5,997 research outputs found
The effect of small streamwise velocity distortion on the boundary layer flow over a thin flat plate with application to boundary layer stability theory
Researchers show how an initially linear spanwise disturbance in the free stream velocity field is amplified by leading edge bluntness effects and ultimately leads to a small amplitude but linear spanwise motion far downstream from the edge. This spanwise motion is imposed on the boundary layer flow and ultimately causes an order-one change in its profile shape. The modified profiles are highly unstable and can support Tollmein-Schlichting wave growth well upstream of the theoretical lower branch of the neutral stability curve for a Blasius boundary layer
Dependence of the open-closed field line boundary in Saturn's ionosphere on both the IMF and solar wind dynamic pressure:comparison with the UV auroral oval observed by the HST
We model the open magnetic field region in Saturn's southern polar ionosphere during two compression regions observed by the Cassini spacecraft upstream of Saturn in January 2004, and compare these with the auroral ovals observed simultaneously in ultraviolet images obtained by the Hubble Space Telescope. The modelling employs the paraboloid model of Saturn's magnetospheric magnetic field, whose parameters are varied according to the observed values of both the solar wind dynamic pressure and the interplanetary magnetic field (IMF) vector. It is shown that the open field area responds strongly to the IMF vector for both expanded and compressed magnetic models, corresponding to low and high dynamic pressure, respectively. It is also shown that the computed open field region agrees with the poleward boundary of the auroras as well as or better than those derived previously from a model in which only the variation of the IMF vector was taken into account. The results again support the hypothesis that the auroral oval at Saturn is associated with the open-closed field line boundary and hence with the solar wind interaction
Search of X-ray emission from roAp stars: The case of gamma Equulei
The detection of X-ray emission from Ap stars can be an indicator for the
presence of magnetic activity and dynamo action, provided different origins for
the emission, such as wind shocks and close late-type companions, can be
excluded. Here we report on results for gamma Equu, the only roAp star for
which an X-ray detection is reported in ROSAT catalogs. We use high resolution
imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that
allow us to spatially resolve companions down to ~1" and ~0.06" separations,
respectively. The bulk of the X-ray emission is associated with a companion of
gamma Equu identified in our NACO image. Assuming coevality with the primary
roAp star (~900 Myr), the available photometry for the companion points at a
K-type star with ~0.6 M_sun. Its X-ray properties are in agreement with the
predictions for its age and mass. An excess of photons with respect to the
expected background and contribution from the nearby companion is observed near
the optical position of gamma Equu. We estimate an X-ray luminosity of log L_x
[erg/s] = 26.6 and log(L_x/L_bol) = -7.9 for this emission. A small offset
between the optical and the X-ray image leaves some doubt on its association
with the roAp star. The faint X-ray emission that we tentatively ascribe to the
roAp star is difficult to explain as a solar-like stellar corona due to its
very low L_x/L_bol level and the very long rotation period of gamma Equu. It
could be produced in magnetically confined wind shocks implying a mass loss
rate of ~10^(-14) M_sun/yr or from an additional unknown late-type companion at
separation ~0.4". If confirmed by future deeper X-ray observations this
emission could point at the origin for the presence of radioactive elements on
some roAp stars.Comment: Accepted for publication in Astronomy & Astrophysics (5 pages
Model of two-fluid reconnection
A theoretical model of quasi-stationary, two-dimensional magnetic
reconnection is presented in the framework of incompressible two-fluid
magnetohydrodynamics (MHD). The results are compared with recent numerical
simulations and experiment.Comment: 4 pages, 1 figure, accepted to Physical Review Letter
Diffusion of passive scalar in a finite-scale random flow
We consider a solvable model of the decay of scalar variance in a
single-scale random velocity field. We show that if there is a separation
between the flow scale k_flow^{-1} and the box size k_box^{-1}, the decay rate
lambda ~ (k_box/k_flow)^2 is determined by the turbulent diffusion of the
box-scale mode. Exponential decay at the rate lambda is preceded by a transient
powerlike decay (the total scalar variance ~ t^{-5/2} if the Corrsin invariant
is zero, t^{-3/2} otherwise) that lasts a time t~1/\lambda. Spectra are sharply
peaked at k=k_box. The box-scale peak acts as a slowly decaying source to a
secondary peak at the flow scale. The variance spectrum at scales intermediate
between the two peaks (k_box0). The mixing
of the flow-scale modes by the random flow produces, for the case of large
Peclet number, a k^{-1+delta} spectrum at k>>k_flow, where delta ~ lambda is a
small correction. Our solution thus elucidates the spectral make up of the
``strange mode,'' combining small-scale structure and a decay law set by the
largest scales.Comment: revtex4, 8 pages, 4 figures; final published versio
Turbulent transport in tokamak plasmas with rotational shear
Nonlinear gyrokinetic simulations have been conducted to investigate
turbulent transport in tokamak plasmas with rotational shear. At sufficiently
large flow shears, linear instabilities are suppressed, but transiently growing
modes drive subcritical turbulence whose amplitude increases with flow shear.
This leads to a local minimum in the heat flux, indicating an optimal E x B
shear value for plasma confinement. Local maxima in the momentum fluxes are
also observed, allowing for the possibility of bifurcations in the E x B shear.
The sensitive dependence of heat flux on temperature gradient is relaxed for
large flow shear values, with the critical temperature gradient increasing at
lower flow shear values. The turbulent Prandtl number is found to be largely
independent of temperature and flow gradients, with a value close to unity.Comment: 4 pages, 5 figures, submitted to PR
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