Phys. Fluids (1994-Present) - Physics of Fluids

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Preface to Special Topic: Turbulence Physics and Control—Papers from a Workshop in Honor of John Kim's 60th Birthday, Stanford, California, September 2007

Haecheon Choi and Parviz Moin

Phys. Fluids 20, 101501 (2008); http://dx.doi.org/10.1063/1.3005814 (1 page)

Online Publication Date: 31 October 2008

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01.30.-y	Physics literature and publications
47.27.-i	Turbulent flows

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Representing anisotropy of two-point second-order turbulence velocity correlations using structure tensors

Amitabh Bhattacharya, Stavros C. Kassinos, and Robert D. Moser

Phys. Fluids 20, 101502 (2008); http://dx.doi.org/10.1063/1.3005818 (13 pages) | Cited 1 time

Online Publication Date: 31 October 2008

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A locally homogeneous representation for the two-point, second-order turbulent velocity fluctuation R_ij(x,r) = 〈ui′(x)uj′(x+r)〉 is formulated in terms of three linearly independent structure tensors [ Kassinos et al., J. Fluid Mech. 428, 213 (2001) ]: Reynolds stress B_ij, dimensionality D_ij, and stropholysis Qijk∗. These structure tensors are single-point moments of the derivatives of vector stream functions that contain information about the directional and componential anisotropies of the correlation. The representation is a sum of several rotationally invariant component tensors. Each component tensor scales like a power law in r, while its variation in r/r depends linearly on the structure tensors. Continuity and self-consistency constraints reduce the number of degrees of freedom in the model to 17. A finite Re correction is introduced to the representation for separations of the order of Kolmogorov’s length scale. To evaluate our representation, we construct a model correlation by fitting the representation to correlations calculated from direct numerical simulation (DNS) of homogeneous turbulence and channel flow. Comparison of the model correlation to the DNS data shows that the representation can capture the character of the anisotropy of two-point second-order velocity correlation tensors.

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47.27.N-	Wall-bounded shear flow turbulence
47.60.Dx	Flows in ducts and channels

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The sound from mixing layers simulated with different ranges of turbulence scales

Randall R. Kleinman and Jonathan B. Freund

Phys. Fluids 20, 101503 (2008); http://dx.doi.org/10.1063/1.3005823 (12 pages) | Cited 3 times

Online Publication Date: 31 October 2008

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The role of turbulence scales in generating far-field sound in free shear flows is studied via direct numerical simulations of temporally developing, Mach 0.9 mixing layers. Four flows were simulated, starting from the same initial conditions but with Reynolds numbers that varied by a factor of 12. Above momentum thickness Reynolds number Re_{δ_m} ≈ 300, all the mixing layers radiate over 85% of the acoustic energy of the apparently asymptotically high-Reynolds-number value that we are able to compute. Turbulence energy and pressure wavenumber spectra show the expected Reynolds number dependence; the two highest Reynolds number simulations show evidence of an inertial range and Kolmogorov scaling at the highest wavenumbers. Far-field pressure spectra all decay much more rapidly with wavenumber than the corresponding near-field spectra and show significantly less sensitivity to Reynolds number. Low wavenumbers account for nearly all of the radiated acoustic energy. Far-field streamwise wavenumber pressure spectra scale well with the layer momentum thickness, consistent with the insensitivity to Reynolds number of the largest turbulence structures. At higher wavenumbers the streamwise spectra scale best with the Taylor microscale. Interestingly, none of the spanwise far-field pressure spectra scale well with momentum thickness despite doing so in the near-field turbulence. Instead they scale well at all wavenumbers with the turbulence microscale. Implications of these results for large-eddy simulation of jet noise are discussed.

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47.27.W-	Boundary-free shear flow turbulence
47.27.E-	Turbulence simulation and modeling

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Modeling the pressure Hessian and viscous Laplacian in turbulence: Comparisons with direct numerical simulation and implications on velocity gradient dynamics

L. Chevillard, C. Meneveau, L. Biferale, and F. Toschi

Phys. Fluids 20, 101504 (2008); http://dx.doi.org/10.1063/1.3005832 (15 pages) | Cited 16 times

