Jump to Content
Your Recent History
View Cart
Top 20 Most Read Articles
January 2012
The 20 articles with the most full-text downloads during the month, in descending order.
 |
Phys. Fluids 21, 021301 (2009); http://dx.doi.org/10.1063/1.3046290 (17 pages) Online Publication Date: 10 February 2009
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Twenty years ago there was no experimental access to the velocity gradient tensor for turbulent flows. Without such access, knowledge of fundamental and defining properties of turbulence, such as vorticity dissipation, and strain rates and helicity, could not be studied in the laboratory. Although a few direct simulations at very low Reynolds numbers had been performed, most of these did not focus on properties of the small scales of turbulence defined by the velocity gradient tensor. In 1987 the results of the development and first successful use of a multisensor hot-wire probe for simultaneous measurements of all the components of the velocity gradient tensor in a turbulent boundary layer were published by
|
|||
|
Show PACS
|
|||
|
|
Grid-point requirements for large eddy simulation: Chapman’s estimates revisited Phys. Fluids 24, 011702 (2012); http://dx.doi.org/10.1063/1.3676783 (5 pages) Online Publication Date: 6 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Resolution requirements for large eddy simulation (LES), estimated by Chapman [AIAA J. 17, 1293 (1979)], are modified using accurate formulae for high Reynolds number boundary layer flow. The new estimates indicate that the number of grid points (N) required for wall-modeled LES is proportional to ReLx, but a wall-resolving LES requires ÑReLx13/7, where Lx is the flat-plate length in the streamwise direction. On the other hand, direct numerical simulation, resolving the Kolmogorov length scale, requires ÑReLx37/14.
|
|||
|
Show PACS
|
|||
|
|
Phys. Fluids 23, 121702 (2011); http://dx.doi.org/10.1063/1.3671738 (4 pages) Online Publication Date: 23 December 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
A defining feature of the inner-outer interactions in wall-bounded turbulent flows is the imprint of the outer large-scale motions on the inner small scale. Recently, Mathis et al. [“Large-scale amplitude modulation of the small-scale structures in turbulent boundary layers,” J. Fluid Mech. 628, 311 (2009)] quantified this imprint by applying the Hilbert transform to small-scale components of the fluctuating streamwise velocity, u. They found that the wall-normal profile of the amplitude modulation between the large scale and the envelope of the small scale exhibits strong resemblance to the skewness profile of u. In this study, we assess this apparent relationship and show that the Reynolds number trend in the skewness profile of u is strongly related to the amplitude modulation effect of the small scales by the large. This observation also leads to an alternative diagnostic for the amplitude modulation effect, which is one component of the skewness factor based on a scale decomposition.
|
|||
|
Show PACS
|
|||
|
|
The onset of oblique vortex shedding behind a heated circular cylinder in laminar wake regime Phys. Fluids 24, 011701 (2012); http://dx.doi.org/10.1063/1.3675551 (7 pages) Online Publication Date: 5 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Oblique vortex shedding (OVS) behind a heated circular cylinder in air was experimentally investigated. Similar to that in the parallel vortex shedding (PVS), the results show that the non-dimensionalized shedding frequency, Strouhal number, decreases under the influence of cylinder heating for oblique shedding mode. Although the onset Reynolds number of OVS increases with the cylinder temperature, the onset effective Reynolds number remains 63.3 ± 1.3 regardless of the cylinder heating. A general Strouhal-Reynolds-number relationship for OVS has been found based on the effective temperature concept in the present study. The ratio of the critical Reynolds numbers for the onsets of OVS and PVS is found to be an invariant with value of 4/3 for both isothermal and non-isothermal cases despite different length/diameter ratios and end conditions.
