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Feb 2013

Volume 25, Issue 2, Articles (02xxxx)

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Phys. Fluids 25, 025102 (2013); http://dx.doi.org/10.1063/1.4790640 (31 pages)

T. A. Casey, J. Sakakibara, and S. T. Thoroddsen
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A simple criterion for filament break-up in drop-on-demand inkjet printing

S. D. Hoath, S. Jung, and I. M. Hutchings

Phys. Fluids 25, 021701 (2013); http://dx.doi.org/10.1063/1.4790193 (5 pages)

Online Publication Date: 1 February 2013

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Jets from drop-on-demand inkjet print-heads consist of a main drop with a trailing filament, which either condenses into the main drop, or breaks up into satellite drops. Filament behaviour is quantitatively similar to that of larger, free symmetrical filaments, and can be predicted from the aspect ratio and Ohnesorge number. Symmetrical filaments generated from inkjet print-heads show the same behaviour. A simple model, based on competition between the processes of axial shortening and radial necking, predicts the critical aspect ratio below which the jet condenses into a single drop. The success of this simple criterion supports the underlying physical model.
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47.55.df Breakup and coalescence
47.60.Kz Flows and jets through nozzles
64.70.F- Liquid-vapor transitions
47.11.-j Computational methods in fluid dynamics
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Formulation for volume flowrate induced by vortex rings, through circular orifice of arbitrary section profile

Chi-Hoon Cho and Duck-Joo Lee

Phys. Fluids 25, 021702 (2013); http://dx.doi.org/10.1063/1.4791776 (5 pages)

Online Publication Date: 13 February 2013

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A new theoretical formulation that relates the volume flowrate to the vorticity distribution for the circular orifice of an arbitrary section profile is presented. The formulation is based on axisymmetric potential theory involving Green's function of Stokes’ streamfunction. The results showed that the volume flowrate through the orifice opening can be separated into potential and vortical portions, and the kernel of the vortical portion is the homogeneous streamfunction of the orifice. The formulation for the thin circular orifice was verified by comparing analytical solutions.
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47.32.C- Vortex dynamics
02.30.-f Function theory, analysis
47.11.-j Computational methods in fluid dynamics
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Some exact properties of the effective slip over surfaces with hydrophobic patternings

Pierre Six and Ken Kamrin

Phys. Fluids 25, 021703 (2013); http://dx.doi.org/10.1063/1.4790536 (7 pages)

Online Publication Date: 19 February 2013

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Shear flows of viscous fluid layers over nonuniformly hydrophobic surfaces are characterized in the far-field by an effective slip velocity, which relates to the applied stress through some mobility tensor characterizing the surface. Here, we identify two methods to determine the mobility tensor for flat surfaces with arbitrary slip-length variations. A family of “Cross Flow Identities” is then analyzed, which equate mobility components of different unidirectional patternings. We also calculate an analytical mobility solution for a family of continuously varying patterns. We validate the results numerically and discuss implications in various limits.
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47.45.Gx Slip flows and accommodation
47.54.Bd Theoretical aspects
02.10.Ud Linear algebra
02.60.-x Numerical approximation and analysis
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Probing high-Reynolds-number effects in numerical boundary layers

Sergio Pirozzoli and Matteo Bernardini

Phys. Fluids 25, 021704 (2013); http://dx.doi.org/10.1063/1.4792164 (7 pages)

Online Publication Date: 25 February 2013

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We study the high-Reynolds-number behavior of a turbulent boundary layer in the low supersonic regime through very-large-scale direct numerical simulation (DNS). For the first time a Reynolds number is attained in DNS (Reτ = δ/δv ≈ 4000, where δ is the boundary layer thickness and δv is the viscous length scale) at which theoretical predictions and experiments suggest the occurrence of phenomena pertaining to the asymptotic Reynolds number regime. From comparison with previous DNS data at lower Reynolds number we find evidence of a continuing trend toward a stronger imprint of the outer-layer structures onto the near-wall region. This effect is clearly manifested both in flow visualizations, and in energy spectra. More than a decade of nearly-logarithmic variation is observed in the mean velocity profiles, with log-law constants k ≈ 0.394, C ≈ 4.84, and a trend similar to experiments. We find some supporting evidence for the debated existence of a k−1 region in the power spectrum of streamwise velocity fluctuations, which extends up to y+ ≈ 150, and of a k−5/3 spectral range in the outer layer.
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47.27.nb Boundary layer turbulence
47.27.ek Direct numerical simulations
47.11.-j Computational methods in fluid dynamics
02.60.Cb Numerical simulation; solution of equations
47.40.Ki Supersonic and hypersonic flows
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