Online Publication Date: 31 October 2008

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Modeling the velocity gradient tensor A = ∇u along Lagrangian trajectories in turbulent flow requires closures for the pressure Hessian and viscous Laplacian of A. Based on an Eulerian–Lagrangian change in variables and the so-called recent fluid deformation closure, such models were proposed recently [ Chevillard and Meneveau, Phys. Rev. Lett. 97, 174501 (2006) ]. The resulting stochastic model was shown to reproduce many geometric and anomalous scaling properties of turbulence. In this work, direct comparisons between model predictions and direct numerical simulation (DNS) data are presented. First, statistical properties of A are described using conditional averages of strain skewness, enstrophy production, energy transfer, and vorticity alignments, conditioned upon invariants of the velocity gradient. These conditionally averaged quantities are found to be described accurately by the stochastic model. More detailed comparisons that focus directly on the terms being modeled in the closures are also presented. Specifically, conditional statistics associated with the pressure Hessian and the viscous Laplacian are measured from the model and are compared with DNS. Good agreement is found in strain-dominated regions. However, some features of the pressure Hessian linked to rotation-dominated regions are not reproduced accurately by the model. Geometric properties such as vorticity alignment with respect to principal axes of the pressure Hessian are mostly predicted well. In particular, the model predicts that an eigenvector of the rate of strain will be also an eigenvector of the pressure Hessian, in accord with basic properties of the Euler equations. The analysis identifies under what conditions the Eulerian–Lagrangian change in variables with the recent fluid deformation closure works well, and in which flow regimes it requires further improvements.

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47.27.ek	Direct numerical simulations
47.32.-y	Vortex dynamics; rotating fluids
47.27.eb	Statistical theories and models

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On streak breakdown in bypass transition

Philipp Schlatter, Luca Brandt, H. C. de Lange, and Dan S. Henningson

Phys. Fluids 20, 101505 (2008); http://dx.doi.org/10.1063/1.3005836 (15 pages) | Cited 10 times

Online Publication Date: 31 October 2008

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Recent theoretical, numerical, and experimental investigations performed at the Department of Mechanics, KTH Stockholm, and the Department of Mechanical Engineering, Eindhoven University of Technology, are reviewed, and new material is presented to clarify the role of the boundary-layer streaks and their instability with respect to turbulent breakdown in bypass transition in a boundary layer subject to free-stream turbulence. The importance of the streak secondary-instability process for the generation of turbulent spots is clearly shown. The secondary instability manifests itself as a growing wave packet located on the low-speed streak, increasing in amplitude as it is dispersing in the streamwise direction. In particular, qualitative and quantitative data pertaining to temporal sinuous secondary instability of a steady streak, impulse responses both on a parallel and a spatially developing streak, a model problem of bypass transition, and full simulations and experiments of bypass transition itself are collected and compared. In all the flow cases considered, similar characteristics in terms of not only growth rates, group velocity, and wavelengths but also three-dimensional visualizations of the streak breakdown have been found. The wavelength of the instability is about an order of magnitude larger than the local boundary-layer displacement thickness δ^∗, the group velocity about 0.8 of the free-stream velocity U_∞, and the growth rate on the order of a few percent of U_∞/δ^∗. The characteristic structures at the breakdown are quasistreamwise vortices, located on the flanks of the low-speed region arranged in a staggered pattern.

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47.20.Ib	Instability of boundary layers; separation
47.80.Jk	Flow visualization and imaging
47.27.E-	Turbulence simulation and modeling

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Local isotropy of the velocity and vorticity fields in a boundary layer at high Reynolds numbers

James M. Wallace and Lawrence Ong

Phys. Fluids 20, 101506 (2008); http://dx.doi.org/10.1063/1.3005842 (7 pages) | Cited 1 time

Online Publication Date: 31 October 2008

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Measurements of the velocity and vorticity field with a 12-sensor hot-wire probe were carried out in the boundary layer of the test section ceiling of the NASA Ames 80×120 ft² wind tunnel at a turbulence Reynolds number of R_λ ≈ 875. Tests of local isotropy were applied to the data obtained at y/δ = 0.1. In the inertial subrange, which extended over a decade of wave numbers for this experiment, both the velocity and vorticity component one-dimensional k_x spectra agree well with the isotropic spectra of Kim and Antonia [J. Fluid Mech. 251, 219 (1993) ]. This agreement extends into the dissipation range up to wave numbers at which the accuracy of the measurements is limited because of spatial resolution and other sources of error. Additional tests of local isotropy, from the characteristics of the Reynolds shear stress correlation coefficient cospectrum and from the isotropic relationships between the k_x spectra of the streamwise velocity and vorticity components with the k_x spectra of the respective cross-stream components, also show evidence of local isotropy at these higher wave numbers.

