|
Items in cart [1] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 2
24eh Symposium on Naval Hydrodynamics
Fukuoka, JAPAN, ~13 July 2002
Microbubbles: Drag Reduction Mechanism and Applicability
to Ships
Yoshiaki Kodama~, Akira Kakugawa~, Takahito Takahashi~, Shigeki Na-
gaya~, K~7~,v~, C,,olv~m~ CR97Q Committee of the. <;hinh,~ildin~ Re-
. . . ..
I ~~ ~ ~~ berm = ~~~
search Association of Japan. (iNational Maritime Research Institute, Japan)
Abstract
Measurements on the skin friction reduction effect
by microbubbles were reviewed. The data by several
investigators were re-plotted using air layer thickness
ta and compared among them, showing the differ-
ences 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. Some factors related to the ap-
plication of microbubbles to full-scale ships were dis-
cussed, and it was shown that net power saving is ~t-
tainable using existing techniques. Finally, a full-scale
microbubble experiment using a 116m-long ship was
shown, in which 3% drag reduction was obtained, re-
sulting in 2% net power saving.
l.Introduction
Microbubbles, i.e. small bubbles injected into the
turbulent boundary layer on a solid wall, reduce skin
friction significantly. Studies on microbubbles started
by a pioneering work by McCormick and Bhat-
tacharyya (1973~. The significant skin friction reduc-
tion effect of microbubbles makes them look attractive
for the military application, where techniques to realize
significant skin friction reduction are regarded as in-
dispensable for very high-speed ships or torpedoes.
Therefore, in the age of the Cold War, their studies were
mainly carried out in the United States and the former
Soviet Union. In the 1970s, microbubble studies were
carried out intensively in the former Soviet Union, e.g.
Bogdevich (1977~. In the 1980s, the intensive study
was carried out this time in the United States, headed
by a group it the Pennsylvania State University. In
the 16th Symposium on Naval Hydrodynamics Merkle
et al. (1987) gave interesting observations on the rela-
tion between the bubble distribution in the boundary
layer and the skin friction reduction effect, and Merkle
and Deutch (1990) reviewed the history of microbubble
studies and described in detail the research activities at
Penn. State.
In the l990s, the center of the study moved to Ja-
pan, aiming at their application to commercial ships.
Ships that carry heavy loads such as crude oil, ore and
grain, play an important role in the worldwide trans-
portation. Their characteristics are that they are very
large and that they move very slowly. Among the two
major drag components of such ships, the free-surface
wave component, being proportional to the square of
the ship speed, is very small due to their low speed.
Therefore the skin frictional drag component occupies
approximately 80% of the total drag, resulting in its
reduction being very important. Figure 2 shows an
image of the application of microbubbles to a tanker
ship. The University of Tokyo started the microbub-
ble study in Japan. They carried out both experimen-
tal and theoretical studies, jointly in some part with IHI
(Kato et al. 1994, Guin et al. 1996, Kato et al. 1998,
Yoshida et al. 1998, Watanabe et al. 19981. In the Na-
tional Maritime Research Institute, Japan (NMRI),
formerly the Ship Research Institute, the microbubble
studies were carried out, first on the basic characteris-
tics (Takahashi 1997, Kodama 2000), and then on the
scale effect using a SOm-long flat plate ship (Takahashi
20011. Computational studies were also carried out
(Kawamura 2001~.
The purpose of the present paper is to discuss m-
crobubbles for the drag reduction mechanism and the
applicability to full-scale ships. In the 2nd chapter the
skin friction mechanism is discussed. In the 3rd
chapter the applicability of mic~bubbles to ships is
discussed. In the fourth chapter a brief outline of the
-
~ I I 1111, 1 1, 1. ~11! ill 1
_
1 _
~ _
_~!
Figure 1 An image of microbubbles applied to a
tanker ship
OCR for page 3
2
full-scale microbubble experiment carried out last year
in Japan is described.
2. Skin friction reduction by microbubbles
In this chapter we discuss how much microbubbles
reduce skin friction in detail, mainly by viewing the
authors experiment using a small circulating water tun-
nel (Kodama 20001.
As shown in Fig.l, the skin friction reduction ~-
fect of microbubbles increases as the amount of i~-
jected air volume ncreases, and reaches 80%, but it
should be noted that the measurement was made at only
50mm to 65mm downstream of the point of bubble
injection. As shown in Fig.l, in the practical applica-
tion of microbubbles it is natural to assume that bubbles
are injected at a small portion of the hull surface, to
minimize installation cost and fouling possibility, and
move along the surface thus covering a wide area effi-
ciently. Therefore, for practical purposes, it is impor-
tant not only to find out how much the bubbles reduce
skin friction but also to find out how long the skin fric-
tion reduction effect persists in the downstream direc-
tion from the point of injection.
2.1 Experiment at The Pennsylvania State Univ.
In the beginning a standard microbubble experi-
mental data is shown. Madavan et al. (1984) carried
out microbubble experiments using a water tunnel with
a test section of BC x HC x LC = 114mm
X508mmX760mm size ("c" for "channel"~. They
used the top or bottom wall for air injection and skin
friction measurement. Fig.2 shows the device in case
the bottom wall was used for the experiment. Air was
injected through a sintered metal porous plate (PP here-
after) of Ba X Ha =102mm X 178mm size ("a" for
"air") and 5 Am nominal pore diameter. The skin
now
CON
nova
UPPER Was
Em=_
\TUNNEL L - IS,
~~ - .~ .~ · . ~rr~i
It Il ~ ~ 1 . by
Sll~Ea ~~1 ~" ~
POROUS PLAN ~ "~t ~ - 41N GAUGE - ~ ON LEG
"S. IN ~" HE "91U
tWA - -FILLY)
Figure 2 The layout for gas injection and skin friction
measurement (plate-on-bottom) (Madavan et
al. 1984)
friction was measured using a drag balance of
B X L=102mm X 254mm size located immediately
downstream of the air injection plate, insulting that the
center of the drag balance is 266mm downstream of the
center of the air injection plate.
Figure 3 shows the measured skin friction reduc-
tion effect in case the air injection and skin friction
measurement were carried out at the top wall
(plate-on-top). The horizontal axis shows the air n-
jection rate Qa non-dimensionalized by the injection
area Sa and the free-stream speed U,~, and the ver-
tical axis shows the ratio of the measured skin friction
in the bubble condition to that in the non-bubble condi-
tion. The skin friction reduction data at various flow
speed collapses nearly to a single line, and increases as
the air injection rate increases, reaching 80% at the
maximum.
Since Merkle and Deutsch 1990 experimentally
showed that the length Pa of the injection plate has
little influence on the skin friction reduction, the use of
the injection area Sa to nondimensionalize Qa has
little physical meaning,. Instead, they suggested to
use Cv, the ratio of volumetric flo w rates
CV -Q QQ (1)
a w
0.4
0.2
~ It,&
~ 4. 2m/s
· 9. 3m/s
0 12. 4m/s
0 17.4m/s
o . . . . . . . 1
0 0.02 0.04 0.06 0.08
QISU
sir ~
Figure 3 Skin friction reduction by microbubbles
(Madavan et al. 1984)
2
OCR for page 4
3
where QW is the volumetric water flow rate in the
boundary layer. But we have shown (see Fig.25) that
the boundary layer thickness has little significance, and
therefore, following Fukuda et al. 1999, prefer the use
of the dimensional air flow thickness ta
pa_ Qa (mm) (2)
In Fig.3 the corresponding ta horizontal axis is also
shown.
2.2 Kawakita's experiment in a cavitation tunnel
(l)Array-of-holes plate (AMP)
Although a porous plate is the most popular means
for air injection, it has some problems. One is that the
actual size of the holes is unknown except for the pore
size shown in a catalog. The other is the
non-uniformity of generated bubbles. The bubbles are
observed to be generated not uniformly over the surface
but at limited number of points distributed
non-uniformly. The third problem is that the pressure
loss across the plate is significant, i.e. 0.2 to 0.5
kgf/cm2 depending on the injection rate. The fourth is
possible fouling at sea. The former two problems
cause the repeatability of experiments questionable, and
the latter two make the practical use of a porous plate
difficult.