|
|||
|
Show PACS
|
|||
|
|
A new unstable mode in the wake of a circular cylinder Phys. Fluids 23, 121701 (2011); http://dx.doi.org/10.1063/1.3664869 (4 pages) Online Publication Date: 2 December 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The flow past a circular cylinder looses stability at Re ∼ 47, via the primary wake (PW) mode. Linear stability analysis of the steady base flow, in two dimensions, is conducted using a stabilized finite element formulation. A new mode, referred to as the secondary wake (SW) mode, is discovered which is found to be unstable for Re ≥ 110.8. The relative roles of the PW and SW mode in the development of Karman vortex shedding are also investigated.
|
|||
|
Show PACS
|
|||
|
|
Three-dimensional swirling flows in a tall cylinder driven by a rotating endwall Phys. Fluids 24, 014101 (2012); http://dx.doi.org/10.1063/1.3673608 (9 pages) Online Publication Date: 4 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The onset and nonlinear dynamics of swirling flows in relatively tall cylinders driven by the rotation of an endwall are studied numerically. These flows are distinguished from the more widely studied swirling flows in shorter cylinders; the instability in the taller cylinders is direct to three-dimensional flows rather than to unsteady axisymmetric flows. The simulations are in very good agreement with recent experiments in terms of the critical Reynolds number, frequency, and azimuthal wavenumber of the flows, but there is disagreement in the interpretation of these flows. We show that these flows are indeed rotating waves and that they have the same vorticity distributions as the flows measured using particle image velocimetry in the experiments. Identifying these as rotating waves gives a direct connection with prior linear stability analysis and the three-dimensional flows found in shorter cylinders as secondary instabilities leading to modulated rotating waves.
|
|||
|
Show PACS
|
|||
|
|
Amplification factors in shock-turbulence interactions: Effect of shock thickness Phys. Fluids 24, 011705 (2012); http://dx.doi.org/10.1063/1.3676449 (6 pages) Online Publication Date: 11 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Amplification factors of streamwise velocity are investigated in canonical shock-turbulence interactions. The ratio of laminar shock thickness to the Kolmogorov length scale is suggested as the appropriate parameter to understand data from simulations and experiments. The different regimes of the interaction suggested in the literature can also be understood in terms of this parameter.
|
|||
|
Show PACS
|
|||
|
|
An experimental study of transitional pulsatile pipe flow Phys. Fluids 24, 014103 (2012); http://dx.doi.org/10.1063/1.3673611 (17 pages) Online Publication Date: 6 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The transitional regime of a sinusoidal pulsatile flow in a straight, rigid pipe is investigated using particle image velocimetry. The main aim is to investigate how the critical Reynolds number is affected by different pulsatile conditions, expressed as the Womersley number and the oscillatory Reynolds number. The transition occurs in the region of Re = 2250-3000 and is characterized by an increasing number of isolated turbulence structures. Based on velocity fields and flow visualizations, these structures can be identified as puffs, similar to those observed in steady flow transition. Measurements at different Womersley numbers yield similar transition behavior, indicating that pulsatile effects do not play a role in the regime that is investigated. Variations of the oscillatory Reynolds number also appear to have little effect, so that the transition here seems to be determined only by the mean Reynolds number. For larger mean Reynolds numbers, a second regime is observed: here, the flow remains turbulent throughout the cycle. The turbulence intensity varies during the cycle, but has a phase shift with respect to the mean flow component. This is caused by a growth of kinetic energy during the decelerating part and a decay during the accelerating part of the cycle. Flow visualization experiments reveal that the flow develops localized turbulence at several random axial positions. The structures quickly grow to fill the entire pipe in the decelerating phase and (partially) decay during the accelerating phase.
