521 research outputs found
Turbulent Details Simulation for SPH Fluids via Vorticity Refinement
A major issue in Smoothed Particle Hydrodynamics (SPH) approaches is the
numerical dissipation during the projection process, especially under coarse
discretizations. High-frequency details, such as turbulence and vortices, are
smoothed out, leading to unrealistic results. To address this issue, we
introduce a Vorticity Refinement (VR) solver for SPH fluids with negligible
computational overhead. In this method, the numerical dissipation of the
vorticity field is recovered by the difference between the theoretical and the
actual vorticity, so as to enhance turbulence details. Instead of solving the
Biot-Savart integrals, a stream function, which is easier and more efficient to
solve, is used to relate the vorticity field to the velocity field. We obtain
turbulence effects of different intensity levels by changing an adjustable
parameter. Since the vorticity field is enhanced according to the curl field,
our method can not only amplify existing vortices, but also capture additional
turbulence. Our VR solver is straightforward to implement and can be easily
integrated into existing SPH methods
Flow Structure Detection with Smoothed Particle Hydrodynamics
We discuss how existing flow structure detection methods can be realised in Smoothed Particle Hydrodynamcs (SPH) simulations. We demonstrate the use of the Delta criterion for the detection of instantaneous Eulerian flow structures. The standard calculation of the velocity gradient tensor (VGT) results too noisy gradient field. We propose a correction method based on the idea of XSPH that yields a much smoother VGT field, enabling significantly more accurate structure detection. We also demonstrate on test cases the process in which the instantaneous Eulerian tools are used to locate Lagrangian coherent flow structures
An improved SPH scheme for cosmological simulations
We present an implementation of smoothed particle hydrodynamics (SPH) with
improved accuracy for simulations of galaxies and the large-scale structure. In
particular, we combine, implement, modify and test a vast majority of SPH
improvement techniques in the latest instalment of the GADGET code. We use the
Wendland kernel functions, a particle wake-up time-step limiting mechanism and
a time-dependent scheme for artificial viscosity, which includes a high-order
gradient computation and shear flow limiter. Additionally, we include a novel
prescription for time-dependent artificial conduction, which corrects for
gravitationally induced pressure gradients and largely improves the SPH
performance in capturing the development of gas-dynamical instabilities. We
extensively test our new implementation in a wide range of hydrodynamical
standard tests including weak and strong shocks as well as shear flows,
turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas
clouds. We jointly employ all modifications; however, when necessary we study
the performance of individual code modules. We approximate hydrodynamical
states more accurately and with significantly less noise than standard SPH.
Furthermore, the new implementation promotes the mixing of entropy between
different fluid phases, also within cosmological simulations. Finally, we study
the performance of the hydrodynamical solver in the context of radiative galaxy
formation and non-radiative galaxy cluster formation. We find galactic disks to
be colder, thinner and more extended and our results on galaxy clusters show
entropy cores instead of steadily declining entropy profiles. In summary, we
demonstrate that our improved SPH implementation overcomes most of the
undesirable limitations of standard SPH, thus becoming the core of an efficient
code for large cosmological simulations.Comment: 21 figures, 2 tables, accepted to MNRA
Particle hydrodynamics with tessellation techniques
Lagrangian smoothed particle hydrodynamics (SPH) is a well-established
approach to model fluids in astrophysical problems, thanks to its geometric
flexibility and ability to automatically adjust the spatial resolution to the
clumping of matter. However, a number of recent studies have emphasized
inaccuracies of SPH in the treatment of fluid instabilities. The origin of
these numerical problems can be traced back to spurious surface effects across
contact discontinuities, and to SPH's inherent prevention of mixing at the
particle level. We here investigate a new fluid particle model where the
density estimate is carried out with the help of an auxiliary mesh constructed
as the Voronoi tessellation of the simulation particles instead of an adaptive
smoothing kernel. This Voronoi-based approach improves the ability of the
scheme to represent sharp contact discontinuities. We show that this eliminates
spurious surface tension effects present in SPH and that play a role in
suppressing certain fluid instabilities. We find that the new `Voronoi Particle
Hydrodynamics' described here produces comparable results than SPH in shocks,
and better ones in turbulent regimes of pure hydrodynamical simulations. We
also discuss formulations of the artificial viscosity needed in this scheme and
