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## Delayed chromosomal instability induced by DNA damage.

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /11/1993
EN

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DNA damage induced by ionizing radiation can result in gene mutation, gene amplification, chromosome rearrangements, cellular transformation, and cell death. Although many of these changes may be induced directly by the radiation, there is accumulating evidence for delayed genomic instability following X-ray exposure. We have investigated this phenomenon by studying delayed chromosomal instability in a hamster-human hybrid cell line by means of fluorescence in situ hybridization. We examined populations of metaphase cells several generations after expanding single-cell colonies that had survived 5 or 10 Gy of X rays. Delayed chromosomal instability, manifested as multiple rearrangements of human chromosome 4 in a background of hamster chromosomes, was observed in 29% of colonies surviving 5 Gy and in 62% of colonies surviving 10 Gy. A correlation of delayed chromosomal instability with delayed reproductive cell death, manifested as reduced plating efficiency in surviving clones, suggests a role for chromosome rearrangements in cytotoxicity. There were small differences in chromosome destabilization and plating efficiencies between cells irradiated with 5 or 10 Gy of X rays after a previous exposure to 10 Gy and cells irradiated only once. Cell clones showing delayed chromosomal instability had normal frequencies of sister chromatid exchange formation...

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## Microsatellite instability in preinvasive and invasive head and neck squamous carcinoma.

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /06/1996
EN

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To investigate the extent and significance of microsatellite instability in head and neck carcinogenesis we analyzed DNA extracted from normal squamous epithelium, severe dysplasia, and corresponding carcinoma specimens from 20 patients by multiplex polymerase chain reaction. Loci on chromosomes 3p, 5p, 5q, 8p, 9p, 9q, 11q, 17p, 17q, 18p, 18q were selected for analysis. Our results show that three of the dysplasias (15.0%) and six of the invasive carcinoma (30.0%) manifested instability at multiple loci. Two of the dysplastic lesions had identical alterations in the corresponding carcinomas and one showed instability differences in only two of eight loci. Normal squamous epithelium lacked microsatellite instability. No apparent association between smoking, alcohol use, or family history of cancer and instability was found in this small cohort. Invasive carcinomas with instability were relatively more poorly differentiated and had a higher stage and a high proliferative fraction. Our study indicates that microsatellite instability is 1) noted in a small subset of dysplastic lesions of head and neck squamous epithelium and 2) present in approximately one-third of invasive lesions, usually with aggressive characteristics, and may clinically be a late event associated with tumor progression.

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## Origin of Microsatellite Instability in Gastric Cancer

Fonte: American Society for Investigative Pathology
Publicador: American Society for Investigative Pathology

Tipo: Artigo de Revista Científica

Publicado em /07/1999
EN

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Microsatellite instability (MSI) is observed in 13–44% of gastric carcinoma. The etiology of MSI in gastric carcinoma has not been clearly defined. To assess the role of mismatch repair in the development of MSI in gastric cancer, expression of hMSH2 and hMLH1 was explored. We examined 117 gastric carcinomas for MSI and observed instability at one or more loci in 19 (16%) of these tumors. Of the 19 tumors with MSI, nine exhibited low-rate MSI (MSI-L) with instability at <17% of loci, whereas the remaining 10 exhibited high-rate MSI (MSI-H) with instability at >33% of loci examined. Immunohistochemical staining for hMLH1 and hMSH2 was performed on eight of the tumors with MSI-H, five with MSI-L, and 15 tumors without MSI. All eight tumors with MSI-H showed loss of staining for either hMLH1 (n = 5) or hMSH2 (n = 3). In contrast, tumors with MSI-L or without MSI all showed normal hMSH2 and hMLH1 protein expression patterns. Moreover, all eight of the tumors with MSI-H also showed instability at BAT-26, whereas none of the MSI-L tumors or tumors without instability showed instability at BAT-26. These findings suggest that the majority of high-level MSI in gastric cancer is associated with defects of the mismatch repair pathway. Although larger studies are needed...

