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May 2010

Volume 22, Issue 5, Articles (05xxxx)

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Phys. Fluids 22, 051301 (2010); http://dx.doi.org/10.1063/1.3407662 (20 pages)

H. J. S. Fernando, D. Zajic, S. Di Sabatino, R. Dimitrova, B. Hedquist, and A. Dallman
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Bubble entrapment through topological change

S. T. Thoroddsen, K. Takehara, and T. G. Etoh

Phys. Fluids 22, 051701 (2010); http://dx.doi.org/10.1063/1.3407654 (4 pages) | Cited 7 times

Online Publication Date: 3 May 2010

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When a viscous drop impacts onto a solid surface, it entraps a myriad of microbubbles at the interface between liquid and solid. We present direct high-speed video observations of this entrapment. For viscous drops, the tip of the spreading lamella is separated from the surface and levitated on a cushion of air. We show that the primary mechanism for the bubble entrapment is contact between this precursor sheet of liquid with the solid and not air pulled directly through cusps in the contact line. The sheet makes contact with the solid surface, forming a wetted patch, which grows in size, but only entraps a bubble when it meets the advancing contact line. The leading front of this wet patch can also lead to the localized thinning and puncturing of the liquid film producing strong splashing of droplets.
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47.55.D- Drops and bubbles
68.08.Bc Wetting

Locality properties of the energy flux in magnetohydrodynamic turbulence

J. Andrzej Domaradzki, Bogdan Teaca, and Daniele Carati

Phys. Fluids 22, 051702 (2010); http://dx.doi.org/10.1063/1.3431227 (4 pages) | Cited 3 times

Online Publication Date: 24 May 2010

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The scale locality functions, originally introduced by Kraichnan for hydrodynamic turbulence, are computed from results of direct numerical simulations of forced magnetohydrodynamic turbulence. It is found that asymptotically the dynamics is dominated by local interactions, but the locality is much weaker than in hydrodynamic turbulence, which is characterized by the scaling exponent of 4/3. Specifically, in magnetohydrodynamic turbulence, two distinct exponents are observed, 1/3 and 2/3. Despite that, direct numerical simulation results reported in this paper exhibit strong coupling between large scales from the forcing band and smallest resolved scales because the locality is too weak to achieve decoupling for the numerical resolution available.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.27.ek Direct numerical simulations
47.11.-j Computational methods in fluid dynamics
02.60.Cb Numerical simulation; solution of equations

The role of electric charge in microdroplets impacting on conducting surfaces

Weiwei Deng and Alessandro Gomez

Phys. Fluids 22, 051703 (2010); http://dx.doi.org/10.1063/1.3431739 (4 pages) | Cited 2 times

Online Publication Date: 24 May 2010

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A rich phenomenology is revealed by temporally resolved image sequences of electrically charged ethanol microdroplets impacting on a conductive surface at temperatures bracketing the liquid boiling point. Notable phenomena include the flattening of the sessile droplets with reduced contact angle, increased evaporation rates for substrate temperatures below the fluid boiling point, and the hindrance of droplet rebound at the Leidenfrost temperature. Scaling considerations are presented to rationalize the observed behavior and to generalize conclusions to a broader droplet size range.
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47.55.dp Cavitation and boiling
47.65.-d Magnetohydrodynamics and electrohydrodynamics
68.03.Cd Surface tension and related phenomena
47.80.Jk Flow visualization and imaging

Quantifying the interaction between large and small scales in wall-bounded turbulent flows: A note of caution

Philipp Schlatter and Ramis Örlü

Phys. Fluids 22, 051704 (2010); http://dx.doi.org/10.1063/1.3432488 (4 pages) | Cited 10 times

Online Publication Date: 24 May 2010

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Turbulent flow close to solid walls is dominated by an ensemble of fluctuations of large and small spatial scales. Recent work by Mathis et al. [J. Fluid Mech. 628, 311 (2009) ; Phys. Fluids 21, 111703 (2009)] introduced and used a decoupling procedure based on the Hilbert transformation applied to the filtered small-scale component of the fluctuating streamwise velocity. This method is employed as a robust tool to quantify a dominant amplitude modulation of the small scales by the large scales found in the outer part of the boundary layer. In the present study, however, we demonstrate by means of experimental and synthetic signals that the correlation coefficient used to quantify the amplitude modulation is related to the skewness of the original signal, and hence, for the Reynolds numbers considered here, may not be an independent tool to unambiguously detect or quantify the effect of large-scale amplitude modulation of the small scales.
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47.27.nb Boundary layer turbulence

Anomalous memory effects on transport of inertial particles in turbulent jets

F. Picano, G. Sardina, P. Gualtieri, and C. M. Casciola

Phys. Fluids 22, 051705 (2010); http://dx.doi.org/10.1063/1.3432439 (4 pages) | Cited 7 times

Online Publication Date: 26 May 2010

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The letter focuses on a new phenomenology found in the far field of turbulent-free jets, where small inertial particles exhibit a local concentration peak on the axis. This finding contrasts with the prediction of classical models based on turbulent kinetic energy gradient transport assumptions, whereby particles should move away from the local kinetic energy maxima. This behavior is universal, i.e., it occurs no matter the details of the specific jet, and takes place irrespective of the inertia of the particles. As anomalous signature of the near field dynamics, it cannot be predicted on purely dimensional grounds. A new form of similarity allows to collapse the local particle flux profile on a universal curve.
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47.55.Kf Particle-laden flows
47.27.wg Turbulent jets
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