Analysis of air injection system for drag reduction in

Int. J.MAr.Sci.Eng., 5(2), 65-75, Summer & Autumn 2015

ISSN 2251-6743

© IAU

Analysis of air injection system for drag reduction in high

speed vessels using numerical simulation software ANSYS-

Fluid Flow

M. Soltaninejad;

*F. Azarsina; A. H. Javid

Department of Naval Architecture, Faculty of Marine Science and Technology, Science and Research branch,

Islamic Azad University, Tehran, Iran

Received 7 March 2015; Revised 5 May 2015; Accepted 11 June 2015

ABSTRACT: Many existing phenomena in nature are considered new design ideas in various fields of

industry. Bionics is the application of biological methods and systems found in nature to the study and

design of engineering systems and modern technology. By performing bionic review, the researchers found

the penguins by delivering air locked under their wings and creating air bubbles, the drag significantly

reduces. This motivates to study this factor in the marine industry by researchers and scientists. The overall

drag of a marine vessel is directly proportional to the frictional drag. The reduction of frictional drag can be

achieved by creating an air layer between vessel`s hull and flow around it. Creation of a cavity and cross

channel of air can be easily just in order to reduce hull drag, while the air from a hole is injecting, the drag

reduction up to 20 percent might be achieved. Experiments indicate that the pattern of wave drag reduces

due to the air using and changes in pressure fields. In this research, by the use of theoretical relationships

and the results of experiments, frictional drag reduction with different methods of air injection was

investigated and then by the use of numerical software simulation (ANSYS-fluid flow), air injection on the

floating model. The achieved results for drag reduction with decrease of wetted surface area at speeds of 4,

6 and 8 m/s are presented. By comparing the results of experiments(Harley high speed craft model) with

software analysis and software simulation was validated. Due to high cost of manufacturing a model with

air injection accessories and towing tank tests, with the acceptable precision results of this research,

numerical software simulation (ANSYS-Fluid Flow) is more quick and efficient.

Keywords: High Speed Boat; Air Injection; Drag Reduction; Hydrodynamic Performance

INTRODUCTION

1In order to promote the presence of navy in the

maritime borders, the use of high speed crafts is

inevitable. Hence the usage of new technologies

in order to increase the maneuverability and

speed of high speed crafts, while localizing these

technologies to be native and synchronized with

the world's marine industry is vital.

The use of different methods of air injection is

one of the ideas to reduce the drag coefficient

and maneuverability in many marine vehicles.

The air lubrication techniques include the air

cavity, micro bubbles and air cushion. These

*Corresponding Author Email: [email protected]

methods can create appropriate changes in the

amount of vessel`s drag coefficient and the flow

pattern around the vessel, which at the end cause

higher speeds and improvement of

maneuverability. On the other hand the main

goal of drag reduction for ships is reduction of

ships fuel consumption, NOx, Sox and CO2

emissions (Davenport et al., 2011).

Results of most researches show that the use of

air injection has many abilities to improve the

performance of ship hull with wide underwater

surfaces such as the case for catamaran, tanker

and barge. The following procedures are known

as air-injection techniques:

mailto:%20%[email protected]

M. Soltaninejad et al.

66

Air cavity

Micro bubbles

Air cushion

Air cavity ships (ACS) are advanced marine

vehicles that use air injection at the wetted hull

surfaces to improve a vessel’ shydro dynamic

characteristics.

ACS FEATURES:

Air cavity ships are already produced in

series

15-40 per cent drag reduction is achieved

Less than 3 per cent of the total ship power is

needed to support the air cavity

Low wash wake is generated due to smoothed

pressure gradients in the presence of the air

cavity

Overloads in rough seas are reduced due to a

damping effect of the air cavity

Fouling growth on the hull in warm seas is

lessened due to decreased wetted surface

ACS is a convenient platform for effective

landing and shallow-water operations

(Matveev, 2003).

In micro bubbles Drag Reduction, air is injected

into the boundary layer, usually through a slot,

porous material or a perforated plate. The air is

separated into bubbles that reside predominantly

in the boundary layer of the hull. The dispersed

bubbles act to reduce the density of the air water

mixture and to modify turbulent momentum

transport.