Instead of a porous plate, Kodama et al. 2000
manufactured a plate with holes of lmm diameter with
3mm spanwise Smm streamwise pitches, and named it
an array-of-holes plate (AHP hereafter). Kawakita
and Takano 2000 carried out mic~bubble experiments
in cavitation tunnel using an array-of-holes plate of the
same type. Their experiments will now be bdescribed
in detail.
(2)Test facility
Fig. 4 shows the layout of the test section. The
dimensions of the test section are BC x HC = 500mm
X500mm. On the upper wall they put a flat plate of
300mm width, 2150mm length upstream of the injec-
tion 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. the
measurement side, of the flat plate was 180mm, result-
ing that the distance between the flat plate and the
lower wall was 320mm, which can be regarded as large
enough to make the wall effect in bubble conditions
negligible. The boundary layer thickness in the meas-
urement locations were 25 to 40mm, according to the
LDV measurement.
Air was injected through an array-of-holes plate of
Ba x L`a =237mmX 20mm. Skin friction sensors (for
detail, see 2.3) were placed at xa =0.Sm, 1.0m, and
1.5m from the center of the injection plate, along the
line 75mm from the center line, in order to avoid the
wake of the propeller dynamorreter.
~:
=
.~
~si ~ ~
~ ~_e
~ -k ~ ~
Figure 4 Layout of test apparatus of Kawakita's
periment in their cavitation tunnel (Kawakita
and Takano 2000)
(3) Cf / Cfo of AHP
They plotted the measured Cf /Cfo as a function
of CX, the average void ratio in the boundary layer
_ _ Qa
Q +Q
a w
(3)
where the boundary layer thickness was estimated us-
ing an empirical formula. CX is the same as Cv
shown in eq.~31. OC can be converted to ta . Using
eqs. (2) and (3), and their definition of the boundary
layer thickness, it results
ta = 0.122xRex-l/7 <4
~ _
1—r'
~ v~
where x=2.15m. The results are shown in Fig.5. In
all the results, the skin friction reduction is almost lin-
ear with ta except for the small plateau region at
small ta values, and it seems the plateau region be-
3
OCR for page 5
4
comes larger at higher speeds. At U=Sm/s, there is no
clear difference among the data at three xa locations,
but at U=7m/s and 1 Om/s the reduction becomes
smaller at larger xa locations. In contrast to the
independence of the skin friction reduction effect on
flow speed in the experiments by Merkle and Deutsch
1990, their results show strong dependence on the flow
speed, with smaller reduction at higher speeds.
1.1
1
o.s
S
0.8
can
0.7
0.6
0.5
1.1
8 ~
- OXa=O.Sm
~ Xa=1 . Om
XXa=1. 5m
.................. .
O 0.5 1
~ (m m )
(a) U=Sm/s
1.5 2
~08 . REX
cow .
0.7 - ° Xa=0 . 5m
to Xa= 1 . Om
0.6 XXa=1. 5m
0.5 . . . . . . . . . . . . . . .
0 0.5
1
0.9
0.8
cam
0.7
0.6
(b) U=7m/s
~ , ~
- OXa=0.5m
~ Xa=1 . Om
XXPI=l .5m
1 1.5 2
t a(m m )
0 0.5 1
t_a (mm)
(c) U=lOm/s
1.5 2
Figure S Skin friction reduction by microbubbles (Ka-
wakita and Takano 2000)
(4) Cf / Cfo of PP and AHP
It is interesting to compare the skin friction reduc-
tion effect by a porous plate (PP) and that by an ~-
ray-of-holes plate (AHP). Fig.6 shows the compari-
son of the reduction using PP at xa=0.27m and at
U=4.2m/s and 9.3m/s (Madavan et al. 1984) and that
using AHP at xa =O.Sm and at U=Sm/s and lOm/s
(Kawakita and Takano 2000). In spite of slight dif-
ference in xa and U. they agree with each other well,
which shows that the skin friction reduction effect by
the two plates are nearly the same, at least at a point
immediately downstream of bubble injection.
0.8
0.6
cam
0.4
0.2
. . -
0&
to
AMadavan PP 4.2m/s Xa=0.27m O
O Madavan PP 9. 3m/s Xa=0 . 27m
AKawaki ta AHP 5m/s Xa=0 . 5m
X Kook i to AllP 1 Om/.c x~=n .Sm
0 1 2 3
t_a (mm)
4 5
Figure 6 Comparison of skin friction reduction using PP
(Madavan 1984) and AHP (Kawakita and Ta-
kano 2000)
2.3 Experiment at NMRI using Small High-speed
Water Tunnel (HSWT)
(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. 7. (Kodama 2000y,
The tunnel was designed following the design by
Guin et al. 1996. Bubbles are injected at the air injec-
tion chamber in the test section and removed by buoy-
ancy in the dump tank to allow continuous operation.
The test section of Be x HC x LC = lOOmm X 15mm
X 3000mm size has been designed to be nearly
two-dimensional in the transverse plane in order to
avoid the side wall effect and be consistent with other
test facilities for basic viscous flow experiment, and to
be long enough in the streamwise direction to measure
4
OCR for page 6
the persistence of the skin friction reduction effect.
The maximum flow speed at the test section is 12m/sec
The air injection chamber is located 1038mm down-
stream of the test section inlet, to have fully developed
flow at the point of injection. This location is called
Position 1 ~ xa =Om), followed by Positions 2
~ xa =O.Sm), Position 3 (xa =O.Sm), and Position 4
(xa=l.Sm), where various measurement was carried
out.
Due to the narrow test section and high speed, the
pressure gradient in the streamwise direction is not
small, e.g. 0.16 atm/m at U=7m/s. Therefore the local
pressure at each measurement location was taken into
account to determine Qa. Hereafter the flow speed
U denotes the bulk or average flow speed in the test
section.
Flow direction Air injection chamber (Posinonl)
. .'. . . '. —%, P2 P3 PA
~=e~e~
it ~ 1 net son son son 46i,1
Wire meshes 3000mm
F Test sector ( Is loo ) ~ ~
. - ~' n mm x mm
Electro-magnetic DOw meter Pump
Figure 7 Small High-speed Water Tunnel at NMRI for
microbubble experiment
(2)Air injection
Air is injected either through a porous plate (PP)
made of sintered bronze with nominal pore udius of
2 ,u m shown in Figs. 8 and 9, or through an ~-
ray-of-holes plate (AMP) shown in Fig. 10. The di-
ameter of the holes is lmm and they have 3mm span-
wise pitch and 5mm streamwise pitch. Both PP and
AHP have the injection area of Ba x La = 72mm
X 72mm.
Air
Porous Pant| -
~1 ~ 1 1.1. 1.1
v
Flow ~ ~
,
_ - -l
_~~G—_
_ ~
__
__
__ · an_ ~ ~ ~~
~_~
__
_~9
_ ~~ 1
_"~" 1
_-,,. 1
_ - ~ ~ ~ 1
_ _ 1
. .
(a)Section view (b)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)
(3)Skin friction sensors
Skin friction was measured using commercial skin
friction sensors (Sankei Engineering co., ltd.) shown in
Fig. 11. The sensor is a force gauge type (Kato 1994)
with a sensing disk of lOmm diameter and the mam-
mum load of 2 grams weight or 250 Nlm2. The
sensors were placed along the center line of the test
section. Static and dynamic calibration of the skin
friction sensors was made (Kodama et al. 2000~.
Figure 10 Array-of-holes plate
$~2mm - Ammo
/ Sensor floating disk
\\\\\\\\\\\\\
O O a O O O
Flow ~
Figure 11 Skin friction sensor
(4) Cfo correction for wall effect
When air is injected in the test section, the water
flow rate is kept constant, so the total flow speed in-
creases due to the volumetric increase of the flow, and
Cfo, the skin friction in the non-bubble condition,
should also increase and therefore needs correction.
