Jump to Content
View Cart
Top 20 Most Read Articles
May 2009
The 20 articles with the most full-text downloads during the month, in descending order.
 |
Turbulent boundary layers up to Reθ = 2500 studied through simulation and experiment Phys. Fluids 21, 051702 (2009); http://dx.doi.org/10.1063/1.3139294 (4 pages) Online Publication Date: 20 May 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Direct numerical simulations (DNSs) and experiments of a spatially developing zero-pressure-gradient turbulent boundary layer are presented up to Reynolds number Reθ = 2500, based on momentum thickness θ and free-stream velocity. For the first time direct comparisons of DNS and experiments of turbulent boundary layers at the same (computationally high and experimentally low) Reθ are given, showing excellent agreement in skin friction, mean velocity, and turbulent fluctuations. These results allow for a substantial reduction of the uncertainty of boundary-layer data, and cross validate the numerical setup and experimental technique. The additional insight into the flow provided by DNS clearly shows large-scale turbulent structures, which scale in outer units growing with Reθ, spanning the whole boundary-layer height.
|
|||
|
Show PACS
|
|||
|
|
Numerical study of eigenmode forcing effects on jet flow development and noise generation mechanisms Phys. Fluids 21, 045106 (2009); http://dx.doi.org/10.1063/1.3112686 (22 pages) Online Publication Date: 23 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The effect of nonlinear interaction of instability eigenmodes on jet flow transition and its near acoustic field for a high-subsonic round jet at a Reynolds number of Re = 4.5×105 and a Mach number of Ma = 0.9 is investigated using large-eddy simulations. At the inflow, helical perturbations of azimuthal wavenumbers |n| = 4,…,8 determined from linear stability theory are superimposed on a laminar base flow in order to trigger transition to turbulence. The disturbance amplitude is varied parametrically in the range from 1.5% to 4.5% of the jet exit velocity Uj. Thereby we aim to characterize sources of noise generation and, in particular, underlying mode interactions. With increasing forcing amplitude, the transitional behavior of the jet changes which affects the mean flow and also the acoustic near-field, which are both analyzed in detail. As the forcing amplitude is increased, the axial root-mean-square peak levels along the jet centerline are reduced by approximately 7%. Simultaneously, pronounced dual-peak distributions are generated along the jet lip line which are related to the localization of vortex pairings of the jet column mode. For low-amplitude excitation the azimuthal turbulent kinetic energy spectra show that the unexcited, naturally least stable axisymmetric mode n = 0 and the helical mode n = 1 dominate the early nonlinear regimes between z ≈ 6r0 and 9r0 where r0 is the jet radius. An analysis of the Fourier mode amplitude clarifies that this energy rise is linked to the helical mode n = 1. For higher forcing amplitudes, in addition to the varicose mode n = 0 interactions between the excited even mode n = 4 and higher azimuthal harmonics thereof dominate the azimuthal energy spectra. These differences in the early nonlinear development of the eigenmodes are found to alter the acoustic near-field. At small angles from the downstream jet axis, the peak acoustic frequency occurs at a Strouhal number based on the angular frequency ω and the jet diameter Dj of St = ωDj/(2πUj) ≈ 0.4. For low-amplitude forcing sound pressure levels are slightly enhanced which can be linked to the dominant low azimuthal wavenumbers identified in the transitional region. In the sideline direction, regardless of the excitation level, broadbanded spectra with maxima in the band 0.7 ≤ St ≤ 0.8 are found which is maintained at intermediate observer angles. For high forcing amplitude, however, a tonal component outside the initially excited frequency range is observed. This peak at St ≈ 0.88 can be explained by weakly nonlinear interactions of initially forced eigenmodes n = 4 and n = 8 together with the jet column mode.
