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INTERNATIONAL CONFERENCEON

SHIP DRAG REDUCTION(SMOOTH-Ships)

ISTANBUL TECHNICAL UNIVERSITY20-21 May 2010

Macka Campus, Istanbul, Turkey

EditorsMustafa Insel

Ismail Hakki HelvaciogluSebnem Helvacioglu

Copyright 2010, SMOOTH Consortium

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Paper No: 13

Flow Analysis of an Air Injection Through Discrete Air Lubrication

M.INSEL, S.GOKCAY, I.H.HELVACIOGLU

Istanbul Technical University and

Trk Loydu

INTERNATIONAL CONFERENCE ON

SHIP DRAG REDUCTION

(SMOOTH-Ships)

20-21 May 2010

Istanbul Technical University

Faculty of Naval Architecture and Ocean EngineeringISTANBUL-TURKEY

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International Conference on Ship Drag Reduction

SMOOTH-SHIPS, Istanbul, Turkey, 20-21 May 2010

Flow analysis of an air injection through discrete air lubrication

M.INSEL, S. GOKCAY & IH.HELVACIOGLUIstanbul Technical University, Faculty of Naval Architecture and Ocean Engineering, Istanbul, Turkey

and

Trk Loydu, Istanbul, Turkey

ABSTRACT: Global warming effects caused investigation of a number of techniques to reduce ship energy

consumption to minimise ship emissions and optimise energy efficiency. By utilising breakdown of resistance

into viscous and wave resistance, ship hull form optimisation has been utilised commonly to reduce the waveresistance component of a ship. However, viscous resistance has not successfully been dealt with up to now.

One of these techniques proposed currently is the air lubrication of hull wetted surface to reduce this

component.

Air lubrication can be established by utilisation of techniques such as air cavity, micro-bubbles, and air

film formation. This paper describes experimental and numerical modelling of air injection at the bottom of

a ship form through a single air injection hole or a series of discrete holes. Flow from a porous media

forming bubbles has also been compared.

Experiments were conducted at two water depths and variations of air for a ship form. Flow visualisation

over the ship bottom have been made thoroughly to understand the flow mechanism. The shape of air film

has been recorded systematically with variation of water speed and the air injection rate.

Numerical investigation of air flow through CFD studies has also been performed. The interaction

between the ship boundary layer and air injection vorticity have been investigated.Total resistance test have been conducted and results clearly indicate that resistance reduction can be

obtained by using this technique even at low speeds.

1 INTRODUCTION

Carbon emission reduction measures are

proposed all over the world including housing,

industry and transportation. Studies to reduce the

carbon emissions have been evaluated ranging from

engine modifications, fuel specifications, and power

consumption reduction measures in maritimetransport. Resistance of ships is key parameter for

such studies. By utilizing the division assumption of

ship resistance into wave and viscous resistance,

different proposals have been made for these

components. Wave resistance reduction of a surface

ship through form optimization is a mature

technique utilized frequently. However the low

speed hull forms are the main source of carbon

emission and savings from wave resistance

optimization are secondary comparing to viscous

resistance. A number of techniques to reduce the

viscous resistance have been proposed by boundary

layer modification through suction, riblets, polymer

injection, micro-bubbles and air films.

Air lubrication is a general term utilized to

express the use of air to reduce resistance. Micro-

bubble injection to modify the boundary layer, air

cavity to form a natural air volume pocket and air

film formation between hull surface and water are

techniques being currently developed.

2 AIR LUBRICATION TECHNIQUES

Three physically different techniques are applied to

lubricate the underwater hull surface with air:

2.1 Air Cavity

It is established by creating an air volume over

the hull bottom by using an air cavity behind a

sudden hull form discontinuity, i.e. a step. A natural

cavity can be established behind a bluff body or step

at high speeds by use of vapours of the ambient

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liquid. However, the required speed for such process

is too high for commercial marine crafts.

It was demonstrated that cavitation can be

stimulated by supplying air into separation zone or

behind a discontinuity. Artificial air cavity ship

concept is based on injecting air into separation zone

behind a wedge shaped step at the bottom tostimulate artificial cavitation (or ventilation) such as

the one given in Figure 1.

