large deformations. However, comparisons of this modeling technique against field-scale test data are remarkably sparse in literature. This study presents three field-scale tests involving vehicular impact into three landscape vehicular anti-ram barriers. Each barrier consisted of a single boulder embedded in compacted American Association of State Highway and Transportation Officials soil and physical testing resulted in one of the following outcomes: minimal boulder/soil movement (Test 1), moderate boulder/soil movement (Test 2), and severe boulder/soil movement and vehicle override (Test 3). For each test, two LS-DYNA models were developed: a model using a traditional finite element method approach for the entire soil region along with a model using a hybrid finite element method-smoothed particle hydrodynamics approach where the near-field soil region was simulated using smoothed particle hydrodynamics. For Tests 1 and 2, both the traditional finite element method approach and the hybrid finite element method-smoothed particle hydrodynamics approach were able to accurately match data collected from the field tests. However, for Test 3, the finite element method-only approach was not able to accurately predict the global response of the system under vehicular impact. On the other hand, the hybrid finite element method-smoothed particle hydrodynamics approach was able to capture global response of the system including boulder rotation, soil upheaval, and vehicle override

Introducing a Hybrid Method of Radiative Transfer in Smoothed Particle Hydrodynamics

We present a new method of incorporating radiative transfer into Smoothed Particle Hydrodynamics (SPH). There have been many recent attempts at radiative transfer in SPH (Stamatellos et al 2005, 2005, Mayer et al 2007, Whitehouse and Bate 2006), however these are becoming increasingly complex, with some methods requiring the photosphere to be mapped (which is often of non-trivial geometric shape), and extra conditions to be applied there (matching atmospheres as in Cai et al (2008), or specifying cooling at the photosphere as in Mayer et al (2007)). The method of identifying the photosphere is usually a significant addition to the total simulation runtime, and often requires extra free parameters, the changing of which will affect the final results. Our method is not affected by such concerns, as the photosphere is constructed implicitly by the algorithm without the need for extra free parameters. The algorithm used is a synergy of two current formalisms for radiative effects: a) the polytropic cooling formalism proposed by Stamatellos et al (2007), and b) flux-limited diffusion, used by many authors to simulate radiation transport in the optically thick regime (e.g. Mayer et al 2007). We present several tests of this method: (1) The evolution of a 0.07 solar mass protoplanetary disc around a 0.5 solarmass star (Pickett et al 2003, Mejia et al 2005, Boley et al 2006, Cai et al 2008); (2) The collapse of a non-rotating 1 solar mass molecular cloud (Masunaga & Inutsuka 2000, Stamatellos et al 2007); (3) The thermal relaxation of temperature fluctuations in an static homogeneous sphere (Masunaga et al 1998, Spiegel 1957, Stamatellos et al 2007)Comment: 4 pages, 6 figures, to appear in the proceedings of the Cool Stars 15 conferenc

Introducing a Hybrid Method of Radiative Transfer for Smoothed Particle Hydrodynamics

A new means of incorporating radiative transfer into smoothed particle hydrodynamics (SPH) is introduced, which builds on the success of two previous methods - the polytropic cooling approximation as devised by Stamatellos et al (2007), and flux limited diffusion (e.g. Mayer et al 2007). This hybrid method preserves the strengths of its individual components, while removing the need for atmosphere matching or other boundary conditions to marry optically thick and optically thin regions. The code uses a non-trivial equation of state to calculate temperatures and opacities of SPH particles, which captures the effects of molecular hydrogen dissociation, atomic hydrogen ionisation, He0 and He+ ionisation, ice evaporation, dust sublimation, molecular absorption, bound-free and free-free transitions and electron scattering. The method is tested in several scenarios, including: (1) the evolution of a 0.07 solar mass protoplanetary disc surrounding a 0.5 solar mass star; (2) the collapse of a 1 solar mass protostellar cloud, and (3) the thermal relaxation of temperature fluctuations in a static homogeneous sphere.Comment: 11 pages, 22 figures, accepted for publication in MNRA

A hybrid stabilization technique for simulating water wave - Structure interaction by incompressible Smoothed Particle Hydrodynamics (ISPH) method

