42,843 research outputs found
Transient Dynamics and Thermal Stress for Nuclear Rocket Heat-exchanger
Transient dynamics and thermal stresses in nuclear rocket heat exchange
Distributed parameter type of control for a bilinear system
Optimal control laws for bilinear system in distributed parameter model - analytical determinatio
The control of absorption cross-section for a nuclear rocket
Control of absorption cross section of nuclear rocket with distributed parameter kinetics using two optimization procedure
Resonant Orbits and the High Velocity Peaks Towards the Bulge
We extract the resonant orbits from an N-body bar that is a good
representation of the Milky Way, using the method recently introduced by Molloy
et al. (2015). By decomposing the bar into its constituent orbit families, we
show that they are intimately connected to the boxy-peanut shape of the
density. We highlight the imprint due solely to resonant orbits on the
kinematic landscape towards the Galactic centre. The resonant orbits are shown
to have distinct kinematic features and may be used to explain the cold
velocity peak seen in the APOGEE commissioning data (Nidever at al., 2012). We
show that high velocity peaks are a natural consequence of the motions of stars
in the 2:1 orbit family and that stars on other higher order resonances can
contribute to the peaks. The locations of the peaks vary with bar angle and,
with the tacit assumption that the observed peaks are due to the 2:1 family, we
find that the locations of the high velocity peaks correspond to bar angles in
the range 10 < theta_bar < 25 (deg). However, some important questions about
the nature of the peaks remain, such as their apparent absence in other surveys
of the Bulge and the deviations from symmetry between equivalent fields in the
north and south. We show that the absence of a peak in surveys at higher
latitudes is likely due to the combination of a less prominent peak and a lower
number density of bar supporting orbits at these latitudes.Comment: 7 Figures, 1 Table, Now includes figures & discussion of higher order
resonances, Minor revisions to text throughout, Conclusions unchange
Resonant Clumping and Substructure in Galactic Discs
We describe a method to extract resonant orbits from N-body simulations
exploiting the fact that they close in a frame rotating with a constant pattern
speed. Our method is applied to the N-body simulation of the Milky Way by Shen
et al. (2010). This simulation hosts a massive bar, which drives strong
resonances and persistent angular momentum exchange. Resonant orbits are found
throughout the disc, both close to the bar itself and out to the very edges of
the disc. Using Fourier spectrograms, we demonstrate that the bar is driving
kinematic substructure even in the very outer parts of the disc. We identify
two major orbit families in the outskirts of the disc that make significant
contributions to the kinematic landscape, namely the m:l = 3:-2 and 1:-1
families resonating with the pattern speed of the bar. A mechanism is described
that produces bimodal distributions of Galactocentric radial velocities at
selected azimuths in the outer disc. It occurs as a result of the temporal
coherence of particles on the 3:-2 resonant orbits, which causes them to arrive
simultaneously at pericentre or apocentre. This resonant clumping, due to the
in-phase motion of the particles through their epicycle, leads to both inward
and outward moving groups which belong to the same orbital family and
consequently produce bimodal radial velocity distributions. This is a possible
explanation of the bimodal velocity distributions observed towards the Galactic
anti-Centre by Liu et al. (2012). Another consequence is that transient
overdensities appear and dissipate (in a symmetric fashion) on timescales equal
to the their epicyclic period resulting in a periodic pulsing of the disc's
surface density.Comment: 11 Figures, 1 Table. Accepted for publication in ApJ. Version 2
reflects minor changes to the text. Animation referenced in Figure 7 is
available at http://hubble.shao.ac.cn/~shen/resonantclumping/DensMovie.mp
Data acquisition and path selection decision making for an autonomous roving vehicle
Problems related to the guidance of an autonomous rover for unmanned planetary exploration were investigated. Topics included in these studies were: simulation on an interactive graphics computer system of the Rapid Estimation Technique for detection of discrete obstacles; incorporation of a simultaneous Bayesian estimate of states and inputs in the Rapid Estimation Scheme; development of methods for estimating actual laser rangefinder errors and their application to date provided by Jet Propulsion Laboratory; and modification of a path selection system simulation computer code for evaluation of a hazard detection system based on laser rangefinder data
Optimization of Dimples in Microchannel Heat Sink with Impinging Jets—Part B: the Influences of Dimple Height and Arrangement
The combination of a microchannel heat sink with impinging jets and dimples (MHSIJD) can effectively improve the flow and heat transfer performance on the cooling surface of electronic devices with very high heat fluxes. Based on the previous work by analysing the effect of dimple radius on the overall performance of MHSIJD, the effects of dimple height and arrangement were numerically analysed. The velocity distribution, pressure drop, and thermal performance of MHSIJD under various dimple heights and arrangements were presented. The results showed that: MHSIJD with higher dimples had better overall performance with dimple radius being fixed; creating a mismatch between the impinging hole and dimple can solve the issue caused by the drift phenomenon; the mismatch between the impinging hole and dimple did not exhibit better overall performance than a well-matched design
Hyperpolarizabilities for the one-dimensional infinite single-electron periodic systems: II. Dipole-dipole versus current-current correlations
Based on Takayama-Lin-Liu-Maki model, analytical expressions for the
third-harmonic generation, DC Kerr effect, DC-induced second harmonic optical
Kerr effect, optical Kerr effect or intensity-dependent index of refraction and
DC-electric-field-induced optical rectification are derived under the static
current-current() correlation for one-dimensional infinite chains. The
results of hyperpolarizabilities under correlation are then compared
with those obtained using the dipole-dipole () correlation. The comparison
shows that the conventional correlation, albeit quite successful for
the linear case, is incorrect for studying the nonlinear optical properties of
periodic systems.Comment: 11 pages, 5 figure
Data acquisition and analysis of range-finding systems for spacing construction
For space missions of future, completely autonomous robotic machines will be required to free astronauts from routine chores of equipment maintenance, servicing of faulty systems, etc. and to extend human capabilities in hazardous environments full of cosmic and other harmful radiations. In places of high radiation and uncontrollable ambient illuminations, T.V. camera based vision systems cannot work effectively. However, a vision system utilizing directly measured range information with a time of flight laser rangefinder, can successfully operate under these environments. Such a system will be independent of proper illumination conditions and the interfering effects of intense radiation of all kinds will be eliminated by the tuned input of the laser instrument. Processing the range data according to certain decision, stochastic estimation and heuristic schemes, the laser based vision system will recognize known objects and thus provide sufficient information to the robot's control system which can develop strategies for various objectives
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