The stern part of the air cavity chamber is

inclined downwards forming a planing surface for

the closure of the cavity. This surface is in contact

with water to reduce air escape from the cavity. The

circulation field behind the step can be filled with air

and air can be kept at this area with small amount of

air supply for high speed craft, meanwhile more air

is needed for lower speed marine vessels.

Figure 1. Artificial air cavityship concept by Chaban et al.

(1993).

High speed air-cavity concepts have been

investigated by Matveev (2003), Butuzov et al

(1999), and Gokcay et al (2004). Air cavity concept

should be modified in order to be applied into low

speed hull forms as circulation field length is greatly

effected by the hull speed.

The cavity limiting length can be expressed from

potential flow as given by Butuzov (1967) and

Matveev (2003) :

37.0

lim

=L (1)

where = free surface wave length which can be

expressed as gU22 = .

Although a high speed hull bottom can be

covered with air by using a single step, this is not

possible for a slow ship. Hence a number of cavities

are required to obtain sufficient air lubrication area

as illustrated in Figure 2. Positioning of the steps

becomes the most important aspect of such forms.

Choi et al (2005) investigates the positioning by

CFD.

Figure 2. Artificial air cavity ship concept for low speed hull

forms by Matveev (2007).

2.2 Micro-Bubbles

The second approach in the air lubrication is supply

of micro-bubbles into the boundary layer. Since the

pioneering work of McCormick and Bhattarcharya

(1973), a number of studies have investigated the

effect of bubble diameter, void ratio on the drag

reduction characteristics such as by Moriguchi &

Kato (2002), Kato et al (1998), Wu et al (2007),

Kodama et al (2005).

Resistance reductions of up to 20% are reported

for a 50 m long large scale model by micro-bubble

injection as shown in Figure 3.

Figure 3. Resistance reduction by micro-bubble injection to 50m long plate by Kodama et al. (2005).

2.3 Air FilmThe paints based on Tributyltin (TBT) concept were

banned due to their adverse environmental effects. A

number of paint concepts have been developed to

replace TBT paints including silicone based paints.

Some of the newly developed paints are

superhydrophobic, i.e. repelled from a mass of water

and very difficult to wet. A water drop on the solid

surface shown in Figure 4 with a contact angle and

on the superhydrophobic surface forms an angle

larger than 150 such as shown in Figure 5.

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Figure 4. Contact angle of a water drop with solid surface

(Wikipedia 2009)

Figure 5. Water drop at a superhydrophobic surface (Wikipedia

2009)Use of hydrophobic paints may help the air

lubrication of hull forms. Air blown at the right

quantity and right position may form an air film over

the surface which in turn reduces the frictional

resistance. This concept has been investigated by

Fukuda et al (2000), Bruin (2007) and Elbring et al

(2007).

3 EXPERIMENTAL SETUP

Experiments with an example model hull form wereconducted for air film application. All tests were

conducted in Istanbul Technical University, Ata

Nutku Ship Model Testing Laboratory.

Flow visualization tests were conducted in

circulation water channel with a testing section of 6

m long, 1.5 m wide and 0.75 m deep. The flow

conditions were observed through the windows at

side and at the bottom. Recordings were made both

through still photographs and high speed camera to

observe the air bubble movements.

All resistance and wave pattern measurementswere made at the large towing tank with 160 m long,

6 m wide and 3.4 m deep testing section. Resistance

measurements were made with mechanical

dynamometer, wave pattern was recorded through

resistance type wave probes and longitudinal wave

traces were recorded at four transverse positions of

the tank.

3.1 Hull Form

A model hull form denoted as M266B has been

utilized in the pilot project to asses the air

lubrication study. Model utilized in the tests was a

tanker form with block coefficient of 0.772 and

principle characteristics of the form are given in

Table 1. Model surface was painted with

International paint Intersleek 900. The model was

equipped with an air supply system consisting of a

compressor, flow regulator/filter, and a flowmeter.

