Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Microbubbles: Drag Reduction Mechanism and Applicability to Ships
Microbubbles: Drag Reduction Mechanism and Applicability to Ships
Pages 2-22
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From page 2... ...
The data by several investigators were re-plotted using air layer thickness ta and compared among them, showing the differences in air injection methods such as porous plates and array-of-holes plates, test section dimensions and the degree of wall effect. The mechanism for the skin friction reduction was briefly discussed, including the density effect, bubble size effect, and the reduction of the Reynolds stress.
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From page 3... ...
Figure 3 shows the measured skin friction reduction effect in case the air injection and skin friction measurement were carried out at the top wall (plate-on-top)
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On the upper wall they put a flat plate of 300mm width, 2150mm length upstream of the injection plate for growing boundary layer and 1 500m length downstream for the measurement. The distance between the upper wall and the bottom side, i.e.
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(l ) Test facility In order to measure the skin friction reduction effect and its persistence in the downstream direction, the authors built a tunnel for microbubble experiments as shown in Fig.
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Test section with air injection chamber Figure 8 Air injection chamber and bubble generation through a porous plate Figure 9 Bubble generation from the porous plate (U=7m/sec)
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The pre sent skin friction reduction values are apparently smaller than those by Madavan 1984, whose reason is not clear. The contribution of the C correction is fo comparable to the reduction effect, and in many cases dominant.
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at ta less than 1, the two results seem to agree well, but beyond that ta value NMRI values tend to saturate in spite of significant contribution of the Cfo correction, while Kawakita's values still go down. This may suggest that at small air njection rate, where the wall effect is not significant, the phenomenon in a channel of small height is the same as that in a channel of large height, but at large air injection rate, where the wall effect is significant, the phenomenon in a small channel becomes different from that in a large channel and further skin friction reduction is prevented due to some unknown mechanisms.
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From page 9... ...
The use of ta to compare results from different test facilities simply neglects this possible influence. 2.4 Experiments using 20m and 40m Hat plate ships by Watanabe 1998 From the practical application point of view, scale effect is the most important factor in the skin friction reduction effect by microbubbles.
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From page 10... ...
oo.8 0.6 0.4 0.2 x t ~ · Xa=1.8m · Xa=5.8m X Xa=13.8m Xa=21.8m Xa=37.8m 0 1 t_a (mm) 2 3 Figure 17 Skin friction reduction of 40m-long flat plate ship (the same data as those in Fig.16)
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(b) xa =1.8m ~ ———————————————1 ¢—-————————————1 ~ :" Figure 21 Skin friction reduction at 12m-long flat plate ship of NMRI (PP, corrected)
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From page 12... ...
(3) Cf /Cfo: Comparison of bow and middle injection In case the drag reduction effect of microbubbles in full scale is estimated using the measurements in circulating tunnels or towing tanks, in which the length scale is considerably smaller than the full scale, the choice of the parameter for air injection rate is important.
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2000) Fig.28 shows the measured local void ratio on the 12m flat plate ship of NMRI with PP, at the speed of V=7m/s and at the air injection rate of ta=0.71mm.
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Since the high local void ratio close to the wall surface is related to the skin friction reduction, there is no doubt that the so-called "density effect" plays an important role. There are several observations that bubbles in the turbulent boundary layer modify (decrease)
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OE-04 1 . 5E-04 Figure 34 Frictional drag reduction of SOm flat plate ship (NMRI, AHP, bow injection)
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U, (14) where Do —D = DFbO DFb In order to estimate the net drag reduction effect, one should subtract the power needed for bubble injection from the power gain due to drag reduction.
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The corresponding reduction n the total drag was9%. The reason for the higher decay of the skin friction reduction effect at the higher speed is suspected to be the higher diffusion of the bubbles away from the wall by the higher turbulence intensity and/or smaller bubble size.
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It was carried out by a team of researchers who worked under the SR239 "Study on Frictional Drag Reduction Methods for Ships" Esearch project organized by the Shipbuilding Research Association of Japan. The following descriptions are from the SR239 Research Report 2002.
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Methods that can measure the interaction of bubbles with the turbulence in the boundary layer need to be developed. Simulation methods that can sim~late the skin friction reduction effect and elucidate the bubble interaction mechanism need to be developed,
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From page 20... ...
Kato, H et al., "Frictional Drag Reduction by injecting 19 , , 19 Bubbly Water into Turbulent Boundary Layer", Cavitation and Gas Liquid Flow in Fluid Machinery and Devices, FED -vol.
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From page 21... ...
et al., "Experimental study on drag reduction by Microbubbles using a 50m-long flat plate ship" TSFP-2~ 2nd Int.
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2002 measured velocity distributions in the channel flow with bubbles using the PIV/LIF technique, and showed that the Reynolds stress decreases and the mean streamwise velocity distribution approaches laminar profile with increased air injection. Since air is injected normal to the wall, there is possibility, in principle, that it increases susceptibility to flow separation, especially at high injection rate.
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