207,655 research outputs found
Near-Earth asteroid (3200) Phaethon. Characterization of its orbit, spin state, and thermophysical parameters
The near-Earth asteroid (3200) Phaethon is an intriguing object: its
perihelion is at only 0.14 au and is associated with the Geminid meteor stream.
We aim to use all available disk-integrated optical data to derive a reliable
convex shape model of Phaethon. By interpreting the available space- and
ground-based thermal infrared data and Spitzer spectra using a thermophysical
model, we also aim to further constrain its size, thermal inertia, and visible
geometric albedo. We applied the convex inversion method to the new optical
data obtained by six instruments and to previous observations. The convex shape
model was then used as input for the thermophysical modeling. We also studied
the long-term stability of Phaethon's orbit and spin axis with a numerical
orbital and rotation-state integrator. We present a new convex shape model and
rotational state of Phaethon: a sidereal rotation period of 3.603958(2) h and
ecliptic coordinates of the preferred pole orientation of (319,
39) with a 5 uncertainty. Moreover, we derive its size
(=5.10.2 km), thermal inertia (=600200 J m
s K), geometric visible albedo
(=0.1220.008), and estimate the macroscopic surface
roughness. We also find that the Sun illumination at the perihelion passage
during the past several thousand years is not connected to a specific area on
the surface, which implies non-preferential heating.Comment: Astronomy and Astrophysics. In pres
CaloGAN: Simulating 3D High Energy Particle Showers in Multi-Layer Electromagnetic Calorimeters with Generative Adversarial Networks
The precise modeling of subatomic particle interactions and propagation
through matter is paramount for the advancement of nuclear and particle physics
searches and precision measurements. The most computationally expensive step in
the simulation pipeline of a typical experiment at the Large Hadron Collider
(LHC) is the detailed modeling of the full complexity of physics processes that
govern the motion and evolution of particle showers inside calorimeters. We
introduce \textsc{CaloGAN}, a new fast simulation technique based on generative
adversarial networks (GANs). We apply these neural networks to the modeling of
electromagnetic showers in a longitudinally segmented calorimeter, and achieve
speedup factors comparable to or better than existing full simulation
techniques on CPU (-) and even faster on GPU (up to
). There are still challenges for achieving precision across
the entire phase space, but our solution can reproduce a variety of geometric
shower shape properties of photons, positrons and charged pions. This
represents a significant stepping stone toward a full neural network-based
detector simulation that could save significant computing time and enable many
analyses now and in the future.Comment: 14 pages, 4 tables, 13 figures; version accepted by Physical Review D
(PRD
Deep learning-based surrogate model for 3-D patient-specific computational fluid dynamics
Optimization and uncertainty quantification have been playing an increasingly
important role in computational hemodynamics. However, existing methods based
on principled modeling and classic numerical techniques have faced significant
challenges, particularly when it comes to complex 3D patient-specific shapes in
the real world. First, it is notoriously challenging to parameterize the input
space of arbitrarily complex 3-D geometries. Second, the process often involves
massive forward simulations, which are extremely computationally demanding or
even infeasible. We propose a novel deep learning surrogate modeling solution
to address these challenges and enable rapid hemodynamic predictions.
Specifically, a statistical generative model for 3-D patient-specific shapes is
developed based on a small set of baseline patient-specific geometries. An
unsupervised shape correspondence solution is used to enable geometric morphing
and scalable shape synthesis statistically. Moreover, a simulation routine is
developed for automatic data generation by automatic meshing, boundary setting,
simulation, and post-processing. An efficient supervised learning solution is
proposed to map the geometric inputs to the hemodynamics predictions in latent
spaces. Numerical studies on aortic flows are conducted to demonstrate the
effectiveness and merit of the proposed techniques.Comment: 8 figures, 2 table
Geometric modeling and optimization over regular domains for graphics and visual computing
The effective construction of parametric representation of complicated geometric objects can facilitate many design, analysis, and simulation tasks in Computer-Aided Design (CAD), Computer-Aided Manufacturing (CAM), and Computer-Aided Engineering (CAE). Given a 3D shape, the procedure of finding such a parametric representation upon a canonical domain is called geometric parameterization. Regular geometric regions, such as polycubes and spheres, are desirable domains for parameterization. Parametric representations defined upon regular geometric domains have many desirable mathematical properties and can facilitate or simplify various surface/solid modeling and processing computation. This dissertation studies the construction of parameterization on regular geometric domains and explores their applications in shape modeling and computer-aided design. Specifically, we studies (1) the surface parameterization on the spherical domain for closed genus-zero surfaces; (2) the surface parameterization on the polycube domain for general closed surfaces; and (3) the volumetric parameterization for 3D-manifolds embedded in 3D Euclidean space. We propose novel computational models to solve these geometric problems. Our computational models reduce to nonlinear optimizations with various geometric constraints. Hence, we also need to explore effective optimization algorithms. The main contributions of this dissertation are three-folded. (1) We developed an effective progressive spherical parameterization algorithm, with an efficient nonlinear optimization scheme subject to the spherical constraint. Compared with the state-of-the-art spherical mapping algorithms, our algorithm demonstrates the advantages of great efficiency, lower distortion, and guaranteed bijectiveness, and we show its applications in spherical harmonic decomposition and shape analysis. (2) We propose a first topology-preserving polycube domain optimization algorithm that simultaneously optimizes polycube domain together with the parameterization to balance the mapping distortion and domain simplicity. We develop effective nonlinear geometric optimization algorithms dealing with variables with and without derivatives. This polycube parameterization algorithm can benefit the regular quadrilateral mesh generation and cross-surface parameterization. (3) We develop a novel quaternion-based optimization framework for 3D frame field construction and volumetric parameterization computation. We demonstrate our constructed 3D frame field has better smoothness, compared with state-of-the-art algorithms, and is effective in guiding low-distortion volumetric parameterization and high-quality hexahedral mesh generation
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