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## Minisatellite instability is found in colorectal tumours with mismatch repair deficiency

Fonte: Nature Publishing Group
Publicador: Nature Publishing Group

Tipo: Artigo de Revista Científica

Publicado em /11/2001
EN

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Microsatellite instability (MSI) in colorectal tumours is demonstrated by PCR amplification of several different microsatellite loci. Minisatellites, which are repeats of longer sequences also found throughout the genome, may also be affected by tumorigenesis. Certain minisatellite alleles contain 2 types of similar repeat unit that are randomly interspersed. The interspersion pattern can be analysed by mapping variant repeat units along an amplified allele, minisatellite variant repeat unit mapping PCR (MVR-PCR). We have applied microsatellite analysis with 10 markers and MVR-PCR for locus D7S21 to 33 cases of colorectal neoplasia, 27 sporadic and 6 from patients suspected of having hereditary non-polyposis colorectal cancer (HNPCC). Of the 27 sporadic cases, 3 were MSI-high on microsatellite analysis and one MSI-low. Instability with MVR-PCR was observed, but only in the MSI-high cases. Four of the HNPCC patients had mismatch repair (MMR) gene mutations in either hMLH1 or hMSH2. All 4 had DNA instability by MVR-PCR, but only two of these had MSI (one high, one low). The other 2 of the 6 patients with suspected HNPCC were negative to mutation analysis. One had features strongly suggestive of HNPCC and was unstable by both microsatellite analysis (MSI-high) and by MVR-PCR. The other tumour...

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## Adjacent Segment Instability after Treatment with a Graf Ligament at Minimum 8 Years’ Followup

Fonte: Springer-Verlag
Publicador: Springer-Verlag

Tipo: Artigo de Revista Científica

EN

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Although there has been some enthusiasm over the early clinical results obtained using the Graf ligament, associated mid- to long-term results are controversial. We retrospectively reviewed 43 patients (67 segments) treated with the Graf ligament for degenerative lumbar stenosis. The minimum followup was 8 years (mean, 10 years; range, 8–14 years). At last followup, we observed angular instability in 19 of the 67 segments (28%) and translational instability in five (7%). The disc height decreased from postoperatively (mean 93% of the preoperative disc) to the final followup (mean 82%). Of the 43 patients, 18 (42%) had adjacent segmental instability at the upper segment, including angular instability in 11 patients, translational instability in four patients, and both in three patients. The adjacent segment instability at the lower segment revealed 13 patients (30%) with angular instability. The data suggest the anticipated mechanical effects of the Graf ligament can be altered by degeneration of the disc and facet joints at instrumented segments and the adjacent segment can be affected, perhaps as a result of abnormal load transmission.

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## Correlation of Dynamic Cartilage Contact Stress Aberrations with Severity of Instability in Ankle Incongruity

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /09/2008
EN

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Joint instability is presumed to cause abnormality in cartilage contact mechanics, which accumulatively damages articular surface over years, leading to osteoarthritis. The purpose of this study was to clarify the effect of instability on dynamic cartilage contact mechanics. Using human ankle cadaver specimens, potentially unstable ankles were modeled by introducing a coronally-directed step-off incongruity of the anterior tibial surface and/or by transecting the anterior talofibular ligament. Specimens were subjected to a duty cycle with quasi-physiologic stance-phase motion and loading. AP tibial forces were modulated, causing a controlled, quantifiable ankle subluxation during the duty cycle. Instantaneous changes in local articular contact stresses in the ankle were continuously measured using a thin and flexible pressure transducer. Tests were repeated while varying the tibial surface condition (anatomic, 1mm step-off, and 2mm step-off), both before/after transection of the anterior talofibular ligament, with various AP force magnitudes, so that situations of various degrees of instability were created for each specimen. Instability events occurred in ankles when the step-off incongruity was introduced, with the abnormality in joint kinematics being greater after ligament transection. Contact stress data revealed that these instability events involved distinctly abrupt increases/decreases in local articular contact stresses...

