5,147 research outputs found
Modeling for Active Control of Combustion and Thermally Driven Oscillations
Organized oscillations excited and sustained by high densities of energy release in combustion chambers have long caused serious problems in development of propulsion systems. The amplitudes often become sufficiently large to cause unacceptable structural vibrations. Because the oscillations are self-excited, they reach limiting amplitudes (limit cycles) only because of the action of nonlinear processes. Traditionally, satisfactory behavior
has been achieved through a combination of trial-and-error
design and testing, with control always involving passive means: geometrical modifications, changes of propellant composition, or devices to enhance dissipation of acoustic energy. Active control has been applied only to small-scale laboratory devices, but the limited success suggests the possibility of serious applications to full-scale propulsion systems. Realization of that potential rests on further experimental work, combined with deeper understanding of the mechanisms causing the oscillations and of the physical behavior of the systems. Effective design of active control systems will require faithful modeling of the relevant processes over broad frequency ranges covering the spectra of natural modes. This paper will cover the general character of the linear and nonlinear behavior of combustion systems, with special attention to acoustics and the mechanisms of excitation.
The discussion is intended to supplement the paper by Doyle et al. concerned primarily with controls issues and the observed behavior of simple laboratory devices
Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation
While time-dependent perturbation theory shows inefficient carrier-phonon
scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic
description of carrier-phonon interaction predicts fast carrier capture and
relaxation. The considered processes do not fulfill energy conservation in
terms of free-carrier energies because polar coupling of localized quantum-dot
states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
Effective environments: Preparation of stationary states with inverse temperature ranging from positive to negative values
In this paper, we discuss how effective environments incorporating periodic
measurements can be used to prepare a two-level system (TLS) in almost
arbitrary thermal states: Concretely, we study a TLS coupled to a spin
environment, the magnetization of which is measured periodically. In ensemble
average these measurements cause a relaxation of the TLS into a thermal
(diagonal) state. By adjusting the time between the measurements and the
detuning of the environmental spins, the creation of very low temperatures as
well as inversion becomes possible. Our analytical results derived for large
environments are numerically shown to be valid even for quite small
environments, down to only a few spins.Comment: 20 pages, 3 figures, accepted for publication in Phys. Rev.
Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems
A microscopic theory is used to study the optical properties of semiconductor
quantum dots. The dephasing of a coherent excitation and line-shifts of the
interband transitions due to carrier-carrier Coulomb interaction and
carrier-phonon interaction are determined from a quantum kinetic treatment of
correlation processes. We investigate the density dependence of both mechanisms
and clarify the importance of various dephasing channels involving the
localized and delocalized states of the system.Comment: 12 pages, 10 figure
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Protection Measures against Product Piracy and Application by the Use of AM
Presently the implications Additive Manufacturing (AM) on intellectual properties are
discussed in public. Here AM is often mentioned as a driver for product piracy as it allows to
produce and to copy objects with any geometries. Imitators need a lot of information to copy
an object accurately. As reverse engineering has been identified as the most important
information source for product imitators, AM can also help to reduce the threat of product
piracy when correctly applied in the product development. Due to the layer wise production
process that allows the manufacturing of very complex shapes and geometries, the reverse-engineering process can be complicated by far. By this, quite contrary to the public opinion,
AM can increase the needed effort of imitators and strongly reduce the economic efficiency of
product piracy. This paper will show different protection measures and a methodological
approach of how to apply these measures to a product. Beside the protective effect some
measures allow a traceability of parts over the product’s lifecycle and thus support the quality
management of AM processes and additively produced parts.Mechanical Engineerin
Propagation of travelling waves in sub-excitable systems driven by noise and periodic forcing
It has been reported that traveling waves propagate periodically and stably
in sub-excitable systems driven by noise [Phys. Rev. Lett. \textbf{88}, 138301
(2002)]. As a further investigation, here we observe different types of
traveling waves under different noises and periodic forces, using a simplified
Oregonator model. Depending on different noises and periodic forces, we have
observed different types of wave propagation (or their disappearance).
Moreover, the reversal phenomena are observed in this system based on the
numerical experiments in the one-dimensional space. As an explanation, we
regard it as the effect of periodic forces. Thus, we give qualitative
explanations to how reversal phenomena stably appear, which seem to arise from
the mixing function of the periodic force and the noise. And the output period
and three velocities (the normal, the positive and the negative) of the
travelling waves are defined and their relationship with the periodic forces,
along with the types of waves, are also studied in sub-excitable system under a
fixed noise intensity.Comment: Some references and information are added in the modified version.
Accepted, The European Physical Journal
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