|
|||
|
Show PACS
|
|||
|
|
Scaling laws for slippage on superhydrophobic fractal surfaces Phys. Fluids 24, 012001 (2012); http://dx.doi.org/10.1063/1.3674300 (13 pages) Online Publication Date: 12 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
We study the slippage on hierarchical fractal superhydrophobic surfaces and find an unexpected rich behavior for hydrodynamic friction on these surfaces. We develop a scaling law approach for the effective slip length, which is validated by numerical resolution of the hydrodynamic equations. Our results demonstrate that slippage does strongly depend on the fractal dimension and is found to be always smaller on fractal surfaces as compared with surfaces with regular patterns. This shows that in contrast to naive expectations, the value of effective contact angle is not sufficient to infer the amount of slippage on a fractal surface: depending on the underlying geometry of the roughness, strongly superhydrophobic surfaces may, in some cases, be fully inefficient in terms of drag reduction. Finally, our scaling analysis can be directly extended to the study of heat transfer at fractal surfaces, in order to estimate the Kapitsa surface resistance on patterned surfaces, as well as to the question of trapping of diffusing particles by patchy hierarchical surfaces, in the context of chemoreception.
|
|||
|
Show PACS
|
|||
|
|
Reynolds number effects on scale energy balance in wall turbulence Phys. Fluids 24, 015101 (2012); http://dx.doi.org/10.1063/1.3673609 (12 pages) Online Publication Date: 17 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The scale energy budget utilizes a modified version of the classical Kolmogorov equation of wall turbulence to develop an evolution equation for the second order structure function [R. J. Hill, “Exact second-order structure-function relationships,” J. Fluid Mech. 468, 317 (2002)]. This methodology allows for the simultaneous characterization of the energy cascade and spatial fluxes in turbulent shear flows across the entire physical domain as well as the range of scales. The present study utilizes this methodology to characterize the effects of Reynolds number on the balance of energy fluxes in turbulent channel flows. Direct numerical simulation data in the range Reτ = 300–934 are compared to previously published results at Reτ = 180 [N. Marati, C. M. Casciola, and R. Piva, “Energy cascade and spatial fluxes in wall turbulence,” J. Fluid Mech. 521, 191 (2004)]. The present results show no Reynolds number effects in the terms of the scale energy budget in either the viscous sublayer or buffer regions of the channel. In the logarithmic layer, the transfer of energy across scales clearly varies with Reynolds number, while the production of turbulent kinetic energy is not dependent on Reynolds number. An envelope of inverse energy cascade is quantified in the buffer region within which energy is transferred from small to larger scales. This envelope is observed in the range 6 < y+ < 37, where all scales except the smallest scales display characteristics of an inverse energy cascade. The cross-over scale lc+, which indicates the transition between production dominated and scale transfer dominated regimes, increases with Reynolds number, implying a larger range of transfer dominated scales, before the dominant mechanism switches to production. At higher Reynolds numbers, two distinct regimes of lc+ as a function of wall-normal location are observed, which was not captured at Reτ = 180. The variations of lc+ match the trends of the shear scale, which is a representation of the mean shear in the flow. Thus, this study demonstrates the utility and importance of the use of higher Reynolds number data in order to accurately characterize and understand the energy dynamics of various scales across the entire boundary layer.
|
|||
|
Show PACS
|
|||
|
|
Non-axisymmetric impact creates pineapple-shaped cavity Phys. Fluids 23, 091106 (2011); http://dx.doi.org/10.1063/1.3640021 (1 page) Online Publication Date: 30 September 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Abstract Unavailable
|
|||
|
Show PACS
|
|||
|
|
Particulate mixing in a turbulent serpentine duct Phys. Fluids 24, 013301 (2012); http://dx.doi.org/10.1063/1.3673610 (14 pages) Online Publication Date: 4 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Direct numerical simulations of particles in a serpentine duct were conducted at bulk flow Stokes numbers between 0.125 and 6. The geometrical curvature causes particles to depart direction from the mean flow. Above a Stokes number of about unity, a reflection layer forms along the outer curve of the bend. Reflectional mixing creates regions of nearly uniform particle mean velocity and kinetic energy. Particles leave the inner bend in a plume that separates from the inner wall at low Stokes number. At higher Stokes number, the plume splits in two, adding an upper part consisting of ballistic particles, that do not follow the geometrical curvature. When the Stokes number is low, the instantaneous 3-D distribution of particles visualizes wall streaks. But at higher Stokes number, particles disperse out of the reflection layer and form large scale puffs in the central portion of the duct.
