Wall-layer structure and drag reduction

Tiederman, W. G.; Luchik, T. S.; Bogard, David G.

doi:10.1017/s0022112085002178

Cited by 121 publications

(54 citation statements)

References 15 publications

(18 reference statements)

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“…Donohue et al 22 examined the effects of polymers on turbulent structures by using the visualization technique; they reported that the streaking spacing and bursting rates in drag-reducing flow are different from that in the Newtonian flow, i.e., the average nondimensional spacing between streaks linearly increases with increasing DR and the viscous sublayer was more stable when polymer solutions were present. A suppression of the burst process and an increment of streak spacing were also reported by Berman 16 and Tiederman et al 23 However, Luchik and Tiederman 19 found that the method for deducing the time between bursts was not accurate in these experiments because they did not marked and counted all of these events. Thus, the flow visualization revealed some important phenomena, but a lack of measure of the corresponding turbulent velocity field limits the interpretation.…”

Section: Introductionsupporting

confidence: 66%

Modeling of viscoelastic turbulent flow in channel and pipe

Yang

Dou

2008

Physics of Fluids

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This paper investigates turbulent flows with or without polymer additives in open channels and pipes. Equations of mean velocity, root mean square of velocity fluctuations, and energy spectrum are derived, in which the shear stress deficit model is used and the non-Newtonian properties are represented by the viscoelasticity ␣ * . The obtained results show that, with ␣ * increment, ͑1͒ the streamwise velocity fluctuations is increased, ͑2͒ the wall-normal velocity fluctuation is attenuated, ͑3͒ the Reynolds stress is reduced, and ͑4͒ there is a redistribution of energy from high frequencies to the low frequencies for the streamwise component, but dimensionless distribution over all frequencies almost remains the same as that in Newtonian fluid flows. Good agreement between the derived equations and experimental data in small drag-reduction regime is achieved, which indicates that the present model is workable for Newtonian/non-Newtonian fluid turbulent flows.

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Section: Introductionsupporting

confidence: 66%

Modeling of viscoelastic turbulent flow in channel and pipe

Yang

Dou

2008

Physics of Fluids

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“…Similar to other drag-reduction techniques, such as polymer injection [18,19,27,28], the oscillating wall motion brings about an increment of the average streak spacing λ + oscill , 5 as seen in figure 10 and in table 2. A reduction of friction of about 40% goes together with an increment of streaks' spacing of about one-third when values are scaled with the relative inner variables, while the distance doubles in physical dimensions.…”

Section: Jot 5 (2004) 024mentioning

confidence: 77%

“…Once the bubble sheet was generated, it was distorted by the turbulence and then lit by the laser plane. For the analysis of the overhead-view images, a definition of low-speed streak was adopted following the works by Oldaker and Tiederman [18] and by Tiederman et al [19]. A low-speed streak was recognized as a clearly identifiable single longitudinal structure of bubbles with streamwise length at least three times the apparent average streaks' spacing.…”

Section: Flow Visualization Setup and Proceduresmentioning

confidence: 99%

Modification of near-wall turbulence due to spanwise wall oscillations

Riccó¹

2004

J. of Turbulence

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Abstract. An experimental investigation of a turbulent boundary layer modified by spanwise wall oscillations is conducted in a water channel by means of the hydrogen-bubble technique.The purpose is to study the dynamics of near-wall turbulent structures to shed new light on the physical mechanisms characterizing the boundary layer perturbed by the wall motion and to comprehend how these changes cause a wall-shear stress reduction. It is likely that flow visualizations conducted at the highest values of maximum wall velocity describe the spatial transient evolution of the flow to the new modified state because of the limited extension of the oscillating wall section. When the oscillatory motion is imposed, the low-speed streaks shift laterally, and cyclically incline to an angle with respect to the streamwise direction. Flow visualizations from the end of the water channel distinctly show that the interaction between these low-velocity pockets and the overriding longitudinal vortices is strongly altered, the latter being only slightly disturbed by the spanwise Stokes layer which develops due to the wall movement. We also notice that the ejection phenomena of low-speed fluid from the viscous sublayer to higher regions of the boundary layer and the sweeping motions of high-speed fluid towards the wall are both significantly weakened. During the first instants of the wall oscillation, the nearwall turbulent structures are already remarkably affected after one oscillating cycle. The drag-reducing flow pattern downstream of the moving wall is also described.

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“…Observations of drag-reducing fluids indicate that, at least near the onset Reynolds number for drag reduction, the effects of the polymer are confined primarily to the near-wall buffer layer, 1,[5][6][7][8] which is the most important region for the production and dissipation of turbulent energy. 9 From experia)…”

Section: Introductionmentioning

confidence: 99%

Polymer drag reduction in exact coherent structures of plane shear flow

et al. 2004

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Recently discovered traveling-wave solutions to the Navier-Stokes equations in plane shear geometries provide model flows for the study of turbulent drag reduction by polymer additives. These solutions, or "exact coherent states" (ECS), qualitatively capture the dominant structure of the near-wall buffer region of shear turbulence, i.e., counter-rotating pairs of streamwise-aligned vortices flanking a low-speed streak in the streamwise velocity. The optimum length scales for the ECS match well the length scales of the turbulent coherent structures and evidence suggests that the ECS underlie the dynamics of these structures. We study here the effect of viscoelasticity on these states. The changes to the velocity field for the viscoelastic ECS, where the FENE-P model calculates the polymer stress, mirror the modifications seen in experiments of fully turbulent flows of polymer solutions at low to moderate levels of drag reduction: drag is reduced, streamwise velocity fluctuations increase while wall-normal fluctuations decrease, and smaller wavelength structures are suppressed. These modifications to the ECS are due to the suppression of the streamwise vortices. The polymer molecules become highly stretched in the wavy, streamwise streaks, where the flow is predominately elongational, then relax as they move from the streaks into and around the streamwise vortices, where the flow is predominately rotational. This relaxation of the polymer molecules produces a force that directly opposes the fluid motion in the vortices, weakening them. Since the pressure fluctuations have their greatest magnitude (i.e., they are most negative) in the cores of the vortices, a reduction in vortex strength leads to a decrease in the magnitude of the pressure fluctuations. The pressure fluctuations redistribute energy from the streamwise velocity fluctuations to the Reynolds shear stress, so a decrease in their magnitude leads to a reduction in turbulent drag. For the viscoelastic ECS, we also find that after the onset of drag reduction (at Weissenberg number, WeϷ 7) there is a dramatic increase in the critical wall-normal length scale at which the ECS can exist. This sharp increase in length scale mirrors experimental observations and is also consistent with the observed shift to higher Reynolds numbers of the transition to turbulence in polymer solutions.

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Wall-layer structure and drag reduction

Cited by 121 publications

References 15 publications

Modeling of viscoelastic turbulent flow in channel and pipe

Modeling of viscoelastic turbulent flow in channel and pipe

Modification of near-wall turbulence due to spanwise wall oscillations

Polymer drag reduction in exact coherent structures of plane shear flow

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