Thus Cfo (Qa), i.e. Cfo in the bubble condition
5
OCR for page 7
6
was corrected using the following formulae.
1 1
Cfo (Qa ) = Cfo (O) [U(O)] (s) 09
~ 0.8
where ~ 0.7
7(f ) = 0.03325pV l /4f 7 / 4r-l l 4 `6' 0.6
as
U(Qa) Total flow speed at Qa. °
r: Hydraulic radius.
The eq.~6) corresponds to the empirical Blasius formula
(Schlichting 1968~. The above two equations result in
on
Cfo (Qa ~ ~U(Qa ~ ]7/4 (7) 0 0.8
Cfo (0) U(O) (a
. ~
- +~
ARC (p 04<
. . . . .' .
a
2 3
t_a (mm)
(a)U=5m/s
. ~
~ 00~
. °~b~:
~ ~ x
and further, using equip), 0.6 ~ O
,~ ~ 0.5 , , to
(1 ~ )~7/4 `8) ° t_a (mm)
(b)U=7m/s
is) Cf /Cfo of PP
The skin friction reduction was measured at three
speeds of U=5, 7, and lOm/sec, at xa =O.Sm (P2),
l.Om (P3), and l.5m (Pip. The results are shown in
Fig. 12. The results by Madavan 1984 at xa =0.27m
are also shown. The dashed line shows the inverse of
eq.~8), i.e. the contribution of the
o.s
0.8
0.7
Cfo correction. 0.6
The Cf / Cfo values at three streamwise locations 05
are approximately the same, except that the values at
xa =O.Sm are smaller in some cases, supporting the
general understanding that the reduction effect becomes
smaller in downstream. It may also be stated that the
reduction becomes smaller at higher speed. 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. It is clear that the correction method needs
further validation.
O Xa=0.5m (P2)
~ Xa=l.Om (P3)
X Xa=1.5m (P4)
CfO (0) /CfO (Qa)
~ Madavan 4. 2m/s
+ Madavan 9. 3m/s
O Xa=0.5m (P2)
~ Xa=l.Om (P3)
X Xa=1.5m (P4)
CfO(0) /CfO(Qa)
O Madavan 4 . 2m/s
~ Madavan 9 . 3m/s
MIX X
Oo ~
o
, . . . . ~ .
0 1
O Xa=0.5m (P2)
· Xa=l.Om (P3)
X Xa=1.5m (P4)
C fO(O)/CfO(Qa)
Madavan 4 . 2m/s
Madavan 9 . 3m/s
2 3
t_a (mm)
(c)U=lOm/s
Figure 12 Skin friction reduction by microbubbles
(NMRI HSWT porous plate)
<6y Cf / Cfo of AHP
The skin friction reduction results similar to Fig.12
but with the array-of-holes plate AHP is shown in Fig.
13. The overall tendency is similar to that of PP., but
there are subtle differences. At large ta' the reduc-
tion effect of AHP tends to saturate. The data scatter
of AHP looks smaller than that of PP.
6
OCR for page 8
1 r
no
08
n.6
0.9
0.8
0.7
0.6
0.5
0.9
0.8
0.7
0.6
0.5
_~
0 1 2 3
t_a (mm)
(a)U=Sm/s
E`,"
°?~
t
I
; . . . . ~ .
(b)U=7m/ s
t_a (mm)
3
.
0 1 2 3
t_a (mm)
(c)U=lOm/s
° Xa=0.5m (P2)
~ Xa=l.Om (P3)
X Xa=1.5m (P4)
CfO (0) /CfO (Qa)
Madavan 4. 2m/s
t Madavan 9. 3m/s
O Xa=0.5m (P2)
· Xa=l.Om (P3)
X Xa=1 . 5m (P4)
CfO (0) /CfO (Qa)
~ Madavan 4. 2m/s
+ Madavan 9. 3m/s
O Xa=0.5m (P2)
· Xa=l.Om (P3)
X Xa=1.5m (P4)
CfO (0) /CfO (Qa)
~ Madavan 4 . 2m/s
t Madavan 9 . 3m/s
Figure 13 Skin friction reduction by microbubbles
(NMRI HSWT array-of-holes plate)
(7)Comparison with Madavan's result (PP)
The skin friction reduction data of PP by Madavan
(Fig.3) and NMRI (Fig.12) are compared, as shown in
Fig.14. It is clear, first of all, that Madavan Ejected
air much more than NMRI. The skin friction reduc-
tion of NMRI is consistently smaller than that of
Madavan. Since there is not much data in Madavan's
result at ta less than two, no further comment is pos-
7
sible.
1.2
0.8
0.6
Cal
0.4
0.2
o
. ~,~ Madavan Xa=0 . 27m 4. 2m/s
Madavan Xa=0 . 27m 9. 3m/s
ON M R I HSWT Xa=O. 5m 5m/s
O NMRI HSWTXa=0.5m lOm/s
~S6~
O O 0 00
0 2 4 6 8 10 12 14
t_a (mm)
Figure 14 Comparison of PP skin friction reduction by
Madavan and NMRI (HSWT)
(8)Comparison with Kawakita's result (AMP)
The skin friction reduction data of AHP by Ka-
wakita (Fig.5) and NMRI (Fig.13) are compared, as
shown in Fig.15. They used the same type of AHP
and measured skin friction at the same downstream
locations using the same kind of sensors. At small air
injection rate, i.e. 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 contribu-
tion of the Cfo correction, while Kawakita's values
still go down. This may suggest that at small air n-
jection 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 injec-
tion rate, where the wall effect is significant, the phe-
nomenon in a small channel becomes different from
that in a large channel and further skin friction reduc-
tion is prevented due to some unknown mechanisms.
no
0.8
0.7
0.6
7
1 1 4)
At\
~ ~ ~,~
' :'K
. . .
0.5
0 1 2 3
t_a (mm)
(a)U=Sm/s
O NMRI Xa=0.5m
~ NMRI Xa=l.Om
X NMRI Xa=1.5m
CfO (0) /CfO (Qa)
0 Kawaki ta Xa=O. 5m
Q Kawaki ta Xa=1. em
OK Kawaki ta Xa=1 . 5m
OCR for page 9
8
in
o.9
0.8
0.7
0.6
0.5
o
1 2 3
t_a (mm)
(b)U=7m/s
O NMRI Xa=O. 5m
~ NMRI Xa=l. Om
X NMRI Xa=1. 5m
CfO (0) /CfO (Qa)
O Kawakita Xa=O. 5m
Kawakita Xa=1. On
Kawakita Xa=1.5m
Figure 15 Comparison of AHP skin friction reduction
by Kawakita and NMRI (HSWT)
(9)Summary
To summarize the comparison of the skin fric-
tion reduction results from three test facilities, we cur-
rently think as follows.
e
a) It is safer to use a large channel for microbubble
experiment.
b) In case a small channel is used, air injection rate
should be limited.
c) The Cfo correction method of eq. (5) needs
further study for validation.
d) The size of the cross section of a test section,
not only the height just discussed but also the
width, may have significant influence on the
microbubble mechanism. The side wall influ-
ence has not been examined so far.
The influence of the width of an injection plate
compared to the width of the channel has not
been examined so far either. There are two
factors that may contribute to the influence.
One is the diffusion of injected bubbles into the
bubble-free region. The other is the interaction
of bubbles thus diffused with the boundary layer
on the side wall. This influence may become
more significant as the location goes further
downstream from the injection point. 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. More specifically,
we want to know how long the reduction effect of n-
jected bubbles persists in the downstream direction.
However, most of existing microbubble experiments
are related to circulating water tunnels whose test sec-
tion length is of the order of a few meters at most. So
we have to do something to bridge the large gap be-
tween a few meters of basic experiments and a few
hundred meters of full-scale ships.
Watanabe et al. 1998 carried out a pioneering ex-
periment on the scale effect of microbubbles by towing
a flat plate ship with maximum length of 40m at the
maximum speed of 7m/s, corresponding to a typical
cruising speed of large tankers, in their towing tank.