|
|||
|
Show PACS
|
|||
|
|
Direct numerical simulation of turbulent channel flow up to Reτ = 590 Phys. Fluids 11, 943 (1999); http://dx.doi.org/10.1063/1.869966 (3 pages)
Full Text:
|
Download PDF
|
||
|
Show Abstract
Numerical simulations of fully developed turbulent channel flow at three Reynolds numbers up to Reτ = 590 are reported. It is noted that the higher Reynolds number simulations exhibit fewer low Reynolds number effects than previous simulations at Reτ = 180. A comprehensive set of statistics gathered from the simulations is available on the web at http://www.tam.uiuc.edu/Faculty/Moser/channel. © 1999 American Institute of Physics. |
|||
|
Show PACS
|
|||
|
|
An analogy of Taylor’s instability criterion in Couette and rotating-magnetic-field-driven flows Phys. Fluids 24, 011704 (2012); http://dx.doi.org/10.1063/1.3675893 (7 pages) Online Publication Date: 11 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The classical stability solution of Taylor for the Couette flow between a rotating inner cylinder and a stationary outer cylinder is used to model the “critical magnetic Taylor number,” Tacr, in a flow of a liquid metal driven by a rotating magnetic field (RMF) in a cylindrical cavity characterized by the parameter H = height/radius. (The magnetic Taylor number is defined as Ta = σωBo2Ro4/(2ρν2), where σ,ν, and ρ are the electrical conductivity, kinematic viscosity, and density of the liquid; ω and
Bo are the magnetic field frequency and induction; Ro is the radius of the cavity; the cr superscript means “critical”) In typical conditions, the RMF flow develops a solid-body-rotating core analogous to the inner rotating cylinder, embedded in a layer in which the swirl decays to zero at the outer wall. Using small-Ekman-number approximations for the core and gap flow, the analogy yields an insightful expression for Tacr. In particular, the model indicates that Tacr depends strongly on the parameter H. Comparisons of the present theoretical results with available realistic data show a good qualitative agreement and plausible quantitative agreement. The model was improved by an empirical adjustment of a coefficient and can be used as simple approximate prediction tool for Tacr in a quite wide range of cylindrical cavity configurations.
|
|||
|
Show PACS
|
|||
|
|
The linear stability of oscillating pipe flow Phys. Fluids 24, 014106 (2012); http://dx.doi.org/10.1063/1.3675899 (10 pages) Online Publication Date: 11 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
An investigation is made of the three-dimensional linear stability of the Stokes layer generated within a fluid contained inside a long oscillating cylinder. Both longitudinal and torsional vibrations are examined and the system of disturbance equations derived using Floquet theory are solved using pseudospectral methods. Critical parameters for instability are obtained for an extensive range of pipe radii and longitudinal and azimuthal wavenumbers. For sufficiently small pipe diameters, three-dimensional perturbations are sometimes found to be more unstable than their two-dimensional counterparts. In contrast, at larger radii, the three-dimensional disturbance modes are less important and the two-dimensional versions are expected to be observed in practice. These results imply constraints on experiments that are designed to exhibit shear modes in oscillatory flow.