|
|||
|
Show PACS
|
|||
|
|
On the unsteady behavior of turbulence models Phys. Fluids 21, 041701 (2009); http://dx.doi.org/10.1063/1.3121303 (4 pages) Online Publication Date: 22 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Periodically forced turbulence is used as a test case to evaluate the predictions of two-equation and multiple-scale turbulence models in unsteady flows. The limitations of the two-equation model are shown to originate in the basic assumption of spectral equilibrium. A multiple-scale model based on a picture of stepwise energy cascade overcomes some of these limitations, but the absence of nonlocal interactions proves to lead to poor predictions of the time variation of the dissipation rate. A new multiple-scale model that includes nonlocal interactions is proposed and shown to reproduce the main features of the frequency response correctly.
|
|||
|
Show PACS
|
|||
|
|
Novel characteristics of valveless pumping Phys. Fluids 21, 053601 (2009); http://dx.doi.org/10.1063/1.3114603 (14 pages) Online Publication Date: 4 May 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
This study investigates the occurrence of valveless pumping in a fluid-filled system consisting of two open tanks connected by an elastic tube. We show that directional flow can be achieved by introducing a periodic pinching applied at an asymmetrical location along the tube, and that the flow direction depends on the pumping frequency. We propose a relation between wave propagation velocity, tube length, and resonance frequencies associated with shifts in the pumping direction using numerical simulations. The eigenfrequencies of the system are estimated from the linearized system, and we show that these eigenfrequencies constitute the resonance frequencies and the horizontal slope frequencies of the system; “horizontal slope frequency” being a new concept. A simple model is suggested, explaining the effect of the gravity driven part of the oscillation observed in response to the tank and tube diameter changes. Results are partly compared with experimental findings.
|
|||
|
Show PACS
|
|||
|
|
Capillary oscillations of a constrained liquid drop Phys. Fluids 21, 032108 (2009); http://dx.doi.org/10.1063/1.3103344 (10 pages) Online Publication Date: 27 March 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
An inviscid spherical liquid drop held by surface tension exhibits linear oscillations of a characteristic frequency and mode shape (Rayleigh oscillations). If the drop is pinned on a circle of contact the mode shapes change and the frequencies are shifted. The linear problem of inviscid, axisymmetric, volume-preserving oscillations of a liquid drop constrained by pinning along a latitude is solved here. The formulation gives rise to an integrodifferential boundary value problem, similar to that for Rayleigh oscillations, and for oscillations of a drop in contact with a spherical bowl [
M. Strani and F. Sabetta, J. Fluid Mech. 141, 233 (1984)
], only more constrained. A spectral method delivers a truncated solution to the eigenvalue problem. A numerical routine has been used to generate the eigenfrequencies/eigenmodes as a function of the location of the pinned circle of constraint. The effect of pinning the drop is to introduce a new low-frequency eigenmode. The center-of-mass motion, important in application, is partitioned among all the eigenmodes but the low-frequency mode is its principal carrier.
|
|||
|
Show PACS
|
|||
|
|
Phys. Fluids 21, 052002 (2009); http://dx.doi.org/10.1063/1.3129592 (9 pages) Online Publication Date: 7 May 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Microfluid mechanics is one of the most exciting research areas in modern fluid mechanics and fluid engineering because of its many potential industrial and biological applications. In the present study, pressure drops (PDs) were measured for water, a 50/50 glycerol/water mixture, and a 0.1% aqueous solution of polyethylene oxide (PEO) 8000 flowing at high velocities through various sizes of micro-orifice. It was found that the measured PD of water and the glycerol/water mixture agrees with the prediction of the Navier–Stokes equation for orifices 100 and 400 μm in diameter, but it is lower for orifices less than 50 μm in diameter. In particular, the measured maximum PD was almost two orders of magnitude lower than the prediction for the 10 and 5 μm diameter orifices. The glycerol/water mixture, possessing a viscosity ten times higher than water, provided nearly the same PDs as water when the reduction was generated. The solution of PEO produced a lower PD than water and the glycerol/water mixture except for the 400 μm diameter orifice. Several factors, including orifice shape, deformation of orifice foil, wall slip, transition, cavitation, and elasticity were considered but the evidence suggests that the reduction in PD may be caused by wall slip or the elasticity induced in a flow of high elongational rate.