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47.32.-y	Vortex dynamics; rotating fluids
47.27.er	Spectral methods
47.27.Gs	Isotropic turbulence; homogeneous turbulence
47.27.nb	Boundary layer turbulence

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Direct numerical simulation of the Ekman layer: A step in Reynolds number, and cautious support for a log law with a shifted origin

Philippe R. Spalart, Gary N. Coleman, and Roderick Johnstone

Phys. Fluids 20, 101507 (2008); http://dx.doi.org/10.1063/1.3005858 (9 pages) | Cited 12 times

Online Publication Date: 31 October 2008

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See Also: RETRACTION

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Results at Ekman Reynolds numbers Re ranging from 1000 to 2828 expand the direct numerical simulation (DNS) contribution to the theory of wall-bounded turbulence. An established spectral method is used, with rules for domain size and grid resolution at each Reynolds number derived from the theory. The Re increase is made possible by better computers and by optimizing the grid in relation to the wall shear-stress direction. The boundary-layer thickness in wall units δ⁺ varies here by a factor of about 5.3, and reaches values near 5000, or 22 times the minimum at which turbulence has been sustained. An equivalent channel Reynolds number, based on the pressure gradient in wall units, would reach about Re_τ = 1250. The principal goal of the analysis, the impartial identification of a log law, is summarized in the local “Karman measure” d(ln z⁺)/dU⁺. The outcome differs from that for Hoyas and Jiménez [Phys. Fluids 18, 011702 (2006) ] and for Hu et al. [AIAA J. 44, 1541 (2006) ] in channel-flow DNS at similar Reynolds numbers, for reasons unknown: Here, the law of the wall is gradually established up to a z⁺ around 400, with little statistical scatter. To leading order, it is consistent with the experiments of Österlund et al. [Phys. Fluids 12, 1 (2000) ] in boundary layers. With the traditional expression, a logarithmic law is not present, in that the Karman measure drifts from about 0.41 at z⁺ ≈ 70 to the 0.37–0.38 range for z⁺ ≈ 500, with Re = 2828. However, if a virtual origin is introduced with a shift of a⁺ = 7.5 wall units, the data support a long logarithmic layer with κ = 0.38 a good fit to d(ln[z⁺+a⁺])/dU⁺. A determination of the Karman constant from the variation of the skin-friction coefficients with Reynolds numbers also yields values near 0.38. The uncertainty is about ±0.01. These values are close to the boundary-layer experiments, but well below the accepted range of [0.40,0.41] and the experimental pipe-flow results near 0.42. The virtual-origin concept is also controversial, although nonessential at transportation or atmospheric Reynolds numbers. Yet, this series may reflect some success in verifying the law of the wall and investigating the logarithmic law by DNS, redundantly and with tools more impartial than the visual fit of a straight line to a velocity profile.

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47.27.N-	Wall-bounded shear flow turbulence
47.27.E-	Turbulence simulation and modeling

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Direct numerical simulation of unsteady flow in channel with rough walls

Kiran Bhaganagar

Phys. Fluids 20, 101508 (2008); http://dx.doi.org/10.1063/1.3005859 (15 pages) | Cited 3 times

Online Publication Date: 31 October 2008

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A fundamental study has been performed to understand the effect of unsteady forcing on turbulence statistics in channel flow with rough walls using direct numerical simulation. Unsteady flows have been generated by applying an unsteady nonzero mean forcing in the form of time varying pressure gradient such that the amplitude of oscillations is between 19% and 26% of mean centerline velocity and covering a range of forcing frequencies. The analysis has revealed unsteady forcing, depending on the forcing frequency, results in enhanced roughness compared to steady channel flow. The rough-wall flow dynamics have been categorized into high-, intermediate-, and low-frequency regimes. In the regime of high-frequency forcing, unsteadiness alters the mean velocity and turbulence intensities only in the inner layer of the turbulent boundary layer. Further, the turbulence intensities are out of phase with each other and also with the external forcing. In the regime of intermediate-frequency forcing, mean velocity and turbulence intensities are altered beyond the inner layer. In the inner layer, the turbulence intensities are out of phase with each other. The Reynolds stress is in phase with the external forcing in the inner layer, but it is out of phase in the outer layer. In the regime of low-frequency forcing, the mean velocity and turbulence intensities are significantly altered throughout the turbulent boundary layer.

