4,082 research outputs found
Double-diffusive instabilities of a shear-generated magnetic layer
Previous theoretical work has speculated about the existence of
double-diffusive magnetic buoyancy instabilities of a dynamically evolving
horizontal magnetic layer generated by the interaction of forced vertically
sheared velocity and a background vertical magnetic field. Here we confirm
numerically that if the ratio of the magnetic to thermal diffusivities is
sufficiently low then such instabilities can indeed exist, even for high
Richardson number shear flows. Magnetic buoyancy may therefore occur via this
mechanism for parameters that are likely to be relevant to the solar
tachocline, where regular magnetic buoyancy instabilities are unlikely.Comment: Submitted to ApJ
The Evolution of a Double Diffusive Magnetic Buoyancy Instability
Recently, Silvers, Vasil, Brummell, & Proctor (2009), using numerical
simulations, confirmed the existence of a double diffusive magnetic buoyancy
instability of a layer of horizontal magnetic field produced by the interaction
of a shear velocity field with a weak vertical field. Here, we demonstrate the
longer term nonlinear evolution of such an instability in the simulations. We
find that a quasi two-dimensional interchange instability rides (or "surfs") on
the growing shear-induced background downstream field gradients. The region of
activity expands since three-dimensional perturbations remain unstable in the
wake of this upward-moving activity front, and so the three-dimensional nature
becomes more noticeable with time.Comment: 9 pages; 3 figures; accepted to appear in IAU symposium 27
Dynamics of charged fluids and 1/L perturbation expansions
Some features of the calculation of fluid dynamo systems in
magnetohydrodynamics are studied. In the coupled set of the ordinary linear
differential equations for the spherically symmetric dynamos, the
problem represented by the presence of the mixed (Robin) boundary conditions is
addressed and a new treatment for it is proposed. The perturbation formalism of
large expansions is shown applicable and its main technical steps are
outlined.Comment: 16 p
Bone marrow examination in newly diagnosed Hodgkin's disease: current practice in the United Kingdom.
In the UK Hodgkin's disease is usually treated by either clinical oncologists or haematologists. A national study of the performance of bone marrow examination in newly diagnosed Hodgkin's disease was undertaken to establish current practice. A total of 620 questionnaires were despatched, and replies were received from 60% of consultants (45% of clinical oncologists and 70% of haematologists). Bone marrow examination was performed in all new cases significantly more often by haematologists than by clinical oncologists (74% vs 40%, P < 0.001). Among haematologists, there was no correlation between the number of new patients seen annually and practice, however clinical oncologists were even less likely to perform routine bone marrow biopsies if they saw more than ten patients per year (P < 0.02). Where bone marrow examination was performed selectively, the most common criteria used were peripheral blood cytopenia and advanced-stage disease. These criteria were applied in the same way by both clinical oncologists and haematologists. Bone marrow biopsy, an invasive and often painful procedure, is currently performed more frequently in Hodgkin's disease than can be recommended on the basis of recent studies in the literature and associated guidelines. There is a significant difference in practice between clinical oncologists and haematologists, and this raises the wider issue of the influence of hospital specialisation on patient management
Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
The turbulent magnetic diffusivity tensor is determined in the presence of
rotation or shear. The question is addressed whether dynamo action from the
shear-current effect can explain large-scale magnetic field generation found in
simulations with shear. For this purpose a set of evolution equations for the
response to imposed test fields is solved with turbulent and mean motions
calculated from the momentum and continuity equations. The corresponding
results for the electromotive force are used to calculate turbulent transport
coefficients. The diagonal components of the turbulent magnetic diffusivity
tensor are found to be very close together, but their values increase slightly
with increasing shear and decrease with increasing rotation rate. In the
presence of shear, the sign of the two off-diagonal components of the turbulent
magnetic diffusion tensor is the same and opposite to the sign of the shear.
This implies that dynamo action from the shear--current effect is impossible,
except perhaps for high magnetic Reynolds numbers. However, even though there
is no alpha effect on the average, the components of the alpha tensor display
Gaussian fluctuations around zero. These fluctuations are strong enough to
drive an incoherent alpha--shear dynamo. The incoherent shear--current effect,
on the other hand, is found to be subdominant.Comment: 12 pages, 13 figures, improved version, accepted by Ap
Numerical Simulation of the Aircraft Wake Vortex Flowfield
The near wake vortex flowfield from a NACA0012 half-wing was simulated using a fully unstructured Navier-Stokes flow solver in three dimensions at a chord Reynolds number of 4.6 million and a Mach number of approximately 0.15. Several simulations were performed to examine the effect of boundary conditions, mesh resolution and turbulence scheme on the formation of wingtip vortex and its downstream propagation. The standard Spalart-Allmaras turbulence model was compared with the Dacles-Mariani and Spalart-Shur corrections for rotation and curvature effects. The simulation results were evaluated using the data from experiment performed at NASA Ames' 32in x 48in low speed wind tunnel
FUM1—A Gene Required for Fumonisin Biosynthesis But Not for Maize Ear Rot and Ear Infection by Gibberella moniliformis in Field Tests
We have analyzed the role of fumonisins in infection of maize (Zea mays) by Gibberella moniliformis (anamorph Fusarium verticillioides) in field tests in Illinois and Iowa, United States. Fumonisin-nonproducing mutants were obtained by disrupting FUM1 (previouslyFUM5), the gene encoding a polyketide synthase required for fumonisin biosynthesis. Maize ear rot, ear infection, and fumonisin contamination were assessed by silk-channel injection in 1999 and 2000 and also by spray application onto maize silks, injection into maize stalks, and application with maize seeds at planting in 1999. Ear rot was evaluated by visual assessment of whole ears and by calculating percentage of symptomatic kernels by weight. Fumonisin levels in kernels were determined by high-performance liquid chromatography. The presence of applied strains in kernels was determined by analysis of recovered isolates for genetic markers and fumonisin production. Two independent fumonisin-nonproducing (fum1-3 and fum1-4) mutants were similar to their respective fumonisin-producing (FUM1-1) progenitor strains in ability to cause ear rot following silk-channel injection and also were similar in ability to infect maize ears following application by all four methods tested. This evidence confirms that fumonisins are not required for G. moniliformis to cause maize ear rot and ear infection
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