149 research outputs found
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
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
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
Influence of a low frequency vibration on a long-wave Marangoni instability in a binary mixture with the Soret effect
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
Avoiding, diagnosing and treating well leg compartment syndrome after pelvic surgery
Background
Patients undergoing prolonged pelvic surgery may develop compartment syndrome of one or both lower limbs in the absence of direct trauma or pre‐existing vascular disease (well leg compartment syndrome). This condition may have devastating consequences for postoperative recovery, including loss of life or limb, and irreversible disability.
Methods
These guidelines represent the collaboration of a multidisciplinary group of colorectal, vascular and orthopaedic surgeons, acting on behalf of their specialty associations in the UK and Ireland. A systematic analysis of the available peer‐reviewed literature was undertaken to provide an evidence base from which these guidelines were developed.
Results
These guidelines encompass the risk factors (both patient‐ and procedure‐related), diagnosis and management of the condition. Key recommendations for the adoption of perioperative strategies to facilitate prevention and effective treatment of well leg compartment syndrome are presented.
Conclusion
All surgeons who carry out abdominopelvic surgical procedures should be aware of well leg compartment syndrome, and instigate policies within their own institution to reduce the risk of this potentially life‐changing complication
Convection in Binary Fluid Mixtures. II. Localized Traveling Waves. (Physical Review E, in press)
Nonlinear, spatially localized structures of traveling convection rolls are
investigated in quantitative detail as a function of Rayleigh number for two
different Soret coupling strengths (separation ratios) with Lewis and Prandtl
numbers characterizing ethanol-water mixtures. A finite-difference method was
used to solve the full hydrodynamic field equations numerically. Structure and
dynamics of these localized traveling waves (LTW) are dominated by the
concentration field. Like in the spatially extended convective states ( cf.
accompanying paper), the Soret-induced concentration variations strongly
influence, via density changes, the buoyancy forces that drive convection. The
spatio-temporal properties of this feed-back mechanism, involving boundary
layers and concentration plumes, show that LTW's are strongly nonlinear states.
Light intensity distributions are determined that can be observed in side-view
shadowgraphs. Detailed analyses of all fields are made using colour-coded
isoplots, among others. In the frame comoving with their drift velocity, LTW's
display a nontrivial spatio-temporal symmetry consisting of time-translation by
half an oscillation period combined with vertical reflection through the
horizontal midplane of the layer. A time-averaged concentration current is
driven by a phase difference between the waves of concentration and vertical
velocity in the bulk of the LTW state. The associated large-scale concentration
redistribution stabilizes the LTW and controls its drift velocity into the
quiescent fluid by generating a buoyancy-reducing concentration "barrier" ahead
of the leading LTW front. The selection of the width of the LTW's is
investigated and comparisons with experiments are presented.Comment: 18 pages and 6 figures as uuencoded Postscript file (using uufiles) 1
color figure as uuencoded Postscript file, a high resolution version of the
color figure (about 10MB) can be requested from [email protected] or
[email protected].: (Barten)present address: PSI, CH-5232 Villigen
PSI, Switzerlan
STRUCTURAL, THERMOPHYSICAL AND MECHANICAL CHARACTERISTICS OF METAL FELT WICKS OF MODERN HEAT PIPES
The article reflects the results of many years’ researches, begun in 1972, of structural, hydrodynamic, thermophysical and mechanical properties of metal felt capillary structures (MFCSs) of heat pipes (HPs). These structures were made from the discrete sintered fibres with length 0.002-0.01 m by a diameter from 20 μm to 70 μm of a copper, stainless steel, or nickel. They were intended for HPs with shells of a copper, stainless steel, nickel, titan, nickel silver, ceramics and heat carriers: nitrogen, ammonia, water, ethanol, methanol, acetone, sodium and others. HPs worked in the range of temperatures from a cryogenic level (- 190 оС) to high temperatures (+ 800 оС). Analytical and experimental researches are executed by authors from the National Technical University of Ukraine “Kyiv Polytechnic Institute” (Kyiv, Ukraine) under the leadership of Dr., Prof. Semena M. On the basis of the conducted researches the functional dependences are obtained for implementation into engineering calculations of heat transfer characteristics of HPs. The proposed methods of the rational HP design with the use of MFCSs and elaborated technology of their fabrication allowed to create and implement into practice a variety of heat pipes and systems on their basis for different fields of applicatio
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