2,956 research outputs found
Computational analysis of shock-induced flow through stationary particle clouds
We investigate the shock-induced flow through random particle arrays using
particle-resolved Large Eddy Simulations for different incident shock wave Mach
numbers, particle volume fractions and particle sizes. We analyze trends in
mean flow quantities and the unresolved terms in the volume averaged momentum
equation, as we vary the three parameters. We find that the shock wave
attenuation and certain mean flow trends can be predicted by the opacity of the
particle cloud, which is a function of particle size and particle volume
fraction. We show that the Reynolds stress field plays an important role in the
momentum balance at the particle cloud edges, and therefore strongly affects
the reflected shock wave strength. The Reynolds stress was found to be
insensitive to particle size, but strongly dependent on particle volume
fraction. It is in better agreement with results from simulations of flow
through particle clouds at fixed mean slip Reynolds numbers in the
incompressible regime, than with results from other shock wave particle cloud
studies, which have utilized either inviscid or two-dimensional approaches. We
propose an algebraic model for the streamwise Reynolds stress based on the
observation that the separated flow regions are the primary contributions to
the Reynolds stress.Comment: 33 pages, 23 figures, 3 table
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
rpSPH: a novel Smoothed Particle Hydrodynamics Algorithm
We suggest a novel discretisation of the momentum equation for Smoothed
Particle Hydrodynamics (SPH) and show that it significantly improves the
accuracy of the obtained solutions. Our new formulation which we refer to as
relative pressure SPH, rpSPH, evaluates the pressure force in respect to the
local pressure. It respects Newtons first law of motion and applies forces to
particles only when there is a net force acting upon them. This is in contrast
to standard SPH which explicitly uses Newtons third law of motion continuously
applying equal but opposite forces between particles. rpSPH does not show the
unphysical particle noise, the clumping or banding instability, unphysical
surface tension, and unphysical scattering of different mass particles found
for standard SPH. At the same time it uses fewer computational operations. and
only changes a single line in existing SPH codes. We demonstrate its
performance on isobaric uniform density distributions, uniform density shearing
flows, the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, the Sod shock
tube, the Sedov-Taylor blast wave and a cosmological integration of the Santa
Barbara galaxy cluster formation test. rpSPH is an improvement these cases. The
improvements come at the cost of giving up exact momentum conservation of the
scheme. Consequently one can also obtain unphysical solutions particularly at
low resolutions.Comment: 17 pages, 13 figures. Final version. Including section of how to
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Vector potential methods
Vector potential and related methods, for the simulation of both inviscid and viscous flows over aerodynamic configurations, are briefly reviewed. The advantages and disadvantages of several formulations are discussed and alternate strategies are recommended. Scalar potential, modified potential, alternate formulations of Euler equations, least-squares formulation, variational principles, iterative techniques and related methods, and viscous flow simulation are discussed
Simulations of Turbulent Flows with Strong Shocks and Density Variations: Final Report
The target of this SciDAC Science Application was to develop a new capability based on high-order and high-resolution schemes to simulate shock-turbulence interactions and multi-material mixing in planar and spherical geometries, and to study Rayleigh-Taylor and Richtmyer-Meshkov turbulent mixing. These fundamental problems have direct application in high-speed engineering flows, such as inertial confinement fusion (ICF) capsule implosions and scramjet combustion, and also in the natural occurrence of supernovae explosions. Another component of this project was the development of subgrid-scale (SGS) models for large-eddy simulations of flows involving shock-turbulence interaction and multi-material mixing, that were to be validated with the DNS databases generated during the program. The numerical codes developed are designed for massively-parallel computer architectures, ensuring good scaling performance. Their algorithms were validated by means of a sequence of benchmark problems. The original multi-stage plan for this five-year project included the following milestones: 1) refinement of numerical algorithms for application to the shock-turbulence interaction problem and multi-material mixing (years 1-2); 2) direct numerical simulations (DNS) of canonical shock-turbulence interaction (years 2-3), targeted at improving our understanding of the physics behind the combined two phenomena and also at guiding the development of SGS models; 3) large-eddy simulations (LES) of shock-turbulence interaction (years 3-5), improving SGS models based on the DNS obtained in the previous phase; 4) DNS of planar/spherical RM multi-material mixing (years 3-5), also with the two-fold objective of gaining insight into the relevant physics of this instability and aiding in devising new modeling strategies for multi-material mixing; 5) LES of planar/spherical RM mixing (years 4-5), integrating the improved SGS and multi-material models developed in stages 3 and 5. This final report is outlined as follows. Section 2 shows an assessment of numerical algorithms that are best suited for the numerical simulation of compressible flows involving turbulence and shock phenomena. Sections 3 and 4 deal with the canonical shock-turbulence interaction problem, from the DNS and LES perspectives, respectively. Section 5 considers the shock-turbulence inter-action in spherical geometry, in particular, the interaction of a converging shock with isotropic turbulence as well as the problem of the blast wave. Section 6 describes the study of shock-accelerated mixing through planar and spherical Richtmyer-Meshkov mixing as well as the shock-curtain interaction problem In section 7 we acknowledge the different interactions between Stanford and other institutions participating in this SciDAC project, as well as several external collaborations made possible through it. Section 8 presents a list of publications and presentations that have been generated during the course of this SciDAC project. Finally, section 9 concludes this report with the list of personnel at Stanford University funded by this SciDAC project
Influence of adaptive mesh refinement and the hydro solver on shear-induced mass stripping in a minor-merger scenario
We compare two different codes for simulations of cosmological structure
formation to investigate the sensitivity of hydrodynamical instabilities to
numerics, in particular, the hydro solver and the application of adaptive mesh
refinement (AMR). As a simple test problem, we consider an initially spherical
gas cloud in a wind, which is an idealized model for the merger of a subcluster
or galaxy with a big cluster. Based on an entropy criterion, we calculate the
mass stripping from the subcluster as a function of time. Moreover, the
turbulent velocity field is analyzed with a multi-scale filtering technique. We
find remarkable differences between the commonly used PPM solver with
directional splitting in the Enzo code and an unsplit variant of PPM in the Nyx
code, which demonstrates that different codes can converge to systematically
different solutions even when using uniform grids. For the test case of an
unbound cloud, AMR simulations reproduce uniform-grid results for the mass
stripping quite well, although the flow realizations can differ substantially.
If the cloud is bound by a static gravitational potential, however, we find
strong sensitivity to spurious fluctuations which are induced at the cutoff
radius of the potential and amplified by the bow shock. This gives rise to
substantial deviations between uniform-grid and AMR runs performed with Enzo,
while the mass stripping in Nyx simulations of the subcluster is nearly
independent of numerical resolution and AMR. Although many factors related to
numerics are involved, our study indicates that unsplit solvers with advanced
flux limiters help to reduce grid effects and to keep numerical noise under
control, which is important for hydrodynamical instabilities and turbulent
flows.Comment: 23 pages, 18 figures, accepted for publication by Astronomy and
Computin
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