Effect of air bubbles entrained from bow on propeller induced pressure fluctuation

(1)

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MITSUBISHi TECHNICAL BULLETIN No. 140

Effect of Air Bubbles Entrained from Bow on

Propeller-induced Pressure Fluctuation

June 1980

(2)

Afdehno Schpppsbouw_ _{en Schetpvart"une}

I

DOCUMENTATiE

TecIì-i.

_{oc-oI__Delft}

Effect of Air Bubbles Entrained from Bow on

DATUMI

Propeller-induced Pressure Fluctuation

Katsuyoshi Takekuma*

The vibration phenomena experienced on a C/ass of cargo liners in their sea trials had very peculiar features which had never been

experienced and could not be explained by the present know/edge on the vibration excitation. They were featured by the excessive vibration and large pressure fluctuations on hull surface with a beating characteristics, the remarkable decrease soon after

the application of helm and the marked change due to a small variation in loading conditions of the ship.

Extensive investigations were carried out to find an effective solution from structural and hydrodynamic point of view. As a result, it was found that a large quantity of air bubbles around the propeller played an important role in these phenomena. The

successful settlement of the vibration problem was attained by fitting a stern tunnel fin by means of which air bubbles could be swept away from the flow above the propeller

Further investigations were carried Out for a proper understanding of hydrodynamic mechanism of the excessive vibration

excita-tion related to air bubbles above the propeller.

1. Introduction

n view of the recent trend of the design requirment for

reduction of ship vibration, propeller-induced vibration has

become one of the most important items to be considered

in the ship design.

Among the causes of propeller-induced ship vibration,

pressure fluctuations induced by a cavitating propeller

oper-ating in the non-uniform flow behind the propeller are

considered to be of primary importance. Extensive investi-gations have been conducted therefore in all the

organiza-tions concerned in various fields, theory, model and full scale test in order to meet the urgent requirement for a

method of prediction and to find out the means for

re-ducing vibration. As a result, theoretical prediction method

of pressure fluctuations induced by the propeller have

become available, and some guidances for the design of hull form and propeller are proposed.

However, there have been reported some cases of

exces-sive vibration problem which occurred on various kinds of

ships13

Most of them have been solved by fitting

some kind of flow control

devices1561, such as a stern

tunnel fin or a set of vortex generators. And the

explana-tions have been made on the hydrodynamic mechanism of the excessive vibration by propeller-induced pressure fluc-tuations.

(1> High wake peak in the flow in the propeller plane was

caused by unsuitable shape of after body, such as

narrow screw aperture, large water line slope at stern frame and transom stern with broad wetted surface

area above the propeller.

(2) Severe cavitation occurred on a propeller operating in the flow with high wake peak. At the same time,

ex-cessive pressure fluctuations occurred and were exerted on the hull surface121.

* Nagasaki Technical Institute, Technical Headquarters

Improvement of flow around a propeller resulting from low wake peak and less degree of turbulence was

ob-tained by flow control devices. The vibration

excita-tion was decreased when the propeller cavitation

became milder in the improved flow.

Propeller-hull vortex11I _{which had been observed in} few cases of the model tests was not considered to play

an important role in excitation of vibration because its

effect was confined to very narrow exciting area on the hull surface.

However, the excessive vibration problem experienced

on a class of cargo liners181, which is the subject of this paper, could neither be explained nor be solved by the

available knowledge of the hydrodynamically induced

vibration as above.

In the course of the design and the model tests on these ships, no vibration problem was expected judging from the wake field and the cavitation pattern on the propeller oper-ating in the behind condition. Therefore excessive vibration experienced on the trial was certainly surprising.

The vibration was characterized by not only the magni-tude but also the peculiar features, such as large pressure

fluctuations accompanied with beating, remarkable varia-tion with small change of bow immersion and significant

decrease soon after application of helm.

The conclusion obtained as the result of extensive

in-vestigations to search the solution was that a large quantity of air bubbles191 in the flow around the propeller played an

important role in the excitation of vibration within the very

limited range of loading conditions of the ship.

A remarkable decrease of vibration was achieved on the

sea trial and was also proved in service by fitting a stern tunnel fin designed with a view to reducing the

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Fn

As the result of basic studies18> including reproduction of the phenomena in a cavitation tunnel and investigations of the flow above the propeller, it was deduced that the un-steady propeller hull vortex above the propeller was

respon-sible for change of pressure fluctuations and that air

bub-bles

in the flow entrained from bow brought about an

increase of the pressure fluctuations together with growth

of the unsteady propeller-hull-vortex.

In the following are outlined the excessive vibration

phenomena experienced on the class of the cargo liners,

basic investigations to clarify the hydrodynamic mechanism

and the solution of the problem by a stern tunnel fin, in

order to

furnish a valuable reference to extraordinary

vibration phenomena.

2. Features of the vibration problem

The class of ships to be discussed in this paper is the

diesel driven, single screw multi-purpose cargo liner. Princi-pal dimensions and the general arrangement are shown in Fig. 1.

