4,987 research outputs found
Hydroelastic vibration analysis of partially liquid-filled shells using a series representation of the liquid
A series representation of the oscillatory behavior of incompressible nonviscous liquids contained in partially filled elastic tanks is presented. Each term is selected on the basis of hydroelastic vibrations in circular cylindrical tanks. Using a complementary energy principle, the superposition of terms is made to approximately satisfy the liquid-tank interface compatibility. This analysis is applied to the gravity sloshing and hydroelastic vibrations of liquids in hemispherical tanks and in a typical elastic aerospace propellant tank. With only a few series terms retained, the results correlate very well with existing analytical results, NASTRAN-generated analytical results, and experimental test results. Hence, although each term is based on a cylindrical tank geometry, the superposition can be successfully applied to noncylindrical tanks
An investigation of hydraulic-line resonance and its attenuation
An investigation of fluid resonance in high-pressure hydraulic lines has been made with two types of fluid dampers (or filters) installed in the line. One type involved the use of one or more closed-end tubes branching at right angles from a main line, and the other type was a fluid muffler installed in-line. These devices were evaluated in forced vibration tests with oscillatory disturbances over a 1000-Hz range applied to one end of the line and with oscillatory pressures measured at various stations along the main pipe. Limited applications of acoustic-wave theory to the branched systems are also included. Results show varying attenuations of pressure perturbations, depending on the number and location of branches and the type of muffler. Up to three branches were used in the branch-resonator study, and the largest frequency range with maximum attenuation was obtained for a three-branch configuration. The widest frequency ranges with significant attenuations were obtained with two types of fluid mufflers
Octave Spanning Frequency Comb on a Chip
Optical frequency combs have revolutionized the field of frequency metrology
within the last decade and have become enabling tools for atomic clocks, gas
sensing and astrophysical spectrometer calibration. The rapidly increasing
number of applications has heightened interest in more compact comb generators.
Optical microresonator based comb generators bear promise in this regard.
Critical to their future use as 'frequency markers', is however the absolute
frequency stabilization of the optical comb spectrum. A powerful technique for
this stabilization is self-referencing, which requires a spectrum that spans a
full octave, i.e. a factor of two in frequency. In the case of mode locked
lasers, overcoming the limited bandwidth has become possible only with the
advent of photonic crystal fibres for supercontinuum generation. Here, we
report for the first time the generation of an octave-spanning frequency comb
directly from a toroidal microresonator on a silicon chip. The comb spectrum
covers the wavelength range from 990 nm to 2170 nm and is retrieved from a
continuous wave laser interacting with the modes of an ultra high Q
microresonator, without relying on external broadening. Full tunability of the
generated frequency comb over a bandwidth exceeding an entire free spectral
range is demonstrated. This allows positioning of a frequency comb mode to any
desired frequency within the comb bandwidth. The ability to derive octave
spanning spectra from microresonator comb generators represents a key step
towards achieving a radio-frequency to optical link on a chip, which could
unify the fields of metrology with micro- and nano-photonics and enable
entirely new devices that bring frequency metrology into a chip scale setting
for compact applications such as space based optical clocks
Mode spectrum and temporal soliton formation in optical microresonators
The formation of temporal dissipative solitons in optical microresonators
enables compact, high repetition rate sources of ultra-short pulses as well as
low noise, broadband optical frequency combs with smooth spectral envelopes.
