2 results
Turbulent drag reduction of boundary layer flow with non-ionic surfactant injection
- Shinji Tamano, Takuya Kitao, Yohei Morinishi
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- Journal:
- Journal of Fluid Mechanics / Volume 749 / 25 June 2014
- Published online by Cambridge University Press:
- 15 May 2014, pp. 367-403
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We experimentally investigated streamwise variations of turbulent dynamics in drag-reducing turbulent boundary layer flows following the injection of non-ionic surfactant solutions, which mainly consisted of oleyldimethylamine oxide. We focus on the comparison of turbulence statistics between injected (i.e. heterogeneous) and premixed (i.e. homogeneous) surfactant solutions, in which the maximum drag reduction ratio of 50 % is the same at the most downstream position for both cases. The wall-normal profiles of turbulence statistics, such as streamwise and wall-normal turbulence intensities, seem to be noticeably different between heterogeneous and homogeneous surfactant solutions. However, streamwise variations in these maxima and the wall-normal locations are essentially similar to one another, except for the maximum of streamwise turbulence intensity, which is not arranged by the amount of drag reduction and is also dependent on the normalization due to outer and inner variables. Such complex behaviour of streamwise turbulence intensity would be caused by the formation of near-wall layered structures that are parallel to the wall. For both heterogeneous and homogeneous surfactant solutions, the streamwise variation in the drag reduction ratio corresponds well to those of the mean velocity in wall units and the wall-normal locations of maxima of streamwise and wall-normal turbulence intensities with both outer and inner scaling. Unlike the Reynolds shear stress, the correlation coefficient of the streamwise and wall-normal turbulent fluctuations is correlated well with the drag reduction ratio. We present plausible pictures of the development of turbulence structures such as hairpin vortices and low-speed streaks for the drag-reducing turbulent boundary layer in heterogeneous and homogeneous surfactant solutions, which are comprehensively derived from the present set of experimental measurements such as flow visualization, planar laser-induced fluorescence, two-component laser-Doppler velocimetry and particle image velocimetry on the streamwise and wall-normal plane and the streamwise and spanwise plane.
Streamwise variation of turbulent dynamics in boundary layer flow of drag-reducing fluid
- Shinji Tamano, Michael D. Graham, Yohei Morinishi
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- Journal:
- Journal of Fluid Mechanics / Volume 686 / 10 November 2011
- Published online by Cambridge University Press:
- 22 September 2011, pp. 352-377
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Direct numerical simulations (DNSs) of a zero-pressure-gradient boundary layer flow of a polymeric fluid have been performed. The FENE-P model was used for the polymer stresses and a wide range of Weissenberg numbers () was addressed. In all cases, the streamwise variations of the level of polymer stretching and the level of drag reduction are anticorrelated. Consistent with earlier studies, the inlet condition for the flow consists of Newtonian velocity data with no polymer stretching, so in the upstream region of the boundary layer the polymer molecules stretch strongly in response, leading to an initial spatial maximum in polymer stretching. Beyond this initial region, the level of drag reduction increases with increasing downstream position, while the polymer stretch is decreasing. At sufficiently high Weissenberg numbers, these variations are monotonic with streamwise position (outside the upstream region), but at , both the polymer stretching and level of drag reduction display a decaying oscillation in the downstream position. The streamwise dependence of the velocity statistics is also shown. In addition, simulations in which the polymer stress is turned off beyond a chosen downstream position were performed; in this case the flow continues to exhibit substantial drag reduction well downstream of the cutoff position. These observations are analysed in light of other recent literature and in particular the observations of ‘active’ and ‘hibernating’ turbulence recently found in minimal channel flow by Xi and Graham. All of these observations suggest that an important role for viscoelasticity in the turbulent drag reduction phenomenon, at least near solid surfaces, is to suppress conventional turbulence, while leaving behind a much weaker form of turbulence that can persist for a substantial length of time (or downstream distance) even in the absence of viscoelastic stresses.