The hull form and propeller were designed to

incorpo-rate the accumulated experience of MHI in order to satisfy

the severe design constraints, such as high performance,

low level of vibration etc. Resistance tests, self-propulsion

tests, wake survey and propeller cavitation tests were

carried out in the Nagasaki Experimental Tank of MHI. As a result, it was concluded that hydrodynamic charac-teristics of the ships relating to vibratory forces were as

good as those of other high speed ships built at MHI which had not suffered from vibration trouble.

However, the excessive vibration occurred on the sea trials of the ist ship. The vibration and pressure fluctua-tions are illustrated in the figures following Fig. 4, in which

V1 is vertical acceleration at the aft end on the poop deck.

I

>

T

V1 = Vertical acceleration at the aft end on the poop deck

F1 I Fore and aft acceleration on the floor of the wheel house

P1 Pressure fluctuations on the hull above the propeller

Fig. i Principal particulars of a class of cargo liners

AP

li

Ii f4

\-/

/

No.1CARt0 HOLD-FP

Fig. 2 Variation of vibration severity with

loading condition

F1 I is fore and aft acceleration on the floor of the wheel

house. P, and P3 are pressure fluctuations on the hull above the propeller. The positions of measurement of the accelerations and the pressure fluctuations are shown in

Fig. 1.

The vibration experienced on these ships was charac-terized not only by magnitude but also peculiar features

as follows:

(1) The amplitude of vibration varied remarkably with

slight changes of loading condition. The loading con-ditions on trials are classified into groups A, B, C, D

and E relating to the magnitude of the vibration as

Group Magnttude Draft forward_(ml Draft aft_(ml

Cond. A Excessive 3.92-3.99 6.94-6.95 B Excessive 3.76-3.83 7.25-7.40 C Moderate 3.15-3.30 3.72-7.96 D Slight 2.85-2.87 7.35-7.43 E Slight 4.88 7.01 MTB 140 June 1980

Length between perpendiculars _{155.0 m} _{Number of containers} _{773 lEU}

Breadth moulded 26.0 m Main engine output 16800 PSX 122 rpm at MCR

Depth moulded 14.2 m Service speed 18.2 kn at 85% MCR with 30% sea margin

Design draft 8.7 m Diameter of propeller 60m

Dead weight 14300 t Number of propeller blades

No 5 CARGO HOLD No 4 CARGO HOLD No 3 CARGO HOLD ,No 2 CARGO HOLD.

(4)

shown in Table i and Fig. 2.

In the conditions with excessive vibration, the ampli-tude of vibration and pressure fluctuations increased with propeller revolution, very steeply between 116

rpm and 118 rpm, as shown in Figs. 3-8.

The ist and 2nd blade components were much larger

than those of higher harmonics, as shown in Fig. 9.

The amplitudes of the vibration and pressure

fluctua-tions displayed a beating characteristic with periods of

5-10 seconds (Fig. lo).

The amplitudes of the vibration and pressure fluctua-tions decreased remarkably when the ship started to

turn after the application of rudder (Fig. 10).

Cavitation erosion on the

propeller blades, which

would have indicated the occurrence of severe cavita-tion and might have been related to excessive

excita-tion on the hull, was not found at all after the trial as

shown in Fig. 11.

3. Investigations for solution of the vibration problem

Taking into account the peculiar features of the vibra-tion phenomena, the extensive investigavibra-tions were made from both structural and hydrodynamic point of view in

the search for a practical solution.

The investigations made are summarized in Fig. 12.

3.1 Structural reinforcement

Heavy reinforcement of the after part of the ship was examined on the 3rd ship, taking account of the results of the exciter test. And tentative reinforcement of the speci-fied area in the engine room and the super-structure was also made on the ist, 2nd and 3rd ships. The vibration in

the reinforced area decreased to some extent but

substan-tial improvement was not attained in the accommodation

area.

3.2 Change of number of blades of propeller

Replacement of the original 5-bladed propeller by a

4-bladed propeller was made on the 3rd ship, taking account

of the results of exciter test. With the 4-bladed propeller

the ist and 2nd blade components of the vibration

de-creased considerably, but habitability in the

accommoda-tion area became worse due to increase of noise level

caused by the 3rd blade component and the higher har-monics of the vibration as shown in Fig. 9. Vibration and

pressure fluctuations are shown in Figs. 13 and 14. 3.3 Full scale observations of propeller cavitation Full scale observations of propeller cavitation were

carried out on the 3rd ship on her first sea trial. The

under-water TV camera fitted to the ship is shown in Fig. 15.

Table i Trial conditions and magnitude of vibration

Ship Note

Trial Condition Magnitude

Trial Condition df Im) dm Im) da Im) Description Class

1 3.83 5.59 7.35 Excessive B

St

2 331 5.60 7.88 Moderate C

No.1 With 5-bladed propeller 1 3.99 5.47 6.94 Excessive A

2nd 2 3.77 5.58 7.39 Excessive B

3 3.30 5.25 7.72 Moderate C

1 3.17 5.57 7.96 Moderate C

ist

2 3.83 5.56 7.29 Excessive B

With 5- bladed propeller

No.2 _{and center line skeg} 1 3.76 5.58 7.40 Excessive B

2nd 2 3.17 5.56 7.95 Moderate C

3 3.97 5.46 6.95 Excessive A

i 3.92 5.43 6.94 Excessive A

With 5-bladed propeller ist 2 3.20 5.58 7.95 Moderate C

3 3.81 5.59 7.36 Excessive B

1 4.88 5.95 7.01 Slight E

No.3 2 3.76 5.56 7.35 Excessive B

2 nd

3.15 5.56 7.96 Moderate C

With 4- bladed propeller

4 2.8/ 5.11 735 Slight D 1 3.77 5.56 7.35 Excessive B 3rd 2 2.87 5.11 7.35 Slight D 1 4.84 5.92 6.99 Slight (El ist 2 3.78 5.57 7.36 Slight (B)

