96,740 research outputs found
A Large Eddy Simulation of Turbulent Compressible Convection: Differential Rotation in the Solar Convection Zone
We present results of two simulations of the convection zone, obtained by
solving the full hydrodynamic equations in a section of a spherical shell. The
first simulation has cylindrical rotation contours (parallel to the rotation
axis) and a strong meridional circulation, which traverses the entire depth.
The second simulation has isorotation contours about mid-way between cylinders
and cones, and a weak meridional circulation, concentrated in the uppermost
part of the shell.
We show that the solar differential rotation is directly related to a
latitudinal entropy gradient, which pervades into the deep layers of the
convection zone. We also offer an explanation of the angular velocity shear
found at low latitudes near the top. A non-zero correlation between radial and
zonal velocity fluctuations produces a significant Reynolds stress in that
region. This constitutes a net transport of angular momentum inwards, which
causes a slight modification of the overall structure of the differential
rotation near the top. In essence, the {\it thermodynamics controls the
dynamics through the Taylor-Proudman momentum balance}. The Reynolds stresses
only become significant in the surface layers, where they generate a weak
meridional circulation and an angular velocity `bump'.Comment: 11 pages, 14 figures, the first figure was too large and is excluded.
Accepted for publication in MNRA
Two-Fluid RANS-RSTM-PDF Model for Turbulent Particulate Flows
A novel three-dimensional (3D) model based on Reynolds turbulence stress model (RSTM) closure of equations of carrier and particulate phases was elaborated for channel turbulent flows. The essence of the model is the direct calculation of normal and shear components of the Reynolds stresses for the particulate phase similar to the carrier fluid. The model is based on the Eulerian approach, which is applied for the 3D RANS modeling of the carrier flow and the particulate phase and the statistical probability dense function (PDF) approach focusing on the mathematical description of the second moments of the particulate phase
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Revue dâEtudes TibĂ©taines Number 16, April 200
Discovering explicit Reynolds-averaged turbulence closures for turbulent separated flows through deep learning-based symbolic regression with non-linear corrections
This work introduces a novel data-driven framework to formulate explicit
algebraic Reynolds-averaged Navier-Stokes (RANS) turbulence closures. Recent
years have witnessed a blossom in applying machine learning (ML) methods to
revolutionize the paradigm of turbulence modeling. However, due to the
black-box essence of most ML methods, it is currently hard to extract
interpretable information and knowledge from data-driven models. To address
this critical limitation, this work leverages deep learning with symbolic
regression methods to discover hidden governing equations of Reynolds stress
models. Specifically, the Reynolds stress tensor is decomposed into linear and
non-linear parts. While the linear part is taken as the regular linear eddy
viscosity model, a long short-term memory neural network is employed to
generate symbolic terms on which tractable mathematical expressions for the
non-linear counterpart are built. A novel reinforcement learning algorithm is
employed to train the neural network to produce best-fitted symbolic
expressions. Within the proposed framework, the Reynolds stress closure is
explicitly expressed in algebraic forms, thus allowing for direct functional
inference. On the other hand, the Galilean and rotational invariance are
craftily respected by constructing the training feature space with independent
invariants and tensor basis functions. The performance of the present
methodology is validated through numerical simulations of three different
canonical flows that deviate in geometrical configurations. The results
demonstrate promising accuracy improvements over traditional RANS models,
showing the generalization ability of the proposed method. Moreover, with the
given explicit model equations, it can be easier to interpret the influence of
input features on generated models
On the role of vortex stretching in energy optimal growth of three dimensional perturbations on plane parallel shear flows
The three dimensional optimal energy growth mechanism, in plane parallel
shear flows, is reexamined in terms of the role of vortex stretching and the
interplay between the span-wise vorticity and the planar divergent components.
For high Reynolds numbers the structure of the optimal perturbations in
Couette, Poiseuille, and mixing layer shear profiles is robust and resembles
localized plane-waves in regions where the background shear is large. The waves
are tilted with the shear when the span-wise vorticity and the planar
divergence fields are in (out of) phase when the background shear is positive
(negative). A minimal model is derived to explain how this configuration
enables simultaneous growth of the two fields, and how this mutual
amplification reflects on the optimal energy growth. This perspective provides
an understanding of the three dimensional growth solely from the two
dimensional dynamics on the shear plane
Entropic Lattice Boltzmann Simulation of the Flow Past Square Cylinder
Minimal Boltzmann kinetic models, such as lattice Boltzmann, are often used
as an alternative to the discretization of the Navier-Stokes equations for
hydrodynamic simulations.
Recently, it was argued that modeling sub-grid scale phenomena at the kinetic
level might provide an efficient tool for large scale simulations. Indeed, a
particular variant of this approach, known as the entropic lattice Boltzmann
method (ELBM), has shown that an efficient coarse-grained simulation of
decaying turbulence is possible using these approaches.
The present work investigates the efficiency of the entropic lattice
Boltzmann in describing flows of engineering interest. In order to do so, we
have chosen the flow past a square cylinder, which is a simple model of such
flows. We will show that ELBM can quantitatively capture the variation of
vortex shedding frequency as a function of Reynolds number in the low as well
as the high Reynolds number regime, without any need for explicit sub-grid
scale modeling. This extends the previous studies for this set-up, where
experimental behavior ranging from to were
predicted by a single simulation algorithm.Comment: 12 pages, 5 figures, to appear in Int. J. Mod. Phys.
On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure
We use the method of Press, Rybicki & Hewitt (1992) to search for time lags
and time leads between different energy bands of the RXTE data for MCG-6-30-15.
We tailor our search in order to probe any reverberation signatures of the
fluorescent iron Kalpha line that is thought to arise from the inner regions of
the black hole accretion disk. In essence, an optimal reconstruction algorithm
is applied to the continuum band (2-4keV) light curve which smoothes out noise
and interpolates across the data gaps. The reconstructed continuum band light
curve can then be folded through trial transfer functions in an attempt to find
lags or leads between the continuum band and the iron line band (5-7keV). We
find reduced fractional variability in the line band. The spectral analysis of
Lee et al. (1999) reveals this to be due to a combination of an apparently
constant iron line flux (at least on timescales of few x 10^4s), and flux
correlated changes in the photon index. We also find no evidence for iron line
reverberation and exclude reverberation delays in the range 0.5-50ksec. This
extends the conclusions of Lee et al. and suggests that the iron line flux
remains constant on timescales as short as 0.5ksec. The large black hole mass
(>10^8Msun) naively suggested by the constancy of the iron line flux is
rejected on other grounds. We suggest that the black hole in MCG-6-30-15 has a
mass of M_BH~10^6-10^7Msun and that changes in the ionization state of the disk
may produce the puzzling spectral variability. Finally, it is found that the
8-15keV band lags the 2-4keV band by 50-100s. This result is used to place
constraints on the size and geometry of the Comptonizing medium responsible for
the hard X-ray power-law in this AGN.Comment: 11 pages, 13 postscript figures. Accepted for publication in Ap
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