Here we study the influence of the resonator mode spectrum on temporal soliton
formation. Using frequency comb assisted diode laser spectroscopy, the measured
mode structure of crystalline MgF2 resonators are correlated with temporal
soliton formation. While an overal general anomalous dispersion is required, it
is found that higher order dispersion can be tolerated as long as it does not
dominate the resonator's mode structure. Mode coupling induced avoided
crossings in the resonator mode spectrum are found to prevent soliton
formation, when affecting resonator modes close to the pump laser. The
experimental observations are in excellent agreement with numerical simulations
based on the nonlinear coupled mode equations, which reveal the rich interplay
of mode crossings and soliton formation
A para-differential renormalization technique for nonlinear dispersive equations
For \alpha \in (1,2) we prove that the initial-value problem \partial_t
u+D^\alpha\partial_x u+\partial_x(u^2/2)=0 on \mathbb{R}_x\times\mathbb{R}_t;
u(0)=\phi, is globally well-posed in the space of real-valued L^2-functions. We
use a frequency dependent renormalization method to control the strong low-high
frequency interactions.Comment: 42 pages, no figure
Spacecraft environments interactions: Protecting against the effects of spacecraft charging
The effects of the natural space environments on spacecraft design, development, and operation are the topic of a series of NASA Reference Publications currently being developed by the Electromagnetics and Environments Branch, Systems Analysis and Integration Laboratory, Marshall Space Flight Center. This primer, second in the series, describes the interactions between a spacecraft and the natural space plasma. Under certain environmental/spacecraft conditions, these interactions result in the phenomenon known as spacecraft charging. It is the focus of this publication to describe the phenomenon of spacecraft charging and its possible adverse effects on spacecraft and to present the key elements of a Spacecraft Charging Effects Protection Plan
Temporal solitons in optical microresonators
Dissipative solitons can emerge in a wide variety of dissipative nonlinear
systems throughout the fields of optics, medicine or biology. Dissipative
solitons can also exist in Kerr-nonlinear optical resonators and rely on the
double balance between parametric gain and resonator loss on the one hand and
nonlinearity and diffraction or dispersion on the other hand. Mathematically
these solitons are solution to the Lugiato-Lefever equation and exist on top of
a continuous wave (cw) background. Here we report the observation of temporal
dissipative solitons in a high-Q optical microresonator. The solitons are
spontaneously generated when the pump laser is tuned through the effective zero
detuning point of a high-Q resonance, leading to an effective red-detuned
pumping. Red-detuned pumping marks a fundamentally new operating regime in
nonlinear microresonators. While usually unstablethis regime acquires unique
stability in the presence of solitons without any active feedback on the
system. The number of solitons in the resonator can be controlled via the pump
laser detuning and transitions to and between soliton states are associated
with discontinuous steps in the resonator transmission. Beyond enabling to
study soliton physics such as soliton crystals our observations open the route
towards compact, high repetition-rate femto-second sources, where the operating
wavelength is not bound to the availability of broadband laser gain media. The
single soliton states correspond in the frequency domain to low-noise optical
frequency combs with smooth spectral envelopes, critical to applications in
broadband spectroscopy, telecommunications, astronomy and low phase-noise
microwave generation.Comment: Includes Supplementary Informatio
Photonic chip based optical frequency comb using soliton induced Cherenkov radiation
By continuous wave pumping of a dispersion engineered, planar silicon nitride
microresonator, continuously circulating, sub-30fs short temporal dissipative
solitons are generated, that correspond to pulses of 6 optical cycles and
constitute a coherent optical frequency comb in the spectral domain. Emission
of soliton induced Cherenkov radiation caused by higher order dispersion
broadens the spectral bandwidth to 2/3 of an octave, sufficient for self
referencing, in excellent agreement with recent theoretical predictions and the
broadest coherent microresonator frequency comb generated to date. In a further
step, this frequency comb is fully phase stabilized. The ability to preserve
coherence over a broad spectral bandwidth using soliton induced Cherenkov
radiation marks a critical milestone in the development of planar optical
frequency combs, enabling on one hand application in e.g. coherent
communications, broadband dual comb spectroscopy and Raman spectral imaging,
while on the other hand significantly relaxing dispersion requirements for
broadband microresonator frequency combs and providing a path for their
generation in the visible and UV. Our results underscore the utility and
effectiveness of planar microresonator frequency comb technology, that offers
the potential to make frequency metrology accessible beyond specialized
laboratories.Comment: Changes: - Added data (new Fig.4) on the first full phase
stabilization of a dissipative Kerr soliton (or dissipative cavity soliton)
in a microresonator - Extended Fig. 8 in the SI - Introduced nomenclature of
dissipative Kerr solitons - Minor other change
E2F Activation of S Phase Promoters via Association with HCF-1 and the MLL Family of Histone H3K4 Methyltransferases
E2F transcriptional regulators control human-cell proliferation by repressing and activating the transcription of genes required for cell-cycle progression, particularly the S phase. E2F proteins repress transcription in association with retinoblastoma pocket proteins, but less is known about how they activate transcription. Here, we show that the human G1 phase regulator HCF-1 associates with both activator (E2F1 and E2F3a) and repressor (E2F4) E2F proteins, properties that are conserved in insect cells. Human HCF-1-E2F interactions are versatile: their associations and binding to E2F-responsive promoters are cell-cycle selective, and HCF-1 displays coactivator properties when bound to the E2F1 activator and corepressor properties when bound to the E2F4 repressor. During the G1-to-S phase transition, HCF-1 recruits the mixed-lineage leukemia (MLL) and Set-1 histone H3 lysine 4 methyltransferases to E2F-responsive promoters and induces histone methylation and transcriptional activation. These results suggest that HCF-1 induces cell-cycle-specific transcriptional activation by E2F proteins to promote cell proliferation
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