N 4

_{and stern tunnel fin}With 5-bladed propeller ₄ 3.11_2.85 5.59_5.14 8.07_7.43 Slight_Slight (Cl_(Dl

1 3.83 5.54 7.25 Slight IB)

2nd

(5)

MTB 140 June 1980 I _J 0- I 1 I i I 130 110 120 130 110 120 130 110 120 N (rpm) Gal 250 F1i III Ship 2 Cond. B 5-bladed propeller Vi 2xBF

I

2XBF 2a B F 0.1 200 150 100 50 I I O I I o I I 130 110 120 130 110 120 130 110 120 130 110 120 130 110 N (rpm)

Fig. 3 Comparison of vibration acceleration and pressure fluctuations (1)

200 150

J

I 8F 2xBF F Ship I Cond. C 5bladed propeller Vi I I I I 0.- J I J 110 120 130 110 120 130 110 120 130 110 120 130 0.1 Gal 400 -350 300 250 200 BF N (rpm)

Fig. 4 Comparison of vertical vibration acceleration at aft end (1)

2x8 F Ship i Cond. A 5bladed propeller Vi 2x B F J i 110 120 130 110 120 130 kg/cm2 0.2 - BF 0.1 0 110 120 Gal 250-200 BF 150 100 50 o I 110 120 Gal 300 250 _BF 200 150 100-O 250 200 150 100 50 I i I I i 130 Gal 110 8F 120 130 F11 110 120 2x 8F 130 kg/cm2 0.2- BF 2x B F _0.2 50 O 120 130 100 50 Gal 300 250 _BF _{2xB F} 150 100 50 o

Ship 2 Cond. B Ship i Cond. C Ship I Cond. A

5-bladed propeller 5-bladed propeller 5-bladed propeller

P3 Pi _kg/cm2 P3

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03'-kg/ cm2 02- 8F 0.1 400 350 300 250 200 150 100 50 O Gal 0- 110 ₁₀ 8F 2x8 F Ship 2 Cand. B 5-bladed propeller Vi N (rpm) Gal Gal 250- 250 Gal 110 120 130 110 120 I

J4

I 110 120 130 110 120 130 Fi i 8F 2xBF j 130 110 120 N (rpm)

Ship 2 Cand C 5-bladed propeller VI kg/ cm2 Gal r I 110 120 130 110 120 130 Ship 2 Cond. A 5-bladed propeller Vi 400-350 300 250 200 150 100 50 BE 2xBF 400 350 300 250 200 150 100 50 o BF

À1

2xBF 130 110 120 130 110 120 130 110 120 130 110 120 130 N (rpm)

kg/cm2 2x 8F _02- BE 2xBF 0.1 0.2-0.1 o 8F 2x8 F 130 110 120 130 o Gal 250 200 150 100 50 o F 200 2x8 F 8F 150 100 50 110 120 130 110 120 130 o 200 150 100 50 o 130

Ship 2 Cand. B Ship 2 Cand. C Ship 2 Cond. A

5-bladed propeller 5-bladed propeller 5-bladed propeller

P3 P3

Fu

8F 2xBF

i i

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MTB 140 June 1980 0.2 al 200 0 J J I 110 120 130 110 120 130 BF

I

2xBF JI 0.2-0.1 o 200 iI BF 2xBF 110 Fn Gal Gal I I I I 110 120 130 110 120 130 0.3-300 250 200 150 100 50 110 120 130 110 120 130 110 120 130 110 120 130 110 120 130 110 120 130 N (rpm)

8F II 2xBF

I

BE

N (rpm)

Ship 3 Cond. B Ship 3 Cond. C Ship 3 Corid. A

Sbladed propeller 5-bladed propeller 5-bladed propeller

VI VI Vt Gal 400-350 BE

i

120 130 110 F12 I I I 120 130 110 120 2x BE 300 250 200 150 loo 50 o Gal -8F

j

I 2xBF I I 300 250 200 150 loo 50 Gal I BE

p

I I I 2xBF I 120 BE 130 110 N (rpm) Fu 2x 120 B F I 130 o 250 200 150 100 So Gal 110

Ship 3 Cond. B Ship 3 Cond. C Ship 3 Corid. A

5bladed propeller 5-bladed propelle r 5-bladed propeller

p3 P3 _{kg! cm2} I P. kg/cm2 k g/cm2 0.2 0.1 o 110 8F 150 loo 50 2x8F 150 loo 50 o 120 130 110 120 130 120 130 2x B F I 130

(8)

Ship 3 Cand. B 4bladed propeller P.1 kg/cm2 at MCR 0.3 0.2 0.1 k g/cm2 0.4 02 0.1

Decrease after start of turn Rudder angle 0H-1° sec

Vi

Rudder angle starboard 15'

