27,729 research outputs found
Subelliptic Li-Yau estimates on three dimensional model spaces
We describe three elementary models in three dimensional subelliptic geometry
which correspond to the three models of the Riemannian geometry (spheres,
Euclidean spaces and Hyperbolic spaces) which are respectively the SU(2),
Heisenberg and SL(2) groups. On those models, we prove parabolic Li-Yau
inequalities on positive solutions of the heat equation. We use for that the
techniques that we adapt to those elementary model spaces. The
important feature developed here is that although the usual notion of Ricci
curvature is meaningless (or more precisely leads to bounds of the form
for the Ricci curvature), we describe a parameter which plays
the same role as the lower bound on the Ricci curvature, and from which one
deduces the same kind of results as one does in Riemannian geometry, like heat
kernel upper bounds, Sobolev inequalities and diameter estimates
Vibration and noise analysis of a gear transmission system
This paper presents a comprehensive procedure to predict both the vibration and noise generated by a gear transmission system under normal operating conditions. The gearbox vibrations were obtained from both numerical simulation and experimental studies using a gear noise test rig. In addition, the noise generated by the gearbox vibrations was recorded during the experimental testing. A numerical method was used to develop linear relationships between the gearbox vibration and the generated noise. The hypercoherence function is introduced to correlate the nonlinear relationship between the fundamental noise frequency and its harmonics. A numerical procedure was developed using both the linear and nonlinear relationships generated from the experimental data to predict noise resulting from the gearbox vibrations. The application of this methodology is demonstrated by comparing the numerical and experimental results from the gear noise test rig
SS Ari: a shallow-contact close binary system
Two CCD epochs of light minimum and a complete R light curve of SS Ari are
presented. The light curve obtained in 2007 was analyzed with the 2003 version
of the W-D code. It is shown that SS Ari is a shallow contact binary system
with a mass ratio and a degree of contact factor f=9.4(\pm0.8%). A
period investigation based on all available data shows that there may exist two
distinct solutions about the assumed third body. One, assuming eccentric orbit
of the third body and constant orbital period of the eclipsing pair results in
a massive third body with and P_3=87.00.278M_{\odot}$. Both of the cases
suggest the presence of an unseen third component in the system.Comment: 28 pages, 9 figures and 5 table
Galilean invariance of lattice Boltzmann models
It is well-known that the original lattice Boltzmann (LB) equation deviates
from the Navier-Stokes equations due to an unphysical velocity dependent
viscosity. This unphysical dependency violates the Galilean invariance and
limits the validation domain of the LB method to near incompressible flows. As
previously shown, recovery of correct transport phenomena in kinetic equations
depends on the higher hydrodynamic moments. In this Letter, we give specific
criteria for recovery of various transport coefficients. The Galilean
invariance of a general class of LB models is demonstrated via numerical
experiments
The quest for companions to post-common envelope binaries. II. NSVS14256825 and HS0705+6700
We report new mid-eclipse times of the two close binaries NSVS14256825 and
HS0705+6700, harboring an sdB primary and a low-mass main-sequence secondary.
Both objects display clear variations in the measured orbital period, which can
be explained by the action of a third object orbiting the binary. If this
interpretation is correct, the third object in NSVS14256825 is a giant planet
with a mass of roughly 12 M_Jup. For HS0705+6700, we provide evidence that
strengthens the case for the suggested periodic nature of the eclipse time
variation and reduces the uncertainties in the parameters of the brown dwarf
implied by that model. The derived period is 8.4 yr and the mass is 31 M_Jup,
if the orbit is coplanar with the binary. This research is part of the
PlanetFinders project, an ongoing collaboration between professional
astronomers and student groups at high schools.Comment: Accepted by Astron. and Astrophy
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Progressive Collapse Resisting Mechanisms of Planar Prestressed Concrete Frame
The paper presents the behavior of six tests of Planar Prestressed Concrete Frame under the loss of a middle column. The six tests consist of two non-prestressed reinforced concrete (RC) specimens and four prestressed concrete (PC) specimens with bonded post-tensioning tendons (BPT). The structural response of the specimens with different flexural reinforcement ratio, span/depth ratio, and effective prestress level has been reported. In addition, the impact of parabolic BPT on the behavior of RC frames to resist progressive collapse is also evaluated. Experimental results indicated that the BPT can not only increase the initial stiffness and yielding load of the RC counterparts, but also increase the ultimate load capacity in the catenary action stage. Moreover, it will impact the load resisting mechanisms and the failure modes. Contrary to the commonly accepted sequential mobilization of compressive arch action and catenary action to resist progressive collapse of RC frames, no effective The paper presents the behavior of six tests of Planar Prestressed Concrete Frame under the compressive arch action is developed in PC frames to mitigate progressive collapse risk. Based on experimental observations, it is found that higher effective prestress in BPT results in enhanced initial stiffness and yielding load but less deformation capacity and ultimate load capacity. It is also found that higher non-prestressed flexural tensile reinforcement ratio could improve the behavior of PC specimens to resist progressive collapse
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