30,930 research outputs found
Gyrokinetic Simulations of Solar Wind Turbulence from Ion to Electron Scales
The first three-dimensional, nonlinear gyrokinetic simulation of plasma
turbulence resolving scales from the ion to electron gyroradius with a
realistic mass ratio is presented, where all damping is provided by resolved
physical mechanisms. The resulting energy spectra are quantitatively consistent
with a magnetic power spectrum scaling of as observed in \emph{in
situ} spacecraft measurements of the "dissipation range" of solar wind
turbulence. Despite the strongly nonlinear nature of the turbulence, the linear
kinetic \Alfven wave mode quantitatively describes the polarization of the
turbulent fluctuations. The collisional ion heating is measured at
sub-ion-Larmor radius scales, which provides the first evidence of the ion
entropy cascade in an electromagnetic turbulence simulation.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Performance of hybrid turbulence models in OpenFOAM for numerical simulations of a confined backward-facing step flow at low Prandtl number
/To date, numerical simulation of complex turbulent flows with separation remains challenging. On the one hand,turbulence models in Reynolds-averaged Navier-Stokes (RANS) equations struggle with correctly representing turbulent momentum transfer in such flows, whereas turbulence-resolving techniques such as large-eddy simulations (LES) carry high computational cost on the other hand. Alternatively, hybrid RANS–LES turbulence models promise to deliver scale-resolving accuracy at acceptable computational cost, yet their accuracy remains highly dependent on simulation setup and flow conditions. Here, we investigate hybrid turbulence models readily available in OpenFOAM, and benchmark their performance to Reynolds-averaged approaches and turbulence-resolving high-fidelity reference data for a confined backward-facing step flow at low Prandtl number and relatively low Reynolds number. Although temperature is generally well predicted by all considered setups, a comparison between RANS and LES shows that turbulence resolution can increase the accuracy for the considered flow case. Results show that scale-adaptive simulation techniques do not produce resolved turbulence and fail to outperform the baseline Reynolds-averaged simulations for the considered case. In contrast, detached-eddy variants do resolve turbulence in the separated shear layer, yet some configurations suffer from modeled-stress depletion due to late development of resolved turbulence. A grid coarsening study compares the degradation of accuracy for each approach, showcasing robustness of the standard RANS approach and the good performance of full LES even at surprisingly coarse resolutions. For each grid, the best-performing setup was either a RANS or an LES approach, but never a hybrid turbulence model setup. Finally, a Reynolds-number sensitivity is presented, indicating that resolved turbulence development is promoted at higher Reynolds numbers, thus leading to setups more amenable to hybrid turbulence models.  
Binary Neutron Star Merger Simulations with a Calibrated Turbulence Model
Magnetohydrodynamic (MHD) turbulence in neutron star (NS) merger remnants can
impact their evolution and multimessenger signatures, complicating the
interpretation of present and future observations. Due to the high Reynolds
numbers and the large computational costs of numerical relativity simulations,
resolving all the relevant scales of the turbulence will be impossible for the
foreseeable future. Here, we adopt a method to include subgrid-scale turbulence
in moderate resolution simulations by extending the large-eddy simulation (LES)
method to general relativity (GR). We calibrate our subgrid turbulence model
with results from very-high-resolution GRMHD simulations, and we use it to
perform NS merger simulations and study the impact of turbulence. We find that
turbulence has a quantitative, but not qualitative impact on the evolution of
NS merger remnants, on their gravitational wave signatures, and on the outflows
generated in binary NS mergers. Our approach provides a viable path to quantify
uncertainties due to turbulence in NS mergers.Comment: 20 pages, 6 figures. Submitted to the special issue "Numerical
Relativity and Gravitational Wave" of Symmetry. Minor changes, matches
accepted versio
CFD simulation of air flow over an object with gable roof, revised with Y+ approach
Aim of this contribution is to provide insight view into analysis focused on obtaining external pressure coefficients on isolated two storey low-rise building with 15° elevation gable roof using Computer Fluid Dynamics simulation and these are compared to values that offering Eurocodes. Final Volume Model consisting of polyhedral mesh will be used for analysis with two different turbulence models. Mesh was created with respect to y+ parameter, where desired value was below one which leads us to fine mesh type. Secondary aim of this contribution is to compare performance of selected turbulence models. For this purpose were chosen Detached Eddy Simulation and Large Eddy Simulation which are part of the Scale Resolving Simulation turbulence models