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## Investigation of the roughness-induced transition: global stability analyses and direct numerical simulations

Fonte: Cambridge University Press
Publicador: Cambridge University Press

EN

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#boundary layers, instability, transition to turbulence#couche limite, instabilité, transition vers la turbulence#Sciences de l'ingénieur: Mécanique: Mécanique des fluides

The linear global instability and resulting transition to turbulence induced by an
isolated cylindrical roughness element of height h and diameter d immersed within an incompressible boundary layer flow along a flat plate is investigated using the joint application of direct numerical simulations and fully three-dimensional global stability analyses. For the range of parameters investigated, base flow computations show that the roughness element induces a wake composed of a central low-speed region surrounded by a three-dimensional shear layer and a pair of low- and high-speed streaks on each of its sides. Results from the global stability analyses highlight the unstable nature of the central low-speed region and its crucial importance in the laminar–turbulent transition process. It is able to sustain two different global instabilities: a sinuous and a varicose one. Each of these globally unstable modes is related to a different physical mechanism. While the varicose mode has its root in the instability of the whole three-dimensional shear layer surrounding the central
low-speed region, the sinuous instability turns out to be similar to the von Kármán instability in the two-dimensional cylinder wake and has its root in the lateral shear layers of the separated zone. The aspect ratio of the roughness element plays a key role on the selection of the dominant instability: whereas the flow over thin cylindrical roughness elements transitions due to a sinuous instability of the near-wake region...

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## Processing of double-R-loops in (CAG)·(CTG) and C9orf72 (GGGGCC)·(GGCCCC) repeats causes instability

Fonte: Oxford University Press
Publicador: Oxford University Press

Tipo: Artigo de Revista Científica

EN

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R-loops, transcriptionally-induced RNA:DNA hybrids, occurring at repeat tracts (CTG)n, (CAG)n, (CGG)n, (CCG)n and (GAA)n, are associated with diseases including myotonic dystrophy, Huntington's disease, fragile X and Friedreich's ataxia. Many of these repeats are bidirectionally transcribed, allowing for single- and double-R-loop configurations, where either or both DNA strands may be RNA-bound. R-loops can trigger repeat instability at (CTG)·(CAG) repeats, but the mechanism of this is unclear. We demonstrate R-loop-mediated instability through processing of R-loops by HeLa and human neuron-like cell extracts. Double-R-loops induced greater instability than single-R-loops. Pre-treatment with RNase H only partially suppressed instability, supporting a model in which R-loops directly generate instability by aberrant processing, or via slipped-DNA formation upon RNA removal and its subsequent aberrant processing. Slipped-DNAs were observed to form following removal of the RNA from R-loops. Since transcriptionally-induced R-loops can occur in the absence of DNA replication, R-loop processing may be a source of repeat instability in the brain. Double-R-loop formation and processing to instability was extended to the expanded C9orf72 (GGGGCC)·(GGCCCC) repeats...

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## The Global Baroclinic Instability in Accretion Disks. II: Local Linear Analysis

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/01/2004

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This paper contains a local linear stability analysis for accretion disks
under the influence of a global radial entropy gradient beta = - d log T / d
log r for constant surface density. Numerical simulations suggested the
existence of an instability in two- and three-dimensional models of the solar
nebula. The present paper tries to clarify, quantify, and explain such a global
baroclinic instability for two-dimensional flat accretion disk models. As a
result linear theory predicts a transient linear instability that will amplify
perturbations only for a limited time or up to a certain finite amplification.
This can be understood as a result of the growth time of the instability being
longer than the shear time which destroys the modes which are able to grow. So
only non-linear effects can lead to a relevant amplification. Nevertheless, a
lower limit on the entropy gradient ~beta = 0.22 for the transient linear
instability is derived, which can be tested in future non-linear simulations.
This would help to explain the observed instability in numerical simulations as
an ultimate result of the transient linear instability, i.e. the Global
Baroclinic Instability.; Comment: 35 pages, 11 figures; ApJ in press

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## Instability of Magnetized Ionization Fronts Surrounding H II Regions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/10/2014

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An ionization front (IF) surrounding an H II region is a sharp interface
where a cold neutral gas makes transition to a warm ionized phase by absorbing
UV photons from central stars. We investigate the instability of a
plane-parallel D-type IF threaded by parallel magnetic fields, by neglecting
the effects of recombination within the ionized gas. We find that weak D-type
IFs always have the post-IF magnetosonic Mach number $\mathcal{M}_{\rm M2} \leq
1$. For such fronts, magnetic fields increase the maximum propagation speed of
the IFs, while reducing the expansion factor $\alpha$ by a factor of
$1+1/(2\beta_1)$ compared to the unmagnetized case, with $\beta_1$ denoting the
plasma beta in the pre-IF region. IFs become unstable to distortional
perturbations due to gas expansion across the fronts, exactly analogous to the
Darrieus-Landau instability of ablation fronts in terrestrial flames. The
growth rate of the IF instability is proportional linearly to the perturbation
wavenumber as well as the upstream flow speed, and approximately to
$\alpha^{1/2}$. The IF instability is stabilized by gas compressibility and
becomes completely quenched when the front is D-critical. The instability is
also stabilized by magnetic pressure when the perturbations propagate in the
direction perpendicular to the fields. When the perturbations propagate in the
direction parallel to the fields...

