39,347 research outputs found
Gyro-induced acceleration of magnetic reconnection
The linear and nonlinear evolution of magnetic reconnection in collisionless
high-temperature plasmas with a strong guide field is analyzed on the basis of
a two-dimensional gyrofluid model. The linear growth rate of the reconnecting
instability is compared to analytical calculations over the whole spectrum of
linearly unstable wave numbers. In the strongly unstable regime (large \Delta
'), the nonlinear evolution of the reconnecting instability is found to undergo
two distinctive acceleration phases separated by a stall phase in which the
instantaneous growth rate decreases. The first acceleration phase is caused by
the formation of strong electric fields close to the X-point due to ion
gyration, while the second acceleration phase is driven by the development of
an open Petschek-like configuration due to both ion and electron temperature
effects. Furthermore, the maximum instantaneous growth rate is found to
increase dramatically over its linear value for decreasing diffusion layers.
This is a consequence of the fact that the peak instantaneous growth rate
becomes weakly dependent on the microscopic plasma parameters if the diffusion
region thickness is sufficiently smaller than the equilibrium magnetic field
scale length. When this condition is satisfied, the peak reconnection rate
asymptotes to a constant value.Comment: Accepted for publication on Physics of Plasma
Forced and self-excited oscillations of an optomechanical cavity
We experimentally study forced and self oscillations of an optomechanical
cavity which is formed between a fiber Bragg grating that serves as a static
mirror and between a freely suspended metallic mechanical resonator that serves
as a moving mirror. In the domain of small amplitude mechanical oscillations,
we find that the optomechanical coupling is manifested as changes in the
effective resonance frequency, damping rate and cubic nonlinearity of the
mechanical resonator. Moreover, self oscillations of the micromechanical mirror
are observed above a certain optical power threshold. A comparison between the
experimental results and a theoretical model that we have recently presented
yields a good agreement. The comparison also indicates that the dominant
optomechanical coupling mechanism is the heating of the metallic mirror due to
optical absorption.Comment: 11 pages, 6 figure
Efficient preliminary floating offshore wind turbine design and testing methodologies and application to a concrete spar design
The current key challenge in the floating offshore wind turbine industry and research is on designing economic floating systems that can compete with fixed-bottom offshore turbines in terms of levelized cost of energy. The preliminary platform design, as well as early experimental design assessments, are critical elements in the overall design process. In this contribution, a brief review of current floating offshore wind turbine platform pre-design and scaled testing methodologies is provided, with a focus on their ability to accommodate the coupled dynamic behaviour of floating offshore wind systems. The exemplary design and testing methodology for a monolithic concrete spar platform as performed within the European KIC AFOSP project is presented. Results from the experimental tests compared to numerical simulations are presented and analysed and show very good agreement for relevant basic dynamic platform properties. Extreme and fatigue loads and cost analysis of the AFOSP system confirm the viability of the presented design process. In summary, the exemplary application of the reduced design and testing methodology for AFOSP confirms that it represents a viable procedure during pre-design of floating offshore wind turbine platforms.Peer ReviewedPostprint (author’s final draft
Issues in the design of switched linear systems : a benchmark study
In this paper we present a tutorial overview of some of the issues that arise in the design of switched linear control systems. Particular emphasis is given to issues relating to stability and control system realisation. A benchmark regulation problem is then presented. This problem is most naturally solved by means of a switched control design. The challenge to the community is to design a control system that meets the required performance specifications and permits the application of rigorous analysis techniques. A simple design solution is presented and the limitations of currently available analysis techniques are illustrated with reference to this example
Analysis of Dynamic Performance Limitations of Fast Response /150 to 200 Hz/ Electrohydraulic Servos
Fast response electrohydraulic valve-controlled piston servo system
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