728 research outputs found
Stick-slip instability for viscous fingering in a gel
The growth dynamics of an air finger injected in a visco-elastic gel (a
PVA/borax aqueous solution) is studied in a linear Hele-Shaw cell. Besides the
standard Saffmann-Taylor instability, we observe - with increasing finger
velocities - the existence of two new regimes: (a) a stick-slip regime for
which the finger tip velocity oscillates between 2 different values, producing
local pinching of the finger at regular intervals, (b) a ``tadpole'' regime
where a fracture-type propagation is observed. A scaling argument is proposed
to interpret the dependence of the stick-slip frequency with the measured
rheological properties of the gel.Comment: 7 pages, 4 figures. Submitted to Europhysics Letter
Viscosity in spherically symmetric accretion
The influence of viscosity on the flow behaviour in spherically symmetric
accretion, has been studied here. The governing equation chosen has been the
Navier-Stokes equation. It has been found that at least for the transonic
solution, viscosity acts as a mechanism that detracts from the effectiveness of
gravity. This has been conjectured to set up a limiting scale of length for
gravity to bring about accretion, and the physical interpretation of such a
length-scale has been compared with the conventional understanding of the
so-called "accretion radius" for spherically symmetric accretion. For a
perturbative presence of viscosity, it has also been pointed out that the
critical points for inflows and outflows are not identical, which is a
consequence of the fact that under the Navier-Stokes prescription, there is a
breakdown of the invariance of the stationary inflow and outflow solutions --
an invariance that holds good under inviscid conditions. For inflows, the
critical point gets shifted deeper within the gravitational potential well.
Finally, a linear stability analysis of the stationary inflow solutions, under
the influence of a perturbation that is in the nature of a standing wave, has
indicated that the presence of viscosity induces greater stability in the
system, than has been seen for the case of inviscid spherically symmetric
inflows.Comment: 7 pages. Minor changes made in the version published in MNRA
Perturbations on steady spherical accretion in Schwarzschild geometry
The stationary background flow in the spherically symmetric infall of a
compressible fluid, coupled to the space-time defined by the static
Schwarzschild metric, has been subjected to linearized perturbations. The
perturbative procedure is based on the continuity condition and it shows that
the coupling of the flow with the geometry of space-time brings about greater
stability for the flow, to the extent that the amplitude of the perturbation,
treated as a standing wave, decays in time, as opposed to the amplitude
remaining constant in the Newtonian limit. In qualitative terms this situation
simulates the effect of a dissipative mechanism in the classical Bondi
accretion flow, defined in the Newtonian construct of space and time. As a
result of this approach it becomes impossible to define an acoustic metric for
a conserved spherically symmetric flow, described within the framework of
Schwarzschild geometry. In keeping with this view, the perturbation, considered
separately as a high-frequency travelling wave, also has its amplitude reduced.Comment: 8 pages, no figur
The cJUN NH2-terminal kinase (JNK) signaling pathway promotes genome stability and prevents tumor initiation
Breast cancer is the most commonly diagnosed malignancy in women. Analysis of breast cancer genomic DNA indicates frequent loss-of-function mutations in components of the cJUN NH2-terminal kinase (JNK) signaling pathway. Since JNK signaling can promote cell proliferation by activating the AP1 transcription factor, this apparent association of reduced JNK signaling with tumor development was unexpected. We examined the effect of JNK deficiency in the murine breast epithelium. Loss of JNK signaling caused genomic instability and the development of breast cancer. Moreover, JNK deficiency caused widespread early neoplasia and rapid tumor formation in a murine model of breast cancer. This tumor suppressive function was not mediated by a role of JNK in the growth of established tumors, but by a requirement of JNK to prevent tumor initiation. Together, these data identify JNK pathway defects as \u27driver\u27 mutations that promote genome instability and tumor initiation
Global summary of baseline household survey results
This report summarises the results of a baseline household-level survey, designed by the CCAFS team and implemented in late 2010/early 2011 in 3 regions: East Africa, West Africa and South Asia. This survey was designed with the intent of developing simple, comparable cross-site household level indicators, for which changes can be evaluated over time, of food security, households assets, agricultural production diversity, agricultural sales diversity, changes being made in farming practices for adaptation, innovation, and/or to help reduce emissions or store