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47.27.ek	Direct numerical simulations
47.27.nb	Boundary layer turbulence
47.27.nd	Channel flow
47.27.eb	Statistical theories and models
47.11.-j	Computational methods in fluid dynamics
47.60.Dx	Flows in ducts and channels

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Control and system identification of a separated flow

Shao-Ching Huang and John Kim

Phys. Fluids 20, 101509 (2008); http://dx.doi.org/10.1063/1.3005860 (11 pages) | Cited 3 times

Online Publication Date: 31 October 2008

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A procedure to construct linear optimal control for separated flows is presented. Unlike previous works in which a system model is derived from the linearized Navier–Stokes equations, we use an approximate linear model for the flow system generated by a system identification method based on input-output data sequences from numerical solutions of the Navier–Stokes equations. The approximate model is used in linear quadratic Gaussian synthesis to compute feedback control laws. Various properties of the identified model are tested and discussed. The closed-loop control is applied to a two-dimensional separated boundary layer, aiming at reducing its separation bubble size.

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47.20.Ib	Instability of boundary layers; separation
47.10.A-	Mathematical formulations
47.55.D-	Drops and bubbles

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Does the sailfish skin reduce the skin friction like the shark skin?

Woong Sagong, Chulkyu Kim, Sangho Choi, Woo-Pyung Jeon, and Haecheon Choi

Phys. Fluids 20, 101510 (2008); http://dx.doi.org/10.1063/1.3005861 (10 pages) | Cited 1 time

Online Publication Date: 31 October 2008

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The sailfish is the fastest sea animal, reaching its maximum speed of 110 km/h. On its skin, a number of V-shaped protrusions pointing downstream exist. Thus, in the present study, the possibility of reducing the skin friction using its shape is investigated in a turbulent boundary layer. We perform a parametric study by varying the height and width of the protrusion, the spanwise and streamwise spacings between adjacent ones, and their overall distribution pattern, respectively. Each protrusion induces a pair of streamwise vortices, producing low and high shear stresses at its center and side locations, respectively. These vortices also interact with those induced from adjacent protrusions. As a result, the drag is either increased or unchanged for most of the cases considered. Some of these cases show that the skin friction itself is reduced but the total drag including the form drag on the protrusion is larger than that of a smooth surface. In a few cases, the drag is decreased only slightly ( ∼ 1%) but this amount is within the experimental uncertainty. Since the shape of present protrusions is similar to that used by Sirovich and Karlsson [Nature (London) 388, 753 (1997) ] where V-shaped protrusions pointing upstream were considered, we perform another set of experiments following their study. However, we do not obtain any drag reduction even with random distribution of those V-shaped protrusions.

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47.27.nb	Boundary layer turbulence
47.32.-y	Vortex dynamics; rotating fluids
47.63.-b	Biological fluid dynamics
87.85.gf	Fluid mechanics and rheology

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Reynolds number effects on the Reynolds-stress budgets in turbulent channels

Sergio Hoyas and Javier Jiménez

Phys. Fluids 20, 101511 (2008); http://dx.doi.org/10.1063/1.3005862 (8 pages) | Cited 23 times

Online Publication Date: 31 October 2008

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Budgets for the nonzero components of the Reynolds-stress tensor are presented for numerical channels with Reynolds numbers in the range Re_τ = 180–2000. The scaling of the different terms is discussed, both above and within the buffer and viscous layers. Above x2+ ≈ 150, most budget components scale reasonably well with uτ3/h, but the scaling with uτ4/ν is generally poor below that level. That is especially true for the dissipations and for the pressure-related terms. The former is traced to the effect of the wall-parallel large-scale motions, and the latter to the scaling of the pressure itself. It is also found that the pressure terms scale better near the wall when they are not separated into their diffusion and deviatoric components, but mostly only because the two terms tend to cancel each other in the viscous sublayer. The budgets, together with their statistical uncertainties, are available electronically from http://torroja.dmt.upm.es/channels.

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47.15.Rq	Laminar flows in cavities, channels, ducts, and conduits
47.60.Dx	Flows in ducts and channels
47.10.-g	General theory in fluid dynamics

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Molecular effects on boundary condition in micro/nanoliquid flows

Umberto Ulmanella and Chih-Ming Ho

Phys. Fluids 20, 101512 (2008); http://dx.doi.org/10.1063/1.3006031 (9 pages) | Cited 12 times

Online Publication Date: 31 October 2008

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We experimentally investigated molecular effects of the slip/no-slip boundary condition of Newtonian liquids in micro- and nanochannels as small as 350 nm. The slip was measurable for channels smaller than approximately 2 μm. The amount of slip is found to be independent of the channel size, but is a function of the shear rate, the type of liquid (polar or nonpolar molecular structure), and the morphology of the solid surface (molecular-level smoothness).