"I sec

III!I

II

F 2xBF 3xBF 4xBF5xBF 8F .-Max. Rudder angle O Ill Ship 2 Cond, B 5.bladed propeller Pl at MCR 2x8 F 10 sec

Beat during straight running

3x8F 4xBF 5uBF BF 2uBF

Fig. 9 Comparison of vibration acceleration and pressure fluctuations

F

9

Vi

6x8 F

Rudder angle port 15'

/llflI/fl

Ship 2 Cond. C Ship i ¡n Service

5-bladed propeller S'bladed propeller

P Fi kg/cm2 at MCR at N= 120 rpm 0.3 0.2 0.1

j

BE Ii 2x8F 3xBF 4xBF 5xBF 6xBF Ship 3 Cand. C 4-bladed propeller Pi at MCR Mean 3xBF 4xBF SxBF 30 20 10 Gal 50 Gal 100-- L'

\

BE 2xBF 3uBF

Fig. 11 Propeller blade after the sea trial

Fig. 10 Temporal change of vibration

I I I I I I I I I I and pressure fluctuations

Ship 3 in Service 4-bladed propeller Fu at N=ll6rpm A 2ußF 3xBF 4xBF

llI

!ihII

(9)

kg/cm2 0.2 -0.1 100 50 o 150-100 50 120 Vibration measurement

¡ri the trials

BF 2xBF

Ship 3 Cond. B & E

Gal 4-bladed propeller

VI

I Far less vibration in heavy load cond Change of vibration due to load conditions

Decrease of vibration after application of helm

Fig. 12 Flow chart of investigations in search of solution

Ship 3 Cond.B & E Ship 3 Cond. C Ship 3 Cond. D

4bladed propeller 4bladed propeller 4-bladed propeller

P3 p3 p3 kg/cm2 _____.%"__ 120 130 10 120 Fi Gal 0F 2x B F Gal BF

t

Forward draft air bubbles kg.. cm' 100 50 Disappearance of air bubbles soon after start Of turning

Solution

Stern tunnel fin

Removal of air bubbles

N (rpm)

Decrease of vibration Increase of noise Ship 3 Gond. D 4-bladed propeller VI 2x B F 2uBF _0.2 0.1

-

BF

t---2xBF O I I 130 10 120 130 110 12i 130 110 120 130 N (rpm) Fu Fi i 2xBF _Gal BF 2xBF 100 50

[b

I o 130 110 120 130 110 120 130 110 120 130 Gal 150 100 50 o 8F Ship 3 Gond. C 4-bladed propeller VI 2xBF 150 100 50 O Gal BF 110 120 130 110 120 130 110 120 Miß 140 June 1980

Hydrodynamic investigations Structural investigations

Full scale tests Model test Euciter test

Cavitation Flow visualization Reinforcement of

observation of the flow the after part

J Eacessive vibration-Much air bubbles

t Less vibration Few air bubbles Not effective

Vibration measurement

in service

Entrainment of air bubbles near bow

Replacement of 5- bladed propeller by 4-bladed one 0.2 - BF 0.1

0

110 120 130 110 o 130 110 120 2xBF 8F 130 110 120 130 130 110 120 130

(10)

In the condition with excessive vibration, the propeller

could not be observed at all because of a large quantity of air bubbles flowing toward the propeller.

In

the condition with

less vibration, which included the ship running straight at low speed and when turning

at high speed, the propeller could be observed clearly with

fewer air bubbles and cavitation pattern was found to be

moderate as expected from the model tests.

A clear and thin vortex tube was also observed to ex-tend down to the propeller blades at intervals like a pro.

peller hull vortex.

3.4 Vibration measurement on the ship

The measurements of the vibration were made on the

ist ship in her maiden voyage.

The vibration was far less in heavily loaded conditions and changed remarkably due to the variation of loading

conditions as shown in Table 2. For example, the fore and

aft acceleration on the floor of the wheel house F1 was

about 60 Gal in lightly loaded condition and was only 10

Gal in a heavily loaded condition at 115 rpm.

3.5 Vibration measurement on the ship on the sea

trial

On the sea trial of the 3rd ship, vibration and pressure

10.0 E 9.0 8.0 7.0 6.0 5.0 4.0 3.0 o Note a Excessive Excessive ° _Moderate Slight Very slight

Table 2 Service conditions of ship No. i in maiden voyage

Fig. 15 Underwater TV camera fitted

to the 3rd ship

Service conditions of No. i ship in maiden voyage

Sea trial conditions

6.0 7.0 8.0 90 10.0

d0 (m)

Fig. 16 Load conditions and magnitude of vibration

Condition O. Position Revolutions of propeller lrpml Condition Evaluation Aft peak tank Displ't (t) df(m) dm (ml da (m)

