260 research outputs found
The g-mode Excitation in the Proto Neutron Star by the Standing Accretion Shock Instability
The so-called "acoustic revival mechanism" of core-collapse supernova
proposed recently by the Arizona group is an interesting new possibility.
Aiming to understand the elementary processes involved in the mechanism, we
have calculated the eigen frequencies and eigen functions for the g-mode
oscillations of a non-rotating proto neutron star. The possible excitation of
these modes by the standing accretion shock instability, or SASI, is discussed
based on these eigen functions. We have formulated the forced oscillations of
-modes by the external pressure perturbations exerted on the proto neutron
star surface. The driving pressure fluctuations have been adopted from our
previous computations of the axisymmetric SASI in the non-linear regime. We
have paid particular attention to low l modes, since these are the modes that
are dominant in SASI and that the Arizona group claimed played an important
role in their acoustic revival scenario. Here l is the index of the spherical
harmonic functions, . Although the frequency spectrum of the non-linear
SASI is broadened substantially by non-linear couplings, the typical frequency
is still much smaller than those of g-modes, the fact leading to a severe
impedance mismatch. As a result, the excitations of various -modes are
rather inefficient and the energy of the saturated g-modes is erg
or smaller, with the g_2-mode being the largest in our model. Here the g_2-mode
has two radial nodes and is confined to the interior of the convection region.
The energy transfer rate from the g-modes to out-going sound waves is estimated
from the growth of the g-modes and found to be erg/s in the model
studied in this paper.Comment: 24 pages, 6 figure
Zigzag edge modes in Z2 topological insulator: reentrance and completely flat spectrum
The spectrum and wave function of helical edge modes in Z_2 topological
insulator are derived on a square lattice using Bernevig-Hughes-Zhang (BHZ)
model. The BHZ model is characterized by a "mass" term M (k) that is
parameterized as M (k) = Delta - B k^2. A topological insulator realizes when
the parameters Delta and B fall on the regime, either 0 < Delta /B < 4 or 4 <
Delta /B < 8. At Delta /B = 4, which separates the cases of positive and
negative (quantized) spin Hall conductivities, the edge modes show a
corresponding change that depends on the edge geometry. In the (1,0)-edge, the
spectrum of edge mode remains the same against change of Delta /B, although the
main location of the mode moves from the zone center for Delta /B < 4, to the
zone boundary for Delta /B > 4 of the 1D Brillouin zone. In the (1,1)-edge
geometry, the group velocity at the zone center changes sign at Delta /B = 4
where the spectrum becomes independent of the momentum, i.e. flat, over the
whole 1D Brillouin zone. Furthermore, for Delta/B < 1.354..., the edge mode
starting from the zone center vanishes in an intermediate region of the 1D
Brillouin zone, but reenters near the zone boundary, where the energy of the
edge mode is marginally below the lowest bulk excitations. On the other hand,
the behavior of reentrant mode in real space is indistinguishable from an
ordinary edge mode.Comment: 19 pages, 33 figure
Gravitational radiation from collapsing magnetized dust
In this article we study the influence of magnetic fields on the axial
gravitational waves emitted during the collapse of a homogeneous dust sphere.
We found that while the energy emitted depends weakly on the initial matter
perturbations it has strong dependence on the strength and the distribution of
the magnetic field perturbations. The gravitational wave output of such a
collapse can be up to an order of magnitude larger or smaller calling for
detailed numerical 3D studies of collapsing magnetized configurations
Relativistic stars with purely toroidal magnetic fields
We investigate the effects of the purely toroidal magnetic field on the
equilibrium structures of the relativistic stars. The master equations for
obtaining equilibrium solutions of relativistic rotating stars containing
purely toroidal magnetic fields are derived for the first time. To solve these
master equations numerically, we extend the Cook-Shapiro-Teukolsky scheme for
calculating relativistic rotating stars containing no magnetic field to
incorporate the effects of the purely toroidal magnetic fields. By using the
numerical scheme, we then calculate a large number of the equilibrium
configurations for a particular distribution of the magnetic field in order to
explore the equilibrium properties. We also construct the equilibrium sequences
of the constant baryon mass and/or the constant magnetic flux, which model the
evolution of an isolated neutron star as it loses angular momentum via the
gravitational waves. Important properties of the equilibrium configurations of
the magnetized stars obtained in this study are summarized as follows ; (1) For
the non-rotating stars, the matter distribution of the stars is prolately
distorted due to the toroidal magnetic fields. (2) For the rapidly rotating
stars, the shape of the stellar surface becomes oblate because of the
centrifugal force. But, the matter distribution deep inside the star is
sufficiently prolate for the mean matter distribution of the star to be