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## Numerical instability due to relativistic plasma drift in EM-PIC simulations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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The numerical instability observed in the Electromagnetic-Particle-in-cell
(EM-PIC) simulations with a plasma drifting with relativistic velocities is
studied using both theory and computer simulations. We derive the numerical
dispersion relation for a cold plasma drifting with a relativistic velocity and
find an instability attributed to the coupling between the beam modes of the
drifting plasma and the electromagnetic modes in the system. The characteristic
pattern of the instability in Fourier space for various simulation setups and
Maxwell Equation solvers are explored by solving the corresponding numerical
dispersion relations. Furthermore, based upon these characteristic patterns we
derive an asymptotic expression for the instability growth rate. The asymptotic
expression greatly speeds up the calculation of instability growth rate and
makes the parameter scan for minimal growth rate feasible even for full three
dimensions. The results are compared against simulation results and good
agreement is found. These results can be used as a guide to develop possible
approaches to mitigate the instability. We examine the use of a spectral solver
and show that such a solver when combined with a low pass filter with a cutoff
value of $|\vec{k}|$ essentially eliminates the instability while not modifying
modes of physical interest. The use of spectral solver also provides minimal
errors to electromagnetic modes in the lowest Brillouin zones.; Comment: Version 2 differs from 1 in that: in expressions of version 1 we
inadvertently omitted a term (-1)^{sum_i\nu_i}...

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## Ion-temperature-gradient sensitivity of the hydrodynamic instability caused by shear in the magnetic-field-aligned plasma flow

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 06/05/2014

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The cross-magnetic-field (i.e., perpendicular) profile of ion temperature and
the perpendicular profile of the magnetic-field-aligned (parallel) plasma flow
are sometimes inhomogeneous for space and laboratory plasma. Instability caused
by a gradient in either the ion-temperature profile or by shear in the parallel
flow has been discussed extensively in the literature. In this paper,
hydrodynamic plasma stability is investigated, real and imaginary frequency are
quantified over a range of the shear parameter, the normalized wavenumber, and
the ratio of density-gradient and ion-temperature-gradient scale lengths, and
the role of inverse Landau damping is illustrated for the case of combined
ion-temperature gradient and parallel-flow shear. We find that increasing the
ion-temperature gradient reduces the instability threshold for the hydrodynamic
parallel-flow shear instability, also known as the parallel Kelvin-Helmholtz
instability or the D'Angelo instability. We also find that a kinetic
instability arises from the coupled, reinforcing action of both free-energy
sources. For the case of comparable electron and ion temperature, we illustrate
analytically the transition of the D'Angelo instability to the kinetic
instability as the shear parameter...

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## Accurate simulations of the dynamical bar-mode instability in full General Relativity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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We present accurate simulations of the dynamical bar-mode instability in full
General Relativity focussing on two aspects which have not been investigated in
detail in the past. Namely, on the persistence of the bar deformation once the
instability has reached its saturation and on the precise determination of the
threshold for the onset of the instability in terms of the parameter
$\beta={T}/{|W|}$. We find that generic nonlinear mode-coupling effects appear
during the development of the instability and these can severely limit the
persistence of the bar deformation and eventually suppress the instability. In
addition, we observe the dynamics of the instability to be strongly influenced
by the value $\beta$ and on its separation from the critical value $\beta_c$
marking the onset of the instability. We discuss the impact these results have
on the detection of gravitational waves from this process and provide evidence
that the classical perturbative analysis of the bar-mode instability for
Newtonian and incompressible Maclaurin spheroids remains qualitatively valid
and accurate also in full General Relativity.; Comment: RevTeX4, 23 pages, 19 figures. Version in print

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## Two evolutional paths of an axisymmetric gravitational instability in the dust layer of a protoplanetary disk

