35,821 research outputs found
Toroidal Imploding Detonation Wave Initiator for Pulse Detonation Engines
Imploding toroidal detonation waves were used to initiate detonations in propane–air and ethylene–air mixtures inside of a tube. The imploding wave was generated by an initiator consisting of an array of channels filled with acetylene–oxygen gas and ignited with a single spark. The initiator was designed as a low-drag initiator tube for use with pulse detonation engines. To detonate hydrocarbon–air mixtures, the initiator was overfilled so that some acetylene oxygen spilled into the tube. The overfill amount required to detonate propane air was less than 2% of the volume of the 1-m-long, 76-mm-diam tube. The energy necessary to create an implosion strong enough to detonate propane–air mixtures was estimated to be 13% more than that used by a typical initiator tube, although the initiator was also estimated to use less oxygen. Images and pressure traces show a regular, repeatable imploding wave that generates focal pressures in excess of 6 times the Chapman–Jouguet pressure.Atheoretical analysis of the imploding toroidal wave performed using Whitham’s method was found to agree well with experimental data and showed that, unlike imploding cylindrical and spherical geometries, imploding toroids initially experience a period of diffraction before wave focusing occurs. A nonreacting numerical simulation was used to assist in the interpretation of the experimental data
Mode-locked dysprosium fiber laser: picosecond pulse generation from 2.97 to 3.30 {\mu}m
Mode-locked fiber laser technology to date has been limited to sub-3 {\mu}m
wavelengths, despite significant application-driven demand for compact
picosecond and femtosecond pulse sources at longer wavelengths. Erbium- and
holmium-doped fluoride fiber lasers incorporating a saturable absorber are
emerging as promising pulse sources for 2.7--2.9 {\mu}m, yet it remains a major
challenge to extend this coverage. Here, we propose a new approach using
dysprosium-doped fiber with frequency shifted feedback (FSF). Using a simple
linear cavity with an acousto-optic tunable filter, we generate 33 ps pulses
with up to 2.7 nJ energy and 330 nm tunability from 2.97 to 3.30 {\mu}m
(3000--3400 cm^-1)---the first mode-locked fiber laser to cover this spectral
region and the most broadly tunable pulsed fiber laser to date. Numerical
simulations show excellent agreement with experiments and also offer new
insights into the underlying dynamics of FSF pulse generation. This highlights
the remarkable potential of both dysprosium as a gain material and FSF for
versatile pulse generation, opening new opportunities for mid-IR laser
development and practical applications outside the laboratory.Comment: Accepted for APL Photonics, 22nd August 201
Planar Detonation Wave Initiation in Large-Aspect-Ratio Channels
In this study, two initiator designs are presented that are able to form planar detonations with low input energy in large-aspect-ratio channels over distances corresponding to only a few channel heights. The initiators use a single spark and an array of small channels to shape the detonation wave. The first design, referred to as the static initiator, is simple to construct as it consists of straight channels which connect at right angles. However, it is only able to create planar waves using mixtures that can reliably detonate in its small-width channels. An improved design, referred to as the dynamic initiator, is capable of detonating insensitive mixtures using an oxyacetylene gas slug injected into the initiator shortly before ignition, but is more complex to construct. The two versions are presented next, including an overview of their design and operation. Design drawings of each initiator are available elsewhere [7]. Finally, photographs and pressure traces of the resulting planar waves generated by each device are shown
Elastic interaction between colloidal particles in confined nematic liquid crystals
The theory of elastic interaction of micron size axially symmetric colloidal
particles immersed into confined nematic liquid crystal has been proposed.
General formulas are obtained for the self energy of one colloidal particle and
interaction energy between two particles in arbitrary confined NLC with strong
anchoring condition on the bounding surface. Particular cases of dipole-dipole
interaction in the homeotropic and planar nematic cell with thickness are
considered and found to be exponentially screened on far distances with decay
length . It is predicted that bounding surfaces in
the planar cell crucially change the attraction and repulsion zones of usual
dipole-dipole interaction. As well it is predicted that \textit{the decay
length} in quadrupolar interaction is \textit{two times smaller} than for the
dipolar case.Comment: 4 pages,2 figure
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