Table 1. M266B Main dimensions

Model M266B Scale 30.5

Loading Condition Full Load Model Ship

Length waterline LWL (m) 4.185 127.6

Length wetted surface LWS (m) 4.311 131.5

Breadth B (m) 0.620 18.90

Draught midship T (m) 0.262 7.98

Displacement Vol. (m3) 0.509 14431

Block Coefficient CB 0.772 0.772

3.2 Single Discrete Hole Air Supply

Discrete air feed holes can be utilized to supply air

under the hull form. The pressure field and incoming

water/air flow interact and special flow regimes are

obtained. Figure 8 shows air feeding hole position

schematically, meanwhile Figure 7 demonstrates the

complicated air flow through this single discrete

hole.

Figure 6. Air supply through a single discrete hole

Air fed through a single hole separates into two

arms forming a V section with an angle depending

on water flow speed and air feed quantity. The area

between the V arms is filled with an unstable air

film. This area may be fully filled with air film,

with no air at all, air film breaking into sections with

partially formed air film.

Contact

Angle

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Figure 7. Air lubrication from a single discrete hole

Figure 8, 9 and 10 demonstrates the effect of air

supply rate through the single for a single speed of

0.42 m/s corresponding to Froude Number of 0.068.

As the air rate increased, the V angle increase.

Figure 8. Air lubrication with Fn:0.067 and air supply rate of0.5 m3/h


The effect of model speed is illustrated in

Figures 10-15. As the water speed increases, V angle

of the air decreases. Air film between the V arms

also is reduced by the increase of model speed. The

V angle trend by change of speed and air feed rate is

given in Figure 18. Additionally, the area between

the V arms is also with less air as the water speed

increases.




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Figure 16. V Angle of the single hole air injection

3.3 Multiple Discrete Holes Air Supply

The hypothesis to increase the effective air film area

by use of air injection through multiple holes wasinvestigated using a number of holes illustrated in

Figure 17.

Figure 17. Air lubrication with Fn and air supply rate of 0.5m3/h

When the holes are arranged in the same

horizontal section (Figure 1), the air supply from the

center hole is less effective. Formation of V arm atthe center is cancelled and only outer holes forms V

arm at the outer side where the pressure is smaller.

When holes are arranged longitudinally, each

hole air supply form their own V shaped air film.

The V shaped air film patterns do not mix,

separation of V shapes with fully wetted hull surface

sections is clearly visible from Figure 19.

Figure 18. Air supply through 3 horizontal holes

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Figure 19. Air supply through 2 longitudinal holes

3.4 Porous Media Air Supply

Covering the underwater surface of the hull with fullair layer may not be possible with simple discrete

holes. Hence air film derived from air bubble clouds

have been considered for this purpose. Generation of

air micro-bubbles through electrolysis have been

utilized frequently. However the energy

consumption and low air flow rate obtained prohibits

practical applications of such an approach. Instead

air bubbles obtained from pumping of air through

porous media have been utilized in a number of

research and full scale applications.

A horizontal strip of porous media have beenapplied to the ship bottom near the bow from one

side of the hull to the other side in the current work

as shown in Figure 20. Air flow rates between 0.5

m3/h to 1.0 m3/h have been applied to observe the

resistance changes. Even though the bubbles are

formed discretely, these bubbles may be combined

to form an air film.

Figure 20. Air through porous media

4 NUMERICAL MODELLING

The flow around an air injection at the bottom of

a hull form was conducted using CFD package. A

cylindrical hole is defined in right angle to the flow

as defined in Figure 21 and 22. Volume of Flow

model was utilized in unsteady flow solutions. Same

air inflow quantities and water speeds were

simulated to generate validation with the

experiments

Figure 21. Computational domain of the air injection flow

Figure 22. Computational grid of the air injection flow

Figure 23. Velocity streamlined behind a single hole air

injection at 1 m/s water speed and 1m3/h air flow rate.

Figure 23, 24 and 25 demonstrates calculated flow

field around a single hole air injection. V form of the

flow is well predicted the velocity streamlines, and

phase division of mixed flow.