Air Cushion Vehicles are essentially hovercraft

with rectangular platforms supported by a

cushion of pressurized air, the escape of which is

impeded by flexible skirts attached around the

whole periphery of the platform. The pressurized

air, which supports 100% of the weight of the

vehicle, is usually provided by dedicated lift

fans. Propulsion is usually provided by air

propellers.

The platforms reduced contact with the water

results in low resistance at high speed. The

principal specific attribute of the Air Cushion

Vehicles is its amphibious capability which

enables it to operate from a variety of

unprepared beaches and with minimal terminal

facilities also enables them to operate in shallow

waters, even over sand banks, and over marsh

land. This, in some instances, can significantly

reduce the time in transit by reducing the length

of a route.

The air cushion allows these craft to operate

efficiently at high-speed (50+ kts) as it

considerably reduces frictional resistance

(Ceccio et al., 2010b; Ceccio, 2010a; Arndt et

al., 2009).

MATERIALS AND METHODS

Governing Equations Total drag inserted on a vessel which moves

with speed of U in the water with density of ρ

can be defined as the sum of several resistance

components.

DT=DF+DA+DM+DR (1)

DTtotal drag

DF frictional drag

DA air drag

DM momentum drag

DR residual drag

DF and DR depend on different dimensionless

numbers and DA and DM, may be unrealistic in

the test model. For both the model and the

prototype surface effect ships (SES), frictional

drag by measuring the wetted surface area, as a

function of the correlation line model ITTC-

1957, that is, CF as a function of Reynolds

number is estimated.

DF=1/2CFρSW U2 (2)

CF=0.075/ [(log10Re-2)]2, Re= (ULWL)/υ (3)

According to (3), the Reynolds number depends

on ship water line length LWL and water viscosity

υ.

For the prototype surface effect ship, the air drag

ideally isexpressed as:

DA=1/2 CA SAρA U2 (4)

CA air drag coefficient can be calculated in wind

tunnel

SA transverse area

ρA air density.

In towing tank tests, the equipment usually

installed on the top of the model causes air flow

mix-up. Hence for the models with high speed, it

is necessary to measure DA directly. For example

Int. J. Mar.Sci.Eng., 5(2), 65-75, Summer & Autumn 2015

67

air drag can be calculated, by slowly taking the

model out of the water and then measure the

drag force.

The momentum drag, results of surface effect

ship fans lift, located on the model. For a given

fan with the input area Ai and air flow Q, the

momentum drag is equal to:

DM=QAi U (5)

The models residual drag (the summation of

wave drag and form drag) depends on waves

generated by ship. DR is essentially a function of

Froude number Fr.

Fr=U/√ (gLWL) (6)

Therefore, ship resistance for a given number of

Fr scales up with measured drag DR as a function

of λ3where λ is the scale of geometric similarity

of model (Harris and Grilli, 2007).

Experimental Data

Design of Harley SES hull form was registered

in 1996 by the Harley’s company. This ship is a

catamaran with rigid hulls having two air

cavities (i.e., cushions); each pressurized from

airflow inlets at the bow. The propulsion thrust

was provided by the twin super-cavitation

propeller. (Fig. 1)

Fig. 1: Harley SES concept hullform (Harris and

Grilli, 2007)

The 2.3 m HSC-SES model, shown in Fig. 2,

was used for towing tank tests at the clear fresh

water (Harris and Grilli, 2007); the tank has a

total length of 200 m, a 12 m width, and a 7 m

water depth. The air blower was positioned on

the carriage and air ducts connected the air

blower to the air cushion inlets on the model.

The model was free to heave, pitch and roll. The

tank water is 15 degrees Celsius. The tank is

limited to a top speed of 9 m/s and tow forces of

±250 N. No turbulence stimulators were used,

but for nearly all tests, the Reynolds number was

greater than 5×106 (the slowest tests at 2 and 4

m/s corresponded to Reynolds numbers of

2.6×106 and 5.2×10

6 respectively). Waterline

length (which determines the Reynolds number

and thus the frictional drag coefficient) was not

measured directly; observations indicated that it

is roughly equal to the length of the air cushions,

or 1.5 m, 65% of the length overall. The length

of each air cushion is 149 cm with a beam of 23

cm; the separation between the two air cushions

is approximately 23 cm. The waterline length,

LWL, is slightly greater than the length of the

cushions. The cushion area is then 0.68 m2.

