Jump to Content
View Cart
Top 20 Most Read Articles
January 2011
The 20 articles with the most full-text downloads during the month, in descending order.
 |
Simple models of turbulent flows Phys. Fluids 23, 011301 (2011); http://dx.doi.org/10.1063/1.3531744 (20 pages) Online Publication Date: 18 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Stochastic Lagrangian models provide a simple and direct way to model turbulent flows and the processes that occur within them. This paper provides an introduction to this approach, aimed at the nonspecialist, and providing some historical perspective. Basic models for the Lagrangian velocity (i.e., the Langevin equation) and composition are described and applied to the simple but revealing case of dispersion from a line source in grid turbulence. With simple extensions, these models are applied to inhomogeneous turbulent reactive flows, where they form the core of probability density function (PDF) methods. The use of PDF methods is illustrated for the case of a lifted turbulent jet flame. Lagrangian time series are now accessible both from experiments and from direct numerical simulations, and this information is used to scrutinize and improve stochastic Lagrangian models. In particular, we describe refinements to account for the observed strong Reynolds-number effects including intermittency. It is emphasized that all models of turbulence are necessarily approximate and incomplete, and that simple models are valuable in many applications in spite of their limitations.
|
|||
|
Show PACS
|
|||
|
|
Instability regimes in flowing suspensions of swimming micro-organisms Phys. Fluids 23, 011901 (2011); http://dx.doi.org/10.1063/1.3529411 (18 pages) Online Publication Date: 6 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The effects of an external shear flow on the dynamics and pattern formation in a dilute suspension of swimming micro-organisms are investigated using a linear stability analysis and three-dimensional numerical simulations, based on the kinetic model previously developed by [
D. Saintillan and M. J. Shelley, Phys. Fluids 20, 123304 (2008)
]. The external shear flow is found to damp the instabilities that occur in these suspensions by controlling the orientation of the particles. We demonstrate in our simulations that the rate of damping is direction-dependent: it is fastest in the flow direction, but slowest in the direction perpendicular to the shear plane. As a result, transitions from three- to two- to one-dimensional instabilities are observed to occur as shear rate increases, and above a certain shear rate the instabilities altogether disappear. The density patterns and complex flows that arise at long time in the suspensions are also analyzed from the numerical simulations using standard techniques from the literature on turbulent flows. The imposed shear flow is found to have an effect on both density patterns and flow structures, which typically align with the extensional axis of the external flow. The disturbance flows in the simulations are shown to exhibit similarities with turbulent flows, and in particular two of the seemingly universal characteristics of turbulent flows also occur, namely: (i) the bias of Q-R plots toward the second and fourth quadrants, corresponding to stable focus/stretching and unstable node/saddle/saddle flow topologies, respectively, and (ii) the alignment of the vorticity vector with the intermediate strain-rate eigenvector. However, the flows described herein also significantly differ from turbulent flows owing to the strong predominance of large scales, as exemplified by the very rapid decay of the kinetic energy spectrum, an effect further enhanced after the transitions to two- and one-dimensional instabilities.
|
|||
|
Show PACS
|
|||
|
|
Theoretical model for k−3 spectra in dispersed multiphase flows Phys. Fluids 23, 011701 (2011); http://dx.doi.org/10.1063/1.3530438 (4 pages) Online Publication Date: 3 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The spectrum of a signal consisting of a sum of localized random bursts can exhibit, under certain conditions, an intermediate subrange evolving as the power −3 of the wavenumber k. These bursts should have a smooth and regular pattern, their strength and size should be statistically independent, and their size should be uniformly distributed between two finite wavelengths. This is probably the explanation for the k−3 subrange that is commonly observed in the velocity spectra of bubble-induced turbulence, which results from the interaction of the localized velocity disturbances caused by the bubbles.