(l)Tested ship and devices
The tested ship had a long parallel middle part of
60cm width, plus streamlined bow and stern parts.
The length of the bow part was 2.0m. The draft was
0.05m. The bow injection plate was located 1.2m
from the bow end. It was a porous plate with 15 ,um
pore diameter, and of size Ba x La = 250mm
X70mm. For skin friction measurement they used
commercial skin friction sensors of Sankei Engineering
co., ltd., the same type as those used by MHI and
NMRI, but with a larger sensing disk of 25 mm diame-
ter and the ma~mum load of 10 grams weight.
(2) Cf /Cfo
The streamwise distribution of the measured local
skin friction reduction at the ship speed V=7m/s is
shown in Fig. 16, where the horizontal axis shows xa,
the distance from the bow injection plate where air was
injected, and the injected air rate is expressed with ta
defined in eq.~21. It is seen that the reduction effect
persists almost all the way up to the downstream end.
1.2
1
o 0.8
0.6
(J
0.4
0.2
0 0 n 0 ~
Q ~ ~ X
X X X
° t_a=1.0mm
~ t_a=1.9mm
x t a=2.9mm
0 10
20 30 40
Xa (m)
Figure 16 Streamwise distribution of skin friction re-
8
OCR for page 10
9
auction of 40m-long flat plate ship (Fig.6
of Watanabe et al. 1998~.
The same data is shown in Fig. 17 with ta in the
horizontal axis. It is seen that the reduction increases
non-linearly with ta. They also measured the total
drag and its reduction.
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)
2.5 Experiments using a 12m flat plate ship at
NMRI
(l)Tested ship and devices
Watanabe's experiment was carried out in a towing
tank whose length was approximately 250m, and
therefore at the speed of 7m/s the time for the meas-
urement was very short. Also one cannot be sure
whether the limited width of 60cm had significant ~n-
fluence on the measured results or not. Therefore we
decided to do a similar experiment but with the ship
width of lm, in our 400m-long towing tank, and, as a
first step, we made a 12m-long flat plate ship. It is
shown in Figs. 18 and 19.
The tested ship had a long parallel middle part of
60cm width, plus streamlined bow and stern parts.
The length of the bow part was 3.0m and that of the
stern part was 2.0m. The plan form of the two parts
are, respectively
(Bow part)
(Stern part)
y = 14 (3- x)4 - ~ 2 (3 - x)3 + 0.5 (9)
y=-1x2+1x (10)
8 2
The bow part has the point of inflection at x= 1. Om, and
the both parts connect smoothly to the parallel part.
The draft was 0.045m. The corners were rounded
with a bilge circle of 20mm radius. Turbulence
stimulators of 2mm height were placed with tom pitch
in the girthwise direction, at O.5m from the bow end.
The bow injection plate was located 3.0m from the bow
end. It was a porous plate with 2 An pore diameter,
the same type as that used in the experiment shown in
Section 2.3, and of size Ba x La = 500mm X lOOmm.
For skin friction measurement we used commercial
skin friction sensors of Sankei Engineering co., ltd.,
with a sensing disk of 25mm diameter, the same kind as
that used by Watanabe.
Bow trim guide Stern trim guide
For~auge ~ TDwing carriage
~ ~L , 47 , ~ ~ w~tcr p~une
Porous plate
for air injection Skin faction sensors
_ \ ~ / \
Flow controller ~ / / \
0~: ~ im \
81 ~ ~,Et°' ~11 33 ~ ~0
_ 1 _ _-
. '1
1 2.000 mm
Figure 18 12m-long flat plate ship of NMRI (PP)
(a) Perspective view (b)Air injection plate (PP)
Figure 19 Photographs of 12m flat plate ship of NMRI
(2)Correction of skin friction values measured with
lOgf sensors
After the 12m-long ship experiment, we carried
out the 50m-long ship experiment using the same skin
friction sensors. In that case we repeated tests by ex-
changing the sensors in order to find out data scatter
due to different sensors. We found out that the meas-
ured values by lOgf sensors were significantly higher
than those by 2gf sensors, and that the latter were in
good agreement with Schoenherr values. We sus-
pected that the over-prediction by the lOgf sensors was
caused by insufficient rigidity, and obtained correction
9
OCR for page 11
10
curves by fitting quadratic polynomials which go to
unity at zero shear stress, to the data in bubble and
non-bubble conditions, as shown in Fig.20, in which
measurement locations are shown with the combination
of the sensors used ("S" denotes lOgf full-scale sensors
and "s" denotes lgf full-scale sensors). All the data
shown in this section have thus been corrected. 1.2
It should be noted that the over-prediction of this
kind causes over-prediction of skin friction reduction.
So we asked the authors of Watanabe et al. 1988, who
used the same kind of sensors of lOgf full scale, if they
had similar experience, but the answer was that their
measured shear stress was in good agreement with the
Schoenherr value. The reason for the discrepancy is
still unknown.
1.2
cc in
O .v
o 0.8
0.6
0.4
o
0.2
1.2
o 1.0
~¢ 0.8
4~
_'
0.6
in, 0.4
a
hi, 0.2
0.0
_ 0,o
_ -
- ~~o~
0.0 0.2 0.4 0.6
t w o f lOgf (unit: gf/cm^2)
(a) V=5m/s
, W~
~i,~ ~~
0.0 0.S 1.0 1.5
t w 0 f lOgf (unit: gf/~2)
(b)V=7m/s
then increases with ta' which shows that skin friction
reduction decreases where the air sheet breaks up. We
see similar tendency at xa =5.8m.
1.0
~ ~ 1
0.8 - 03 .. ~
o
0.6
0.4
0.2
0.0
~ Xa=0 5m (S1 & s4)
is Xa= 1 em (S2 & s5)
c Xa=58m (S3 & s6)
—- - Curve fit (Xa=0 . 5m)
—- ~Curve fit (Xa=1 .8m)
- - - Curve fit (Xa=5.8m)
1.2
1.0
0.8
0.6
cat
0.4
0.2
0.0
1.2
~ Xa=0 . 5m (S 1 & s4)
`` Xa=1.8m (S2 & s5)
5 Xa=5 . em (S3 & s6)
—- -Curve fit (Xa=0.5m)
- - - Curve fit (Xa=1. em)
Curve fit (Xa=5.8m) 0.4
1.0
Q8
'I 06
cat
Q2
DO
~ "Xa=5.8m (V=5m/s)
F ;.xa=~5.8m.(v=7'm/s) ,
0.00 1.00 2.00, 3.00
t_a tom)
Figure 20 Correction of skin friction measured with (C)xa=s.8m
sensors of lOgf full scale.
(3) Cf /Cfo
The measured skin friction reduction values thus
corrected are shown in Fig.21. At xa =O.Sm, the skin
friction reduces to zero, because the point was covered
with a large air sheet, especially at V=5m/s. At
xa =1.8m, the Cf /Cfo value first decreases and
10
l
~ Xa=0 . 5m (V=5m/s)
· Xa=0 . 5m (V=7m/s)
· ~ · A _
0.00 1.00 2.00 3.00
t_a (mm)
(a) xa =O.Sm
: A' ~
L\Xa= 1 . em (V=5m/s)
AXa=1.8m (V=7m/s)
. · . . . . .
l
4.00
0.00 1.00 2.00 3.00 4.00
t_a (mm)
(b) xa =1.8m
~ ———————————————1
¢—-————————————1
~ :"
Figure 21 Skin friction reduction at 12m-long flat plate
ship of NMRI (PP, corrected).
(4)Comparisonof Cf /Cfo with Watanabe's results
Fig.22 shows the present data compared with those
by Watanabe et al. 1998 shown in Fig.17, at V=7m/s.
The both data were obtained using PP and at the same
streamwise locations. The two results agree well.
