Effect of Bubble Size on the Microbubble Drag Reduction of a Turbulent Boundary Layer

2003 ◽  
Author(s):  
Takafumi Kawamura ◽  
Yasuhiro Moriguchi ◽  
Hiroharu Kato ◽  
Akira Kakugawa ◽  
Yoshiaki Kodama
Keyword(s):  
Drag Reduction ◽  
Flow Velocity ◽  
Bubble Diameter ◽  
Average Diameter ◽  
Main Flow ◽  
Frictional Drag ◽  
The Third ◽  
Local Shear ◽  
Small Bubbles ◽  
Local Shear Stress

Three different methods have been investigated for generating microbubbles to control the bubble diameter separately from the main flow velocity. The first two methods achieve this by adjusting the local shear stress where bubbles are generated, while the third method uses foaming of dissolved air to generate very small bubbles. The average diameter of bubbles was successfully controlled by the first two method within the range of 0.5–2 mm for the fixed main flow velocity of U = 3 m/s, while the very small bubbles of 20–40 μm were generated by the third method. The influence of the bubble diameter on the frictional drag reduction was found to be insignificant for the diameter range of 0.5–2 mm, while we also obtained experimental results suggesting that smaller bubbles on the order of 10 μm in diameter can be effective for the drag reduction.

Experimental Investigation on Influence of Inclination and Curved Surface of Ship Bottom in Air Lubrication Method

2019 ◽  
Author(s):  
Chiharu Kawakita ◽  
Tatsuya Hamada
Keyword(s):  
Shear Stress ◽  
Drag Reduction ◽  
Energy Saving ◽  
Frictional Drag ◽  
Void Fraction Distribution ◽  
Air Lubrication ◽  
Fraction Distribution ◽  
Local Shear ◽  
Reduction Effect ◽  
Local Shear Stress

Abstract The air lubrication method, which mixes millimeter bubbles into the flow around the hull and reduces frictional resistance, is expected to have a large energy saving effect among a number of marine energy saving technologies. Concerning the frictional drag reduction effect using the air lubrication method, in this study, the frictional drag reduction effect was experimentally investigated for gas-liquid two phase flow considering the influence of inclination and curved surface of the ship bottom. Measurement of local shear stress and measurement of void fraction distribution near the wall surface were carried out and the correlation between local shear stress and local void fraction distribution was grasped.

A Controllable Microbubble Emitter

2005 ◽  
Author(s):  
Neal A. Brown ◽  
Martin Wosnik
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Water Flow ◽  
Bubble Diameter ◽  
Proof Of Concept ◽  
Surface Drag ◽  
Frictional Drag ◽  
Micro Holes ◽  
Bubble Number ◽  
Theoretical Analyses

Controlled emission of microbubbles into a water flow boundary layer appears to be a promising means to significant reduction of frictional drag on ships. Theoretical analyses and hypotheses require that particularly small (∼ 100 micrometers or less) gas bubbles be emitted and retained in particular laminae close to the wetted surface. Drag reduction economy requires that the quantity of gas emitted be very small. Here a design of a controllable microbubble emitter which meets both demands above is put forth. The two key requirements governing the design are pulsed operation, which expels a known volume of air during each cycle, and a known number of uniformly-sized micro-holes, which determines bubble number and therefore bubble diameter. A first, proof-of-concept experiment with a modified pulsed-pressure design of the proposed microbubble emitter was carried out and shows promise.

Frictional drag reduction using small bubbles in a Couette-Taylor flow

2015 ◽  
Vol 20 (4) ◽  
pp. 652-669 ◽  
Author(s):  
R. Maryami ◽  
S. Farahat ◽  
M. JavadPour ◽  
M. Shafiei Mayam
Keyword(s):  
Drag Reduction ◽  
Frictional Drag ◽  
Taylor Flow ◽  
Small Bubbles

Mass Transfer From a Bubble in a Vertical Pipe

2011 ◽  
Author(s):  
Shogo Hosoda ◽  
Ryosuke Sakata ◽  
Kosuke Hayashi ◽  
Akio Tomiyama
Keyword(s):  
Carbon Dioxide ◽  
Mass Transfer ◽  
Bubble Diameter ◽  
Vertical Pipe ◽  
Oblate Spheroidal ◽  
Wide Range ◽  
Bubble Sizes ◽  
Small Bubbles ◽  
Taylor Bubbles