|
|||
|
Show PACS
|
|||
|
|
Flow past two rotating cylinders Phys. Fluids 23, 014102 (2011); http://dx.doi.org/10.1063/1.3528260 (14 pages) Online Publication Date: 3 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Flow past two uniformly rotating cylinders with the same rotation rates in a side-by-side configuration is studied experimentally. The experiments are carried out at Reynolds numbers, Re, of 100, 200, 300, 400, and 500 and nondimensional rotation rates, α, varying from 0 to 5. The spacing ratios, T/D, are 1.8, 2.5, 4.0, and 7.5. Two possibilities of rotations are considered with the cylinder surfaces in between the two cylinders moving upstream in one case (inward rotation case) and downstream in the other (outward rotation case). The diagnostics is done by flow visualization using hydrogen bubble technique and quantitative measurements using particle image velocimetry (PIV). We present, using extensive flow visualization, the global view of the wake structure at Re of 200 for various rotation rates, and two senses. Vortex shedding suppression is studied through flow visualization and/or PIV at various Re’s, T/D’s, and two senses. Vortex shedding is found to be suppressed in the inward rotation cases at all Re and T/D’s. The value of α corresponding to vortex shedding suppression, αs, in the inward rotation case is ∼ 2.0 for Re of 200–500 at all T/D’s. The value of αs for Re of 100 in the case of inward rotation shows an increasing trend with T/D from T/D = 1.8 to 4.0 with αs changing from 1.2 to 1.7; further increase of T/D does not change αs. For outward rotation cases, vortex shedding suppression is clearly observed for Re of 100 and for all values of T/D; however, for higher Re, vortex shedding suppression is observed for T/D of 4.0 and 7.5 only. The measurements of αs in this case showed a decreasing trend with increasing T/D. Symmetry breaking is reported for inward rotation case near α = 1.35 for T/D = 2.5 at Re of 200 where the wake pattern changes from in-phase to antiphase mode.
|
|||
|
Show PACS
|
|||
|
|
Hairpin vortex organization in wall turbulence Phys. Fluids 19, 041301 (2007); http://dx.doi.org/10.1063/1.2717527 (16 pages) Online Publication Date: 18 April 2007
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Coherent structures in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. Above the viscous wall layer, the hairpin vortex paradigm of Theodorsen coupled with the quasistreamwise vortex paradigm have gained considerable support from multidimensional visualization using particle image velocimetry and direct numerical simulation experiments. Hairpins can autogenerate to form packets that populate a significant fraction of the boundary layer, even at very high Reynolds numbers. The dynamics of packet formation and the ramifications of organization of coherent structures (hairpins or packets) into larger-scale structures are discussed. Evidence for a large-scale mechanism in the outer layer suggests that further organization of packets may occur on scales equal to and larger than the boundary layer thickness.
|
|||
|
Show PACS
|
|||
|
|
Propulsive force measurements and flow behavior of undulatory swimmers at low Reynolds number Phys. Fluids 22, 121901 (2010); http://dx.doi.org/10.1063/1.3529236 (9 pages) Online Publication Date: 22 December 2010
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The swimming behavior of the nematode Caenorhabditis elegans is investigated in aqueous solutions of increasing viscosity. Detailed flow dynamics associated with the nematode’s swimming motion as well as propulsive force and power are obtained using particle tracking and velocimetry methods. We find that C. elegans delivers propulsive thrusts on the order of a few nanonewtons. Such findings are supported by values obtained using resistive force theory; the ratio of normal to tangential drag coefficients is estimated to be approximately 1.4. Over the range of solutions investigated here, the flow properties remain largely independent of viscosity. Velocity magnitudes of the flow away from the nematode body decay rapidly within less than a body length and collapse onto a single master curve. Overall, our findings support that C. elegans is an attractive living model to study the coupling between small-scale propulsion and low Reynolds number hydrodynamics.
|
|||
|
Show PACS
|
|||
|
|
A wall-layer model for large-eddy simulations of turbulent flows with/out pressure gradient Phys. Fluids 23, 015101 (2011); http://dx.doi.org/10.1063/1.3529358 (12 pages) Online Publication Date: 6 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
In this work, modeling of the near-wall region in turbulent flows is addressed. A new wall-layer model is proposed with the goal to perform high-Reynolds number large-eddy simulations of wall bounded flows in the presence of a streamwise pressure gradient. The model applies both in the viscous sublayer and in the inertial region, without any parameter to switch from one region to the other. An analytical expression for the velocity field as a function of the distance from the wall is derived from the simplified thin-boundary equations and by using a turbulent eddy coefficient with a damping function. This damping function relies on a modified van Driest formula to define the mixing-length taking into account the presence of a streamwise pressure gradient. The model is first validated by a priori comparisons with direct numerical simulation data of various flows with and without streamwise pressure gradient and with eventual flow separation. Large-eddy simulations are then performed using the present wall model as wall boundary condition. A plane channel flow and the flow over a periodic arrangement of hills are successively considered. The present model predictions are compared with those obtained using the wall models previously proposed by
Spalding, Trans. ASME, J. Appl. Mech 28, 243 (2008)
and
Manhart et al., Theor. Comput. Fluid Dyn. 22, 243 (2008)
. It is shown that the new wall model allows for a good prediction of the mean velocity profile both with and without streamwise pressure gradient. It is shown than, conversely to the previous models, the present model is able to predict flow separation even when a very coarse grid is used.
