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• W250714786 abstract "In order to clarify the skin friction reduction mechanism by microbubbles, both experiment and numerical simulation of turbulent channel flow with bubbles were carried out in the same parameter range, and were compared. In the experiment, silicone oil ten times more viscous than water has been used, in order to lower the Reynolds number to that of numerical simulation. The surface tension of the Silicone oil is 1/3 of that of water. In the numerical simulation, two numerical methods have been used. One is the Force Coupling Method (FCM), in which bubbles are assumed to be rigid and the influence of bubbles to the flow is simulated by body forces. The other is the Front-Tracking Method (FTM), in which fluid phase and gas phase are solved simultaneously and bubble shape is allowed to deform by being expressed with polynomials In the experiment the Reynolds number (Re) is 2,777 to 4,500, and the Weber number (We) is 200. The experimental values of local skin friction shows that the flow is semi-laminar at Re=2,777 in the non-bubble condition, and that, by injecting bubbles, the skin friction increases to the turbulent flow value, which means that bubble injection stimulates the flow and turns it to fully turbulent. At Re=3,734(3,811) the flow is already fully turbulent in the non-bubble condition, and therefore adding bubbles has little influence on the flow, resulting in little change in the skin friction. At even higher Reynolds number of nearly 4,500, adding bubbles decreases skin friction slightly. The Reynolds number of the numerical simulation using FTM is 3,000. The time history of skin friction shows that at the wall where bubbles are clustered by buoyancy local skin friction tends to show slight decrease by adding deformable bubbles (We=100). However, by adding less deformable bubbles (We=50) local skin friction slightly increases. Computation at We=200 has blown up. The local skin friction in the FCM computation at Re=4,000 shows 2.3% increase by adding (rigid) bubbles. Therefore it may be stated that, at Re=3,000, addition of deformable bubbles tends to decrease skin friction, while addition of less deformable bubbles, or rigid bubbles, tends to increase skin friction. In order to carry out numerical simulation of turbulent shear flow with bubbles at higher Reynolds number, the simulation of the homogeneous turbulent shear flow (HTSF) with deformable bubbles has been carried out. The result shows that large turbulent Reynolds number, smaller shear Reynolds number and large Weber number have positive influence on the microbubble drag reduction. The increase of the turbulent Reynolds number of HTFS corresponds to the increase of the Reynolds number of the turbulent channel flow (TCF), and therefore the result agrees with the experimental result that added bubbles decrease skin friction at higher Reynolds number but not at lower Reynolds number. The Weber number dependence of the HTSF result can be compared directly with that of TCF. Thus the result confirms that bubble deformation acts to decrease skin friction. *1) University of Twente, the Netherlands *2) Hokkaido University *3) The University of Tokyo" @default.
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• W250714786 date "2007-12-28" @default.
• W250714786 modified "2023-10-16" @default.
• W250714786 title "Microscopic Mechanisms for Skin Friction Reduction by Microbubbles" @default.
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