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47.60.Dx	Flows in ducts and channels
47.45.Gx	Slip flows and accommodation

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Stability of a channel flow subject to wall blowing and suction in the form of a traveling wave

Changhoon Lee, Taegee Min, and John Kim

Phys. Fluids 20, 101513 (2008); http://dx.doi.org/10.1063/1.3006057 (8 pages) | Cited 4 times

Online Publication Date: 31 October 2008

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Inspired by the recent finding by Min et al. [J. Fluid Mech. 558, 309 (2006) ], the stability of a channel flow subject to wall blowing and suction in the form of a traveling wave is investigated by combined use of the Floquet analysis, direct numerical simulation, and singular value decomposition analysis. Results show that stability highly depends on the phase speed of the traveling wave; most disturbances become highly unstable when the phase speed is around 40% of the centerline velocity, while streamwise streak-type three-dimensional disturbances become stabilized with transient growth suppressed when the phase speed exceeds the centerline velocity for both subcritical and supercritical Reynolds numbers. This destabilization is interpreted by investigation of wave interactions. An upstream-traveling wave, which reduces mean drag, does not stabilize the flow.

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47.60.Dx	Flows in ducts and channels
47.20.-k	Flow instabilities
47.11.-j	Computational methods in fluid dynamics

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Turbulent dispersion from line sources in grid turbulence

Sharadha Viswanathan and Stephen B. Pope

Phys. Fluids 20, 101514 (2008); http://dx.doi.org/10.1063/1.3006069 (25 pages) | Cited 8 times

Online Publication Date: 31 October 2008

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Probability density function (PDF) calculations are reported for the dispersion from line sources in decaying grid turbulence. The calculations are performed using a modified form of the interaction by exchange with the conditional mean (IECM) mixing model. These flows pose a significant challenge to statistical models because the scalar length scale (of the initial plume) is much smaller than the turbulence integral scale. Consequently, this necessitates incorporating the effects of molecular diffusion in order to model laboratory experiments. Previously, Sawford [Flow Turb. Combust. 72, 133 (2004) ] performed PDF calculations in conjunction with the IECM mixing model, modeling the effects of molecular diffusion as a random walk in physical space and using a mixing time scale empirically fit to the experimental data of Warhaft [ J. Fluid Mech. 144, 363 (1984) ]. The resulting transport equation for the scalar variance contains a spurious production term. In the present work, the effects of molecular diffusion are instead modeled by adding a conditional mean scalar drift term, thus avoiding the spurious production of scalar variance. A laminar wake model is used to obtain an analytic expression for the mixing time scale at small times, and this is used as part of a general specification of the mixing time scale. Based on this modeling, PDF calculations are performed, and comparison is made primarily with the experimental data of Warhaft on single and multiple line sources and with the previous calculations of Sawford. A heated mandoline is also considered with comparison to the experimental data of Warhaft and Lumley [ J. Fluid Mech. 88, 659 (1978) ]. This establishes the validity of the proposed model and the significant effect of molecular diffusion on the decay of scalar fluctuations. The following are the significant predictions of the model. For the line source, the effect of the source size is limited to early times and can be completely accounted for by simple transformations. The peak centerline ratio of the rms to the mean of the scalar increases with the Reynolds number (approximately as Rλ1/3), whereas this ratio tends to a constant (approximately 0.4) at large times independent of R_λ. In addition, the model yields a universal long-time decay exponent for the temperature variance.

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47.27.eb	Statistical theories and models
47.15.Tr	Laminar wakes
47.10.-g	General theory in fluid dynamics

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Discrete conservation principles in large-eddy simulation with application to separation control over an airfoil

Donghyun You, Frank Ham, and Parviz Moin

Phys. Fluids 20, 101515 (2008); http://dx.doi.org/10.1063/1.3006077 (11 pages) | Cited 7 times

Online Publication Date: 31 October 2008

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An unstructured-grid large-eddy simulation (LES) technique is used to investigate the turbulent flow separation over an airfoil with and without synthetic-jet control. Numerical accuracy and stability on arbitrary shaped mesh elements at high Reynolds numbers are achieved using a finite-volume discretization of the incompressible Navier–Stokes equations based on higher-order conservation principles—i.e., in addition to mass and momentum conservation, kinetic energy conservation in the inviscid limit is used to guide the selection of the discrete operators and solution algorithm. Two different stall configurations, which consist of flow over a NACA 0015 airfoil at 16.6° and 20° angles of attack, are simulated at Reynolds number of 896 000 based on the airfoil chord length and freestream velocity. In the case of 16.6° angle of attack where flow separates around a midchord location, LES results show excellent agreement with the experimental data for both uncontrolled and controlled cases. LES confirms the experimental finding that synthetic jets, which are produced through a slot across the entire span on suction surface at 12% chord location, effectively delay the onset of flow separation and cause a significant increase in the lift coefficient. In the case of 20° angle of attack where flow separates near the leading edge, LES predicts reasonable results comparable to experimental data when grid resolution is sufficient to predict the separated shear layer. In this case, the synthetic-jet actuation at 12% chord location is found marginally effective in controlling leading-edge separation.

