224 research outputs found
Effects of friction and heat conduction on sound propagation in ducts
The theory of sound propagation is examined in a viscous, heat-conducting fluid, initially at rest and in a uniform state, and contained in a rigid, impermeable duct with isothermal walls. Topics covered include: (1) theoretical formulation of the small amplitude fluctuating motions of a viscous, heat-conducting and compressible fluid; (2) sound propagation in a two dimensional duct; and (3) perturbation study of the inplane modes
Inflow/outflow boundary conditions and global dynamics of spatial mixing layers
The numerical simulation of incompressible spatially-developing shear flows poses a special challenge to computational fluid dynamicists. The Navier-Stokes equations are elliptic and boundary equations need to be specified at the inflow and outflow boundaries in order to compute the fluid properties within the region of interest. It is, however, difficult to choose inflow and outflow conditions corresponding to a given experimental situation. Furthermore the effects that changes in the boundary conditions or in the size of the computational domain may induce on the global dynamics of the flow are presently unknown. These issues are examined in light of recent developments in hydrodynamic stability theory. The particular flow considered is the spatial mixing layer but it was expected that similar phenomena were bound to occur in other cases such as channel flow, the boundary layer, etc. A short summary of local/global and absolute/convective instability concepts is given. The results of numerical simulations are presented which strongly suggest that global resonances may be triggered in domains of finite streamwise extent although the evolution of the perturbation vorticity field is everywhere locally convective. A relationship between finite domains and pressure sources which might help in devising a scheme to eliminate these difficulties is discussed
Liquid droplet generation
A pre-prototype segment of a droplet sheet generator for a liquid droplet radiator was designed, constructed and tested. The ability to achieve a uniform, non-diverging droplet sheet is limited by manufacturing tolerances on nozzle parallelism. For an array of 100, 100 micrometer diameters nozzles spaced 5 stream diameters apart, typical standard deviations in stream alignment were plus or minus 10 mrad. The drop to drop fractional speed variations of the drops in typical streams were similar and independent of position in the array. The absolute value of the speed dispersion depended on the amplitude of the disturbance applied to the stream. A second generation preliminary design of a 5200 stream segment of a droplet sheet generator was completed. The design is based on information developed during testing of the pre-prototype segment, along with the results of an acoustical analysis for the stagnation cavity pressure fluctuations used to break-up the streams into droplets
Stability of clay particle-coated microbubbles in alkanes against dissolution induced by heating
We investigated the dissolution and morphological dynamics of air bubbles in alkanes stabilized by fluorinated colloidal clay particles when subjected to temperature changes. A quasi-steady model for bubble dissolution with time-dependent temperature reveals that increasing the temperature enhances the bubble dissolution rate in alkanes, opposite to the behavior in water, due to the differing trends in gas solubility. Experimental results for uncoated air bubbles in decane and hexadecane confirm this prediction. Clay-coated bubbles in decane and hexadecane are shown to be stable in air-saturated oil at constant temperature, where dissolution is driven mainly by the Laplace pressure. When the temperature increases from ambient, the particle-coated bubbles are prone to dissolution as the oil phase becomes under-saturated. The interfacial layer of particles is observed to undergo buckling and crumpling, without shedding of clay particles. Increasing the concentration of particles is shown to enhance the bubble stability by providing a higher resistance to dissolution and buckling. When subjected to complex temperature cycles, the clay-coated bubbles can remain stable in conditions for which uncoated bubbles dissolve completely. These results underpin the design of ultra-stable oil foams stabilized by solid particles with improved shelf life under changing environmental conditions
Nonlinear global modes in hot jets
International audienceSince the experiments of Monkewitz et al. (J. Fluid Mech. vol. 213, 1990, p. 611), sufficiently hot circular jets have been known to give rise to self-sustained synchronized oscillations induced by a locally absolutely unstable region. In the present investigation, numerical simulations are carried out in order to determine if such synchronized states correspond to a nonlinear global mode of the underlying base flow, as predicted in the framework of Ginzburg - Landau model equations. Two configurations of slowly developing base flows are considered. In the presence of a pocket of absolute instability embedded within a convectively unstable jet, global oscillations are shown to be generated by a steep nonlinear front located at the upstream station of marginal absolute instability. The global frequency is given, within 10% accuracy, by the absolute frequency at the front location and, as expected on theoretical grounds, the front displays the same slope as a k--wave. For jet flows displaying absolutely unstable inlet conditions, global instability is observed to arise if the streamwise extent of the absolutely unstable region is sufficiently large: While local absolute instability sets in for ambient-to-jet temperature ratios S = 0.453, global modes only appear for S = 0.3125. In agreement with theoretical predictions, the selected frequency near the onset of global instability coincides with the absolute frequency at the inlet. For lower S, it gradually departs from this value. © 2006 Cambridge University Press