|
|||
|
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
|
|||
|
|
Negative Magnus lift on a rotating sphere at around the critical Reynolds number Phys. Fluids 24, 014102 (2012); http://dx.doi.org/10.1063/1.3673571 (15 pages) Online Publication Date: 5 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
Negative Magnus lift acting on a sphere rotating about the axis perpendicular to an incoming flow was investigated using large-eddy simulation at three Reynolds numbers of 1.0 × 104, 2.0 × 105, and 1.14 × 106. The numerical methods used were first validated on a non-rotating sphere, and the spatial resolution around the sphere was determined so as to reproduce the laminar separation, reattachment, and turbulent transition of the boundary layer observed in the vicinity of the critical Reynolds number. The rotating sphere exhibited a positive or negative Magnus effect depending on the Reynolds number and the imposed rotating speed. At Reynolds numbers in the subcritical or supercritical regimes, the direction of the Magnus lift force was independent of the rotational speed. In contrast, the lift force was negative in the critical regime when particular rotating speeds were imposed. This negative Magnus effect was investigated in the context of suppression or promotion of boundary layer transition around the separation point.
|
|||
|
Show PACS
|
|||
|
|
Direct simulation Monte Carlo method for an arbitrary intermolecular potential Phys. Fluids 24, 011703 (2012); http://dx.doi.org/10.1063/1.3676060 (6 pages) Online Publication Date: 9 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
A scheme to implement an arbitrary intermolecular potential into the direct simulation Monte Carlo method is proposed. To illustrate the scheme, two benchmark problems are solved employing the Lennard-Jones potential. Since the computational effort of the new scheme is comparable with that of the hard sphere model of molecules, it can completely substitute the widely used models such as variable hard spheres and variable soft spheres.
|
|||
|
Show PACS
|
|||
|
|
Dynamic evolution of fingering patterns in a lifted Hele–Shaw cell Phys. Fluids 23, 123101 (2011); http://dx.doi.org/10.1063/1.3659140 (11 pages) Online Publication Date: 7 December 2011
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
We present a study on pattern formation in a Newtonian liquid during lifting of a circular Hele–Shaw cell. When a confined layer of oil is subject to such a stretch flow, air penetrates into the liquid from the sides and a fingering instability, a variant of the classical Saffman–Taylor instability, evolves. This setting has the particularity that the finger growth takes place in a conserved volume of liquid and that the dimensionless surface tension, the control parameter which governs the Saffman–Taylor instability, is changing with time. This leads to a constantly evolving pattern, which we investigate with regard to number of fingers and finger amplitude. We distinguish in the pattern at each instant growing fingers and stagnant fingers. Systematically varying the properties of the viscous oil and the geometry of the Hele–Shaw cell, we show that the number of growing fingers is at each moment well described by a simple approach based on linear stability analysis and depends only on the dimensionless surface tension. In contrast, the finger amplitude and consequently the total number of fingers (growing and stagnant fingers) depend also on the cell confinement. We demonstrate that the finger amplitude has a distinct influence on the debonding force. Higher finger amplitude and number of fingers lead to lower forces.
|
|||
|
Show PACS
|
|||
|
|
Uniformly valid asymptotic flow analysis in curved channels Phys. Fluids 24, 013601 (2012); http://dx.doi.org/10.1063/1.3673568 (25 pages) Online Publication Date: 6 January 2012
Full Text:
Read Online (HTML)
|
Download PDF
|
||
|
Show Abstract
The laminar incompressible flow in a two-dimensional curved channel having at its upstream and downstream extremities two tangent straight channels is considered. A global interactive boundary layer (GIBL) model is developed using the approach of the successive complementary expansions method (SCEM) which is based on generalized asymptotic expansions leading to a uniformly valid approximation. The GIBL model is valid when the non dimensional number μ = δ
 is O(1) and gives predictions in agreement with numerical Navier-Stokes solutions for Reynolds numbers Re ranging from 1 to 104 and for constant curvatures δ =  ranging from 0.1 to 1, where H is the channel width and Rc the curvature radius. The asymptotic analysis shows that μ, which is the ratio between the curvature and the thickness of the boundary layer of any perturbation to the Poiseuille flow, is a key parameter upon which depends the accuracy of the GIBL model. The upstream influence length is found asymptotically and numerically to be O( ). |
|||
|
Show PACS
|
|||
This Publication
Scitation
SPIN
Google Scholar
PubMed