OCR for page 12
11
12
10
Qua
'A Q6
cat
Q4
02
, ~ ~ ~ O ~
Il O
~ Xa= 1 . 8m NMRI PP _
OXa=5 . 8m NMRI PP
- QXa= 1 . am IHI PP
~ XX~ ; Am TlIT PP
0.0 ID 2.0 3.0 40
t_a (mm)
Figure 22 Comparison of skin friction reduction results
by NMRI and Watanabe et al. (PP)
2.6 Experiments using a 50m flat plate ship at
NMRI
The test was carried out twice, in 1999 and in 2000,
with different layout.
(l)Tested ship and devices
In 1999 the 12m ship was extended to 50m by
adding the parallel part. At the air injection point at
3.0m from the bow end, an array-of-holes plate (AMP)
was set, in order to obtain data for the full-scale test
described later. The skin friction sensors were placed
at x=3.5m, 4.8m, 8.8m, 16.8m, 24.8m, 28.8m, 32.8m,
and 46.8m, where x denotes distance from the bow end.
Both 2gf and 10gf sensors were used. The data by the
10gf sensors were corrected in the way described in the
previous section.
P1 tC Arms ply t4- 8rn) p3 ($~)
\l !
HE _m
.
(A' r ~ eject `on :at the A
.
(a)Bow half
palm Dime pa (~.~3 P(i~30~3~,
'
W. ~ ,
I ~ I I~ ~ I ~1~ I I I I~1 i i ~ I ~ I , ~ I
:N
Or tnj`cltio'~ ~~ Ted, Ale
(b)S tern half
Figure 23 50m-long flat plate ship of NMRI (AMP)
In 2000 another AHP was set at x=3 1 m, in order to
investigate the influence of boundary layer thickness on
skin friction reduction effect. This time the sensors
were placed at x=3.5m, 4.8m, 8.8m, 31.5m, 32.8m, and
36.8m, i.e., three sensors were placed at the same rela-
tive distance downstream of each injection plate.
Only 2gf sensors were used. Fig.23 shows the layout
of the 50m flat plate ship used in the 2000 test.
(2' Cf /Cfo: Ha distribution
The streamwise distribution of the measured skin
friction reduction at two speeds is shown in Fig.24.
The data of the two tests are shown with the same
symbols, because they agreed well with each other.
The skin friction reduction is greater at V=Sm/s than
V=7m/s. At V=Sm/s the reduction persists to the
downstream end, while, at V=7m/s, the reduction is
almost comparable with that of V=Sm/s immediately
downstream of the injection point, but reduces more
quickly downstream.
I.2
I.0
0.8
to
t0.6
cat
0.4
0.2
-
~ '
~Q ~
~ V= 5m/s
· V=7m/s
1 . ~ . ~ . ~ . ~ ~
0 10 20 30
Xa (m)
40 50
Figure 24 Streamwise distribution of skin friction ~-
duction by microbubbles on 50m flat
plate ship of NMRI with air injection at
x=3.0m (AMP)
(3) Cf /Cfo: Comparison of bow and middle injec-
tion
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 impor-
tant. For example, Merkle and Deutsch 1990 used the
average void ratio in the boundary layer as such pa-
rameter. In this case, the amount of injected air
needed increases with the length scale, and can result
that the application of microbubbles to full-scale ships
11
OCR for page 13
12
is not feasible.
To find out how important, or unimportant, the
boundary layer thickness is to the skin friction reduc-
tion effect, is also useful for designing air injectors for a
full-scale ship. Thus, in the experiment of the 50m
flat plate ship, AHPs were placed at x=3.0m (Bow) and
31.0m (Middle). The comparison of the measured
skin friction reduction for the two injection cases are
shown in Fig.25 (V=7m/s). The reduction by the
middle injection is slightly greater. That is perhaps
because the boundary layer is thicker there and it helps
the bubbles to stay near the wall. Although there is
slight difference, the magnitude of the reduction is ap-
proximately the same, and it may be stated that the re-
duction effect is mainly a function of the distance from
the injection point and that the influence of the bound-
ary layer is small.
0.8
Q6
0.4
1 ~ ~ ~ a ~ ~ ~ 1
° Xa=0 . 5m (Bow)
O Xa=5 . em (Bow)
XXa=0.5m (Middle)
Xa=5.8m (Middle)
X O
X
0 1 2 3 4
ta (mm)
5
Figure 25 Skin friction reduction in bow (x=3.0m) and
middle (x=31 m) injection cases of the
50m flat plate ship. V=7m/s.
(4' Cf / Cfo; comparison of AHP and PP
1.2
1.0
0.e
Cal
1, o.e
0.4
0.2
1~.~.~t
it: · ~ ~
°t_a=2rnrn AHP NMRI
X<_a=4mm AHP NMRI
Qt_a=1.9mm PP Watanabe
· t a=2.9mm PP Watanabe
0 10 20 30
Xa (m)
40 50
Figure 26 Cf /Cfo; Comparison of AHP (NMRI) and
PP (Watanabe). V=7m/s
The streamwise distribution of the skin friction
reduction by NMRI using AHP is compared with that
by Watanabe et al. using PP at V=7rn/s, as shown in
Fig.26. It is clearly seen that the reduction of PP is
significantly larger than that of AHP. It seems that,
although the question on the accuracy of the lOgf sen-
sors has not been answered yet, PP reduces skin friction
considerably more than AHP, especially at large down-
stream distance from the injection point.
3. Local void ratio distribution of microbubbles
It is now a generally accepted idea that there must
be many bubbles close to the solid wall, in order to
have significant skin friction reduction. The consen-
sus on the degree of closeness has not been reached, but
Merkle et al. 1987 says, based on their observation of
bubble behavior in plate-on-top and plate-on-bottom
conditions and the skin friction reduction, that if there
is no bubble in the range y+ less than 100 no skin
friction reduction occurs, for example.
Here, we show only one example of the measure-
ment carried out at NMRI (Kodama et al. 20001.
Fig.27 shows the system we used to measure the local
void ratio as a function of the normal distance from the
wall, designed after the one used by Guin (Guin et al.
1996~. The mixture of air and water is taken into a
tube facing the stream by suction, and the volume of air
and water is measured separately in the two charribers.
The local void ratio Car is then calculated using
eq.~11), which is similar to eq.~3) for average void ra-
tio.
fV = Qa
Qa +Qw Ally
>\ acuum meter
A'r:~ Cl~mberA
ve o Chamber B
| Electromag-/ ,~ ~ ~ ~ I
Valve B l Vatcaunukm
[low ~~` ° 0 0 opt ° O ° no ~
Suction tube
Vacuum pump
Figure 27 Local void ratio measurement system at
NMRI (Kodama et al. 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.
12
OCR for page 14
13
At xa =0.Sm there is a sharp and high peak in the vi-
cinity of the wall, but at xa =5.8m the peak decreases
significantly, which corresponds to the decrease in the
skin friction reduction effect. It should be noted that
this measurement required several runs in the towing
tank to accumulate sufficiently the air and water vol-
umes.
n an
- ~ ~
0.30 t,O
0.20
0.10
0.00
O °Xa=0.5m
; °g AXa=5. 8m
) o
~^
\. · . °~1
0 10 20 30 40 50 60 7 0 80 90
y (mm)
AXa=5 . em
Figure 28 Local void ratio distribution on the 12m flat
plate ship of NMRI with PP at V=7m/s
and ta =0.71mm.
4. Skin friction reduction mechanism of micro-
bubbles
The skin friction reduction mechanism of micro-
bubbles is not yet fully understood. 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) the turbu-
lence intensity and thus decrease skin friction. For
example, Merkle and Deutsch 1990 showed that the
turbulence fluctuations at the wall is reduced by the
presence of bubbles near the wall, by using a hot film
mounted flush with the surface.
More recently, Nag aya et al. used PIV/LIF to
measure turbulence in the water part of bubbly flows in
the channel shown in Fig.7, by selectively obtaining the
image of the tracers that emitted fluorescent light
whose wavelength was different from the original laser
sheet. According to their measurement, at U=5m/s,
both u and v velocity fluctuations increased but
the Reynolds stress decreased, with increasing void
ratio, as shown in Fig.29.