A numerical study of bifurcations in a barotropic shear flow
In the last few years, more and more evidence has emerged suggesting that transition to turbulence may be viewed as a succession of bifurcations to deterministic chaos. Most experimental and numerical observations have been restricted to Rayleigh-Benard convection and Taylor-Couette flow between concentric cylinders. An attempt is made to accurately describe the bifurcation sequence leading to chaos in a 2-D temporal free shear layer on the beta-plane. The beta-plane is a locally Cartesian reduction of the equations describing the dynamicss of a shallow layer of fluid on a rotating spherical planet. It is a valid model for large scale flows of interest in meteorology and oceanography
Determining the Spectral Signature of Spatial Coherent Structures
We applied to an open flow a proper orthogonal decomposition (pod) technique,
on 2D snapshots of the instantaneous velocity field, to reveal the spatial
coherent structures responsible of the self-sustained oscillations observed in
the spectral distribution of time series. We applied the technique to 2D planes
out of 3D direct numerical simulations on an open cavity flow. The process can
easily be implemented on usual personal computers, and might bring deep
insights on the relation between spatial events and temporal signature in (both
numerical or experimental) open flows.Comment: 4 page
Quantitative analysis of the dripping and jetting regimes in co-flowing capillary jets
We study a liquid jet that breaks up into drops in an external co-flowing
liquid inside a confining microfluidic geometry. The jet breakup can occur
right after the nozzle in a phenomenon named dripping or through the generation
of a liquid jet that breaks up a long distance from the nozzle, which is called
jetting. Traditionally, these two regimes have been considered to reflect the
existence of two kinds of spatiotemporal instabilities of a fluid jet, the
dripping regime corresponding to an absolutely unstable jet and the jetting
regime to a convectively unstable jet. Here, we present quantitative
measurements of the dripping and jetting regimes, both in an unforced and a
forced state, and compare these measurements with recent theoretical studies of
spatiotemporal instability of a confined liquid jet in a co-flowing liquid. In
the unforced state, the frequency of oscillation and breakup of the liquid jet
is measured and compared to the theoretical predictions. The dominant frequency
of the jet oscillations as a function of the inner flow rate agrees
qualitatively with the theoretical predictions in the jetting regime but not in
the dripping regime. In the forced state, achieved with periodic laser heating,
the dripping regime is found to be insensitive to the perturbation and the
frequency of drop formation remains unaltered. The jetting regime, on the
contrary, amplifies the externally imposed frequency, which translates in the
formation of drops at the frequency imposed by the external forcing. In
conclusion, the dripping and jetting regimes are found to exhibit the main
features of absolutely and convectively unstable flows respectively, but the
frequency selection in the dripping regime is not ruled by the absolute
frequency predicted by the stability analysis.Comment: 10 pages, 12 figures, to appear in Physics of Fluid
Noise sensitivity of sub- and supercritically bifurcating patterns with group velocities close to the convective-absolute instability
The influence of small additive noise on structure formation near a forwards
and near an inverted bifurcation as described by a cubic and quintic Ginzburg
Landau amplitude equation, respectively, is studied numerically for group
velocities in the vicinity of the convective-absolute instability where the
deterministic front dynamics would empty the system.Comment: 16 pages, 7 Postscript figure
A shallow-water theory of river bedforms in supercritical conditions
A supercritical free-surface turbulent stream flowing over an erodible bottom can generate a characteristic pattern of upstream migrating bedforms known as antidunes. This morphological instability, which is quite common in fluvial environments, has attracted speculative and applicative interests, and has always been modelled in 2D or 3D mathematical frameworks. However, in this work we demonstrate that antidune instability can be described by means of a suitable one-dimensional model that couples the Dressler equations to a mechanistic model of the sediment particle deposition/entrainment. The results of the linear stability analysis match the experimental data very well, both for the instability region and the dominant wavelength. The analytical tractability of the 1D modeling allows us (1) to elucidate the key physical processes which drive antidune instability, (2) to show the secondary role played by sediment inertia, (3) to obtain the dispersion relation in explicit form, and (4) to demonstrate the absolute nature of antidune instabilit
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