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47.11.-j	Computational methods in fluid dynamics
47.20.Ib	Instability of boundary layers; separation
47.27.E-	Turbulence simulation and modeling
47.32.Ff	Separated flows
47.40.-x	Compressible flows; shock waves
47.85.Gj	Aerodynamics
47.11.Df	Finite volume methods

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Time evolving simulations as a tentative reproduction of the Reynolds experiments on flow transition in circular pipes

P. Orlandi

Phys. Fluids 20, 101516 (2008); http://dx.doi.org/10.1063/1.3006124 (12 pages) | Cited 1 time

Online Publication Date: 31 October 2008

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Time developing numerical simulations of the Navier–Stokes equations in circular pipes can be performed to make an attempt to reproduce Reynolds’ 1883 experiments [ Philos. Trans. R. Soc. London 174, 935 (1883) ]. However, it should be demonstrated that these simulations are equivalent to space developing simulations. For Reynolds it was rather difficult to estimate exactly the inlet conditions. With high probability these were different from a Poiseuille profile with superimposed clean disturbances, the conditions often assigned in stability and transitional studies. To be close to the Reynolds experiment, in the present study, a turbulent field has been assigned as the initial condition. The lowest transition Reynolds numbers have been evaluated by decreasing the Reynolds numbers, and determining, by pressure gradient time history, and flow visualizations when the turbulent flow survives. This paper, in honor of John Kim, who used direct numerical simulation to understand the role of the near wall structures in turbulent plane channels, shows that the flow remains turbulent only when the near wall structures survive.

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47.60.Dx	Flows in ducts and channels
47.27.N-	Wall-bounded shear flow turbulence
47.10.ad	Navier-Stokes equations
47.80.Jk	Flow visualization and imaging

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Direct intervention of hairpin structures for turbulent boundary-layer control

Yong-Duck Kang, Kwing-So Choi, and Ho Hwan Chun

Phys. Fluids 20, 101517 (2008); http://dx.doi.org/10.1063/1.3006346 (13 pages) | Cited 2 times

Online Publication Date: 31 October 2008

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Direct intervention of large-scale, outer-layer structures of a turbulent boundary layer has been carried out by counteracting the upwash motion of hairpin vortices with jets issued from a nozzle placed outside the boundary layer. The methodology of this turbulent boundary-layer control is similar in concept to the opposition control of near-wall turbulence, where the induced velocity field of vortical motion during the turbulence activities is opposed by suction and blowing at the wall. Unlike wall-based turbulence control techniques whose time and length scales reduce with an increase in the Reynolds number, scales of the proposed control are those of the outer layer, making this control technique highly practical. Here we show some results from a direct intervention of hairpin structures in a turbulent boundary layer, demonstrating that this is a promising technique for turbulence control.

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47.27.nb	Boundary layer turbulence
47.27.nf	Flows in pipes and nozzles
47.27.wg	Turbulent jets
47.60.Kz	Flows and jets through nozzles
47.32.-y	Vortex dynamics; rotating fluids

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Variations of von Kármán coefficient in canonical flows

Hassan M. Nagib and Kapil A. Chauhan

Phys. Fluids 20, 101518 (2008); http://dx.doi.org/10.1063/1.3006423 (10 pages) | Cited 32 times

Online Publication Date: 31 October 2008

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The overlap parameters for the logarithmic law are obtained for available turbulent pipe and channel flow data using composite profiles fitted to the mean velocity. The composite profile incorporates κ, B, and Π as the varying parameters and their resulting behavior with Reynolds number is examined for these flows and compared to results from boundary layers. The von Kármán coefficient in channel flow is smaller than the well-established value for zero pressure gradient turbulent boundary layers of 0.384, while in pipe flows it is consistently higher. In contrast, the estimates of the wake parameter Π are the smallest for channel flows and largest for boundary layers. Further, the Superpipe data are reanalyzed to reveal that κ = 0.41 is a better value for the von Kármán constant in pipe flow. The collective behavior of κ in boundary layers, pipes, and channels reveals that the von Kármán coefficient is not universal and exhibits dependence not only on the pressure gradient but also on the flow geometry.