|
|||
|
Show PACS
|
|||
|
|
Introduction: 25th Annual Gallery of Fluid Motion (Salt Lake City, Utah, 2007) Phys. Fluids 20, 091101 (2008); http://dx.doi.org/10.1063/1.2974812 (1 page) Online Publication Date: 25 September 2008
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Abstract Unavailable
|
|||
|
Show PACS
|
|||
|
|
A new deconvolution method for large eddy simulation Phys. Fluids 21, 045107 (2009); http://dx.doi.org/10.1063/1.3123530 (3 pages) Online Publication Date: 24 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
In this paper we present and we briefly discuss a new way to derive approximate deconvolution subgrid models for the large eddy simulation of turbulent flows.
|
|||
|
Show PACS
|
|||
|
|
Time dependence of effective slip on textured hydrophobic surfaces Phys. Fluids 21, 052001 (2009); http://dx.doi.org/10.1063/1.3127123 (8 pages) Online Publication Date: 6 May 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
In this paper, we present results on water flow past randomly textured hydrophobic surfaces with relatively large surface features of the order of 50 μm. Direct shear stress measurements are made on these surfaces in a channel configuration. The measurements indicate that the flow rates required to maintain a shear stress value vary substantially with water immersion time. At small times after filling the channel with water, the flow rates are up to 30% higher compared with the reference hydrophilic surface. With time, the flow rate gradually decreases and in a few hours reaches a value that is nearly the same as the hydrophilic case. Calculations of the effective slip lengths indicate that it varies from about 50 μm at small times to nearly zero or “no slip” after a few hours. Large effective slip lengths on such hydrophobic surfaces are known to be caused by trapped air pockets in the crevices of the surface. In order to understand the time dependent effective slip length, direct visualization of trapped air pockets is made in stationary water using the principle of total internal reflection of light at the water-air interface of the air pockets. These visualizations indicate that the number of bright spots corresponding to the air pockets decreases with time. This type of gradual disappearance of the trapped air pockets is possibly the reason for the decrease in effective slip length with time in the flow experiments. From the practical point of usage of such surfaces to reduce pressure drop, say, in microchannels, this time scale of the order of 1 h for the reduction in slip length would be very crucial. It would ultimately decide the time over which the surface can usefully provide pressure drop reductions.
|
|||
|
Show PACS
|
|||
|
|
Topology and breakdown of Görtler vortices on an oscillating cylinder Phys. Fluids 20, 091102 (2008); http://dx.doi.org/10.1063/1.2973206 (1 page) Online Publication Date: 25 September 2008
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Abstract Unavailable
|
|||
|
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
|
|||
|
|
Phys. Fluids 21, 045109 (2009); http://dx.doi.org/10.1063/1.3115068 (10 pages) Online Publication Date: 30 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The dynamic global-coefficient subgrid-scale eddy-viscosity model by
You and Moin [Phys. Fluids 19, 065110 (2007)
] is generalized for large-eddy simulation of turbulent flow with scalar transport. The model coefficient for subgrid-scale scalar flux which is constant in space but varies in time is dynamically determined based on the “global conservation” of the transport equation for scalar variance. Large-eddy simulations of turbulent flow with passive scalar transport through a channel and over a backward-facing step show that the present model has a similar predictive capability as the dynamic Smagorinsky model. The present dynamic model is especially suitable for large-eddy simulation of turbulent flow with scalar transport in complex geometries since it does not require any spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and requires only a single-level test filter. The present model is not more complicated in implementation and not more expensive in terms of computational cost than the dynamic Smagorinsky model.
|
|||
|
Show PACS
|
|||
|
|
Swirl flow focusing: A novel procedure for the massive production of monodisperse microbubbles Phys. Fluids 21, 042003 (2009); http://dx.doi.org/10.1063/1.3123533 (8 pages) Online Publication Date: 30 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
A volume-of-fluid numerical method is used to predict the dynamics of microbubble formation in an axisymmetric flow-focusing microfluidic device for a gas-liquid system. Numerical results show that, in all the cases analyzed, the introduction of swirl into the focusing liquid stabilizes the tapering gas-liquid meniscus from which a steady gas ligament issues. Consequently, a drastic reduction in the size of bubbles generated by the device is achieved under similar gas and liquid flow rates.