|
|||
|
Show PACS
|
|||
|
|
Experimental study of freely falling thin disks: Transition from planar zigzag to spiral Phys. Fluids 23, 011702 (2011); http://dx.doi.org/10.1063/1.3541844 (4 pages) Online Publication Date: 24 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Using a stereoscopic vision method, we have experimentally investigated the time evolution of a free thin disk motion with six degrees of freedom for the first time. It is found that, as the dimensionless moment of inertia I∗ decreases, the trajectory of the disk transits from planar to nonplanar. New types of free falling motions were identified for small I∗ values, including the spiral state and the transitional state. An extended Re−I∗ phase diagram corresponding to different flow regimes was given. The underlying physics associated with the transition is found to be connected to the interactions between the moving object and induced vortices.
|
|||
|
Show PACS
|
|||
|
|
Slip length for longitudinal shear flow over a dilute periodic mattress of protruding bubbles Phys. Fluids 22, 121703 (2010); http://dx.doi.org/10.1063/1.3531683 (4 pages) Online Publication Date: 30 December 2010
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
An analytical formula for the frictional slip length associated with transverse shear flow over a bubble mattress comprising a dilute periodic array of parallel circular-arc grooves protruding into the fluid has recently been presented by
Davis and Lauga [Phys. Fluids 21, 011701 (2009)]
. This letter derives an analytical formula for the slip length associated with longitudinal shear flow over the same surface. The formula is in excellent agreement with a phenomenological result based on finite element simulations given by
Teo and Khoo [Microfluid. Nanofluid. 9, 499 (2010)]
.
|
|||
|
Show PACS
|
|||
|
|
Phys. Fluids 23, 012001 (2011); http://dx.doi.org/10.1063/1.3537833 (8 pages) Online Publication Date: 11 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Previous studies dedicated to modeling drag reduction and stability of the air-water interface on superhydrophobic surfaces were conducted for microfabricated coatings produced by placing hydrophobic microposts/microridges arranged on a flat surface in aligned or staggered configurations. In this paper, we model the performance of superhydrophobic surfaces comprised of randomly distributed roughness (e.g., particles or microposts) that resembles natural superhydrophobic surfaces, or those produced via random deposition of hydrophobic particles. Such fabrication method is far less expensive than microfabrication, making the technology more practical for large submerged bodies such as submarines and ships. The present numerical simulations are aimed at improving our understanding of the drag reduction effect and the stability of the air-water interface in terms of the microstructure parameters. For comparison and validation, we have also simulated the flow over superhydrophobic surfaces made up of aligned or staggered microposts for channel flows as well as streamwise or spanwise ridges configurations for pipe flows. The present results are compared with theoretical and experimental studies reported in the literature. In particular, our simulation results are compared with work of Sbragaglia and Prosperetti, and good agreement has been observed for gas fractions up to about 0.9. The numerical simulations indicate that the random distribution of surface roughness has a favorable effect on drag reduction, as long as the gas fraction is kept the same. This effect peaks at about 30% as the gas fraction increases to 0.98. The stability of the meniscus, however, is strongly influenced by the average spacing between the roughness peaks, which needs to be carefully examined before a surface can be recommended for fabrication. It was found that at a given maximum allowable pressure, surfaces with random post distribution produce less drag reduction than those made up of staggered posts.