greenhouse gases (mitigation), and gender indicators (e.g. men’s versus women’s reception of weather-related information). A standardised survey, guidelines and training materials were implemented across 12 countries in West Africa (Senegal, Mali, Burkina Faso, Niger, and Ghana), East Africa (Kenya, Uganda, Ethiopia, and Tanzania), and South Asia (India, Nepal and Bangladesh). Thus we have a total of 15 core sites covering 108 villages and 2095 households. This summary report gives an overview of the farming practices, and asset, livelihoods and food security status of rural households in these sites. It describes what changes farmers have been making in recent years with respect to crop management, livestock practices, use of inputs and other agricultural practices. We explore what kinds of weather/climate and associated information these households are receiving, how and by whom. This information
provides important baseline information, as these households will be revisited in 5-10 years’ time in order to evaluate the changes in these indicators. This will give us important information as to if, how, and which households are adapting to a changing climate while improving their food security status
Secular instability in quasi-viscous disc accretion
A first-order correction in the -viscosity parameter of Shakura and
Sunyaev has been introduced in the standard inviscid and thin accretion disc. A
linearised time-dependent perturbative study of the stationary solutions of
this "quasi-viscous" disc leads to the development of a secular instability on
large spatial scales. This qualitative feature is equally manifest for two
different types of perturbative treatment -- a standing wave on subsonic
scales, as well as a radially propagating wave. Stability of the flow is
restored when viscosity disappears.Comment: 15 pages, 2 figures, AASTeX. Added some new material and upgraded the
reference lis
Implications of nonlinearity for spherically symmetric accretion
We subject the steady solutions of a spherically symmetric accretion flow to
a time-dependent radial perturbation. The equation of the perturbation includes
nonlinearity up to any arbitrary order, and bears a form that is very similar
to the metric equation of an analogue acoustic black hole. Casting the
perturbation as a standing wave on subsonic solutions, and maintaining
nonlinearity in it up to the second order, we get the time-dependence of the
perturbation in the form of a Li\'enard system. A dynamical systems analysis of
the Li\'enard system reveals a saddle point in real time, with the implication
that instabilities will develop in the accreting system when the perturbation
is extended into the nonlinear regime. The instability of initial subsonic
states also adversely affects the temporal evolution of the flow towards a
final and stable transonic state.Comment: 14 pages, ReVTeX. Substantially revised with respect to the previous
version. Three figures and a new section (Sec. VI) adde
Linearized perturbation on stationary inflow solutions in an inviscid and thin accretion disc
The influence of a linearized perturbation on stationary inflow solutions in
an inviscid and thin accretion disc, has been studied here, and it has been
argued, that a perturbative technique would indicate that all possible classes
of inflow solutions would be stable. The choice of the driving potential,
Newtonian or pseudo-Newtonian, would not particularly affect the arguments
which establish the stability of solutions. It has then been surmised that in
the matter of the selection of a particular solution, adoption of a
non-perturbative technique, based on a more physical criterion, as in the case
of the selection of the transonic solution in spherically symmetric accretion,
would give a more conclusive indication about the choice of a particular branch
of the flow.Comment: 7 pages. Includes an additional treatment on the perturbation as a
travelling wave. Minor alterations have been made in the version published in
MNRAS. Some typos in equations have been correcte
Realizability of stationary spherically symmetric transonic accretion
The spherically symmetric stationary transonic (Bondi) flow is considered a
classic example of an accretion flow. This flow, however, is along a
separatrix, which is usually not physically realizable. We demonstrate, using a
pedagogical example, that it is the dynamics which selects the transonic flow.Comment: 4 pages in REVTeX with 2 figures. Typos have been corrected and some
alterations have been made in the version published in Physical Review
- …