Nagasaki -. Manila 111 15750 4.54 592 7.29 Severe Empty

2 Manila-° Hong Kong 115 16270 4.93 6.09 7.24 Slight Empty

3 Manila -a Hong Kong 115 16690 5.07 6.21 7.35 Slight Empty

4 Manila-. Hong Kong 115 17030 4.64 6.33 8.02 Severe Full

5 Hong Kong -vKaohsiung 114 15510 4.20 5.82 7.44 Severe Empty

6 Hong Kong -°Kaohsiung 113 16740 5.09 6.22 7.35 Slight Empty

7 Kaohsiung - Keelung 113 17490 5.50 6.47 7.43 Slight Empty

8 Keelung- Mou 114 18800 6.12 6.89 7.65 Slight Empty

9 Mou - Kobe 114.5 19970 6.74 7.72 7.38 Very slight Empty

10 Kobe -v Yokohama 113 23820 8.46 8.47 8.48 Very slight Empty

11 Kobe-°Yokohama 113 24190 8.11 8.56 9.00 Slight Full

12 Yokohama - Cristobal 115 26540 9.27 9.29 9.30 Very slight Empty

13 Yokohama -a Cristobal 115 26890 8.95 9.37 9.38 Very slight Full

14 Yokohama- Cristobal 115 26050 8.45 9.09 9.73 Very slight Full

15 Yokohama -a Cristobal 122 25650 8.74 9.01 9.27 Very slight Empty

(11)

MTB 140 June 1980

Cond. A

Cond. B

Fig. 17 Behaviour of air bubbles around a propeller

Cand B

Fig. 20 Bow wave on the sea trials

Fig. 18 Photograph of the sea trial (1)

(12)

fluctuations were measured at various loading conditions to make sure of the vibration data measured on the

pre-vious ships including data of the ist ship in her maiden

voyage.

As a result,

it was confirmed that the forward draft

had a definite effect on the vibration on the ships.

Rela-tion between loading condiRela-tions and magnitude of vibraRela-tion is shown in Fig. 16.

3.6 Visualization of the flow on the models

Visualization of the flow on the models was carried out not only around the propeller but also near the bulbous

bow.

A large quantity of air bubbles was observed also on the model around the propeller at conditions corresponding to the excessive vibration on the ships (Fig. 17).

Air bubbles were entrained into the flow around the hull

AP

Modification of stern frame

Cond. B (df=3.6m. d0=7.4m) V=20.2 kn

AP

Modification of bulbous bow

O.5lD _{Cond. B (df3.6m, d0=7.4m)}

..abt. 19' V5=20.2kn

Oblique running condition

Cond. B (df= 36m. d07.4m) V,=20.2kn

Drift angle starboard 3'

9y FP

With stern tunnel fin

Cond. B (df3.6m. da7.4m)

Vs=20,2kfl

Drift angle port 3'

Straight running at MCR

Cond. A (df=3.92m. d0=6.94m) Cond. C (df=3.30m, do=7.72rn)

V5=20.2kn '/5=20.2kn

near 5% Lpp from the F.P., where water which had come up on the upper nose of the bulbous bow fell down on to

the free surface191. Bow waves observed on the sea trials

are shown in Figs. 18-20. The air bubbles flowed towards the stern along the bottom and concentrated above the propeller where the flow velocity was generally relatively

slow.

After some accumulation, the air bubbles were sucked down intermittently into the propeller, sometimes forming

vortex tubes by the suction effect of the propeller.

At the condition with less vibration, air bubbles were

few or could not be observed.

The forward draft, or in other words the immersion

of the bulbous bow, was found to be the most important

factor in this phenomenon.

At the conditions with excessive vibration, the air

bub-bles disappeared suddenly within 2° or 3° of drift angle just

after the beginning of a turning motion of the ship. Such

behaviours of air bubbles are illustrated in Fig. 21, and

their variation with loading conditions corresponds well

with those observed during sea trials, as shown in Fig. 22.

3.7 Solution of the vibration problem

As the results of the investigations described above, it was concluded that a large quantity of air bubbles around

the propeller was closely related to the excessive vibration

and that their removal would be the key to the reduction

of the vibration.

Some countermeasures were examined to remove the

air bubbles above the propeller. As a result, a stern tunnel fin was fitted to the 4th ship as the most practical solution.

A remarkable decrease of the vibration was

demon-5.0 45 4.0 3.5 o u-3.0 2.5 Without

S.T. fin Vibration Amount ofair bubbles Swinging mo-tion of tufts

Q

Excessive Excessive Moderate Slight Much Much A little Invisible Significant Moderate Small Small

Fig. 21 Behaviour of air bubbles observed in flow Fig. 22 Relation between vibration, concentration of

visualization tests _{air bubbles and loading condition}

7.0 7.5 8.0 7.0 7.5 8.0

Shìp Model

Aft draft (m)

Vibration in sea trial (Ship) Amount of air bubbles

E ---' (N> 116 118rpm) swinging motion of tufts

\ (Model: V520.2kn)

Severe vibration

A in service

(Ship and model)

B

with stern tunnel fin

o

\,._(Ship

_{and model)}

with stern tunnel

finN

AP ('f AP ('f

AP ₂ 91/2 FP

(13)

MTB 140 June 1980 _4t1è P,4 110 120 130 Gal 100 o FM 2xBF 120 130 110 120 130 P2 2x B F BF 2xBF u

rr

r 110 120 130 110 120 130 o L

---- i

-'--

o 110 120 130 110 120 130 110 120 130 110 120 130 N (rpm)

Gal loo 50-o BF 110 120

strated on her sea trial and also in service, as shown in

Figs. 23 and 24.

The, same successful reduction of vibration was also

shown on the other sister ships with the same type of stern

tunnel fin.