Mass transfer from single carbon dioxide bubbles in a vertical pipe is measured using a stereoscopic image processing method to develop a mass transfer correlation applicable to a wide range of bubble and pipe diameters. The pipe diameters are 12.5, 18.2 and 25.0 mm and the bubble diameter ranges from 5 to 26 mm. The ratio, λ, of bubble diameter to pipe diameter is therefore varied from 0.2 to 1.8, which covers various bubble shapes such as spherical, oblate spheroidal, wobbling, cap, and Taylor bubbles. Measured Sherwood numbers, Sh, strongly depend on bubble shape, i.e., Sh of Taylor bubbles clearly differs from those of spheroidal and wobbling bubbles. Hence two Sherwood number correlations, which are functions of the Peclet number and the diameter ratio λ, are deduced from the experimental data: one is for small bubbles (λ < 0.6) and the other for Taylor bubbles (λ > 0.6). The applicability of the proposed correlations for the prediction of bubble dissolution process is examined through comparisons between measured and predicted long-term bubble dissolution processes. The predictions are carried out by taking into account the presence of all the gas components in the system of concern, i.e. nitrogen, oxygen and carbon dioxide. As a result, good agreements for the dissolution processes for various bubble sizes and pipe diameters are obtained. It is also demonstrated that it is possible to evaluate an equilibrium bubble diameter and instantaneous volume concentration of carbon dioxide in a bubble using a simple model based on a conservation of gas components.

Drag Reduction for Blunt Body With Cross Flow by Extremum-Seeking Control

2008 ◽  
Author(s):  
Nobuhiko Kamagata ◽  
Susumu Horio ◽  
Koichi Hishida
Keyword(s):  
Control System ◽  
Drag Reduction ◽  
Shear Layer ◽  
Flow Velocity ◽  
Drag Force ◽  
Lift Force ◽  
Flow Direction ◽  
Extremum Seeking ◽  
Extremum Seeking Control ◽  
Separated Shear Layer

The active flow control, which can adapt to variation of flow velocity and/or direction, is an effective technique to achieve drag reduction. The present study has investigated a separated shear layer and established two control systems; the system reduces drag force and lift force by controlling the separated shear layer to reattachment for variation of flow velocity and /or direction. The adaptive control system to the variation of flow velocity was constructed by using a hot wire anemometer as a sensor to detect flow separation. The system to flow direction was constructed by using pressure transducers as a sensor to estimate drag force and lift force. The extremum-seeking control was introduced as a controller of the both systems. It is indicated from the experimental results that adaptive drag/lift control system to various flow velocity ranging from 3 to 7 m/s and various flow direction ranging from 0 to 30 deg. was established.

Air Layer on Superhydrophobic Surface for Frictional Drag Reduction

2020 ◽  
Vol 64 (02) ◽  
pp. 118-126
Author(s):  
Bradley C. Peifer ◽  
Christopher Callahan-Dudley ◽  
Simo A. Makiharju
Keyword(s):  
Drag Reduction ◽  
Superhydrophobic Surface ◽  
Energy Savings ◽  
Surface Hydrophobicity ◽  
Net Energy ◽  
Frictional Drag ◽  
Gas Flux ◽  
Still Images ◽  
Painted Surface ◽  
Air Layer Drag Reduction

We examined the feasibility of combining a superhydrophobic surface (SHS) and air layer drag reduction (ALDR) to achieve the frictional drag reduction (DR) shown achievable with traditional ALDR, but at a reduced gas flux to increase the achievable net energy savings. The effect of a commercial SHS coating on the gas flux required to maintain a stable air layer (AL) for DR was investigated and compared with that of a painted non-SHS at Reynolds numbers up to 5.1 X 106. Quantitative electrical impedance measurements and more qualitative image analysis were used to characterize surface coverage and to determine whether a stable AL was formed and maintained over the length of the model. Analysis of video and still images for both the SHS and painted surface gives clear indications that the SHS is able to maintain AL consistency at significantly lower gas flux than required on the non-SHS painted surface. Hydrophobicity of the surfaces was characterized through droplet contact angle measurements, and roughness of all the flow surfaces was measured. The results from these preliminary experiments seem to indicate that for conditions explored (up to Rex = 5.1 X 106), there is a significant decrease in the amount of gas required to establish a uniform AL (and hence presumably achieve ALDR) on the SHS when compared with a hydraulically smooth painted non-SHS.

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