prolate. (3) The stronger toroidal magnetic fields lead to the mass-shedding of
the stars at the lower angular velocity. (4) For some equilibrium sequences of
the constant baryon mass and magnetic flux, the stars can spin up as they lose
angular momentum.Comment: 13 figures, 7 tables, submitted to PR
Unsteady aerodynamic and optimal kinematic analysis of a micro flapping wing rotor
Inspired by the high performance of rotary and insect flapping wings capable of vertical take-off and landing and hovering (VTOLH), a novel flapping wing rotor (FWR) has been developed by combining the above two types of wing motions. The FWR offers an alternative configuration for micro air vehicles (MAV) of such high flight performance. Unlike the well-studied aerodynamics of rotary and insect-like flapping wing with prescribed wing motion, the aerodynamic lift and efficiency of the FWR associated with optimal kinematics of motion has not been studied in a systematic manner before. This investigation is therefore focused on the FWR optimal kinematic motion in terms of aerodynamic lift and efficiency. Aerodynamic analysis is conducted for a FWR model of aspect ratio 3.6 and wing span 200 mm in a range of kinematic parameters. The analysis is based on a quasi-steady aerodynamic model with empirical coefficients and validated by CFD results at Re∼3500. For comparison purpose, the analysis includes rotary and insect-like flapping wings in hovering status with the FWR at an equilibrium rotation speed when the thrust equals to drag. The results show that the rotary wing has the greatest power efficiency but the smallest lift coefficient. Whereas the FWR can produce the greatest aerodynamic lift with power efficiency between rotary and insect-like flapping wings. The results provide a quantified guidance for design option of the three types of high performance MAVs together with the optimal kinematics of motion according to flight performance requirement
Scalar perturbations of higher dimensional rotating and ultra-spinning black holes
We investigate the stability of higher dimensional rotating black holes
against scalar perturbations. In particular, we make a thorough numerical and
analytical analysis of six-dimensional black holes, not only in the low
rotation regime but in the high rotation regime as well. Our results suggest
that higher dimensional Kerr black holes are stable against scalar
perturbations, even in the ultra-spinning regime.Comment: 7 pages, ReVTeX
Relativistic r-modes in Slowly Rotating Neutron Stars: Numerical Analysis in the Cowling Approximation
We investigate the properties of relativistic -modes of slowly rotating
neutron stars by using a relativistic version of the Cowling approximation. In
our formalism, we take into account the influence of the Coriolis like force on
the stellar oscillations, but ignore the effects of the centrifugal like force.
For three neutron star models, we calculated the fundamental -modes with
and 3. We found that the oscillation frequency of the
fundamental -mode is in a good approximation given by , where is defined in the corotating frame at the
spatial infinity, and is the angular frequency of rotation of the
star. The proportional coefficient is only weakly dependent on
, but it strongly depends on the relativistic parameter ,
where and are the mass and the radius of the star. All the fundamental
-modes with computed in this study are discrete modes with distinct
regular eigenfunctions, and they all fall in the continuous part of the
frequency spectrum associated with Kojima's equation (Kojima 1998). These
relativistic -modes are obtained by including the effects of rotation higher
than the first order of so that the buoyant force plays a role, the
situation of which is quite similar to that for the Newtonian -modes.Comment: 22 pages, 8 figures, accepted for publication in Ap
Gust response and body freedom flutter of a flying-wing aircraft with a passive gust alleviation device
The effectiveness of a passive gust alleviation device (PGAD) mounted at the wingtip of aircraft in conventional and flying-wing configurations have been studied in previous research. However the PGAD influence on the aeroelastic stability in particular the body freedom flutter (BFF) of a flying-wing aircraft remains as a concern. This present investigation is focused on evaluating the beneficial effect of PGAD on both gust load alleviation and BFF of a small flying-wing aircraft of high aspect ratio wing made of composite. A small range of (1-cos) type of gust load has been considered to select a representative critical gust load case for the study. A parametric study indicates that there is a narrow band of optimal key parameters for the PGAD design. Subsequently a set of optimal parameters is selected to further the analysis of the PGAD mechanism. The case study results show that the PGAD can make the bending moment at the wing root due to gust reduced by 16%. In addition, the BFF speed of the flying-wing aircraft is increased by 4.2%. The investigation reveals that the PGAD mode and its interaction with the wing bending mode and short period oscillation of the aircraft can have beneficial aeroelastic effect on both gust alleviation and flutter suppression
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