23,878 research outputs found
Gravitational waves from rotating and precessing rigid bodies. 2: General solutions and computationally useful formulae
The classical mechanics results for free precession which are needed in order to calculate the weak field, slow-motion, quadrupole-moment gravitational waves are reviewed. Within that formalism, algorithms are given for computing the exact gravitational power radiated and waveforms produced by arbitrary rigid-body freely-precessing sources. The dominant terms are presented in series expansions of the waveforms for the case of an almost spherical object precessing with a small wobble angle. These series expansions, which retain the precise frequency dependence of the waves, may be useful for gravitational astronomers when freely-precessing sources begin to be observed
Detectability of Gravitational Waves from SN 1987A
We discuss the potential for detection of gravitational waves from a rapidly
spinning neutron star produced by supernova 1987A taking the parameters claimed
by Middleditch et al. (2000) at face value. Asssuming that the dominant
mechanism for spin down is gravitational waves emitted by a freely precessing
neutron star, it is possible to constrain the wobble angle, the effective
moment of inertai of the precessing crust and the crust cracking stress limit.
Our analysis, suggests that, if the interpretation of the Middleditch data is
correct, the compact remnant of SN 1987A may well provide a reliable and
predictable source of gravitational waves well within the capability of LIGO
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From an axiological standpoint
I maintain that intrinsic value is the fundamental concept of axiology. Many contemporary philosophers disagree; they say the proper object of value theory is final value. I examine three accounts of the nature of final value: the first claims that final value is non‐instrumental value; the second claims that final value is the value a thing has as an end; the third claims that final value is ultimate or non‐derivative value. In each case, I argue that the concept of final value described is either identical with the classical notion of intrinsic value or is not a plausible candidate for the primary concept of axiology
Formation of Strain-Induced Quantum Dots in Gated Semiconductor Nanostructures
Elastic strain changes the energies of the conduction band in a
semiconductor, which will affect transport through a semiconductor
nanostructure. We show that the typical strains in a semiconductor
nanostructure from metal gates are large enough to create strain-induced
quantum dots (QDs). We simulate a commonly used QD device architecture, metal
gates on bulk silicon, and show the formation of strain-induced QDs. The
strain-induced QD can be eliminated by replacing the metal gates with
poly-silicon gates. Thus strain can be as important as electrostatics to QD
device operation operation.Comment: 5 pages, 3 figures, plus supplementary informatio
A disk-shaped domain integral method for the computation of stress intensity factors using tetrahedral meshes
A novel domain integral approach is introduced for the accurate computation of pointwise J-integral and stress intensity factors (SIFs) of 3D planar cracks using tetrahedral elements. This method is efficient and easy to implement, and does not require a structured mesh around the crack front. The method relies on the construction of virtual disk-shaped integral domains at points along the crack front, and the computation of domain integrals using a series of virtual triangular and line elements. The accuracy of the numerical results computed for through-the-thickness, penny-shaped, and elliptical crack configurations has been validated by using the available analytical formulations. The average error of computed SIFs remains below 1% for fine meshes, and 2–3% for coarse ones. The results of an extensive parametric study suggest that there exists an optimum mesh-dependent domain radius at which the computed SIFs are the most accurate. Furthermore, the results provide evidence that tetrahedral elements are efficient, reliable and robust instruments for accurate linear elastic fracture mechanics calculations
On the use of quarter-point tetrahedral finite elements in linear elastic fracture mechanics
This paper discusses the reproduction of the square root singularity in quarter-point tetrahedral (QPT) finite elements. Numerical results confirm that the stress singularity is modeled accurately in a fully unstructured mesh by using QPTs. A displacement correlation (DC) scheme is proposed in combination with QPTs to compute stress intensity factors (SIF) from arbitrary meshes, yielding an average error of 2–3%. This straightforward method is computationally cheap and easy to implement. The results of an extensive parametric study also suggest the existence of an optimum mesh-dependent distance from the crack front at which the DC method computes the most accurate SIFs
A finite element framework for modeling internal frictional contact in three-dimensional fractured media using unstructured tetrahedral meshes
AbstractThis paper introduces a three-dimensional finite element (FE) formulation to accurately model the linear elastic deformation of fractured media under compressive loading. The presented method applies the classic Augmented Lagrangian(AL)-Uzawa method, to evaluate the growth of multiple interacting and intersecting discrete fractures. The volume and surfaces are discretized by unstructured quadratic triangle-tetrahedral meshes; quarter-point triangles and tetrahedra are placed around fracture tips. Frictional contact between crack faces for high contact precisions is modeled using isoparametric integration point-to-integration point contact discretization, and a gap-based augmentation procedure. Contact forces are updated by interpolating tractions over elements that are adjacent to fracture tips, and have boundaries that are excluded from the contact region. Stress intensity factors are computed numerically using the methods of displacement correlation and disk-shaped domain integral. A novel square-root singular variation of the penalty parameter near the crack front is proposed to accurately model the contact tractions near the crack front. Tractions and compressive stress intensity factors are validated against analytical solutions. Numerical examples of cubes containing one, two, twenty four and seventy interacting and intersecting fractures are presented
Computer program simulates design, test, and analysis phases of sensitivity experiments
Modular program with a small main program and several specialized subroutines provides a general purpose computer program to simulate the design, test and analysis phases of sensitivity experiments. This program allows a wide range of design-response function combinations and the addition, deletion, or modification of subroutines
Simulating Capacitances to Silicon Quantum Dots: Breakdown of the Parallel Plate Capacitor Model
Many electrical applications of quantum dots rely on capacitively coupled
gates; therefore, to make reliable devices we need those gate capacitances to
be predictable and reproducible. We demonstrate in silicon nanowire quantum
dots that gate capacitances are reproducible to within 10% for nominally
identical devices. We demonstrate the experimentally that gate capacitances
scale with device dimensions. We also demonstrate that a capacitance simulator
can be used to predict measured gate capacitances to within 20%. A simple
parallel plate capacitor model can be used to predict how the capacitances
change with device dimensions; however, the parallel plate capacitor model
fails for the smallest devices because the capacitances are dominated by
fringing fields. We show how the capacitances due to fringing fields can be
quickly estimated.Comment: 4 pages, 3 figures, to be published in IEEE Trans. Nan
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