3,935 research outputs found
Active stabilization to prevent surge in centrifugal compression systems
This report documents an experimental and analytical study of the active stabilization of surge in a centrifugal engine. The aims of the research were to extend the operating range of a compressor as far as possible and to establish the theoretical framework for the active stabilization of surge from both an aerodynamic stability and a control theoretic perspective. In particular, much attention was paid to understanding the physical limitations of active stabilization and how they are influenced by control system design parameters. Previously developed linear models of actively stabilized compressors were extended to include such nonlinear phenomena as bounded actuation, bandwidth limits, and robustness criteria. This model was then used to systematically quantify the influence of sensor-actuator selection on system performance. Five different actuation schemes were considered along with four different sensors. Sensor-actuator choice was shown to have a profound effect on the performance of the stabilized compressor. The optimum choice was not unique, but rather shown to be a strong function of some of the non-dimensional parameters which characterize the compression system dynamics. Specifically, the utility of the concepts were shown to depend on the system compliance to inertia ratio ('B' parameter) and the local slope of the compressor speedline. In general, the most effective arrangements are ones in which the actuator is most closely coupled to the compressor, such as a close-coupled bleed valve inlet jet, rather than elsewhere in the flow train, such as a fuel flow modulator. The analytical model was used to explore the influence of control system bandwidth on control effectiveness. The relevant reference frequency was shown to be the compression system's Helmholtz frequency rather than the surge frequency. The analysis shows that control bandwidths of three to ten times the Helmholtz frequency are required for larger increases in the compressor flow range. This has important implications for implementation in gas turbine engines since the Helmholtz frequencies can be over 100 Hz, making actuator design extremely challenging
The physics of streamer discharge phenomena
In this review we describe a transient type of gas discharge which is
commonly called a streamer discharge, as well as a few related phenomena in
pulsed discharges. Streamers are propagating ionization fronts with
self-organized field enhancement at their tips that can appear in gases at (or
close to) atmospheric pressure. They are the precursors of other discharges
like sparks and lightning, but they also occur in for example corona reactors
or plasma jets which are used for a variety of plasma chemical purposes. When
enough space is available, streamers can also form at much lower pressures,
like in the case of sprite discharges high up in the atmosphere.
We explain the structure and basic underlying physics of streamer discharges,
and how they scale with gas density. We discuss the chemistry and applications
of streamers, and describe their two main stages in detail: inception and
propagation. We also look at some other topics, like interaction with flow and
heat, related pulsed discharges, and electron runaway and high energy
radiation. Finally, we discuss streamer simulations and diagnostics in quite
some detail.
This review is written with two purposes in mind: First, we describe recent
results on the physics of streamer discharges, with a focus on the work
performed in our groups. We also describe recent developments in diagnostics
and simulations of streamers. Second, we provide background information on the
above-mentioned aspects of streamers. This review can therefore be used as a
tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure
Skyrmion Gas Manipulation for Probabilistic Computing
The topologically protected magnetic spin configurations known as skyrmions
offer promising applications due to their stability, mobility and localization.
In this work, we emphasize how to leverage the thermally driven dynamics of an
ensemble of such particles to perform computing tasks. We propose a device
employing a skyrmion gas to reshuffle a random signal into an uncorrelated copy
of itself. This is demonstrated by modelling the ensemble dynamics in a
collective coordinate approach where skyrmion-skyrmion and skyrmion-boundary
interactions are accounted for phenomenologically. Our numerical results are
used to develop a proof-of-concept for an energy efficient
() device with a low area imprint ().
Whereas its immediate application to stochastic computing circuit designs will
be made apparent, we argue that its basic functionality, reminiscent of an
integrate-and-fire neuron, qualifies it as a novel bio-inspired building block.Comment: 41 pages, 20 figure
Design guidelines for assessing and controlling spacecraft charging effects
The need for uniform criteria, or guidelines, to be used in all phases of spacecraft design is discussed. Guidelines were developed for the control of absolute and differential charging of spacecraft surfaces by the lower energy space charged particle environment. Interior charging due to higher energy particles is not considered. A guide to good design practices for assessing and controlling charging effects is presented. Uniform design practices for all space vehicles are outlined
Streamer branching in air: physical model and simulations in fully 3d spatial domain
Fully coupled self-consistent model of streamer branching is presented. The model incorporates inhomogeneities in gas media representing, e.g., large molecular clusters, micro dust particles, etc., which act as centers for localized space charge build-up. The results of the performed simulations show that charge accumulation at such centers leads to their electrostatic interactions with the streamer head and causes its splitting. Quantitative analysis of the dynamics of charge carriers densities, generated space charges and magnitudes of the electric fields associated with the branching process is presented
Finite element analysis of plasma dust-acoustic waves
For dust acoustic solitary waves, we propose a finite element formulation of the fluid dusty plasma equations. To solve this continuum problem, a Petrov-Galerkin weak form with unwinding is applied. We consider an unmagnetised dusty plasma with negatively charged dust and Boltzmann distributions for electrons and ions. Nonlinearity of electron and ion number density as functions of the electrostatic potential is included. A fully-implicit time-integration is used (backward-Euler method) which requires the derivative of the weak form. A three-field formulation is introduced, with dust number-density, electrostatic potential and dust velocity being the unknown fields. We test the formulation with two numerical (2D and 3D)examples where convergence with mesh size is assessed. These establish the new formulation as a predictive tool in dusty plasmas
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