|
|||
|
Show PACS
|
|||
|
|
Visualizing the very-large-scale motions in turbulent pipe flow Phys. Fluids 23, 011703 (2011); http://dx.doi.org/10.1063/1.3533016 (4 pages) Online Publication Date: 26 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Time-resolved stereoscopic particle image velocimetry is used to investigate the structure of the very-large-scale motions (VLSMs) in fully developed turbulent pipe flow. The motions are visualized using snapshot proper orthogonal decomposition. It is shown that the structures can be reconstructed using a small number of the most energetic modes. The results strongly suggest a possible connection between the origin of the VLSM and linear stability analysis. The structures are seen to be highly three-dimensional, meandering azimuthally and radially. At this Reynolds number (ReD = 12 500), they occasionally extend from the near-wall region to the wake region of the pipe.
|
|||
|
Show PACS
|
|||
|
|
Phys. Fluids 23, 013602 (2011); http://dx.doi.org/10.1063/1.3537791 (12 pages) Online Publication Date: 11 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The velocity field of the trailing vortex behind a wing at different angles of attack has been measured through the stereo particle image velocimetry technique in a water tunnel for Reynolds numbers between 20 000 and 40 000, and for several distances to the wing tip. After filtering out the vortex meandering, the radial profiles of the axial and the azimuthal velocity components and of the radial profiles of the vorticity were compared to the theoretical models for trailing vortices by [
G. K. Batchelor, J. Fluid Mech. 20, 645 (1964)
] and by [
D. W. Moore and P. G. Saffman, Proc. R. Soc. London, Ser. A 333, 491 (1973)
], whose main features are conveniently summarized. We take into account the downstream evolution of these profiles from just a fraction of the wing chord to more than ten chords. The radial profiles of the vorticity and the azimuthal velocity are shown to fit quite well to Moore and Saffman’s trailing vortex model, while Batchelor’s model does not fit so well, especially in the tails of the profiles. At the downstream distances considered, the radial profiles of the axial velocity do not adjust so well to Moore and Saffman’s model as the azimuthal velocity profiles do, but the disagreement with Batchelor’s model is quite manifested, especially at the axis. Thus, the details of the flow structure are in better agreement with the predictions of Moore and Saffman’s model. The downstream evolution of several key features of the measured velocity profiles is also in agreement with the predictions of Moore and Saffman’s model, within the dispersion of the experimental data, but up to the largest axial distance considered in this work we cannot decide if they follow the asymptotic behavior predicted by this model.
|
|||
|
Show PACS
|
|||
|
|
Electrokinetic instability: The sharp interface limit Phys. Fluids 23, 014101 (2011); http://dx.doi.org/10.1063/1.3532950 (14 pages) Online Publication Date: 3 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
An instability between two miscible liquid regions of identical mechanical properties but different electrical conductivities stressed by an external electric field parallel to the interface is studied. The problem is of interest due to its applications to mixing in microchannels. It is shown that the problem can be modeled by considering a sharp interface and an appropriate jump condition for the electrical conductivity. The transport of the electrical conductivity is governed by a diffusive equation. An infinite domain case and a shallow channel case are considered. It is shown that any velocity perturbation at the interface leads to a varying electrical conductivity in its vicinity due to the electromechanical coupling in the jump condition for the electrical conductivity. This in turns leads to a bulk charge density that gives a body force in the fluid equations. The body force generates a cellular motion that results in the instability. The results compare favorably with the experimental data and the numerical analysis for the diffuse interface case by
Chen et al. [J. Fluid Mech. 524, 263 (2005)]
. The critical condition for the instability is given in terms of a nondimensional parameter PΣ, which is a product of the Péclet number and another nondimensional parameter that depends on the conductivity ratio of the two liquids.
|
|||
|
Show PACS
|
|||
|
|
Electromagnetically driven oscillatory shallow layer flow Phys. Fluids 23, 013601 (2011); http://dx.doi.org/10.1063/1.3531729 (10 pages) Online Publication Date: 6 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
We report experimental observations of the laminar flow in a thin horizontal layer of electrolyte, generated by a time-periodic Lorentz force produced by an alternate, unidirectional electric current and the field of a small permanent magnet. The force drives a periodically oscillating dipolar vortex which displays some spatial and temporal symmetries. The attention is focused on the motion of the oscillatory layer in vertical planes perpendicular to both the bottom wall and the injected current. For different frequencies of the injected current, velocity fields were obtained using particle image velocimetry in the zone of more intense magnetic field as well as close to the edges of the magnet where the inhomogeneity of the field is more pronounced. Velocity profiles as functions of the normal coordinate are determined in characteristic points at different phases and oscillation frequencies. Experimental results are compared with a simple analytical solution and a full three-dimensional numerical simulation that reproduces satisfactorily the experimental observations. Under the explored conditions and available experimental resolution, results indicate that except in the zone above of the lateral edges of the magnet, no recirculating flows appear and vertical velocity components are negligible.