Oh
n 5
O-
-2 -1 0 1 2
up
Figure 29 Measured Reynolds stress distribution across
the channel height of HSWT shown in Fig. 7
Nagaya et al. 2002. U=Sm/s. OC: average
void ratio.
If there are mechanisms other than the density ef-
fect for the skin friction reduction by microbubbles,
there is a chance that one can maximize the reduction
by controlling the bubble size. However, Moriguchi
and Kato 2002 changed the bubble diameter from
0.5mm to 2mm at the same flow speed, by changing the
channel height only at injection point, but the reduction
value did not change.
The existence of bubbles makes both measurement
and computation difficult. Kawamura and Kodama
2001 carried out DNS of bubbly turbulent channel flow
at Rer=360, where the bubble motion and deforma-
tion were expressed using VOF, but they got only skin
friction increase.
Further study is need to clarify then mechanism.
5. Frictional drag reduction by microbubbles
(l)Total drag
The measured total drag of the 12m flat plate ship
in the non-bubble condition is shown in Fig.30. Using
Prohaska's method with n=4 and using the data below
Fn =O.15, the form factor has been determined to be
1+ K=1.1678.
The measured total drag of the 50m flat plate ship
in the non-bubble condition is shown in Fig.31. The
two tests carried out in 1999 and 2000 gave almost the
same result. At very low speed the data in 1999 show
laminar behavior.
13
OCR for page 15
14
0.0045
0.0040
~ 0.0035
Cal
~ 0.0030
· Ct
CfO Schoenherr
I\ (l+K)CfO (1+K=1.1678)
\~e
\
.
qb —— (12)
°° b
Air injection plate ash
0.0
0.3 0.4
En
0.5
0.6
Figure 30 Total drag of 12m flat plate ship of NMRI in
non-bubble condition.
0.0035
0.0030
0.0025
4~
Cal
0.0020
\~
· Ct 2000 (1+K=1.143)
° Ct 1999 (1+K=1.146)
C fO (SchoellheIt)
(1+K) CfO (1+K=1 . 146)
0°~° ·e o.
0.0015 . ._
0.00 O. 10 0.20 0.30 0.40
Fn=V/sqrt (gL)
Figure 31 Total drag of 50m flat plate ship of NMRI in
non-bubble condition.
(2)Reduction of frictional drag by microbubbles
The reduction of total drag by microbubbles was
measured and the reduction was attributed to the reduc-
tion of the frictional drag component Rf . Rfo, the
frictional drag of the area Sb (see Fig.32), which is
downstream of the air injection plate, in the non-bubble
condition was estimated using Schoenherr's formula
(29~.
The result is shown in Fig.33 for the 12m flat plate
ship of NMRI (PP), and in Fig.34 for the 50m ship of
NMRI (AHP). The horizontal axis shows qb, the
injected air rate nondimensionalized by ship's speed
and Sb. It is seen that the reduction is smaller at
higher speed and that the reduction per unit qb is
much higher with the 50m ship than with the 12m ship.
The reason for the latter may be that the skin friction
reduction effect of microbubble persists for a long dis-
tance downstream.
14
07 Ship's hull bottom
Figure 32 Sb : Area to be covered with bubbles
1.0
0.9
0.8
0.7
Q6
no
0 ~ ~
0 ~
o
o
0 V=5m/s
~ V=7m/s °
0.00E+00 1. OOE-04 2. OOE-04 3. OOE-04 4. OOE-04
qb
Figure 33 Frictional drag reduction of 12m flat plate
ship (NMRL PP)
as
Q8
~0.7
0.6
0.5
O ~ to ^~^
~ ~ A&6
- 0.
°V=5m/s 1999 °O
·V=5m/s 2000
aV=7m/s 1999
~&V=7m/s 2000
O . OE+OO 5 . OE-05
qb
1 . OE-04 1 . 5E-04
Figure 34 Frictional drag reduction of SOm flat plate
ship (NMRI, AHP, bow injection)
6. Applicability of microbubbles to ships
This chapter aims at discussing the applicability of
microbubbles to ships. First, an equations for the net
power saving of a full-scale ship is derived by consid-
ering the skin friction induction by microbubbles and
the power needed to inject them. Second, issues on
OCR for page 16
15
the practical application of microbubbles to ships are
discussed, using the parameters showing in the equation
derived. Finally, the net power saving value is esti-
mated using available experimental results and typical
specifications of a full-scale ship.
( 1 ) Equation for net power saving
We assume that a ship runs at speed U with
power WO, "O" denoting the non-bubble condition, and
by Ejecting bubbles the ship runs at the same speed where p: water density,
U with different, hopefully reduced, power W .
WO and W are expressed as
WO = DoU, W = DU, (13)
where Do, D: ship's drags. ~—/\D) and ~—/\W),
the drag and power reductions, are related as
(-AW)= (-AD)U, (14)
where
Do —D = DFbO DFb -
In order to estimate the net drag reduction effect,
one should subtract the power needed for bubble injec-
tion from the power gain due to drag reduction. The
pumping power Wpump is expressed as
Wpump (PgZ + p)Q (20)
g: gravity acceleration,
z: water depth at injection point,
p: dynamic pressure at injection point,
Q: volumetric air injection rate.
By introducing nondimensional parameters
CQ _ Q : volumetric air injection coefficient,
USb
rz _—: water depth ratio ~ L': ship length)
AW_W-WO, /\D-D-DO (15)
(21)
(22)
Fn—U / it;: Froude number, (23)
C _ P / I PU2 (24)
Let DF be the frictional drag and define P 2
rF - DF /D. (16)
Let DFb be the frictional drag of the surface covered
with bubbles. The ratio DFb/DF should be ~p-
proximately equal to
rs-Sb/S (17)
where
S: wetted surface area of the ship,
Sb: surface area covered with bubbles,
but by selecting the Sb location near the bow we can
expect, using a parameter mb > 1,
DFb ID = rFrSmb- (18)
Since the drag reduction occurs only in DFb,
15
Wpump of eq.~20) is expressed as
Wpump = 2 pU35bCQ~ Z2 +CP), (25)
where p: water density.
Finally the net power saving ratio rnpS is ex-
pressed as, using eqs.(l3) through (25),
(-/\W) - Wpump `26)
nps W
= rS [rF mb (1——~——~ 2 + C p )] '
DFbO Do n
(27)
where DO has been nondimensionalizedas
CDO - DO /—PU25 . (28)
OCR for page 17
16
For example, rnpS = 0.05 means 5% net power saving.
It is the purpose of microbubble studies to maximize
this parameter.
(2)Issues on the application of microbubbles to ships
This chapter discusses the issues on the applicability of
microbubbles to full-scale ships mainly by discussing
each parameter in eq.~27~.
a) l—DFb / DFbO
This parameter represents the reduction of the fric-
tional drag by microbubbles and therefore is the most
important. It is not the local value but the integrated
value over the entire surface covered with bubbles.
Fig.34 shows the values for the 50m flat plate ship
in the bow injection case. The frictional drag in that
area to be covered with bubbles in the non-bubble con-
dition was estimated using the Schoenherr experimental
formula shown below.
logy ReCFS h I) = 0~242 log ~ (29)
The figure again shows that the skin friction reduction
is smaller at higher speed. This fact suggests that m-
crobubbles should be applied to slow-speed ships.