4. Basic investigations into the excessive hydrodynamic excitation

After the successful settlement of the vibration problem,

investigations have been conducted in order to clarify the hydrodynamic mechanism in the excessive excitation

re-lated to air bubbles above the propeller. Investigations

made are summarized in Fig. 25.

4.1 Entrainment of air bubbles near the bow

An attempt was made to clarify

the hydrodynamic mechanism of the behaviour of the air bubbles near the

N (rpm) 130 110 2a 8F 120 130 50 o N (rpm)

Gal

50

--o 130 110 120 130 110 120 130 Gal BF 2xBF 110 120 130110 120 130

bow by injecting water jets on to a simple two-dimensional model similar to the cross section near the bow. When the

water jets were directed from above along the model a

large concentration of air bubbles was trapped at the

bot-tom, as shown n Fig.26191.

This phenomenon is known as Coanda's effect, and the

behaviour of the air bubbles near the bulbous bow is

con-sidered to be governed by the same mechanismlDL Namely, water which had come up on the upper nose of the bulbous

bow fell down on the free surface (Fig,. 27), and a large

amount of air bubbles was entrained into the flow with

water fall from the nose of the bulbous bow. Air bubbles adhered to the hull surface by Coanda's effect and flowed

towards the stern along the bottom.

4.2 Flow around the propeller

The flow pattern around the propeller was examined by

0. 1 O Gal loo kg/cm BF P2 8F 2aBF 110 -120 Fu BF 130

0- __i,

110 120 130 110 120 kg/cm2 P2 BF P2 2x BF kg/cm2 P2 BF P2 2x8 F 0.1 -0 0 Max. _JMeaneanC\ 3xBF 4xBF Max. Mean A BF 2aBF 3xBF Fu BF Fi i 8F Fi 2x B F F1 i 2a B F

Ship 4 Cond. B Ship 4 Cond. C Ship 4 Cond. D

5bladed propeller 5bladed propeller 5-bladed propeller

with stern tunnel fin with stern tunnel fin with stern tunnel fin

P2 P2 at MCR at MCR kg/cm2 kg/cm2 0.1 -Gal 50 50 2 110

Ship 4 Cond. B Ship 4 Cand. C Ship 4 Cond. D

5-bladed propeller 5-bladed propeller 5.bladed propeller

with stern tunnel fin with stern tunnel fin _{with stern tunnel fin}

(14)

Explanation on the phenomenon

(1)Water on the upper nose of the bulbous bow (2) Water fall on to the free

surface

(3)Air bubble entrainment into the flow

Wider extent of Excessive vibration

the low speed with propeller

region running

Flow visualization with tufts Larger swinging motion of tufts

Excessive vibration

Effect of stern tunnel fin Improvement of flow

Removal of air bubbles above the

propeller

Fig. 25 Flow chart of the basic investigations into the excessive hydrodynamic excitation

Water fall and entrainment of air bubbles after an inclined cylinder

Fig. 26 Entrainment of air bubbles by water jet

Mechanism of air bubble entrainment F low around the propeller Reproduction of phenomena

region above the propeller PHV

of cavity volume

cavitation

Large pressure fluctuations with beating

Flow improved by a stern funnel fin Moderate cavitation pattern

Low pressure fluctuations

pitot tube and flow visualization. The influence of the

load-ing condition, of air bubbles above the propeller, of the speed and of the suction effect of a propeller was

investi-gated (Fig. 25). Wake fraction contour curves are illustrated

in Figs. 28-32.

Wake distribution in the propeller plane

The wake peak in the propeller plane was higher in

the condition with excessive vibration. Difference of wake peaks due to the loading conditions was within the range W = 0.05-0.10.

Flow pattern above the propeller

The low speed region above the propeller was wider

¡n

the condition with excessive vibration. With the

propeller running, the variation of the flow patterns with loading condition was about the same as above.

13

(4)Air bubble adhesion on the hull surface (Coandas effect)

Injection of air bubbles

Sufficient quantity '-- Growth of PHV

Too much quantify' Decrease of

Influence of air bubbles

(5)Flow with air bubbles pressure fluctuations

towards the Stern Increase of air bubbles Deterioration

of flow

Removal of air bubbles Improvement

of flow

Experiment on behaviour of Wake peak in the propeller plane

air bubbles around a simple

two dimensional model

Higher wake peak Excessive vibration

Low speed

Unsteady Increase

Flow pattern above the propeller _Unstable

Lu concentration of air bubbles water Much concentration of air bubbles water

(15)

MTB 140 June 1980

Cond. A

Cand. B

Fig. 27 Bow wave in model test Flow visualization with tufts

lt was shown by flow visualization with tufts that a large quantity of air bubbles concentrated above the propeller was accompanied by a significant swinging

motion of tufts attached to the hull above and

for-ward of the propeller in the conditions with excessive

vibration. In the conditions with less vibration, a

swing-ing motion of tufts was far less significant with few or

no air bubbles.

