Journal ArticleDOI
Wall-layer structure and drag reduction
TLDR
In this article, the authors measured the spanwise spacing and bursting rate of the wall-layer structure of a turbulent channel flow of water and showed that when the additives are confined entirely to the linear sublayer of the water flow and there is no evidence of drag reduction, the span-wise streak spacing increases and the average bursting rate decreases.Abstract:
When drag-reducing additives are confined entirely to the linear sublayer of a turbulent channel flow of water, both the spanwise spacing and bursting rate of the wall-layer structure are the same as those for a water flow and there is no evidence of drag reduction. Drag reduction is measured downstream of the location where the additives injected into the sublayer begin to mix in significant quantities with the buffer region (10 y + The superscript + denotes a dimensionless quantity scaled with the kinematic viscosity ν and the wall shear velocity v * = (τ w /ρ) ½ . of the channel flow. At streamwise locations where drag reduction does occur and where the injected fluid is not yet uniformly mixed with the channel flow, the dimensionless spanwise streak spacing increases and the average bursting rate decreases. The decrease in bursting rate is larger than the corresponding increase in streak spacing. The wall-layer structure is like the structure in the flow of a homogeneous, uniformly mixed, drag-reducing solution. Thus, the additives have a direct effect on the flow processes in the buffer region and the linear sublayer appears to have a passive role in the interaction of the inner and outer portions of a turbulent wall layer.read more
Citations
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Journal ArticleDOI
Direct numerical simulation of the turbulent channel flow of a polymer solution
TL;DR: In this paper, the authors present a direct numerical simulation of a fully turbulent channel flow of a dilute polymer solution, where the polymer chains are modeled as finitely extensible and elastic dumbbells.
Journal ArticleDOI
Flow Turbulence over Fixed and Weakly Mobile Gravel Beds
Vladimir Nikora,Derek G. Goring +1 more
TL;DR: In this paper, three sets of measurements with acoustic Doppler velocimeters in an irrigation canal were used: two with subcritical bed shear stress (static beds) and one with the bedshear stress τo close to critical τoc (weakly mobile bed), and the analyses included vertical distributions of local mean velocities, turbulence intensities, turbulent shear stresses, velocity auto- and cross-spectra, the quadrant method, and high-order velocity moments.
Journal ArticleDOI
Timescale and structure of ejections and bursts in turbulent channel flows
T. S. Luchik,W. G. Tiederman +1 more
TL;DR: In this article, Eulerian burst-detection schemes were developed through extensions of the uv quadrant 2, VITA, and u-level techniques to identify and to group those ejections from a single burst into a single-burst detection.
Journal ArticleDOI
Direct numerical simulation of viscoelastic turbulent channel flow exhibiting drag reduction: effect of the variation of rheological parameters
TL;DR: Sureshkumar et al. as discussed by the authors used a semi-implicit, time-splitting technique which uses spectral approximations in the spatial domain to predict drag reduction for a variety of rheological parameters.
Journal ArticleDOI
Effects of bacterial exopolymer adhesion on the entrainment of sand
W. Brian Dade,John D. Davis,Peter D. Nichols,Arthur R. M. Nowell,David Thistle,Melanie B. Trexler,David C. White +6 more
TL;DR: In this paper, it was shown that either pure exopolymer alone or ex-polymer generated during in situ growth increases erosion resistance of fine quartz sand and that the degree of erosion resistance increases in proportion to the concentration of ex-opolymer component uronic acids, which in turn is dependent on relative nitrogen content of peptone-based growth media.
References
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Journal ArticleDOI
The structure of turbulent boundary layers
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Organized Motion in Turbulent Flow
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Journal ArticleDOI
The characteristics of low-speed streaks in the near-wall region of a turbulent boundary layer
C. R. Smith,S. P. Metzler +1 more
TL;DR: In this article, a high-speed video system and hydrogen bubble-wire flow visualization was used to investigate the characteristics of low-speed streaks which occur in the near-wall region of turbulent boundary layers.