|
|||
|
Show PACS
|
|||
|
|
The effective slip length and vortex formation in laminar flow over a rough surface Phys. Fluids 21, 052105 (2009); http://dx.doi.org/10.1063/1.3121305 (10 pages) Online Publication Date: 11 May 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The flow of viscous incompressible fluid over a periodically corrugated surface is investigated numerically by solving the Navier–Stokes equation with the local slip and no-slip boundary conditions. We consider the effective slip length which is defined with respect to the level of the mean height of the surface roughness. With increasing corrugation amplitude the effective no-slip boundary plane is shifted toward the bulk of the fluid, which implies a negative effective slip length. The analysis of the wall shear stress indicates that a flow circulation is developed in the grooves of the rough surface provided that the local boundary condition is no-slip. By applying a local slip boundary condition, the center of the vortex is displaced toward the bottom of the grooves and the effective slip length increases. When the intrinsic slip length is larger than the corrugation amplitude, the flow streamlines near the surface are deformed to follow the boundary curvature, the vortex vanishes, and the effective slip length saturates to a constant value. Inertial effects promote vortex flow formation in the grooves and reduce the effective slip length.
|
|||
|
Show PACS
|
|||
|
|
Phys. Fluids 21, 041901 (2009); http://dx.doi.org/10.1063/1.3122802 (18 pages) Online Publication Date: 22 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
We developed a method to extract the energetically dominant flow features in a complicated fish wake according to an energetic point of view, and applied singular-value decomposition (SVD) to two-dimensional instantaneous fluid velocity, vorticity and λ2 (vortex-detector) data. We demonstrate the effectiveness and merits of the use of SVD through an example regarding the wake of a fish executing a fast-start turn. The energy imparted into the water by a swimming fish is captured and portrayed through SVD. The analysis and interpretation of complicated data for the fish wake are greatly improved, and thus help to characterize more accurately a complicated fish wake. The velocity vectors and Galilean invariants (i.e., vorticity and λ2) resulting from SVD extraction are significantly helpful in recognizing the energetically dominant large-scale flow features. To obtain successful SVD extractions, we propose useful criteria based on the Froude propulsion efficiency, which is biologically and physically related. We also introduce a novel and useful method to deduce the topology of dominant flow motions in an instantaneous fish flow field, which is based on combined use of the topological critical-point theory and SVD. The concept and approach proposed in this work are useful and adaptable in biomimetic and biomechanical research concerning the fluid dynamics of a self-propelled body.
|
|||
|
Show PACS
|
|||
|
|
Fractal Kelvin–Helmholtz breakups Phys. Fluids 20, 091109 (2008); http://dx.doi.org/10.1063/1.2976423 (1 page) Online Publication Date: 25 September 2008
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Abstract Unavailable
|
|||
|
Show PACS
|
|||
|
|
A simple model for the streamwise fluctuations in the log-law region of a boundary layer Phys. Fluids 21, 055105 (2009); http://dx.doi.org/10.1063/1.3140075 (9 pages) Online Publication Date: 22 May 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
We build on the work of
Davidson et al. [Phys. Fluids 18, 065112 (2006)
] and propose an elementary model for the log-law region of a boundary layer. The model is remarkably simple, contains only one free parameter (which we set equal to unity) and, unlike Davidson et al., assumes very little about the shape of the boundary-layer eddies. The physical content of the model is simple: we assume that the two-point statistics of the streamwise velocity fluctuations know about the presence of the wall only to the extent that, over a range of eddy sizes, it imposes a kinetic energy scale proportional to the square of the shear velocity. Little else is assumed, other than classic Kolmogorov phenomenology for the small scales. Despite its naivety, and the minimal number of free parameters, the model is a good fit to experimental data for the k−1 law of the one-dimensional, longitudinal spectrum, Φuu(k), and also to Φuu(k) in the inertial range. It is also an excellent fit to experimental data for the real-space analog of the k−1 law; that is, the logarithmic law for the longitudinal structure function. In addition, the model predicts the cross-stream variation of the variance of the streamwise velocity fluctuations, 〈ux2〉, and, to within an additive constant, it too is a reasonable fit to the data. Our model differs from the classic analyses of
Townsend [J. Fluid Mech. 165, 163 (1986)
] and
Perry et al. [J. Fluid Mech. 165, 163 (1986)
] in three respects. First, we do not invoke the attached eddy hypothesis, which assumes that the vortical structures in the log-layer extend to the wall. Second, we predict the precise shape of Φuu(k) for all values of k and not just the k−1 region. Third, there are three adjustable parameters in the theory of Perry et al., whereas we have only one. It is instructive that a model which contains very little information about the morphology of the vorticity field, and has only one free parameter, is able to reproduce both the structural form for, and magnitude of, many features of the one-dimensional spectrum and the second-order structure function. This suggests that these two measures of the scale-by-scale energy distribution are insensitive to the detailed structure of the vorticity field. We argue that this is indeed the case.