The maximum reduction at V=7m/sec reaches over
20%. 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.
byrs
We should efficiently cover a wide surface area with
bubbles.
e) rz / Fn2
This ratio, being equal to go / U 2, means that the
pumping power Educes quadratically with speed. So
the higher the speed, the better the performance. Note
that the skin friction reduction effect decreases with
speed at the same time. It would also be possible to
reduce rz by utilizing the downward flow near the
bow.
f) Cp
By choosing the location for bubble injection or
designing the local shape, one can reduce Cp and
thus improve the net power gain.
g) Hull form
As Fig. 1 suggests, the hull form of a large tanker is
regarded suitable for microbubbles, because its flat and
very wide bottom will help the bubbles injected at the
bow stay close to the bottom by buoyancy. It is also
important to consider effects of non-horizontal surface,
pressure gradient, and surface curvature.
h) Sea water
Bubbles generated in sea water are in general
smaller than those in fresh water with which almost all
the laboratory experiments were carried out. This will
affect the trajectories and the skin friction reduction
effect.
i) Propeller performance in bubbly flow
As Fig.1 shows, there is a good chance that the
bubbles go into a propeller operating at the stern end.
Ichikawa and Matsumoto 1995, measured lift and drag
of a 2D NNCA4412 wing section in various average
void ratio (Fig.35), and found that lift decreased and
1 B
|.6
I.4
'.2
~ 1
0 8
0 6
o.`
0.2.
n
~ .-
l _ OX .
_ ~ 2X
_ _ ~ 51
.... .... ....
C) rF
The wave-making drag, the other major drag com-
ponent, increases quadratically with speed, and there-
fore the smaller the speed, the greater rF becomes,
and the better-suited to microbubbles. With a dis-
placement-type ship such as a large tanker at the cruis-
ing speed of 14knots (7m/sec), rF reaches 0.8 ~ °,,,,~,; r.~ - C(C - .) 5 2)
d) CQ /CDO
We should minimize the air volume for a given skin
friction reduction.
16
_.w
0.3
0.2-g
0.2
0.15
0.1
0.05
O
0 ~ 10 15 .
- ~e of Sty (I ]
(a)Lift (b)Drag
Figure 35 Lift and drag of NACA4412 wing section in
bubbly flow (Ichikawa 1995~.
OCR for page 18
drag increased by the presence of bubbles. Therefore
we should expect that the propeller performance suffers
when bubbles go into the propeller, and we should
avoid it.
The influence of inflowing bubbles on propeller vibra-
tion should also be investigated.
j) CFD prediction methods
In order to predict the drag reduction performance of
bubbles at full scale, one must first simulate the flow
around a full-scale ship, which is already very difficult.
Second, one must predict bubble trajectories by consid-
ering the bubbles as a group, which means the two-way
coupling approach.
(3)Estimation of rnpS
rnpS is now estimated. Observing the plots in
Fig.34 are almost linear at 7m/s, let us define the slope
a b —(1 DFb / DFbO )/ CQ
CF, the skin friction of a ship, is traditionally related
to CFS h ' the skin friction of a flat plate with the same
length and area, as
CF = (1 + K)CFSCh, (31)
where 1 + K iS called the form factor. Using the
two equations and eq.~16), rnpS of eq.~27) becomes
rnpS = rS rF CQ [mbab - (} K TIC ~ F. 2 + Cp )]
Using the parameter values shown in Table 1, in
which dimensions are of a large tanker, the bubble area
is the front half of the hull surface, mb is given as the
ratio of CFS h of 50m and 100m, rF and 1 + K are
typical values of a large tanker. Using the values it
results that
rnpS = 0.046, (33)
which means 5% net power saving. If z can be E-
duced to Sm somehow, it becomes that rnpS =0.069.
Table 1 Parameters for estimating rnpS
L (m)
B (m)
Z (m)
U (mls)
rS
100
20
7
7
_ 0.5
mb 1 1.093
1 0.8
Cp T °
1+K 1 1.3
ab 2373
Figure 36 Training ship "SEIUN-MARU", Institute for
Sea Training. 5,884 GT, Loa=1 16m,
B=17.9m, 10,500PS, l9.5kt.
7. Full-scale microbubble experiment
In September 2001, a full-scale microbubble ~-
periment was carried out using a training ship
"SEIUN-MARU" of the Institute for Sea Training
(Fig.36~. 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. The detail of the microbubble
study in the project will be published as Kodama et al.
2002 and Nagamatsu et al. 2002.
Air, which was supplied by six compressors on
deck, was injected through three separate horizontal
ducts welded on the bow side, as shown in Fig.37.
Each duct has holes of AHP type.
The tests were carried out by switching on and off
the air injection while keeping the propeller rps con-
stant, and the change in the ship speed was converted
into power change at constant speed. Fig.38 shows
the speed-power curve in bubble and non-bubble condi-
tions. Air was injected at full power (max) or half
power (1/2 max) from all the three locations (All), and
in all the bubble injection cases, contrary to the expec-
tation, the engine power increased at constant speed.
OCR for page 19
18
Figure 37 The bubble injectors and a TV camera
BI 1, 2, 3: Bubble injectors No. 1, 2, 3
C: Underwater TV camera
From the measurement of thrust and torque in the
bubble conditions, it was suspected that due to the bub-
ble entrainment, propeller performance was deterio-
rated. Therefore, another series of tests were carried
out, in which air was injected from a single injector,
one by one. And when air was injected only from BIT,
the propeller performance was hardly deteriorated, and
3% power saving was obtained, as shown in Fig.39.
In this condition, since BI1 from which air was injected
was only lm below the water surface, the 2% net power
5000
4000
3000
3
o
~ 2000
i=
Cot
1000
O _
o No bubbles
~ x A llmax i
- a--- AH 1/2max
him
./
~ ~
. . . . . . . . .
9 10 1 1 12 13 14 15 16 17 18 19
V(kt)
Figure 38 Speed-power curve in bubble and non-bubble
conditions
18
~ without bubbles ,
I · with bubbles
5
,= 1 7 0 0
o
ED
~ 1600
i=
500
400
1~.
-.
kW}
kW
. A.
4;7—'I
;;4
.
V3 curve . ,
1.. ~ .
.
'i
. . s
3.6 13.8 14.0 14.2 14.4 14.6
Ship speed V (kt)
Figure 39 3% power saving at about 14 knots with max.
air injection from BIT.
saving was obtained, by taking into account the hydro-
static pressure at BIT. The reason that the bubbles
generated at such a shallow location reduced the drag
efficiently is that there was downward flow that carried
the bubbles down to the hull bottom. This is a good
news because the power needed to inject bubbles can
be reduced significantly in that way.
8. Conclusions
Microbubbles can be called "a big child -- imma-
ture, but with bright future". They reduce skin friction
significantly both in small scales and in large scales,
and the SEIUN-MARU experiment has proved that
they can actually reduce drag of a full scale ship with
length over lOOm. But at the same time many prob-
lems need to be solved before they can be put to practi-
cal use.
The mechanism for skin friction reduction by
microbubbles is not yet fully understood. This causes
uncertainty in the way the experiment is carried out and
leaves many related problems, such as scale effects,
unsolved. To date no numerical simulation has suc-
cessfully simulated the mechanism and the skin friction
reduction effect of microbubbles. This is a serious
disadvantage, since numerical simulation can give us
detailed information on the flow, which experiment
cannot. 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,
OCR for page 20
too.
The application of Microbubbles to full-scale ships
poses another series of problems to be solved, such as
scale effects, design of bubble injectors, deterioration
of propeller performance due to inflowing bubbles, and
prediction methods for bubble trajectories along a ship
hull.
The information on the bubble behavior at full
scale is perhaps the most lacking and the most needed.
It was a good news that drag reduction by Microbubbles
was confirmed in the full scale test using
SEIUN-MARU. Further full scale test should be
conducted, especially using a ship that has a wide and
flat bottom, where bubbles are expected to perform
well.
Acknowledgements
A part of the present study was carried out as the
SR239 project organized by the Shipbuilding Research
Association of Japan, funded by the Nippon Foundation.
The full scale experiment using SEIUN-MARU was
carried out solely as the SR239 project. The authors
thank the members of the microbubble working group
of the SR239 project, especially Prof H. Kato from
Toyo Univ., Profs. T. Suzuki and Y. Toda from Osaka
Univ., Prof. T. Nagamatsu from Kagoshima Univ., Mr.
S. Yamatani from the Institute for Sea Training, Mr. T.
Lanai from the Shipbuilding Research Center, Mr. S.
