120 research outputs found
Thick Domain Walls Intersecting a Black Hole
We discuss the gravitationally interacting system of a thick domain wall and
a black hole. We numerically solve the scalar field equation in the
Schwarzschild spacetime and show that there exist scalar field configurations
representing thick domain walls intersecting the black hole.Comment: 14 pages, 8 figure
Effect of Mixed Precision Computing on H-Matrix Vector Multiplication in BEM Analysis
Hierarchical Matrix (H-matrix) is an approximation technique which splits a
target dense matrix into multiple submatrices, and where a selected portion of
submatrices are low-rank approximated. The technique substantially reduces both
time and space complexity of dense matrix vector multiplication, and hence has
been applied to numerous practical problems.
In this paper, we aim to accelerate the H-matrix vector multiplication by
introducing mixed precision computing, where we employ both binary64 (FP64) and
binary32 (FP32) arithmetic operations. We propose three methods to introduce
mixed precision computing to H-matrix vector multiplication, and then evaluate
them in a boundary element method (BEM) analysis. The numerical tests examine
the effects of mixed precision computing, particularly on the required
simulation time and rate of convergence of the iterative (BiCG-STAB) linear
solver. We confirm the effectiveness of the proposed methods.Comment: Accepted manuscript to International Conference on High Performance
Computing in Asia-Pacific Region (HPCAsia2020), January 15--17, 2020,
Fukuoka, Japa
A new equation to estimate basal energy expenditure of patients with diabetes
[Background & aims]Predictive equations for basal energy expenditure (BEE) derived from Caucasians tend to overestimate BEE in non-Caucasians. The aim of this study was to develop a more suitable method to estimate BEE in Japanese patients with diabetes using indices readily measured in clinical practice. [Methods]BEE was measured by indirect calorimetry under a strict basal condition in 68 Japanese patients with type 1 or type 2 diabetes. The best fitting equation was investigated by multiple regression analysis using of age, sex, and anthropometric indices. The resultant new equation was tested in a separate group of 60 Japanese patients with type 1 or type 2 diabetes, and the accuracy compared with existing equations. [Results]The best-fit equation was BEE [kcal/day] = 10 × (body weight)[kg] – 3 × (age)[y] + 125 (if male) + 750. Adjusted coefficient of determination was 81.0%. Root mean squared errors and accurate prediction in the validation set were 103 kcal/day and 78% for the new equation; 184 and 50 for Harris-Benedict; 209 and 38 for Oxford; 205 and 42 for Liu; and 140 and 63 for Ganpule. [Conclusions]This new equation is simpler and estimates BEE more accurately in Japanese patients with diabetes than the presently used equations do
Instability of a membrane intersecting a black hole
The stability of a Nambu-Goto membrane at the equatorial plane of the
Reissner-Nordstr{\o}m-de Sitter spacetime is studied. The covariant
perturbation formalism is applied to study the behavior of the perturbation of
the membrane. The perturbation equation is solved numerically. It is shown that
a membrane intersecting a charged black hole, including extremely charged one,
is unstable and that the positive cosmological constant strengthens the
instability.Comment: 12 pages, 3 figures, to be published in Physical Review
Further restrictions on the topology of stationary black holes in five dimensions
We place further restriction on the possible topology of stationary
asymptotically flat vacuum black holes in 5 spacetime dimensions. We prove that
the horizon manifold can be either a connected sum of Lens spaces and "handles"
, or the quotient of by certain finite groups of
isometries (with no "handles"). The resulting horizon topologies include Prism
manifolds and quotients of the Poincare homology sphere. We also show that the
topology of the domain of outer communication is a cartesian product of the
time direction with a finite connected sum of 's
and 's, minus the black hole itself. We do not assume the existence of
any Killing vector beside the asymptotically timelike one required by
definition for stationarity.Comment: LaTex, 22 pages, 9 figure
Determination of the Major Impurity Radiators in the Reheat Mode Discharges in the Compact Helical System
Radiation brightness and impurity behaviors have been studied for reheat mode discharges in the Compact Helical System (CHS) by three different types of impurity diagnostics. Total radiation power measured by a pyroelectric detector significantly reduces after entering the reheat mode, whereas the line-averaged radiation brightness measured by an absolute extreme ultraviolet (AXUV) photodiode array increases especially for a center viewing chord due to the impurity accumulation in the plasma core. One possible reason for this opposite behavior between the two bolometric detectors is the reduced sensitivity of the AXUV photodiode for lower energy photons in vacuum ultraviolet (VUV) region. This speculation is supported by temporal evolutions of VUV spectra measured by a grazing incidence spectrometer. These results demonstrate that the comparison of three impurity diagnostics would be beneficial to the determination of the major impurity radiators and a comprehensive understanding of impurity behaviors in the reheat mode discharges
Thick domain walls around a black hole
We discuss the gravitationally interacting system of a thick domain wall and
a black hole. We numerically solve the scalar field equation in the
Schwarzschild spacetime and obtain a sequence of static axi-symmetric solutions
representing thick domain walls. We find that, for the walls near the horizon,
the Nambu--Goto approximation is no longer valid.Comment: 18 pages, 11 figures, one reference adde
Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma
In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities
Response of plasma toroidal flow to the transition between nested and stochastic magnetic field in LHD
Response of the plasma toroidal flow to the forward and backward transition between the nested and the stochastic magnetic field is studied using the charge exchange spectroscopy in the large helical device (LHD). Abrupt damping of toroidal flow associated with a transition from nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 after the exchange of the neutral beam injection (NBI) direction from co- to counter-direction in LHD. The stochastization of magnetic field occurs only in a narrow range of magnetic shear near 0.5 and spontaneousback-transition from stochastic to nested magnetic field (healing) is observed in the steady-state phase of magnetic shear. When the NBI direction is changed from counter- to co-direction, the healing of magnetic field occurs associated with the increase of magnetic shear
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