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Mar 2013

Volume 25, Issue 3, Articles (03xxxx)

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Phys. Fluids 25, 031302 (2013); http://dx.doi.org/10.1063/1.4793543 (13 pages)

Gretar Tryggvason, Sadegh Dabiri, Bahman Aboulhasanzadeh, and Jiacai Lu
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Experimental manipulation of wall turbulence: A systems approach

B. J. McKeon, A. S. Sharma, and I. Jacobi

Phys. Fluids 25, 031301 (2013); http://dx.doi.org/10.1063/1.4793444 (34 pages)

Online Publication Date: 19 March 2013

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We review recent progress, based on the approach introduced by McKeon and Sharma [J. Fluid Mech. 658, 336–382 (2010)10.1017/S002211201000176X], in understanding and controlling wall turbulence. The origins of this analysis partly lie in nonlinear robust control theory, but a differentiating feature is the connection with, and prediction of, state-of-the-art understanding of velocity statistics and coherent structures observed in real, high Reynolds number flows. A key component of this line of work is an experimental demonstration of the excitation of velocity response modes predicted by the theory using non-ideal, but practical, actuation at the wall. Limitations of the approach and promising directions for future development are outlined.
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47.27.nb Boundary layer turbulence
47.85.ld Boundary layer control
02.30.Yy Control theory
02.50.-r Probability theory, stochastic processes, and statistics
47.27.De Coherent structures
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Multiscale considerations in direct numerical simulations of multiphase flows

Gretar Tryggvason, Sadegh Dabiri, Bahman Aboulhasanzadeh, and Jiacai Lu

Phys. Fluids 25, 031302 (2013); http://dx.doi.org/10.1063/1.4793543 (13 pages)

Online Publication Date: 19 March 2013

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Direct Numerical Simulations of multiphase flows have progressed rapidly over the last decade and it is now possible to simulate, for example, the motion of hundreds of deformable bubbles in turbulent flows. The availability of results from such simulations should help advance the development of new and improved closure relations and models of the average or large-scale flows. We review recent results for bubbly flow in vertical channels, discuss the difference between upflow and downflow and the effect of the bubble deformability and how the resulting insight allowed us to produce a simple description of the large scale flow, for certain flow conditions. We then discuss the need for the development of numerical methods for more complex situations, such as where the flow creates spontaneous thin films and threads, or where additional physical processes take place at a rate that is very different from the fluid flow. Recent work on capturing localized small-scale processes using embedded analytical models, focusing on the mass transfer from bubbles in liquids with low mass diffusivity, suggests one approach. We conclude by discussing immediate needs for progress on the theoretical framework for describing the large-scale motion of multiphase flows and the need for multiscale methods to capture physical processes taking place at diverse length and time scales.
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47.55.D- Drops and bubbles
47.60.Dx Flows in ducts and channels
47.11.-j Computational methods in fluid dynamics
47.27.E- Turbulence simulation and modeling
47.27.ek Direct numerical simulations
47.27.nd Channel flow
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Liquid meniscus friction on a wet plate: Bubbles, lamellae, and foams

Isabelle Cantat

Phys. Fluids 25, 031303 (2013); http://dx.doi.org/10.1063/1.4793544 (21 pages)

Online Publication Date: 19 March 2013

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Many microfluidics devices, coating processes, or diphasic flows involve the motion of a liquid meniscus on a wet wall. This motion induces a specific viscous force, that exhibits a nonlinear dependency in the meniscus velocity. We propose a review of the theoretical and experimental work made on this viscous force, for simple interfacial properties. The interface is indeed assumed either perfectly compressible (mobile interface) or perfectly incompressible (rigid interface). We show that, in the second case, the viscous force exerted by the wall on the meniscus is a combination of two power laws, scaling such as Ca1/3 and Ca2/3, with Ca the capillary number. We provide a prediction for the stress exerted on a foam sliding on a wet solid and compare it with experimental data, for the incompressible case.
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47.55.dd Bubble dynamics
47.55.nb Capillary and thermocapillary flows
68.03.Cd Surface tension and related phenomena
82.70.Rr Aerosols and foams
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