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/05/2006

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Nonlinear numerical simulations are performed to investigate the density
evolution in the dust layer of a protoplanetary disk due to the gravitational
instability and dust settling toward the midplane. We assume the region where
the radial pressure gradient at equilibrium is negligible so that the
shear-induced instability is avoided, and also restrict to an axisymmetric
perturbation as a first step of nonlinear numerical simulations of the
gravitational instability. We find that there are two different evolutional
paths of the gravitational instability depending on the nondimensional gas
friction time, which is defined as the product of the gas friction time and the
Keplerian angular velocity. If the nondimensional gas friction time is equal to
0.01, the gravitational instability grows faster than dust settling. On the
other hand, if the nondimensional gas friction time is equal to 0.1, dust
aggregates settle sufficiently before the gravitational instability grows. In
the latter case, an approximate analytical calculation reveals that dust
settling is faster than the growth of the gravitational instability regardless
of the dust density at the midplane. Thus, the dust layer becomes extremely
thin and may reach a few tenth of the material density of the dust before the
gravitational instability grows.; Comment: 4 pages...

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## Thermal instability and multi-phase interstellar medium in the first galaxies

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/12/2014

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We examine the linear stability and nonlinear growth of the thermal
instability in isobarically contracting gas with various metallicities and FUV
field strengths. When the H2 cooling is suppressed by FUV fields (G_0>10^-3) or
the metallicity is high enough (Z/Zs>10^-3), the interstellar medium is
thermally unstable in the temperature range 100-7000 K owing to the cooling by
CII and OI fine-structure lines. In this case, a bi-phasic medium with a
bimodal density probability distribution is formed as a consequence of the
thermal instability. The characteristic scales of the thermal instability
become smaller with increasing metallicity. Comparisons of the nonlinear
simulations with different resolution indicates that the maximum scale of the
thermal instability should be resolved with more than 60 cells to follow
runaway cooling driven by the thermal instability. Under sufficiently weak FUV
fields and with low metallcity, the density range of the thermal instability
shrinks owing to dominance of H2 cooling. As the FUV intensity is reduced,
bi-phasic structure becomes less remarkable and disappears eventually. Our
basic results suggest that, in early galaxies, i) the thermal instability has
little effect for the medium with Z/Zs<10^-4...

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## Essence of inviscid shear instability: a point view of vortex dynamics

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 19/05/2006

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The essence of shear instability is fully revealed both mathematically and
physically. A general sufficient and necessary stable criterion is obtained
analytically within linear context. It is the analogue of Kelvin-Arnol'd
theorem, i.e., the stable flow minimizes the kinetic energy associated with
vorticity. Then the mechanism of shear instability is explored by combining the
mechanisms of both Kelvin-Helmholtz instability (K-H instability) and resonance
of waves. It requires both concentrated vortex and resonant waves for the
instability. The waves, which have same phase speed with the concentrated
vortex, have interactions with the vortex to trigger the instability. We call
this mechanism as "concentrated vortex instability". The physical explanation
of shear instability is also sketched. Finally, some useful criteria are
derived from the theorem. These results would intrigue future works to
investigate the other hydrodynamic instabilities.; Comment: Revtex4, 4 pages, 1 figure, extends the second part of
physics/0512208

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## The instability of counter-propagating kernel gravity waves in a constant shear flow

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 29/11/2007

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The mechanism describing the recently developed notion of kernel gravity
waves (KGWs) is reviewed and such structures are employed to interpret the
unstable dynamics of an example stratified plane parallel shear flow. This flow
has constant vertical shear, is infinite in the vertical extent, and
characterized by two density jumps of equal magnitude each decreasing
successively with height, in which the jumps are located symmetrically away
from the midplane of the system. We find that for a suitably defined
bulk-Richardson number there exists a band of horizontal wavenumbers which
exhibits normal-mode instability. The instability mechanism closely parallels
the mechanism responsible for the instability seen in the problem of
counter-propagating Rossby waves. In this problem the instability arises out of
the interaction of counter-propagating gravity waves. We argue that the
instability meets the Hayashi-Young criterion for wave instability. We also
argue that the instability is the simplest one that can arise in a stratified
atmosphere with constant shear flow. The counter propagating gravity waves
mechanism detailed here explains why the Rayleigh criteria for shear flow
instability in the unstratified case does not need to be satisfied in the
stratified case. This illustrates how the Miles-Howard theorem may support
destabilization through stratification. A normal mode analysis of a foamy layer
consisting of two density jumps of unequal magnitude is also analyzed. The
results are considered in terms of observations made of sea-hurricane
interfaces.; Comment: Submitted to Physics of Fluids...

