40,289 research outputs found
Bifurcation Boundary Conditions for Switching DC-DC Converters Under Constant On-Time Control
Sampled-data analysis and harmonic balance analysis are applied to analyze
switching DC-DC converters under constant on-time control. Design-oriented
boundary conditions for the period-doubling bifurcation and the saddle-node
bifurcation are derived. The required ramp slope to avoid the bifurcations and
the assigned pole locations associated with the ramp are also derived. The
derived boundary conditions are more general and accurate than those recently
obtained. Those recently obtained boundary conditions become special cases
under the general modeling approach presented in this paper. Different analyses
give different perspectives on the system dynamics and complement each other.
Under the sampled-data analysis, the boundary conditions are expressed in terms
of signal slopes and the ramp slope. Under the harmonic balance analysis, the
boundary conditions are expressed in terms of signal harmonics. The derived
boundary conditions are useful for a designer to design a converter to avoid
the occurrence of the period-doubling bifurcation and the saddle-node
bifurcation.Comment: Submitted to International Journal of Circuit Theory and Applications
on August 10, 2011; Manuscript ID: CTA-11-016
Improved silicon nitride for advanced heat engines
The results of a four year program to improve the strength and reliability of injection-molded silicon nitride are summarized. Statistically designed processing experiments were performed to identify and optimize critical processing parameters and compositions. Process improvements were monitored by strength testing at room and elevated temperatures, and microstructural characterization by optical, scanning electron microscopes, and scanning transmission electron microscope. Processing modifications resulted in a 20 percent strength and 72 percent Weibull slope improvement of the baseline material. Additional sintering aids screening and optimization experiments succeeded in developing a new composition (GN-10) capable of 581.2 MPa at 1399 C. A SiC whisker toughened composite using this material as a matrix achieved a room temperature toughness of 6.9 MPa m(exp .5) by the Chevron notched bar technique. Exploratory experiments were conducted on injection molding of turbocharger rotors
Coronal rain in magnetic arcades: Rebound shocks, Limit cycles, and Shear flows
We extend our earlier multidimensional, magnetohydrodynamic simulations of
coronal rain occurring in magnetic arcades with higher resolution,
grid-adaptive computations covering a much longer ( hour) timespan. We
quantify how in-situ forming blob-like condensations grow along and across
field lines and show that rain showers can occur in limit cycles, here
demonstrated for the first time in 2.5D setups. We discuss dynamical,
multi-dimensional aspects of the rebound shocks generated by the siphon inflows
and quantify the thermodynamics of a prominence-corona-transition-region like
structure surrounding the blobs. We point out the correlation between
condensation rates and the cross-sectional size of loop systems where
catastrophic cooling takes place. We also study the variations of the typical
number density, kinetic energy and temperature while blobs descend, impact and
sink into the transition region. In addition, we explain the mechanisms leading
to concurrent upflows while the blobs descend. As a result, there are plenty of
shear flows generated with relative velocity difference around 80 km s
in our simulations. These shear flows are siphon flows set up by multiple blob
dynamics and they in turn affect the deformation of the falling blobs. In
particular, we show how shear flows can break apart blobs into smaller
fragments, within minutes
Averaged null energy condition in Loop Quantum Cosmology
Wormhole and time machine are very interesting objects in general relativity.
However, they need exotic matters which are impossible in classical level to
support them. But if we introduce the quantum effects of gravity into the
stress-energy tensor, these peculiar objects can be constructed
self-consistently. Fortunately, loop quantum cosmology (LQC) has the potential
to serve as a bridge connecting the classical theory and quantum gravity.
Therefore it provides a simple way for the study of quantum effect in the
semiclassical case. As is well known, loop quantum cosmology is very successful
to deal with the behavior of early universe. In the early stage, if taken the
quantum effect into consideration, inflation is natural because of the
violation of every kind of local energy conditions. Similar to the inflationary
universe, the violation of the averaged null energy condition is the necessary
condition for the traversable wormholes. In this paper, we investigate the
averaged null energy condition in LQC in the framework of effective
Hamiltonian, and find out that LQC do violate the averaged null energy
condition in the massless scalar field coupled model.Comment: 5 page
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