On the Skin Friction Reduction Mechanisms of Microbubbles (Keynote)

2003 ◽  
Author(s):  
Yoshiaki Kodama
Keyword(s):  
Skin Friction ◽  
Reynolds Shear Stress ◽  
Flow Speed ◽  
Friction Reduction ◽  
Injection Point ◽  
Reduction Mechanisms ◽  
Layer Increase ◽  
Reduction Effect ◽  
Scale Experiment ◽  
Skin Friction Reduction

The skin friction reduction effect of microbubbles in various flow conditions are reviewed. At location immediately downstream from an air injection point, the reduction does not depend on the flow speed, but at further downstream, it becomes smaller at higher flow speed, and decays with downstream distance. Experiments in a towing tank using a very long flat plate show that the reduction persists up to nearly 50m. In a full scale experiment using a 105m-long ship, 5.5% drag reduction was obtained. In case the bubbles are away from the wall, the skin friction increases. An optical technique has shown that bubbles in a boundary layer increase velocity fluctuations but decrease the Reynolds shear stress. Three possible skin friction reduction mechanisms are listed.

scholarly journals A Full-scale Experiment on Microbubbles for Skin Friction Reduction Using "SEIUN MARU"

2002 ◽  
Vol 2002 (192) ◽  
pp. 15-28 ◽  
Author(s):  
Tetsuo Nagamatsu ◽  
Yoshiaki Kodama ◽  
Akira Kakugawa ◽  
Michio Takai ◽  
Kyoji Murakami ◽  
...  
Keyword(s):  
Skin Friction ◽  
Full Scale ◽  
Friction Reduction ◽  
Full Scale Experiment ◽  
Scale Experiment ◽  
Skin Friction Reduction

scholarly journals Skin Friction Reduction Characteristics of Nonsmooth Surfaces Inspired by the Shapes of Barchan Dunes

2017 ◽  
Vol 2017 ◽  
pp. 1-12 ◽  
Author(s):  
Xiao-wen Song ◽  
Ming-xiao Zhang ◽  
Peng-zhe Lin
Keyword(s):  
Skin Friction ◽  
Friction Reduction ◽  
Test Results ◽  
Image Velocimetry ◽  
New Type ◽  
Barchan Dunes ◽  
Reduction Effect ◽  
Skin Friction Reduction ◽  
Reduction Characteristics ◽  
Reduction Percentage

A new type of nonsmooth surface inspired by the shape of barchan dunes has been proposed and is intended to reduce skin friction, a major cause of overall drag. Simulations were carried out to obtain skin friction reduction characteristics for the nonsmooth surface using the commercial computational fluid dynamics software Fluent. A realizable k-ε model was employed to assess the influence of the nonsmooth structure on turbulent flow and velocity fields. The numerical simulation results showed that the new nonsmooth surface possesses the desired skin friction reduction effect and that the maximum skin friction reduction percentage reached 33.63% at a fluid speed of 30 m/s. Various aspects of the skin friction reduction mechanism were discussed, including the distribution of velocity vectors and shear stress contours and the variations in boundary layer thickness. The accuracy of the flow field for the nonsmooth unit was further verified by particle image velocimetry test results. The new bionic nonsmooth surface, which exceeds the limitations of existing nonsmooth bionic structures, can effectively reduce skin friction and should provide insights into engineering applications in the future.

scholarly journals A Full-scale Experiment on Microbubbles for Skin Friction Reduction Using "SEIUN MARU"

2002 ◽  
Vol 2002 (192) ◽  
pp. 1-13 ◽  
Author(s):  
Yoshiaki Kodama ◽  
Akira Kakugawa ◽  
Takahito Takahashi ◽  
Satoru Ishikawa ◽  
Chiharu Kawakita ◽  
...  
Keyword(s):  
Skin Friction ◽  
Full Scale ◽  
Friction Reduction ◽  
Full Scale Experiment ◽  
Scale Experiment ◽  
Skin Friction Reduction

Application of Air Microblowing Through a Porous Wall for Skin Friction Reduction on a Flat Plate

2010 ◽  
Vol 5 (3) ◽  
pp. 38-46
Author(s):  
Vladimir I. Kornilov ◽  
Andrey V. Boiko
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Skin Friction ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Porous Wall ◽  
Friction Reduction ◽  
Local Skin ◽  
Skin Friction Reduction

The effect of air microblowing through a porous wall on the properties of a turbulent boundary layer formed on a flat plate in an incompressible flow is studied experimentally. The Reynolds number based on the momentum thickness of the boundary layer in front of the porous insert is 3 900. The mass flow rate of the blowing air per unit area was varied within Q = 0−0.0488 кg/s/m2 . A consistent decrease in local skin friction, reaching up to 45−47 %, is observed to occur at the maximal blowing air mass flow rate studied.

Multiple slot skin friction reduction

1975 ◽  
Vol 12 (9) ◽  
pp. 753-754 ◽  
Author(s):  
F. G. Howard ◽  
J. N. Hefner ◽  
A. J. Srokowski
Keyword(s):  
Skin Friction ◽  
Friction Reduction ◽  
Skin Friction Reduction

Skin friction reduction by large air bubbles in a horizontal channel flow

2007 ◽  
Vol 33 (2) ◽  
pp. 147-163 ◽  
Author(s):  
Yuichi Murai ◽  
Hiroshi Fukuda ◽  
Yoshihiko Oishi ◽  
Yoshiaki Kodama ◽  
Fujio Yamamoto
Keyword(s):  
Skin Friction ◽  
Channel Flow ◽  
Friction Reduction ◽  
Horizontal Channel ◽  
Air Bubbles ◽  
Skin Friction Reduction

Proposal of control laws for turbulent skin friction reduction based on resolvent analysis

2019 ◽  
Vol 866 ◽  
pp. 810-840 ◽  
Author(s):  
Aika Kawagoe ◽  
Satoshi Nakashima ◽  
Mitul Luhar ◽  
Koji Fukagata
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Drag Reduction ◽  
Skin Friction ◽  
Friction Reduction ◽  
Suboptimal Control ◽  
Spectral Space ◽  
Control Laws ◽  
Wall Shear ◽  
Skin Friction Reduction

This paper evaluates and modifies the so-called suboptimal control technique for turbulent skin friction reduction through a combination of low-order modelling and direct numerical simulation (DNS). In a previous study, Nakashima et al. (J. Fluid Mech., vol. 828, 2017, pp. 496–526) employed resolvent analysis to show that the efficacy of suboptimal control was mixed across spectral space when the streamwise wall shear stress (case ST) was used as a sensor signal, i.e. specific regions of spectral space showed drag increment. This observation suggests that drag reduction may be attained if control is applied selectively in spectral space. DNS results presented in the present study, however, do not show a significant effect on the flow with selective control. A posteriori analyses attribute this lack of efficacy to a much lower actuation amplitude in the simulations compared to model assumptions. Building on these observations, resolvent analysis is used to design and provide a preliminary assessment of modified control laws that also rely on sensing the streamwise wall shear stress. Control performance is then assessed by means of DNS. The proposed control laws generate as much as $10\,\%$ drag reduction, and these results are broadly consistent with resolvent-based predictions. The physical mechanisms leading to drag reduction are assessed via conditional sampling. It is shown that the new control laws effectively suppress the near-wall quasi-streamwise vortices. A physically intuitive explanation is proposed based on a separate evaluation of clockwise and anticlockwise vortices.

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