|
|||
|
Show PACS
|
|||
|
|
Passive hydrodynamic synchronization of two-dimensional swimming cells Phys. Fluids 23, 011902 (2011); http://dx.doi.org/10.1063/1.3532954 (19 pages) Online Publication Date: 11 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Spermatozoa flagella are known to synchronize when swimming in close proximity. We use a model consisting of two-dimensional sheets propagating transverse waves of displacement to demonstrate that fluid forces lead to such synchronization passively. Using two distinct asymptotic descriptions (small amplitude and long wavelength), we derive the synchronizing dynamics analytically for arbitrarily shaped waveforms in Newtonian fluids, and show that phase-locking will always occur for sufficiently asymmetric shapes. We characterize the effect of the geometry of the waveforms and the separation between the swimmers on the synchronizing dynamics, the final stable conformations, and the energy dissipated by the cells. For two closely swimming cells, synchronization always occurs at the in-phase or opposite-phase conformation, depending solely on the geometry of the cells. In contrast, the work done by the swimmers is always minimized at the in-phase conformation. As the swimmers get further apart, additional fixed points arise at intermediate values of the relative phase. In addition, computations for large amplitude waves using the boundary integral method reveal that the two asymptotic limits capture all the relevant physics of the problem. Our results provide a theoretical framework to address other hydrodynamic interactions phenomena relevant to populations of self-propelled organisms.
|
|||
|
Show PACS
|
|||
|
|
Building water bridges in air: Electrohydrodynamics of the floating water bridge Phys. Fluids 22, 122104 (2010); http://dx.doi.org/10.1063/1.3518463 (9 pages) Online Publication Date: 15 December 2010
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The interaction of electrical fields and liquids can lead to a phenomenon that defies intuition. Some famous examples can be found in electrohydrodynamics as Taylor cones, whipping jets, or noncoalescing drops. A less famous example is the floating water bridge: a slender thread of water held between two glass beakers in which a high voltage difference is applied. Surprisingly, the water bridge defies gravity even when the beakers are separated at distances up to 2 cm. In this paper, experimental measurements and simple models are proposed and discussed for the stability of the bridge and the source of the flow, revealing an important role of polarization forces on the stability of the water bridge. On the other hand, the observed flow can only be explained due to the non-negligible free charge present in the surface. In this sense, the floating water bridge can be considered as an extreme case of a leaky dielectric liquid [
J. R. Melcher and G. I. Taylor, Annu. Rev. Fluid Mech. 1, 111 (1969)
].
|
|||
|
Show PACS
|
|||
|
|
Experimental investigation into segregating granular flows down chutes Phys. Fluids 23, 013301 (2011); http://dx.doi.org/10.1063/1.3536658 (10 pages) Online Publication Date: 6 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
We experimentally investigated how a binary granular mixture made up of spherical glass beads (size ratio of 2) behaved when flowing down a chute. Initially, the mixture was normally graded, with all the small particles on top of the coarse grains. Segregation led to a grading inversion, in which the smallest particles percolated to the bottom of the flow, while the largest rose toward the top. Because of diffusive remixing, there was no sharp separation between the small-particle and large-particle layers, but a continuous transition. Processing images taken at the sidewall, we were able to measure the evolution of the concentration and velocity profiles. These experimental profiles were used to test a recent theory developed by
Gray and Chugunov [J. Fluid Mech. 569, 365 (2006)]
, who derived a nonlinear advection diffusion equation that describes segregation and remixing in dense granular flows of binary mixtures. We found that this theory was able to provide a consistent description of the segregation/remixing process under steady uniform flow conditions. To obtain the correct depth-averaged concentration far downstream, it was very important to use an accurate approximation to the downstream velocity profile through the avalanche depth. The S-shaped concentration profile in the far field provided a useful way of determining what we refer to as the Péclet number for segregation, a dimensionless number that quantifies how large the segregation is compared to diffusive remixing. While the theory was able to closely match the final fully developed concentration profile far downstream, there were some discrepancies in the inversion region (i.e., the region in which the mixing occurs). The reasons for this are not clear. The difficulty to set up the experiment with both well controlled initial conditions and a steady uniform bulk flow field is one of the most plausible explanations. Another interesting lead is that the flux of segregating particles, which was assumed to be a quadratic function of the concentration in small beads, takes a more complicated form.