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47.60.Dx	Flows in ducts and channels
47.27.N-	Wall-bounded shear flow turbulence

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Subsonic jet noise reduction by fluidic control: The interaction region and the global effect

E. Laurendeau, P. Jordan, J. P. Bonnet, J. Delville, P. Parnaudeau, and E. Lamballais

Phys. Fluids 20, 101519 (2008); http://dx.doi.org/10.1063/1.3006424 (15 pages) | Cited 5 times

Online Publication Date: 31 October 2008

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A microjet arrangement comprising both penetration (or immersion) and convergence (jets oriented such that two jets of a pair interact with one another) is used to control a subsonic turbulent jet with a view to noise reduction. The acoustic effect of the so-called fluidevron system is comparable to chevrons and nonconverging microjets as far as the noise reduction is concerned. Detailed experimental measurements are performed for a main jet with Mach and Reynolds numbers of 0.3 and 310 000, respectively. A direct numerical simulation study is performed for a model, plane mixing-layer problem using the immersed-boundary method, in order to help understand the topological features of the fluidevron–mixing-layer interaction. In terms of modifications produced in the flow, two relatively distinct regions are identified: the near-nozzle region, 0<(x/D)<1, where the dynamics are dominated by the fluidevron–main-jet interaction, and the region (x/D)>1, where the jet recovers many of the uncontrolled-jet flow characteristics, but with globally reduced turbulence levels and a longer potential core. The flow structure produced in the near-nozzle region is found to comprise an ejection of fluid from the main jet; the ejection process leads to very high fluctuation levels. This highly turbulent fluid, on being reassimilated by the mixing-layer downstream of the interaction point, has a spectacular local impact on turbulent kinetic energy production and on the entrainment: the former is reduced by 70%, and the latter boosted by 30% over the range 0.2<(x/D)<3. The impact of the flow control on the integral scales of the turbulence is assessed, as these are central to acoustic-analogy-based source models. A significant reduction is found in the radial integral scales, and these are then weighted by the local fluctuation energy in order to assess the impact of the control on the source mechanisms of the flow (considered in the context of Lighthill’s formulation of the problem). Considerable reductions are shown between the base line and controlled flows in terms of these energy-weighted space scales.

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47.27.W-	Boundary-free shear flow turbulence
47.40.Dc	General subsonic flows
47.60.Kz	Flows and jets through nozzles

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Critical mass and a new periodic four-ring vortex wake mode for freely rising and falling spheres

M. Horowitz and C. H. K. Williamson

Phys. Fluids 20, 101701 (2008); http://dx.doi.org/10.1063/1.2992126 (4 pages) | Cited 3 times

Online Publication Date: 10 October 2008

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In this letter, we study the dynamics and vortex wakes of spheres rising or falling freely through a fluid. Since this problem was first considered by Newton in 1726, the conditions under which a sphere will vibrate are still not understood clearly. In our experiments, all falling spheres (where the relative density, m^∗>1) descend rectilinearly. Although previous studies conclude that all rising spheres (m^∗<1) vibrate, we find instead that there exists a critical value of the relative density (for example, mcrit∗ = 0.36, for Reynolds numbers 400–500) above which there is a significant regime where rising spheres do not vibrate. Lighter spheres undergo large-amplitude periodic oscillations, confined to a single vertical plane. We discover a new mode of vortex formation comprising four vortex rings formed in each cycle, distinct from previous vortex modes for fixed and tethered bodies.

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47.32.cf	Vortex reconnection and rings
47.27.wb	Turbulent wakes
47.55.-t	Multiphase and stratified flows

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Enhanced particle filtration in straight microchannels using shear-modulated inertial migration

Ali Asgar S. Bhagat, Sathyakumar S. Kuntaegowdanahalli, and Ian Papautsky

Phys. Fluids 20, 101702 (2008); http://dx.doi.org/10.1063/1.2998844 (4 pages) | Cited 33 times

Online Publication Date: 15 October 2008

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In this work, we introduce a novel method for enhanced particle filtration using shear-modulated inertial migration in straight microchannels. Depending on their size, inertial lift causes particles to migrate toward microchannel walls. Using microchannels with high aspect ratio cross sections, the fluidic shear can be modulated, resulting in preferential equilibration of particles along the longer microchannel walls. Due to large lift forces generated in these high aspect ratio channels, complete particle filtration can be achieved in short distances even at low flow rates (Re<50). Based on this principle, we use a straight microfluidic channel with a rectangular cross section to passively and continuously filter 1.9 μm polystyrene particles. Overall, the proposed technique is versatile and can be easily integrated with on-chip microfluidic systems for filtration of a wide range of particle sizes, from micro- to nanoparticles.