The Smoothed Particle Hydrodynamics (SPH) method is emerging as a potential tool for studying water wave related problems, especially for violent free surface flow and large deformation problems. The incompressible SPH (ISPH) computations have been found not to be able to maintain the stability in certain situations and there exist some spurious oscillations in the pressure time history, which is similar to the weakly compressible SPH (WCSPH). One main cause of this problem is related to the non-uniform and clustered distribution of the moving particles. In order to improve the model performance, the paper proposed an efficient hybrid numerical technique aiming to correct the ill particle distributions. The correction approach is realized through the combination of particle shifting and pressure gradient improvement. The advantages of the proposed hybrid technique in improving ISPH calculations are demonstrated through several applications that include solitary wave impact on a slope or overtopping a seawall, and regular wave slamming on the subface of open-piled structure

Towards a Mini-App for Smoothed Particle Hydrodynamics at Exascale

The smoothed particle hydrodynamics (SPH) technique is a purely Lagrangian method, used in numerical simulations of fluids in astrophysics and computational fluid dynamics, among many other fields. SPH simulations with detailed physics represent computationally-demanding calculations. The parallelization of SPH codes is not trivial due to the absence of a structured grid. Additionally, the performance of the SPH codes can be, in general, adversely impacted by several factors, such as multiple time-stepping, long-range interactions, and/or boundary conditions. This work presents insights into the current performance and functionalities of three SPH codes: SPHYNX, ChaNGa, and SPH-flow. These codes are the starting point of an interdisciplinary co-design project, SPH-EXA, for the development of an Exascale-ready SPH mini-app. To gain such insights, a rotating square patch test was implemented as a common test simulation for the three SPH codes and analyzed on two modern HPC systems. Furthermore, to stress the differences with the codes stemming from the astrophysics community (SPHYNX and ChaNGa), an additional test case, the Evrard collapse, has also been carried out. This work extrapolates the common basic SPH features in the three codes for the purpose of consolidating them into a pure-SPH, Exascale-ready, optimized, mini-app. Moreover, the outcome of this serves as direct feedback to the parent codes, to improve their performance and overall scalability.Comment: 18 pages, 4 figures, 5 tables, 2018 IEEE International Conference on Cluster Computing proceedings for WRAp1

A numerical investigation into the correction algorithms for SPH method in modeling violent free surface flows

A quantitative comparison of the usual and recent numerical treatments which are applied to the Smoothed Particle Hydrodynamics (SPH) method are presented together with a new free-surface treatment. A series of numerical treatments are studied to refine the numerical procedures of the SPH method particularly for violent flows with a free surface. Two dimensional dam-break and sway-sloshing problems in a tank are modeled by solving Euler's equation of motion utilizing weakly compressible SPH method (WCSPH). Initially, the dam-break benchmark problem is studied by adopting only conventional basic equations of SPH without any numerical remedy and then by considering numerical treatments of interest one after another. In the WCSPH method, the precise calculation of the densities of the particles is vital for the solution, accordingly a density correction algorithm is presented as a basic numerical treatment. Subsequently, Monaghan's (1994) [1] XSPH velocity variant algorithm, artificial particle displacement (APD) algorithm (Shaldoo et al., 2011) [2], and a hybrid combination of velocity updated XSPH (VXSPH) and APD algorithms are implemented separately, but all with the density correction algorithm as a default treatment. The effects of each of these treatments on the pressure and on the free surface profiles are analyzed by comparing our numerical findings with experimental and numerical results in the literature. After the detailed scrutiny on the dam-break problem, sway-sloshing problem is handled with the VXSPH+APD algorithm which has been noted to provide the most reliable and accurate results in the dam-break problem. For the sway-sloshing problem, the time histories of free surface elevations on the left side wall of the rectangular tank are compared with experimental and numerical results available in the literature. It was shown that the VXSPH+APD treatment significantly improves the accuracy of the numerical simulations for violent flows with a free surface and lead to the results which are in very good agreement with experimental and numerical findings of literature in terms of both the kinematic and the dynamic point of view