|
|||
|
Show PACS
|
|||
|
|
“Black hole” nucleation in a splash of milk Phys. Fluids 20, 091106 (2008); http://dx.doi.org/10.1063/1.2973667 (1 page) Online Publication Date: 25 September 2008
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Abstract Unavailable
|
|||
|
Show PACS
|
|||
|
|
The influence of hydrodynamic slip on the electrophoretic mobility of a spherical colloidal particle Phys. Fluids 21, 042001 (2009); http://dx.doi.org/10.1063/1.3116664 (14 pages) Online Publication Date: 17 April 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Recent theoretical studies have suggested a significant enhancement in electro-osmotic flows over hydrodynamically slipping surfaces, and experiments have indeed measured O(1) enhancements. In this paper, we investigate whether an equivalent effect occurs in the electrophoretic motion of a colloidal particle whose surface exhibits hydrodynamic slip. To this end, we compute the electrophoretic mobility of a uniformly charged spherical particle with slip length λ as a function of the zeta (or surface) potential of the particle ζ and diffuse-layer thickness κ−1. In the case of a thick diffuse layer, κa⪡1 (where a is the particle size), simple arguments show that slip does lead to an O(1) enhancement in the mobility, owing to the reduced viscous drag on the particle. On the other hand, for a thin-diffuse layer κa⪢1, the situation is more complicated. A detailed asymptotic analysis, following the method of O’Brien [
J. Colloid Interface Sci. 92, 204 (1983)
], reveals that an O(κλ) increase in the mobility occurs at low-to-moderate zeta potentials (with ζ measured on the scale of thermal voltage kBT/e ≈ 25 mV). However, as ζ is further increased, the mobility decreases and ultimately becomes independent of the slip length—the enhancement is lost—which is due to the importance of nonuniform surface conduction within the thin-diffuse layer, at large ζ and large, but finite, κa. Our asymptotic calculations for thick and thin-diffuse layers are corroborated and bridged by computation of the mobility from the numerical solution of the full electrokinetic equations (using the method of O’Brien and White [
J. Chem. Soc., Faraday Trans. 2 74, 1607 (1978)
]). In summary, then, we demonstrate that hydrodynamic slip can indeed produce an enhancement in the electrophoretic mobility; however, such enhancements will not be as dramatic as the previously studied κa→∞ limit would suggest. Importantly, this conclusion applies not only to electrophoresis but also to electro-osmosis over highly charged surfaces, wherein any inhomogeneities (e.g., due to curvature, roughness, charge patterning, or a variation in slip length) will drive nonuniform surface conduction, which prevents the significant slip-driven flow enhancements predicted for a uniform highly charged surface.
|
|||
|
Show PACS
|
|||
|
|
Viscous jet drawing air into a bath Phys. Fluids 20, 091107 (2008); http://dx.doi.org/10.1063/1.2973677 (1 page) Online Publication Date: 25 September 2008
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Abstract Unavailable
|
|||
|
Show PACS
|
|||
This Publication
Scitation
Google Scholar
PubMed