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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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back to top Turbulent Flows

Implicit large-eddy simulation of passive scalar mixing in statistically stationary isotropic turbulence

A. J. Wachtor, F. F. Grinstein, C. R. DeVore, J. R. Ristorcelli, and L. G. Margolin

Phys. Fluids 25, 025101 (2013); http://dx.doi.org/10.1063/1.4783924 (19 pages) | Cited 3 times

Online Publication Date: 4 February 2013

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Turbulent mixing of a passive scalar by forced isotropic turbulence with a prescribed mean scalar gradient is studied in the context of implicit large-eddy simulation. The simulation strategy uses a multi-dimensional compressible flux-corrected transport algorithm, with low wavenumber momentum forcing imposed separately for the solenoidal and dilatational velocity components. Effects of grid resolution on the flow and scalar mixing are investigated at turbulent Mach numbers 0.13 and 0.27. Turbulence metrics are used to show that an implicit large-eddy simulation can accurately capture the mixing transition and asymptotic self-similar behaviors predicted by previous theoretical, laboratory, and direct numerical simulation studies, including asymptotically constant scalar variance and increasing velocity-to-scalar Taylor micro-scales ratio as function of effective Reynolds number determined by grid resolution. The results demonstrate the feasibility of predictive under-resolved simulations of high Reynolds number turbulent scalar mixing using implicit large-eddy simulation.
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47.27.Gs Isotropic turbulence; homogeneous turbulence
47.27.wj Turbulent mixing layers
47.40.Dc General subsonic flows
47.11.-j Computational methods in fluid dynamics
47.27.ep Large-eddy simulations
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Scanning tomographic particle image velocimetry applied to a turbulent jet

T. A. Casey, J. Sakakibara, and S. T. Thoroddsen

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

Online Publication Date: 21 February 2013

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We introduce a modified tomographic PIV technique using four high-speed video cameras and a scanning pulsed laser-volume. By rapidly illuminating adjacent subvolumes onto separate video frames, we can resolve a larger total volume of velocity vectors, while retaining good spatial resolution. We demonstrate this technique by performing time-resolved measurements of the turbulent structure of a round jet, using up to 9 adjacent volume slices. In essence this technique resolves more velocity planes in the depth direction by maintaining optimal particle image density and limiting the number of ghost particles. The total measurement volumes contain between 1 ×106 and 3 ×106 velocity vectors calculated from up to 1500 reconstructed depthwise image planes, showing time-resolved evolution of the large-scale vortical structures for a turbulent jet of Re up to 10 000.
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47.80.Jk Flow visualization and imaging
42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy
47.27.wg Turbulent jets
47.32.C- Vortex dynamics
47.80.Cb Velocity measurements

Experimental study of skin friction drag reduction on superhydrophobic flat plates in high Reynolds number boundary layer flow

Elias Aljallis, Mohammad Amin Sarshar, Raju Datla, Vinod Sikka, Andrew Jones, and Chang-Hwan Choi

Phys. Fluids 25, 025103 (2013); http://dx.doi.org/10.1063/1.4791602 (14 pages)

Online Publication Date: 21 February 2013

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In this paper, we report the measurement of skin friction drag on superhydrophobic-coated flat plates in high Reynolds (Re) number boundary layer flows, using a high-speed towing tank system. Aluminum flat plates with a large area (4 feet × 2 feet, 3/8 in. thick) and sharpened leading/trailing edges (1 in. long) were prepared as a boundary layer flow model. Spray coating of hydrophobic nanoparticles was applied to make two different types of superhydrophobic coatings: one with low contact angle and high contact angle hysteresis, and the other with high contact angle and low contact angle hysteresis. Skin friction drag of the superhydrophobic plates was measured in the flow speed up to 30 ft/s to cover transition and turbulent flow regimes (105 < ReL < 107), and was compared to that of an uncoated bare aluminum plate. A significant drag reduction was observed on the superhydrophobic plate with high contact angle and low contact angle hysteresis up to ∼30% in transition regime (105 < ReL < 106), which is attributed to the shear-reducing air layer entrapped on the superhydrophobic surface. However, in fully turbulence regime (106 < ReL < 107), an increase of drag was observed, which is ascribed to the morphology of the surface air layer and its depletion by high shear flow. The texture of superhydrophobic coatings led to form a rugged morphology of the entrapped air layer, which would behave like microscale roughness to the liquid flow and offset the drag-reducing effects in the turbulent flow. Moreover, when the superhydrophobic coating became wet due to the removal of air by high shear at the boundary, it would amplify the surface roughness of solid wall and increase the drag in the turbulent flow. The results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the effective drag reduction using superhydrophobic surfaces, especially in high Re number turbulent flow regimes.
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47.27.nb Boundary layer turbulence
47.55.dr Interactions with surfaces
47.80.Jk Flow visualization and imaging
68.03.Cd Surface tension and related phenomena
68.08.Bc Wetting
47.27.Cn Transition to turbulence
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Reynolds-number scaling of turbulent channel flow