Effects of near wall modeling in the Improved-Delayed-Detached-Eddy-Simulation (IDDES) methodology
The present study aims to assess the effects of two different underlying RANS models on overall behavior of the IDDES methodology when applied to different flow configurations ranging from fully attached (plane channel flow) to separated flows (periodic hill flow). This includes investigating prediction accuracy of first and second order statistics, response to grid refinement, grey area dynamics and triggering mechanism. Further, several criteria have been investigated to assess reliability and quality of the methodology when operating in scale resolving mode. It turns out that irrespective of the near wall modeling strategy, the IDDES methodology does not satisfy all criteria required to make this methodology reliable when applied to various flow configurations at different Reynolds numbers with different grid resolutions. Further, it is found that using more advanced underlying RANS model to improve prediction accuracy of the near wall dynamics results in extension of the grey area, which may delay the transition to scale resolving mode. This systematic study for attached and separated flows suggests that the shortcomings of IDDES methodology mostly lie in inaccurate prediction of the dynamics inside the grey area and demands further investigation in this direction to make this methodology capable of dealing with different flow situations reliably
Surface-sampled simulations of turbulent flow at high Reynolds number
A new approach to turbulence simulation, based on a combination of large-eddy
simulation (LES) for the whole flow and an array of non-space-filling
quasi-direct numerical simulations (QDNS), which sample the response of
near-wall turbulence to large-scale forcing, is proposed and evaluated. The
technique overcomes some of the cost limitations of turbulence simulation,
since the main flow is treated with a coarse-grid LES, with the equivalent of
wall functions supplied by the near-wall sampled QDNS. Two cases are tested, at
friction Reynolds number Re=4200 and 20,000. The total grid node count
for the first case is less than half a million and less than two million for
the second case, with the calculations only requiring a desktop computer. A
good agreement with published DNS is found at Re=4200, both in terms of
the mean velocity profile and the streamwise velocity fluctuation statistics,
which correctly show a substantial increase in near-wall turbulence levels due
to a modulation of near-wall streaks by large-scale structures. The trend
continues at Re=20,000, in agreement with experiment, which represents
one of the major achievements of the new approach. A number of detailed aspects
of the model, including numerical resolution, LES-QDNS coupling strategy and
sub-grid model are explored. A low level of grid sensitivity is demonstrated
for both the QDNS and LES aspects. Since the method does not assume a law of
the wall, it can in principle be applied to flows that are out of equilibrium.Comment: Author accepted version. Accepted for publication in the
International Journal for Numerical Methods in Fluids on 26 April 201
Resolving the fine-scale structure in turbulent Rayleigh-Benard convection
We present high-resolution direct numerical simulation studies of turbulent
Rayleigh-Benard convection in a closed cylindrical cell with an aspect ratio of
one. The focus of our analysis is on the finest scales of convective
turbulence, in particular the statistics of the kinetic energy and thermal
dissipation rates in the bulk and the whole cell. The fluctuations of the
energy dissipation field can directly be translated into a fluctuating local
dissipation scale which is found to develop ever finer fluctuations with
increasing Rayleigh number. The range of these scales as well as the
probability of high-amplitude dissipation events decreases with increasing
Prandtl number. In addition, we examine the joint statistics of the two
dissipation fields and the consequences of high-amplitude events. We also have
investigated the convergence properties of our spectral element method and have
found that both dissipation fields are very sensitive to insufficient
resolution. We demonstrate that global transport properties, such as the
Nusselt number, and the energy balances are partly insensitive to insufficient
resolution and yield correct results even when the dissipation fields are
under-resolved. Our present numerical framework is also compared with
high-resolution simulations which use a finite difference method. For most of
the compared quantities the agreement is found to be satisfactory.Comment: 33 pages, 24 figure
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