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47.60.Dx	Flows in ducts and channels
07.10.Cm	Micromechanical devices and systems
47.27.N-	Wall-bounded shear flow turbulence

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Optical levitation and transport of microdroplets: Proof of concept

Peter T. Nagy and G. Paul Neitzel

Phys. Fluids 20, 101703 (2008); http://dx.doi.org/10.1063/1.3005394 (4 pages) | Cited 4 times

Online Publication Date: 31 October 2008

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A new technique for microfluidic transport of liquid droplets in “lab-on-a-chip” (LOC) applications is described in which droplets are levitated above or between solid planar surfaces through the use of thermocapillarity, the variation in a liquid’s surface tension with temperature. Levitated liquid droplets are not in contact with solid surfaces and so may be transported from point-to-point along arbitrary paths with little friction. In addition, the lack of liquid-solid contact virtually eliminates the potential for sample-to-sample contamination. The new technique therefore addresses three issues associated with existing LOC devices employing microchannels: droplet pathway constraints, transport speed, and sample-to-sample contamination.

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47.85.Np	Fluidics
47.61.-k	Micro- and nano- scale flow phenomena
47.55.N-	Interfacial flows
47.55.D-	Drops and bubbles
68.03.Cd	Surface tension and related phenomena
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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Dewetting of nanometer thin films under an electric field

Guo-Hui Hu, Ai-Jin Xu, Zhen Xu, and Zhe-Wei Zhou

Phys. Fluids 20, 102101 (2008); http://dx.doi.org/10.1063/1.2998845 (5 pages) | Cited 1 time

Online Publication Date: 14 October 2008

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The dewetting of a nanoscale water film under the action of an electric field is studied with molecular dynamics simulation. Results show that the onset of film rupture is induced by a spontaneous instability mechanism. After the rupture, the rim of the film recedes with a dynamic contact angle. The transient streamlines at a typical moment show that the liquid molecule near the rim moves almost vertically upwards, driven by the repulsive force from the solid surface. The oscillatory behavior of the density profile, resulting from the interaction between attractive and repulsive potentials, is observed near the solid surface. The analyses of the dewetting process demonstrate that the applied electric field will increase the wettability of graphite walls, thus suppressing the rupture, reducing the dynamic contact angle, and raising the liquid density adjacent to both the solid and liquid-vacuum surfaces. Owing to the polarity of water, the positive voltage produces stronger influences than the negative one.

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68.15.+e	Liquid thin films
68.08.Bc	Wetting
68.03.Cd	Surface tension and related phenomena
61.20.Ja	Computer simulation of liquid structure

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Capillary rise of a liquid between two vertical plates making a small angle

F. J. Higuera, A. Medina, and A. Liñán

Phys. Fluids 20, 102102 (2008); http://dx.doi.org/10.1063/1.3000425 (7 pages) | Cited 3 times

Online Publication Date: 20 October 2008

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The penetration of a wetting liquid in the narrow gap between two vertical plates making a small angle is analyzed in the framework of the lubrication approximation. At the beginning of the process, the liquid rises independently at different distances from the line of intersection of the plates except in a small region around this line where the effect of the gravity is negligible. The maximum height of the liquid initially increases as the cubic root of time and is attained at a point that reaches the line of intersection only after a certain time. At later times, the motion of the liquid is confined to a thin layer around the line of intersection whose height increases as the cubic root of time and whose thickness decreases as the inverse of the cubic root of time. The evolution of the liquid surface is computed numerically and compared with the results of a simple experiment.

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47.60.-i	Flow phenomena in quasi-one-dimensional systems
68.08.Bc	Wetting

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Instabilities and Taylor dispersion in isothermal binary thin fluid films

Z. Borden, H. Grandjean, A. E. Hosoi, L. Kondic, and B. S. Tilley

Phys. Fluids 20, 102103 (2008); http://dx.doi.org/10.1063/1.3005453 (11 pages)

Online Publication Date: 28 October 2008

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Experiments with glycerol-water thin films flowing down an inclined plane reveal a localized instability that is primarily three dimensional. These transient structures, referred to as “dimples,” appear initially as nearly isotropic depressions on the interface. A linear stability analysis of a binary mixture model in which barodiffusive effects dominate over thermophoresis (i.e., the Soret effect) reveals unstable modes when the components of the mixture have different bulk densities and surface tensions. This instability occurs when Fickian diffusion and Taylor dispersion effects are small, and is driven by solutalcapillary stresses arising from gradients in concentration of one component, across the depth of the film. Qualitative comparison between the experiments and the linear stability results over a wide range of parameters is presented.

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