M. P. Schultz and K. A. Flack

Phys. Fluids 25, 025104 (2013); http://dx.doi.org/10.1063/1.4791606 (13 pages)

Online Publication Date: 21 February 2013

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Results of an experimental study of smooth-wall, fully developed, turbulent channel flow are presented. The Reynolds number (Rem) based on the channel height and the bulk mean velocity ranged from 10 000 to 300 000. The present results indicate that the skin-friction coefficient (Cf) closely follows a power law for Rem < 62 000. At higher Reynolds numbers, Cf is best described by a log law. Detailed two-component velocity measurements taken at friction Reynolds numbers of Reτ = 1000–6000 indicate that the mean flow and Reynolds shear stress display little or no Reynolds-number dependence. The streamwise Reynolds normal stress (math+), on the other hand, varies significantly with Reynolds number. The inner peak in math+ is observed to grow with Reynolds number. Growth in math+ farther from the wall is documented over the entire range of Reynolds number giving rise to a plateau in the streamwise Reynolds normal stress in the overlap region of the profile for Reτ = 6000. The wall-normal Reynolds normal stress (math+) displays no Reynolds-number dependence near the wall. Some increase in math+ in the outer layer is noted for Reτ ≤ 4000. The trends in the present Reynolds stress results agree qualitatively with recent experimental results from pipe and boundary layer flows.
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47.27.nd Channel flow
47.60.Dx Flows in ducts and channels
47.27.nb Boundary layer turbulence

Three-dimensional simulation of natural convection in a reservoir sidearm

Karl A. Dittko, Michael P. Kirkpatrick, and Steven W. Armfield

Phys. Fluids 25, 025105 (2013); http://dx.doi.org/10.1063/1.4792709 (26 pages)

Online Publication Date: 27 February 2013

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Numerical simulations were conducted of turbulent natural convection in a shallow tetrahedron domain representing the sidearm of a lake or water reservoir. The tetrahedron cavity is a more realistic, three-dimensional approximation of a lake or reservoir sidearm than the two-dimensional triangle cavity often seen in the literature. Lateral temperature gradients exist due to the varying depth of the cavity, resulting in lateral circulation. These flows are important in a reservoir as they can carry with them particles and various pollutants, transporting and mixing them with the central section. Therefore, study in this area is important in water quality management. The simulations use a Cartesian grid with an Immersed Boundary Method for the sloped bottom surfaces. Heat input is through a solar radiation model consisting of a heat flux from the sloped bottom boundaries and an internal heating source term in the body of the water. Also studied is the night time model where cooling is through a heat flux at the top boundary. Scaling analysis from the literature is extended to suit the new geometry and numerical simulations are used to validate the results. The numerical simulations include calculating horizontal heat transfer profiles, volumetric flow rates, and analysis of complex flow features. The extension to three dimensions results in significant changes to the flow and introduces some complex features, such as helical flow both towards and away from the tip. Some general parameterisations are proposed for the tetrahedron cavity based on the numerical simulations.
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47.27.te Turbulent convective heat transfer
02.60.Gf Algorithms for functional approximation
47.11.-j Computational methods in fluid dynamics
47.27.E- Turbulence simulation and modeling
47.27.wj Turbulent mixing layers
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