Water-

air

mixed

outflow

Air

inflow

Water

inflowCalculati

on plate

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Figure 24. Phase difference behind a single hole air injection at

1.4 m/s water speed and 1m3/h air flow rate

Figure 25. Phase division behind a single hole air injection at

1.25 m/s water speed and 1m3/h air flow rate

The air injection case from 5 holes is demonstrated

in Figures 26 and 27. The air flow is formed into V

shape similar to the experimental measurements.

Figure 26. Vorticity behind a five hole air injection case

Figure 27. Velocity streamlined behind a five hole air injection

5 FORCE MEASUREMENTS

Total resistance measurement tests were been

conducted in 10 conditions as given in Table 2. Two

draughts have been utilized in order to observe the

effect of static pressure effects. Conditions without

air feed, 2 air feed rates for single hole and 2 air feed

rates for porous media have been tested. Total

resistance was measured for each configuration for

model conditions of free to trim, sinkage and surge

but fixed to heel, yaw and sway.

Table 2. Test conditions

Condition FullDraught

LowDraught

Without Air Injection FDN00 HDN00

Single Hole Air Injection at 0.5

m3/hour rate

FDH05 HDH05

Single Hole Air Injection at 1.0

m3/hour rate

FDH10 HDH10

Porous Media Air Injection at 0.5

m3/hour rate

FDP05 HDP05

Porous Media Air Injection at 1.0

m3/hour rate

FDP10 HDP10

4.1 Total Resistance Test

Total resistance measured at the towing post can

be subdivided into components. Traditional way of

subdivision according to ITTC 1978 method is wave

resistance and viscous resistance.

WFWVT RRkRRR ++=+= )1( (2)

where

FR : Frictional resistance

W

R : Wave resistance

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)1( k+ : Form factor

The total resistance tests with single hole given in

Figure 28 clearly indicate that resistance reductions

up to 10% can be obtained for the model speed

range of 0.8 to 1.1 corresponding to Froude number

range of 0.125 to 0.171. The resistance reduction

drops with increasing speed and there is no gain at

the full speed of 1.4 m/s corresponding to Froude

Number of 0.218.

Figure 28. Total resistance measurement results with singlehole air feed system at full draught

Resistance measurements with porous media

have similarly shown a resistance reduction up to

7% for almost all speed range. However resistancereduction disappears around the service speed

(Figure 29).

Figure 29. Total resistance measurement results with singlehole and porous media air feed system at half draught

Resistance measurements with half draught has

similar tendency, but porous media results are better

than the single hole results indicating that single

hole air lubrication is effected from the static

pressure.

6 CONCLUSIONS

Air lubrication technique is an effective method to

reduce the resistance of ships for both low and high

speed craft. The resistance reduction using air cavity

and micro-bubbles are relatively investigated. Air

film lubrication for low speed ships is still notunderstood well. The case study approach in this

work indicates that resistance reduction in the range

of 5% to 10% of the total resistance can be obtained

in the model scale for lower speeds.

Location and air supply rate of air feed has the

prime importance for the optimum performance of

air lubrication technique. Single hole air feed system

is simpler but optimization of location air supply

rate is very critical.

Experimental flow visualization studies have

been indicated that a complex interaction between

the ship boundary layer and air flow shapes the airdistribution over hull bottom. It does not form a

simple layer of air film, instead it diverges from the

hull bottom.

Numerical studies of air feed through discrete

holes have been performed and simulation of flow

can be achieved. The coverage of hull surface

through discrete air injection holes requires a careful

optimization of hole positions and air injection rates.

6 ACKNOWLEDGEMENTS

This work was conducted partly within EU FP6Project titled as Sustainable Methods for Optimal

Design and Operation of Ships with Air LubricaTed

Hulls (SMOOTH) with participation of MARIN,

AKZO Nobel International Coating, Bureau Veritas,

Damen Shipyards, Istanbul Technical University,

Atlas Copco Ketting Marine Centre, New Logistics,

SSPA, DST, Thyssesn Krupp Veerhaven, Imtech.

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