Fig. 2:The full relevant dimensions

For numerical simulations, nonlinear potential

flow was solved by the use of boundary

elements method.

Momentum drag test

The momentum drag was measured during arpm

test as the airflow Q through air ducts of cross-

section Ai changing speed to come to rest was

performed. Then:

DM= -ρa Q2/Ai (7) The amount of air flow according to the

characteristic curve of the fan at different speeds

was calculated. The curve in Fig. 3represents the

momentum drag for model of displacement

(W=289N).

Air drag tests Air drag was measured by raising the model

slightly out of the water and measuringthe drag

force when towed. The mean results vary with

the square of the velocity, which is expected for

a turbulent resistance measurement.

Analysis of air injection system for drag reduction in high speed vessels using numerical simulation software

68

Fig. 3: Measured momentum drag (for W=289 N):

mean (o),±standard deviation (—), and 95%

confidence interval for the mean (red);compared to

theoretical Eq. (7)

The theoretical Eq.(4),with an effective cross-

sectional area, CASA=0.30 m2that was estimated

based on a least-squares fit to the mean

measured data. The large standard deviation for

the air drag measurements is most likely due to

vibration of the tow carriage. The tow carriage is

designed to measure forces over range of ±350 N

range with a model hull in the water, so small

vibrations would probably be damped, as

compared to air drag measurements around 15

N.

Fig. 4 shows the tow force measured during air

drag tests; mean value as well as standard

deviation with 95% confidence interval is

shown.

Because the mean of the measured air drag

closely follows a quadratic fit with speed,the

results seem credible.

Fig. 4: Measured air drag, mean ± standard deviation

with 95% confidence level

Resistance tests

The total drag DT in resistance tests, as a

function of model displacement W, blower speed

(rpm) and towing speed U was measured. Total

drag was corrected for momentum and air drag

using cushion inlet pressures. Correction factors

were used for momentum drag calculation sand

air drag estimates. Note that the corrected drag

does not always decrease with increasing

airflow; This could be due to a number of

physical factors, such as instabilities caused by

high airflows in the cushions, oscillations within

the air ducts, or model proposing. According to

Eq. (1), theme an total drag measurements were

corrected for mean measured momentum (i.e.,

using Eq. (7): -5.57 N, -9.20 N, and -19.9 N, for

2400, 3000, and 4140 RPM tests, respectively)

and air drag (Eq. (4) with CA SA = 0.3

m2).Corrected average results are given in Table

1.

As expected, the corrected hydrodynamic drag

increases with the increase of W, U and

decreases with increase of air flow. This

behavior is generally in all measurements are not

observed, which can be basis of the incorrect

amendments on drag momentum or simply can

be due to changes in wetted surface area with

regard to model`s dynamic draft.

Table 1: Corrected hydrodynamic drag as a function

of air blower speed (rpm) and towing speed U (m/s)

rpm 4 m/s 6 m/s 8 m/s

4140 32.03 43.7 47.33

3000 35.80 49.65 55.40

2400 36.51 55.13 69.06

Finally this tableis usedas a reference to measure

the hydrodynamic drag for thevalidation of

simulation results.

Software simulation

Simulation of the vessel using the software

ANSYS-Fluid Flow is performed.

1. Vessel geometry

A simple float as shown in Fig. s 5 and 6, with a

length of 2m, maximum width of 60 cm, 19 cm

in height and 14 cm designed draft was modeled

proximately with the equivalent wetted surface

area comparison with the Harley craft model.

This is a common example of a high speed boat.


69

Fig. 5: The overview of the vessel

Fig. 6: The facade of the front

To analyze the computational fluid dynamics of

an immersed object, a domain must be set which

is several times the size of the body and use of

finite element software that suits the desired

shape. Then appropriate boundary conditions

must be applied. The domain considered with

the designed draft that is in the form of a large

cube around the float.

Because the float model is symmetrical, to

reduce the size of the elements and calculation

time, half model is used then all data including

the lift and drag forces are doubled. In the

simulation, only the outer shell, just the wetted

surface area in the design draft will be studied

(see Fig.s 7 and 8).

Fig. 7: The facade of the side

Fig. 8: Closer view of the Domain in which the vessel

sheared at considered draft

2. Mesh generation and definitions of problem

domain

For simulation of the flow pattern around the

float, the fluid domain is discretized into very

small elements. The problem is then solved for

each element and then with the convergence of

answers, the desired result obtained.

