184,436 research outputs found
Vortex generation in protoplanetary disks with an embedded giant planet
Vortices in protoplanetary disks can capture solid particles and form
planetary cores within shorter timescales than those involved in the standard
core-accretion model. We investigate vortex generation in thin unmagnetized
protoplanetary disks with an embedded giant planet with planet to star mass
ratio and . Two-dimensional hydrodynamical simulations of a
protoplanetary disk with a planet are performed using two different numerical
methods. The results of the non-linear simulations are compared with a
time-resolved modal analysis of the azimuthally averaged surface density
profiles using linear perturbation theory. Finite-difference methods
implemented in polar coordinates generate vortices moving along the gap created
by Neptune-mass to Jupiter-mass planets. The modal analysis shows that unstable
modes are generated with growth rate of order for azimuthal
numbers m=4,5,6, where is the local Keplerian frequency.
Shock-capturing Cartesian-grid codes do not generate very much vorticity around
a giant planet in a standard protoplanetary disk. Modal calculations confirm
that the obtained radial profiles of density are less susceptible to the growth
of linear modes on timescales of several hundreds of orbital periods.
Navier-Stokes viscosity of the order (in units of )
is found to have a stabilizing effect and prevents the formation of vortices.
This result holds at high resolution runs and using different types of boundary
conditions. Giant protoplanets of Neptune-mass to Jupiter-mass can excite the
Rossby wave instability and generate vortices in thin disks. The presence of
vortices in protoplanetary disks has implications for planet formation, orbital
migration, and angular momentum transport in disks.Comment: 14 pages, 15 figures, accepted for publication in A&
Static Axisymmetric Vacuum Solutions and Non-Uniform Black Strings
We describe new numerical methods to solve the static axisymmetric vacuum
Einstein equations in more than four dimensions. As an illustration, we study
the compactified non-uniform black string phase connected to the uniform
strings at the Gregory-Laflamme critical point. We compute solutions with a
ratio of maximum to minimum horizon radius up to nine. For a fixed
compactification radius, the mass of these solutions is larger than the mass of
the classically unstable uniform strings. Thus they cannot be the end state of
the instability.Comment: 48 pages, 13 colour figures; v2: references correcte
Multi-scale initial conditions for cosmological simulations
We discuss a new algorithm to generate multi-scale initial conditions with
multiple levels of refinements for cosmological "zoom-in" simulations. The
method uses an adaptive convolution of Gaussian white noise with a real space
transfer function kernel together with an adaptive multi-grid Poisson solver to
generate displacements and velocities following first (1LPT) or second order
Lagrangian perturbation theory (2LPT). The new algorithm achieves RMS relative
errors of order 10^(-4) for displacements and velocities in the refinement
region and thus improves in terms of errors by about two orders of magnitude
over previous approaches. In addition, errors are localized at coarse-fine
boundaries and do not suffer from Fourier-space induced interference ringing.
An optional hybrid multi-grid and Fast Fourier Transform (FFT) based scheme is
introduced which has identical Fourier space behaviour as traditional
approaches. Using a suite of re-simulations of a galaxy cluster halo our real
space based approach is found to reproduce correlation functions, density
profiles, key halo properties and subhalo abundances with per cent level
accuracy. Finally, we generalize our approach for two-component baryon and
dark-matter simulations and demonstrate that the power spectrum evolution is in
excellent agreement with linear perturbation theory. For initial baryon density
fields, it is suggested to use the local Lagrangian approximation in order to
generate a density field for mesh based codes that is consistent with
Lagrangian perturbation theory instead of the current practice of using the
Eulerian linearly scaled densities.Comment: 22 pages, 24 figures. MNRAS in press. Updated affiliation
The effects of displacement induced by thermal perturbations on the structure and stability of boundary-layer flows
The free-interaction influence of a thermal expansión process in boundary-layer gas flow is analyzed using the formalism of triple-deck theory. The physical model considered is the forced convection of a gas flowing over a flat plate subject to a heated slab. Both linearized and full nonlinear solutions are obtained using Fourier transform methods and spectral numericaà techniques. The influence of monochromatic thermal perturbation on boundary-layer stability (lower branch) is studied and first-ordcr correction of the lower branch neutral stability curve for the boundary-layer flow has been obtained. The shift of neutral stability is then computed for different values of the therma! perturbation wave number, making unstable some otherwise stable modes
QEBA: Query-Efficient Boundary-Based Blackbox Attack
Machine learning (ML), especially deep neural networks (DNNs) have been
widely used in various applications, including several safety-critical ones
(e.g. autonomous driving). As a result, recent research about adversarial
examples has raised great concerns. Such adversarial attacks can be achieved by
adding a small magnitude of perturbation to the input to mislead model
prediction. While several whitebox attacks have demonstrated their
effectiveness, which assume that the attackers have full access to the machine
learning models; blackbox attacks are more realistic in practice. In this
paper, we propose a Query-Efficient Boundary-based blackbox Attack (QEBA) based
only on model's final prediction labels. We theoretically show why previous
boundary-based attack with gradient estimation on the whole gradient space is
not efficient in terms of query numbers, and provide optimality analysis for
our dimension reduction-based gradient estimation. On the other hand, we
conducted extensive experiments on ImageNet and CelebA datasets to evaluate
QEBA. We show that compared with the state-of-the-art blackbox attacks, QEBA is
able to use a smaller number of queries to achieve a lower magnitude of
perturbation with 100% attack success rate. We also show case studies of
attacks on real-world APIs including MEGVII Face++ and Microsoft Azure.Comment: Accepted by CVPR 202
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