|
|||
|
Show PACS
|
|||
|
|
Numerical simulation of a liquid bridge in a coaxial gas flow Phys. Fluids 23, 012101 (2011); http://dx.doi.org/10.1063/1.3534076 (11 pages) Online Publication Date: 11 January 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The dynamical response of an isothermal liquid bridge to a coaxial gas stream is examined from axisymmetric numerical simulations of the Navier–Stokes equations. The simulation method is previously validated by calculating the temporal evolution of the first oscillation mode in both cylindrical and axisymmetric liquid bridges. The comparison with other theoretical approaches and experiments shows good agreement in most cases, although significant discrepancies are found between the simulation and the experimental values of the damping rate for hexadecane. The simulation of a liquid bridge in a coaxial gas stream shows that a recirculation cell always appears in the liquid driven by the gas viscous stress on the free surface. The recirculation cell speed depends quasilinearly on the gas velocity for the range of gas flow rates considered. If the gas stream and gravity have the same direction, then the speed of the recirculation cell increases considerably due to the free surface deformation of the liquid bridge at equilibrium. This effect does not occur when gravity has the opposite direction because viscous dissipation in the liquid increases in this case. If the gas stream and gravity point downward, the liquid bridge shrinks at the upper part and bulges at the lower owing to the accumulation of momentum there. The same occurs for zero gravity, but noncylindrical liquid bridges deform more than cylindrical shapes with the same slenderness. If one inverts the direction of the gravity force, the interface deformation caused by the gas stream is the opposite, and its magnitude is smaller. The magnitude of the free surface deformation depends almost linearly on the gas stream velocity for both zero and normal gravity conditions.
|
|||
|
Show PACS
|
|||
|
|
Breakup of diminutive Rayleigh jets Phys. Fluids 22, 122003 (2010); http://dx.doi.org/10.1063/1.3524533 (11 pages) Online Publication Date: 8 December 2010
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Discharging a liquid from a nozzle at sufficient large velocity leads to a continuous jet that due to capillary forces breaks up into droplets. Here we investigate the formation of microdroplets from the breakup of micron-sized jets with ultra high-speed imaging. The diminutive size of the jet implies a fast breakup time scale τc =
 of the order of 100 ns, and requires imaging at 14×106 frames/s. We directly compare these experiments with a numerical lubrication approximation model that incorporates inertia, surface tension, and viscosity [
J. Eggers and T. F. Dupont, J. Fluid Mech. 262, 205 (1994)
;
X. D. Shi, M. P. Brenner, and S. R. Nagel, Science 265, 219 (1994)
]. The lubrication model allows to efficiently explore the parameter space to investigate the effect of jet velocity and liquid viscosity on the formation of satellite droplets. In the phase diagram, we identify regions where the formation of satellite droplets is suppressed. We compare the shape of the droplet at pinch-off between the lubrication approximation model and a boundary-integral calculation, showing deviations at the final moment of the pinch-off. In spite of this discrepancy, the results on pinch-off times and droplet and satellite droplet velocity obtained from the lubrication approximation agree with the high-speed imaging results. |
|||
|
Show PACS
|
|||
|
|
Phys. Fluids 22, 121702 (2010); http://dx.doi.org/10.1063/1.3527548 (4 pages) Online Publication Date: 21 December 2010
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Direct numerical simulations at Reynolds numbers ranging from Reλ = 30 to 160 show that the thickness δω of the turbulent/nonturbulent (T/NT) interface in planar jets is of the order of the Taylor scale δω ∼ λ, while in shear free, irrotational/isotropic turbulence is of the order of the Kolmogorov microscale δω ∼ η. It is shown that δω is equal to the radius of the large vorticity structures (LVSs) in this region, δω ≈ RLVS. Thus, the mean shear and the Reynolds number affect the T/NT interface thickness insofar as they define the radial dimension of the LVS near the T/NT interface.
|
|||
|
Show PACS
|
|||
This Publication
Scitation
Google Scholar
PubMed