2,082 research outputs found
Convectively driven shear and decreased heat flux
We report on direct numerical simulations of two-dimensional, horizontally
periodic Rayleigh-B\'enard convection, focusing on its ability to drive
large-scale horizontal flow that is vertically sheared. For the Prandtl numbers
() between 1 and 10 simulated here, this large-scale shear can be induced
by raising the Rayleigh number () sufficiently, and we explore the
resulting convection for up to . When present in our simulations,
the sheared mean flow accounts for a large fraction of the total kinetic
energy, and this fraction tends towards unity as . The shear helps
disperse convective structures, and it reduces vertical heat flux; in parameter
regimes where one state with large-scale shear and one without are both stable,
the Nusselt number of the state with shear is smaller and grows more slowly
with . When the large-scale shear is present with , the
convection undergoes strong global oscillations on long timescales, and heat
transport occurs in bursts. Nusselt numbers, time-averaged over these bursts,
vary non-monotonically with for . When the shear is present with
, the flow does not burst, and convective heat transport is
sustained at all times. Nusselt numbers then grow roughly as powers of ,
but the growth rates are slower than any previously reported for
Rayleigh-B\'enard convection without large-scale shear. We find the Nusselt
numbers grow proportionally to when and to when
. Analogies with tokamak plasmas are described.Comment: 25 pages, 12 figures, 5 video
Destabilizing Taylor-Couette flow with suction
We consider the effect of radial fluid injection and suction on
Taylor-Couette flow. Injection at the outer cylinder and suction at the inner
cylinder generally results in a linearly unstable steady spiralling flow, even
for cylindrical shears that are linearly stable in the absence of a radial
flux. We study nonlinear aspects of the unstable motions with the energy
stability method. Our results, though specialized, may have implications for
drag reduction by suction, accretion in astrophysical disks, and perhaps even
in the flow in the earth's polar vortex.Comment: 34 pages, 9 figure
Temperature Variation and the Solar Oblateness
Dicke and Goldenberg's oblateness measurement may be explained by an equatorial temperature
excess of 30° K, smoothly distributed in optical depths ≤ 0 01 The resulting brightness variation
with solar latitude is concentrated close to the limb, and it is not possible, with data presently available, to distinguish such variation from true oblateness
Higher-order Continuum Approximation for Rarefied Gases
The Hilbert-Chapman-Enskog expansion of the kinetic equations in mean flight
times is believed to be asymptotic rather than convergent. It is therefore
inadvisable to use lower order results to simplify the current approximation as
is done in the traditional Chapman-Enskog procedure, since that is an iterative
method. By avoiding such recycling of lower order results, one obtains
macroscopic equations that are asymptotically equivalent to the ones found in
the Chapman-Enskog approach. The new equations contain higher order terms that
are discarded in the Chapman-Enskog method. These make a significant impact on
the results for such problems as ultrasound propagation. In this paper, it is
shown that these results turn out well with relatively little complication when
the expansions are carried to second order in the mean free time, for the
example of the relaxation or BGK model of kinetic theory.Comment: 20 pages, 2 figures, RevTeX 4 macro
Photofluid Instabilities of Hot Stellar Envelopes
Beginning from a relatively simple set of dynamical equations for a fluid
permeated by a radiative field strong enough to produce significant forces, we
find the structure of plane-parallel equilibria and study their stability to
small acoustic disturbances. In doing this, we neglect viscous effects and
complications of nongreyness. We find that acoutic instabilities occur over a
wide range of conditions below the Eddington limit. This result is in line with
findings reported twenty years ago but it contradicts some more recent reports
of the absence of instabilities. We briefly attempt to identify the causes of
the discrepancies and then close with a discussion of the possible
astrophysical interest of such instabilities.Comment: 10 pages, LaTeX, 5 postscript figures, to be published in Physics
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