315,192 research outputs found
Absolute Time Derivatives
A four dimensional treatment of nonrelativistic space-time gives a natural
frame to deal with objective time derivatives. In this framework some well
known objective time derivatives of continuum mechanics appear as
Lie-derivatives. Their coordinatized forms depends on the tensorial properties
of the relevant physical quantities. We calculate the particular forms of
objective time derivatives for scalars, vectors, covectors and different second
order tensors from the point of view of a rotating observer. The relation of
substantial, material and objective time derivatives is treated.Comment: 26 pages, 4 figures (minor revision
Fine strand-like structure in the solar corona from MHD transverse oscillations
Current analytical and numerical modelling suggest the existence of
ubiquitous thin current sheets in the corona that could explain the observed
heating requirements. On the other hand, new high resolution observations of
the corona indicate that its magnetic field may tend to organise itself in fine
strand-like structures of few hundred kilometres widths. The link between small
structure in models and the observed widths of strand-like structure several
orders of magnitude larger is still not clear. A popular theoretical scenario
is the nanoflare model, in which each strand is the product of an ensemble of
heating events. Here, we suggest an alternative mechanism for strand
generation. Through forward modelling of 3D MHD simulations we show that small
amplitude transverse MHD waves can lead in a few periods time to strand-like
structure in loops in EUV intensity images. Our model is based on previous
numerical work showing that transverse MHD oscillations can lead to
Kelvin-Helmholtz instabilities that deform the cross-sectional area of loops.
While previous work has focused on large amplitude oscillations, here we show
that the instability can occur even for low wave amplitudes for long and thin
loops, matching those presently observed in the corona. We show that the
vortices generated from the instability are velocity sheared regions with
enhanced emissivity hosting current sheets. Strands result as a complex
combination of the vortices and the line-of-sight angle, last for timescales of
a period and can be observed for spatial resolutions of a tenth of loop radius.Comment: Accepted for publication in ApJ
Multiple junction biasing of superconducting tunnel junction detectors
We describe a new biasing scheme for single photon detectors based on
superconducting tunnel junctions. It replaces a single detector junction with a
circuit of three junctions and achieves biasing of a detector junction at
subgap currents without the use of an external magnetic field. The biasing
occurs through the nonlinear interaction of the three junctions, which we
demonstrate through numerical simulation. This nonlinear state is numerically
stable against external fluctuations and is compatible with high fidelity
electrical readout of the photon-induced current. The elimination of the
external magnetic field potentially increases the capability of these types of
photon detectors and eases constraints involved in the fabrication of large
detector arrays.Comment: 15 pages, including 3 figure
Innovative all composite multi-pultrusion truss system for stressed arch deployable shelters
Trusses are one of the successful structural forms that have been utilised, at extended scale, since the nineteen century. Fibre composite materials are relatively new to civil engineering applications. The increased interest in using composites in civil applications can be attributed to advantages when compared to other construction materials that offset their associated costs. Using conventional approaches for truss systems in composite materials can undermine their efficiency. This is mainly due to concentration of stresses at connections which usually govern the truss design.
The Military Modular Shelter System (M2S2) initiative is a research project that aims to develop a fibre composite re-deployable arched shelter system with rigid PVC or fabric cladding. The main frames are formed from modular fibre composite panels that are connected and stressed into position by prestressing cables. Different geometries can be obtained using this system by changing the number of panels per frame and the packer sizes between panels.
This paper presents the development and testing of innovative fibre composite truss modules that were investigated as part of this project. The truss system is based on using multi-pultrusion sections for the chord and vertical members. Truss bracing is provided by a double skin laminated web. This structure offers many advantages including semi-ductile failure that occurred outside the joint area and ease of manufacturing. In spite of being developed for the M2S2 system, the concept is similarly applicable as a general purpose truss system
Stabilization Control of the Differential Mobile Robot Using Lyapunov Function and Extended Kalman Filter
This paper presents the design of a control model to navigate the
differential mobile robot to reach the desired destination from an arbitrary
initial pose. The designed model is divided into two stages: the state
estimation and the stabilization control. In the state estimation, an extended
Kalman filter is employed to optimally combine the information from the system
dynamics and measurements. Two Lyapunov functions are constructed that allow a
hybrid feedback control law to execute the robot movements. The asymptotical
stability and robustness of the closed loop system are assured. Simulations and
experiments are carried out to validate the effectiveness and applicability of
the proposed approach.Comment: arXiv admin note: text overlap with arXiv:1611.07112,
arXiv:1611.0711
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