Ishikawa from Mitsubishi Heavy Industries, Dr. M.
Takai from Sumtomo Heavy Industries, Dr. H.
Kamiirisa from Mitsui Engineering & Shipbuilding Co.,
Ltd., Dr. S. Ogiwara from IHI Marine United, and Y.
Yoshida from IHI Aerospace for providing the
full-scale data and for valuable discussions. The au-
thors also thank Mr. Yabuki, Captain of the
SEIUN-MARU, and Mr. K. Yamashita from Chugoku
Paint, for the cooperation in the full-scale experiment.
References
Fukuda, K. et al., "Frictional Drag Reduction with Air
Lubricant over Super Water Repellent Surface (2nd
report>- Resistance Tests of Tanker and High
Length-to-beam-ratio Ship Models --", J. of Soc. of
Naval Architects of Japan, vol.186, 1999, pp.7~81.
Guin, M.M. et al., "Reduction of Skin Friction by Mi-
crobubbles and its Relation with Near-Wall Bubble
Concentration in a Channel", J. of Marine Science and
Technolo~v, vol. 1, No.5, 1996, pp.241-254.
Kato, H. et al., "Frictional Drag Reduction by injecting
19
, ,
19
Bubbly Water into Turbulent Boundary Layer", Cavita-
tion and Gas Liquid Flow in Fluid Machinery and De-
vices, FED -vol. 190,ASME, 1994, pp 185- 194.
Kato, H. et al., "Effect of Microbubble Cluster on Tur-
bulent Flow Structure" IUTAM Symposium on Me-
chanics of Passive and Active Flow Control, Gottingen,
1998.
Kawakita, C. and Takano, S., "Microbubble Skin Fric-
tion Reduction under the Influence of Pressure Gradi-
ents and Curved Surface", J. of the Society of Naval
Architects of Japan, vol. 1 88, 2000, pp. 1 1- 21 (in Japa-
nese).
Kawamura, T. and Kodama, Y., "Direct numerical
simulation of interactions between bubbles and
wall-turbulence", TSFP-2, Ad Int. Svmp. on Turbu-
lence and Shear Flow Phenomena, Stockholm, 2001.
Kodama, Y. et al., "Experimental study on microbub-
bles and their applicability to ships for skin friction
reduction.", Int. J. of Heat and Fluid Flow vol.21, 2000.
Kodama, Y. et al., " A Full-scale Experiment on Micm-
bubbles for Skin Friction Reduction Using SEIUN
MARU Part 1: The Preparatory Study", to be pub-
lished in J. of the Society of Naval Architects of Japan,
vol. 192, 2002 (in Japanese).
Madavan, N.K. et al.," Reduction of Turbulent Skin
Friction by Microbubbles", Physics of Fluids, Nb1.27,
No.2, pp.35~363, 1984.
McCormick, M.E. and Bhattacharyya, R., "Drag Pe-
duction of a Submersible Hull by Electrolysis", Naval
Engineers Journal, Vol.85, No.2, 1973, pp. 11-16.
Merkle, C. L., Deutsch, S., Pal, S., Cimbala, J., and
Seelig, W., "Microbubble Drag Reduction", 16th SYm-
posium on Naval Hvdrodvnamics, 1987, pp.199-215.
Merkle, C. L and Deutsch, S., "Drag Reduction in Liq-
uid Boundary Layers by Gas Iniection", Progress in
Astronautics and Aeronautics vol.123, 1990, AIAA,
pp.35 1-412.
Moriguchi, Y. and Kato, H.: Influence of Microbubble
diameter and bubble distribution on frictional resistance
reduction by microbubbles, J. of Marine Science and
Technology vol.7, No.2, 2002 (to be published).
Nagamatsu, T. et al., " A Full-scale Experiment on
Microbubbles for Skin Friction Reduction Using
SEIUN MARU Part 2: The Full-scale Experiment
Prepara", to be published in J. of the Society of Naval
OCR for page 21
20
to be published in J. of the Society of Naval Architects
of Japan, vol.192, 2002 (in Japanese).
Nagaya, S., Hishida, K., Kakugawa, A., and Kodama,
Y., "PIV/LIF Measurement of Wall Turbulence Modifi-
cation by Microbubbles", Proceedings of the 3rd SY m-
posium on Smart Control of Turbulence, March 2002,
Tokyo.
Schlichting, H. ,"Boundary-Layer Theory", 6th edi-
tion, McGrawhill, 1968.
SR239 Research Report, "Study on Frictional Drag
Reduction Methods for Ships", Shipbuilding Research
Association of Japan, March 2002.
Takahashi, T. et al., "Streamwise Distribution of the
Skin Friction Reduction by Microbubbles", J. of the
Society of Naval Architects of Japan, vol.182, 1997,
pp. 1-8 (in Japanese).
Takahashi, T. et al., "Experimental study on drag reduc-
tion by Microbubbles using a 50m-long flat plate ship"
TSFP-2~ 2nd Int. Symp. on Turbulence and Shear Flow
Phenomena, Vol.1, June 2001, Stockholm, pp. 175-180.
Yoshida, Y. et al., "Distribution of void fraction in bub-
bly flow through a horizontal channel: Bubbly bound-
ary layer flow, 2nd report", J. of Marine Science Tech-
nology 3, 1998, pp.30-36.
Watanabe, O. et al., "Measurements of Drag Reduction
by Microbubbles Using Very Long Ship Models", J. of
Soc. Naval Architects Japan, vol.183, 1998, pp.53-63
(in Japanese).
20
OCR for page 22
DISCUSSION
Prof. C. M. Lee
Pohang University of Science and Technology,
Korea
I suppose the size of the bubbles should have
some effect on the reduction in the friction drag.
I wonder if you have investigated the effect of
the bubble size.
AUTHORS' REPLY
I understand that the discussion on the effect of
bubble size on the skin friction reduction effect
has not been settled yet. The main reason for
that is that techniques for changing bubble size
without changing the flow property such as flow
speed are very rare. But recently Moriguchi, Y.
and Kato, H. 2002 developed a technique and
changed the bubble diameter from 0.5mm to
2mm, but the skin friction reduction effect did
not change. Kawamura, T. et al. 2002 developed
another technique which generates bubbles of 20
to 40 micron meters diameter, and their
preliminary result shows that the reduction effect
is significantly greater than that by the bubbles
of mm size. So further research is needed to
clarify the effect of bubble size on the reduction
effect.
Kawamura, T., Kakugawa, A., Kodama, Y.,
Moriguchi, Y. and Kato, H.: Controlling
the Size of Microbubbles for Drag
Reduction, Proceedings of the 3rd
International Symposium on Smart Control
of Turbulence, March 2002, Tokyo, Japan,
pp. 121-128, (http://www.turbulence-
control.grjp/symposium/FY200 1 /index.htm
1)
Moriguchi, Y. and Kato, H.: Influence of
microbubble diameter and bubble
distribution on frictional resistance
reduction by microbubbles. J. of Marine
Science and Technology
(to be published).
DISCUSSION
Hoyte C. Raven
MARIN, The Netherlands
The authors are commended with this interesting
paper and important work.
As mentioned in Section 4, microbubbles seem
to reduce the turbulence intensity. Are there
experimental data on the effect on the boundary
layer velocity profile?
microbubble injection
susceptibility to flow separation?
AUTHORS' REPLY
Could it happen that
would increase the
. ~
Nagaya et al. 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. But the authors think that
there was no flow separation in their tested
cases, because of the observed strong persistence
of the skin friction reduction effect in the
downstream direction.
Nagaya, S., Hishida, K., Kakugawa, A., and
Kodama, Y., "PIV/LIF Measurement of
Wall Turbulence Modifi-cation by
Microbubbles", Proceedings of the 3rd
Sym-posium on Smart Control of
Turbulence, March 2002, Tokyo.
(http ://www.turbulence-
control.gr jp/symposium/FY200 1 /index.htm
1)
7 No.2, 2002
Representative terms from entire chapter:
friction reduction