Influence of air bubbles above the propeller

Influence of air bubbles on the flow pattern around

the propeller was examined by artificial increase or

decrease of air bubbles in the flow. When the quantity of air bubbles above the propeller was increased by the

injection of water jet at the bow, the extent of the low

speed region above the propeller and a swinging motion of tufts increased. When the air bubbles above the

pro-peller was decreased by the modification of the fore

part of the ship model, the extent of the low speed

region above the propeller and the swinging motion of

tufts decreased considerably. Variation of the wake

peak in the propeller plane was about the same as

above.

Effect of a stern tunnel fin

When a stern tunnel fin was fitted, the wake peak in

the propeller plane was reduced and the flow velocity

Cand. A Original bow V= 20.2 kn

o

p.9

8

Cond. A Modified bow

V5=20.2 kn

Fig. 28 Wake fraction contour curves at propeller position (1)

Cand, B Original bow

V520.2kn

o

/

0.9

Fig. 29 Wake fraction contour curves at propeller position 2)

Cand D Original bow

V5=20.2 kn

Cand, C Original bow

V520.2kn

Cond. D Original bow

V5= 20.2 kn

with stern tunnel fin

Cand. A Original bow

V5 15.0 kn

Cond. C Original bow

V5 - 15.Okn

0.8

Cond. B Original bow V= 20.2 kn with stern tunnel fin

0.6

Fig. 30 Wake fraction contour curves at propeller position (3)

above the propeller increased remarkably. The

varia-tion of flow pattern with loading condivaria-tion and the

suction effect of the propeller were far less

pronounc-ed. Furthermore, a motion of tufts above and forward

of the propeller decreased remarkably.

The quantity of air bubbles from the bow was

divid-ed into two parts at the forward end of the stern

tun-nel fin. One of them flowed along the upper side of the stern tunnel fin and rose to the water surface. Another

flowed towards above the propeller along the lower

side

of the stern tunnel fin

without concentration

(Fig. 21).

4.3 Reproduction of large pressure fluctuations in a cavitation tunnel

Attempts were made to reproduce in a cavitation tunnel the large pressure fluctuations with a beating characteristic,

(16)

ir

Original bow

with water jet injection

V5 =20.2 ko

o

With concentration of air bubbles

With concentration of air bubbles No air bubbles

0.9

9.?

0.9

taking into account the results of investigations made as above. As a result it was found that such a phenomenon occurred when the velocity above the propeller was

ap-preciably reduced and the wake peak in the propeller plane was very high and sharp. Cavitation pattern on a blade and

the pressure fluctuations are illustrated in Figs. 33-35.

In these tests, the intermittent occurrence of a propeller.

hull.vortex (PHV) was observed13. The upper end of the

PHV moved about with great speed over an area almost the same as the propeller disc. The lower end of the PHV was

connected to a cavity on a blade and seemed to become

unstable, moving from blade to blade. The volume of cavity

on a blade changed considerably, when connecting with the PHV, with increase of pressure fluctuations exerted

widely on the flat plate above the propeller.

Injection of a sufficient quantity of air bubbles in the

flow above the propeller helped the growth of the PHV and

Fig. 31 Wake fraction contour curves above the propeller (with propeller loading) (1)

Cond. B Original bow Cond. C Original bow

J

Cond. D Original bow Cond. B Original bow

V3=2O.2 kn with propeller loading ( =8.4rps) without water jet injection

0.9

No air bubbles

Original bow

without water jet injection

V5 = 15.0 kn

0.9

With stern tunnel fin

Fig. 32 Wake fraction contour curves above the propeller (with propeller loading) (2)

MTB 140 June 1980

sometimes the formation of many PHV's. The pressure fluctuations increased with the presence of the PHV, but

amplification effect of the air bubbles on the pressure

fluctuations was not so remarkable as had been expected

from the vibration phenomena observed on the ship. With further increase in the concentration of air bubbles above

the propeller, the pressure fluctuations decreased.

As the velocity above the propeller increased and the

wake peak in the propeller plane reduced, the PHV became moderate and the pressure fluctuations decreased, as shown

in Figs. 36-38. The pressure fluctuations increased with

growth of PHV by injection of a sufficient quantity of air bubbles into the flow above the propeller. The magnitude of pressure fluctuations scaled up to full size were almost

the same as those which had occurred during the excessive vibration on the ships.

In the flow with considerably increased velocity above

15

Cand. A with propeller loading (n=8.4rps)

Original bow Modified bow

without water jet injection without water jet injection

V5=20.2 bn V5 =20.2 kn

o

(17)

MTB 140 June 1980

In severe wake with fewer air babbles

o

With air injection

o

/í

Without air injection

Fig. 35 Reproduction of pressure fluctuations (1)

the propeller, reduced wake peak in the propeller plane and

no concentration of air bubbles above the propeller, the cavitation pattern was observed to be moderate and the

L

n moderate wake with fewer air bubbles

n =20rp)

With air injection

pressure fluctuations was remarkably decreased, as shown in Figs. 39 and 40.

kg/cm2

0.15

Severe wake (o,, - 1.639,

kg/cm2

0.15-K=O.169. n20rps) _{Moderate wake (o',,}

kg/cm2

1.639, K0.169,

kg/cm

0.15-

0.15-0.10 Without 0.10 With

s. air injection Q- air injection

0.10 Without 0.10

Q- air injection

n-0.05 0.05

0.05 0.05

Fig. 33 Reproduction of cavitation phenomena (1) Fig. 36 Reproduction of cavitation phenomena (3)

In severe wake with sufficient air bubbles In moderate wake with sufficient air bubbles

Fig. 34 Reproduction of cavitation phenomena (2) Fig. 37 Reproduction of cavitation phenomena (4)

1 2 3 4 5 6 xBF 1 2 3 4 5 xBF

o

(18)

In improved flow with fewer air bubbles

Fig. 39 Reproduction of cavitation phenomena (5)

4.4 Explanation of hydrodynarnic mechanism

From the investigations described above, the process of

the hydrodynamic excitation may be explained as follows

(Fig. 41):

Entrainment of a large quantity of air bubbles near the bow at certain values of forward draft.