To reduce computation, the mesh sizes be

considered for the two types of size, but with a

good approximation which do not affect the

negative influence on the answers. The smaller

size (0.5 cm) is for the sensitive boundary

condition parts or the parts that depend on the

shape geometry that have small area or volume.

The other elements have larger size (7 cm) for

the sidesat less sensitive boundaries. (Fig. 9)

(a)

(b)


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Fig. 9: a) the mesh size near the float is much tinier

than other areas; b) far away from the float surface,

the size of elements grows to reach the greatest value

at farthest from float surface.

Next, boundary conditions were defined

according to Fig. 10 named as inlet, outlet,

symmetry, opening, surface and bottom.

(a)

(b)

Fig. 10: The areas naming

RESULTS AND DISCUSSION

To understand the effect of air layer injection

several cases were simulated as follow.

Case I: parameter definitions and examination

of vessel conditions without air layer injection

For fluid flow simulation around the float and to

get the drag coefficients, it is essential to apply

the correct parameters considering the

assumptions and simplifications.

Table 2: simulation inputs for case I

Desired fluid Water

Density of the fluid 1000 kg⁄m3

Reference pressure 1 atmosphere

Fluid speed 8 m⁄s

Calculation method k-ε

Desired area No roughness (smooth)

The process of applying problem parameters is

seen in Fig. 11.The black arrows related to inlet

and outlet, the blue arrows correspond to the

opening boundary condition, the red arrows

correspond to the symmetry boundary condition,

the blue top surface is wall boundary condition

and also yellow float is wall boundary condition.

Fig. 11: Apply boundary conditions ondomain

Now, at the project schematic, all question

marks in front of the (Geometry, Mesh, setup)

steps changed to check mark √ that mean all

steps done correctly and the fluid flow solver

can be run.

Fig. 12: The convergence scantlings diagram

Fig. 13: The drag diagram for the float without air

injection

As shown in the Fig. 13, the amount of drag is


71

negative because the positive direction of speed

and drag force are summed in the Z direction.

In the initial iterations, the flow approach with a

sharp nose of the float, the amount of drag

significantly increased and after a while the fluid

motion near the float, the drag balanced. The

value of drag got39 N for half of wetted surface

area so by doubling it, the total drag is 78 N.

At this point the results, including pressure, fluid

velocity, percent of volumetric fluid etc. can be

observed and have better understanding of the

problem and solution.

In relation to the solution, the most important

parameters are pressure and fluid velocity

around the floating body which determines the

advantage of using a layer of air below the body.

Fig. 14: pressure contours for the float without air

injection layer at symmetry plane

Fig. 15: the minimum pressure on the float`s body

without air injection layer mode

As shown in Fig. s (14) and (15) at the front

point of the float, the fluid pressure substantially

increases due to stagnation on the body and

sharp geometry of that area and then the fluid

pressure drops over the rear parts of the float and

gets the minimum amount. It is noted that the

pressures shown are relative (gauge pressure).

Since the fluid velocity is assumed positive in Z

direction, so in the direction that the fluid meets

the float body, it is negative. Note that the

velocity contours are greatly correlated with

pressure contours; so where fluid pressure is

minimum velocity is maximum and vice versa.

Fig. 16: velocity contours for the float without air

injection layer

Case II: the float with air layer injection

In this case, the vessel motion is simulatedwith

the same velocity, geometry and boundary

conditions of the previous case, only with the

difference that an air inlet injection existsunder

the float body with a diameter of 13.8 cm that

obtained by calibration and 92 cm distance from

the forward due to the Planning and lift

forces.Fig. 17 shows the air inlet.

Fig. 17: air injection inlet in the forward of float

It is clear that due to symmetry of flow on port

and starboard of the vessel, in order to reduce

the size of the elements and calculations, the air

inlet on the other side of the float body exists

and is considered in the calculation. The mesh is

also produced same as in previous section.

In this step, all the settings and boundary

conditions are alike the previous step, with the

difference that the air is injected into the air

cavity so the air phase is also entered in the

problem domain and a two-phase flow is being

solved. The process of applying boundary

Analysis of air injection system for drag reduction in high speed vessels using numerical simulation software

72

conditions with air injection system is seen in

Table 3 and Fig. 18.

