169 research outputs found
Second All-Union Seminar on Hydromechanics and Heat and Mass Exchange in Weightlessness, summaries of reports
Abstracts of reports are given which were presented at the Second All Union Seminar on Hydromechanics and Heat-Mass Transfer in Weightlessness. Topics include: (1) features of crystallization of semiconductor materials under conditions of microacceleration; (2) experimental results of crystallization of solid solutions of CDTE-HGTE under conditions of weightlessness; (3) impurities in crystals cultivated under conditions of weightlessness; and (4) a numerical investigation of the distribution of impurities during guided crystallization of a melt
Long-Wave Instability of Advective Flows in Inclined Layer with Solid Heat Conductive Boundaries
We investigate the stability of the steady convective flow in a plane tilted
layer with ideal thermal conductivity of solid boundaries in the presence of
uniform longitudinal temperature gradient. Analytically found the stability
boundary with respect to the long-wave perturbations, find the critical Grashof
number for the most dangerous among them of even spiral perturbation.Comment: in Russian, 18 pages, 5 figures, submited to Appl. mechanics and
physics, RAS Siberian brunch, Novosibirsk, Russia; Key words: advective flow,
oblique layer, a longitudinal temperature gradient, long-wave instabilit
Buoyant-thermocapillary instabilities of differentially heated liquid layers
URL: http://www-spht.cea.fr/articles/T95/103 Instabilités d'écoulements thermocapillaires en présence de gravitéInternational audienceThe stability of buoyant-thermocapillary-driven flows in a fluid layer subjected to a horizontal temperature gradient is analysed. Our purpose is the modelization of recent experimental results obtained for a fluid of Prandtl number Pr=7, by Daviaud and Vince [Phys. Rev. E, 4432 (1993)] who observed a transition between traveling waves and stationary rolls when the height of fluid is increased. Our model takes into account several effects which were examined separately in previous studies. The relative importance of buoyancy and thermocapillarity is controlled by the ratio W of Marangoni number to Rayleigh number. The fluid layer is bounded below by a rigid plane whose temperature varies linearly along the direction of the thermal gradient. A Biot number is introduced to describe heat transfer at the top free surface. Our stability analysis establishes the existence of a transition between stationary and oscillatory modes when W exceeds a value which is function of the Biot number. Moreover, two types of oscillatory modes have been identified which differ by the range of variation of their critical parameters (wave number, frequency, angle of propagation) versus W
Convection in nanofluids with a particle-concentration-dependent thermal conductivity
Thermal convection in nanofluids is investigated by means of a continuum
model for binary-fluid mixtures, with a thermal conductivity depending on the
local concentration of colloidal particles. The applied temperature difference
between the upper and the lower boundary leads via the Soret effect to a
variation of the colloid concentration and therefore to a spatially varying
heat conductivity. An increasing difference between the heat conductivity of
the mixture near the colder and the warmer boundary results in a shift of the
onset of convection to higher values of the Rayleigh number for positive values
of the separation ratio psi>0 and to smaller values in the range psi<0. Beyond
some critical difference of the thermal conductivity between the two
boundaries, we find an oscillatory onset of convection not only for psi<0, but
also within a finite range of psi>0. This range can be extended by increasing
the difference in the thermal conductivity and it is bounded by two
codimension-2 bifurcations.Comment: 13 pages, 11 figures; submitted to Physical Review
Particle entrapment as a feedback effect
We consider a suspension of polarizable particles under the action of
traveling wave dielectrophoresis (DEP) and focus on particle induced effects.
In a situation where the particles are driven by the DEP force, but no external
forces are exerted on the fluid, the joint motion of the particles can induce a
steady fluid flow, which leads to particle entrapment. This feedback effect is
proven to be non-negligible even for small volume concentration of particles.Comment: 4 pages, 4 figures, submitte
Capture of particles of dust by convective flow
Interaction of particles of dust with vortex convective flows is under
theoretical consideration. It is assumed that the volume fraction of solid
phase is small, variations of density due to nonuniform distribution of
particles and those caused by temperature nonisothermality of medium are
comparable. Equations for the description of thermal buoyancy convection of a
dusty medium are developed in the framework of the generalized Boussinesq
approximation taking into account finite velocity of particle sedimentation.
The capture of a cloud of dust particles by a vortex convective flow is
considered, general criterion for the formation of such a cloud is obtained.
The peculiarities of a steady state in the form of a dust cloud and backward
influence of the solid phase on the carrier flow are studied in detail for a
vertical layer heated from the sidewalls. It is shown that in the case, when
this backward influence is essential, a hysteresis behavior is possible. The
stability analysis of the steady state is performed. It turns out that there is
a narrow range of governing parameters, in which such a steady state is stable.Comment: 14 pages, 10 figures, published in Physics of Fluid
Instability of small-amplitude convective flows in a rotating layer with stress-free boundaries
We consider stability of steady convective flows in a horizontal layer with
stress-free boundaries, heated below and rotating about the vertical axis, in
the Boussinesq approximation (the Rayleigh-Benard convection). The flows under
consideration are convective rolls or square cells, the latter being
asymptotically equal to the sum of two orthogonal rolls of the same wave number
k. We assume, that the Rayleigh number R is close to the critical one, R_c(k),
for the onset of convective flows of this wave number: R=R_c(k)+epsilon^2; the
amplitude of the flows is of the order of epsilon. We show that the flows are
always unstable to perturbations, which are a sum of a large-scale mode not
involving small scales, and two large-scale modes, modulated by the original
rolls rotated by equal small angles in the opposite directions. The maximal
growth rate of the instability is of the order of max(epsilon^{8/5},(k-k_c)^2),
where k_c is the critical wave number for the onset of convection.Comment: Latex, 12 pp., 15 refs. An improved version of the manuscript
submitted to "Mechanics of fluid and gas", 2006 (in Russian; English
translation "Fluid Dynamics"
Analysis of vibration impact on stability of dewetting thin liquid film
Dynamics of a thin dewetting liquid film on a vertically oscillating
substrate is considered. We assume moderate vibration frequency and large
(compared to the mean film thickness) vibration amplitude. Using the
lubrication approximation and the averaging method, we formulate the coupled
sets of equations governing the pulsatile and the averaged fluid flows in the
film, and then derive the nonlinear amplitude equation for the averaged film
thickness. We show that there exists a window in the frequency-amplitude domain
where the parametric and shear-flow instabilities of the pulsatile flow do not
emerge. As a consequence, in this window the averaged description is reasonable
and the amplitude equation holds. The linear and nonlinear analyses of the
amplitude equation and the numerical computations show that such vibration
stabilizes the film against dewetting and rupture.Comment: 19 pages, 11 figure
- …