how judiciously chosen correction forces can be derived in order to maintain a
high degree of particle order and hence a regular Voronoi mesh. This is
especially helpful in simulating self-gravitating fluids with existing gravity
solvers used for N-body simulations.Comment: 26 pages, 24 figures, currentversion is accepted by MNRA
Large-scale wave breaking over a barred beach : SPH numerical simulation and comparison with experiments
Acknowledgements The experiments were supported by the European Community’s Horizon 2020 Programme through the grant to the budget of the integrated infrastructure initiative HYDRALAB+, Contract no. 654110, and were conducted as part of the transnational access project HYBRID. Dr. Corrado Altomare acknowledges funding from the Spanish government and the European Social Found (ESF) under the programme ’Ramón y Cajal 2020’ (RYC2020-030197- /AEI/10.13039/501100011033). Pietro Scandura acknowledges the support received from the University of Catania, Italy by funding the research project ‘Valutazione del rischio idraulico in sistemi complessi (VARIO)’.Peer reviewedPublisher PD
Analyzing the Interaction of Vortex and Gas–Liquid Interface Dynamics in Fuel Spray Nozzles by Means of Lagrangian-Coherent Structures (2D)
Predictions of the primary breakup of fuel in realistic fuel spray nozzles for aero-engine combustors by means of the SPH method are presented. Based on simulations in 2D, novel insights into the fundamental effects of primary breakup are established by analyzing the dynamics of Lagrangian-coherent structures (LCSs). An in-house visualization and data exploration platform is used in order to retrieve fields of the finite-time Lyapunov exponent (FTLE) derived from the SPH predictions aiming at the identification of time resolved LCSs. The main focus of this paper is demonstrating the suitability of FTLE fields to capture and visualize the interaction between the gas and the fuel flow leading to liquid disintegration. Aiming for a convenient illustration at a high spatial resolution, the analysis is presented based on 2D datasets. However, the method and the conclusions can analoguosly be transferred to 3D. The FTLE fields of modified nozzle geometries are compared in order to highlight the influence of the nozzle geometry on primary breakup, which is a novel and unique approach for this industrial application. Modifications of the geometry are proposed which are capable of suppressing the formation of certain LCSs, leading to less fluctuation of the fuel flow emerging from the spray nozzle
A SPH model for incompressible turbulence
A coarse-grained particle model for incompressible Navier-Stokes (NS)
equation is proposed based on spatial filtering by utilizing smoothed particle
hydrodynamics (SPH) approximations. This model is similar to our previous
developed SPH discretization of NS equation ({\it Hu X.Y. & N.A. Adams, J.
Comput. Physics}, 227: 264-278, 2007 and 228: 2082-2091, 2009) and the
Lagrangian averaged NS (LANS-) turbulence model. Other than using
smoothing approaches, this model obtains particle transport velocity by
imposing constant which is associated with the particle density, and
is called SPH- model. Numerical tests on two-dimensional decay and
forced turbulences with high Reynolds number suggest that the model is able to
reproduce both the inverse energy cascade and direct enstrophy cascade of the
kinetic energy spectrum, the time scaling of enstrophy decay and the
non-Guassian probability density function (PDF) of particle acceleration.Comment: 23 pages. 7 figure
Simulating water-entry/exit problems using Eulerian-Lagrangian and fully-Eulerian fictitious domain methods within the open-source IBAMR library
In this paper we employ two implementations of the fictitious domain (FD)
method to simulate water-entry and water-exit problems and demonstrate their
ability to simulate practical marine engineering problems. In FD methods, the
fluid momentum equation is extended within the solid domain using an additional
body force that constrains the structure velocity to be that of a rigid body.
Using this formulation, a single set of equations is solved over the entire
computational domain. The constraint force is calculated in two distinct ways:
one using an Eulerian-Lagrangian framework of the immersed boundary (IB) method
and another using a fully-Eulerian approach of the Brinkman penalization (BP)
method. Both FSI strategies use the same multiphase flow algorithm that solves
the discrete incompressible Navier-Stokes system in conservative form. A
consistent transport scheme is employed to advect mass and momentum in the
domain, which ensures numerical stability of high density ratio multiphase
flows involved in practical marine engineering applications. Example cases of a
free falling wedge (straight and inclined) and cylinder are simulated, and the
numerical results are compared against benchmark cases in literature.Comment: The current paper builds on arXiv:1901.07892 and re-explains some
parts of it for the reader's convenienc
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