Flow with air bubbles towards the stern along the

bot-tom of the hull.

Concentration of air bubbles above the propeller.

Retardation of the flow above the propeller by

pre-sence of air bubbles and the occurrence of PHV. Occurrence of the large pressure fluctuations with

beat-ing due to the volume change of a cavity on a blade

related to the PHV.

Increase of cavity volume

Flow with air bubbles

Concentration of air bubbles Deterioration of flow Occurrence of unsteady PNV Growth of unsteady PNV Entrainment of air bubbles

/

2 3 4 5 6 xBF

Significant increase of pressure fluctuations

accompa-nied with growth of PHV by the sufficient quantity of

air bubbles.

5. Conclusion

Extensive investigations were made to find an effective

solution to, and a clarification of the hydrodynamic mech-anism of the excessive vibration excitation. The results of

investigations are summarized as follows.

The excessive vibration experienced on the class of

cargo-liners, was an extraordinary phenomenon which

could not be explained by the present knowledge of

the hydrodynamic excitation.

The vibration had peculiar features which had never

been experienced; i.e. excessive vibration and large

pressure fluctuations with a beating characteristic, the remarkable decrease soon after the commencement of a turn and the remarkable change due to a small varia-tion in bow immersion.

In the conditions with excessive vibration, a large quan-tity of air bubbles was entrained into the flow near the

Water comes up

Fig. 41 Explanation of hydrodynamic mechanism of extraordinary increase of pressure fluctuations

17 Improved wake (0,, 1.645, Xr=0.1904, n24rps) kg/cm2 0.05 Without air injection 9v2

(19)

MTB 140 June '1980

Huse E. Propeller-Hull Vortex Cavitation, International

Shipping Progress, Vol. 19, No. 212 (Apr. 19721

Chiba N. and Hoshino T., Effect of Unsteady Cavity on

Propel-ler-Induced Hydrodynamic Pressure, Journal of the Society of Naval Architects of Japan, Vol. 139 (May 1976)

Jonk J. and Kooij J. van der, Propeller-Induced Hydrodynamic Hull Forces on a Great Lakes Bulk Carrier, Results of Model

Tests and Full Scale Measurement, Symposium on High Powered

Propulsion of Large Ships, Publication No. 490, N.S,M.B. (Dec.

1974)

Johnsson C.A. and Sontvedt T., Propeller Excitation and Re-sponse of 230,000 TOW Tankers, ONR Symposium, Paris

(August 1932)

Hylarides S., Some Hydrodynamic Considerations of

Propeller-bow, flowed towards aft along the bottom and

con-centrated above the propeller.

A successful solution was obtained by a stern tunnel

fin by means of which the air bubbles were swept away from the region above the propeller.

The flow around the propeller was deteriorated with the concentration of air bubbles above the propeller,

and was much improved by a stern tunnel fin.

Pressure fluctuations with beating was reproduced in a cavitation tunnel, accompanied by an unsteady

propel-ler hull vortex, when the low speed region above the propeller was extensive. A sufficient concentration of

air bubbles in the flow caused an increase of the pres-sure fluctuations together with growth of the unsteady

propeller hull vortex, but with further increase of the

References:

air bubble concentration the pressure fluctuations

decreased.

(7) The pressure fluctuations decreased remarkably, when

the flow around the propeller was improved including

vanishing of air bubbles.

From the investigations made, it was inferred that air bubbles in the flow above the propeller were responsible

for a remarkable increase of pressure fluctuations by

acci-dental juxtaposition of the deterioration of the flow and

the change in pressure fluctuation.

However, a better understanding of the mechanism of the excessive hydrodynamic excitation related to

charac-teristics of the flow with air bubbles, the unsteady PHV and

the unstable behaviour of the cavity on a blade will be the

subject of further investigations in the near future.

Induced Ship Vibrations, Ship Vibration Symposium, SNAME

(1978)

Huse E., Effect of Afterbody Forms and Afterbody Fins on

the Wake Distribution of Single Screw Ships, The Ship Research

Institute of Norway, Report No. 9-31,74 (1974)

Vossnack E. and Voogd A., Developments of Ships After-bodies, Propeller Excited Vibrations, 2nd Lips Propeller

Sympo-sium (19731

Takekuma K., Vibration Problem with a Class of Caigo Liners and the Solution from Fitting a Fin, Symposium on Propeller-Induced Ship Vibration, RINA (Dec. 1979)

Baba E., Observation of Air Entrainment at Protruding Bow, Schiff und Hafen, Jahrgang 31, Heft 10 lOct. 1979)