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## Papaloizou-Pringle Instability of Magnetized Accretion Tori

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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Hot accretion tori around a compact object are known to be susceptible to a
global hydrodynamical instability, the so-called Papaloizou-Pringle (PP)
instability, arising from the interaction of non-axisymmetric waves across the
corotation radius, where the wave pattern speed matches the fluid rotation
rate. However, accretion tori produced in various astrophysical situations
(e.g., collapsars and neutron star binary mergers) are likely to be highly
magnetized. We study the effect of magnetic fields on the PP instability in
incompressible tori with various magnetic strengths and structures. In general,
toroidal magnetic fields have significant effects on the PP instability: For
thin tori (with the fractional width relative to the outer torus radius much
less than unity), the instability is suppressed at large field strengths with
the corresponding toroidal Alfven speed $v_{A\phi}\go 0.2r\Omega$ (where
$\Omega$ is the flow rotation rate). For thicker tori (with the fractional
width of order 0.4 or larger), which are hydrodynamically stable, the
instability sets in for sufficiently strong magnetic fields (with $v_{A\phi}\go
0.2 r\Omega$). Our results suggest that highly magnetized accretion tori may be
subjected to global instability even when it is stable against the usual
magneto-rotational instability.; Comment: 15 pages...

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## Taming of Modulation Instability by Spatio-Temporal Modulation of the Potential

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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Spontaneous pattern formation in a variety of spatially extended nonlinear
system always occurs through a modulation instability: homogeneous state of the
system becomes unstable with respect to growing modulation modes. Therefore,
the manipulation of the modulation instability is of primary importance in
controlling and manipulating the character of spatial patterns initiated by
that instability. We show that the spatio-temporal periodic modulation of the
potential of the spatially extended system results in a modification of its
pattern forming instability. Depending on the modulation character the
instability can be partially suppressed, can change its spectrum (for instance
the long wave instability can transform into short wave instability), can split
into two, or can be completely eliminated. The latter result is of especial
practical interest, as can be used to stabilize the intrinsically unstable
system. The result bears general character, as it is shown here on a universal
model of Complex Ginzburg-Landau equations in one and two spatial dimension
(and time). The physical mechanism of instability suppression can be applied to
a variety of intrinsically unstable dissipative systems, like self-focusing
lasers, reaction-diffusion systems...

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## Global stability and control of swirling jets and flames

Fonte: University of Cambridge; Department of Engineering
Publicador: University of Cambridge; Department of Engineering

Tipo: Thesis; doctoral; PhD

EN

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#hydrodynamic instability#vortex breakdown#swirling flows#diffusion flames#passive control#adjoint#structural sensitivity

Large-scale unsteady flow structures play an influential role in the dynamics of many practical flows, such as those found in gas turbine combustion chambers. This thesis is concerned primarily with large-scale unsteady structures that arise due to self-sustained hydrodynamic oscillations, also known as global hydrodynamic instability. Direct numerical simulation (DNS) of the Navier--Stokes equations in the low Mach number limit is used to obtain a steady base flow, and the most unstable direct and adjoint global modes. These are combined, using a structural sensitivity framework, to identify the region of the flow and the feedback mechanisms that are responsible for causing the global instability. Using a Lagrangian framework, the direct and adjoint global modes are also used to identify the regions of the flow where steady and unsteady control, such as a drag force or heat input, can suppress or promote the global instability. These tools are used to study a variety of reacting and non-reacting flows to build an understanding of the physical mechanisms that are responsible for global hydrodynamic instability in swirling diffusion flames. In a non-swirling lifted jet diffusion flame, two modes of global instability are found. The first mode is a high-frequency mode caused by the instability of the low-density jet shear layer in the premixing zone. The second mode is a low-frequency mode caused by an instability of the outer shear layer of the flame. Two types of swirling diffusion flames with vortex breakdown bubbles are considered. They show qualitatively similar behaviour to the lifted jet diffusion flames. The first type of flame is unstable to a low-frequency mode...

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