Table 3: simulation inputs for case II The first fluid Water(1000 kg⁄m3 )

The second fluid Air(1.185 kg⁄m3 )

Reference pressure 1 atmosphere

The first fluid’s speed 8 m⁄s

The second fluid flux (Air) 0.711 kg⁄s

Calculation method k-ε

Desired area No roughness (smooth)

Fig. 18: Apply boundary conditions with inlet cavity

for air injection

As shown in Fig. s (19) and (20) the resulting

curve is oscillating between the values 32 to 38

N, then an average value of 34 N was selected

for the drag. But because of considering half of

the wetted surface area so by doubling it, the

total drag obtained is68 N.

In this way by air injection nearly 10 N of drag

is reduced, in other words, 13% of the total

amount of drag reduced which means

considerable savings in the required propulsive

power and fuel consumption. View the other

contours in the results as shown in Fig. s 21 to

24.

Fig. 19: convergence curve of the fluid flow solver

Fig. 20: results for drag exerted on the float with air

layer injection at speed of 8 m/s

Fig. 21: pressure contour at symmetry sector;

side view

Fig. 22: The pressure contour on the float body; local

pressure drop around the air cavity

Fig. 23: speed contour during air injection


73

Fig. 24: the air volume fraction contour

As shown in Fig. 24, the amount of air volume

fraction is variable from zero to one under the

float body behind the air injection cavity, even

the air layer injection reached to the heel and at

this part, drag reduces.

Drag reduction performed, while air layer caused

less water flow which has got more viscosity

compared to air, in contact with float body, so by

reducing the wetted surface area, the friction

drag was reduced.

Validation

With regard to the values obtained from

experimental tests that were described in section

3, Table 1, similar can be obtained from

software simulation for verification purpose.

Where the fan specs is not available, according

to the software simulation and considering 8m/s

for water velocity and definite air flow, as was

presented in section 5.2, the drag was calculated

68N that in comparison with the experimental

tests at the same velocity it has got 1.5% error

which is acceptable.

Simulation at speeds 4and 6 m ⁄s

The following results obtained assuming water

velocity (vessel velocity in calm water)of 4 and

6 m/s respectively, fan speed 2400 rpm and an

input air flow 0.711 kg ⁄s.

All process and boundary conditions are similar

to 8 m/s and just speed at this stage change to 4

and 6 m/s to simulate the hydrodynamic drag by

software.

According to Fig. 25 the amount of drag, at

speed 6 m/s is 26 N for semi-hull so by doubling

it, the total drag is 52 N. Now by comparison the

drag with the amount of 55 N from the

experimental tests, deduces an eligible error less

than 5.5%.

Fig. 25: The drag graph at speed6 m ⁄ s

Fig. 26: The drag graph at speed of 4 m ⁄ s

According to Fig. 26 the amount of drag, at

speed 4 m/s is 17 N so by doubling it, the total

drag is 34 N. Now by comparison the drag with

the amount of 36.5 N from the experimental

tests, deduces an acceptable error less than 7.3%;

though it might be noted that at lower speeds the

difference between simulation results and test

data becomes larger. The reason is perhaps is

that the two-phase flow solution at higher speeds

(turbulence two-phase flow) is a better

approximation of reality.

Reduction of air cavity area

Now to extend results of simulation, consider air

injection cavity with a one-third of the previous

diameter, then the area of air inlet cavity is nine

times smaller, as shown in Fig. 27.

As shown in Fig. 28, an amount of 37 N for the

semi-hull drag and 74 N for total drag was

obtained, in comparison with the previous value

68N for the drag with larger air input cavity area

(nine times); 6 N drag force difference was

observed.


74

Fig. 27: the air inlet cavity with the area of 1/9 of the

previous cavity

Fig. 28: drag curve with the smaller air cavity

It means by reducing the air inlet cavity, the

wetted surface area and drag is increased. This is

shown in Fig. 29.

Fig. 29: air volume fraction contour in the case of

smaller air inlet cavity

Comparison of the values of the drag between

lowand high speeds was also carried out and it

was realized that the influence of air layer

injection under the floating body for drag

reduction at higher speeds are more than lower

speeds.

As at the beginning of this research noted,

because of the major part of the drag (80%) is

frictional drag, therefore at higher speeds more

friction and frictional drag is reduced using the

air cavity.

So with reduction of wetted surface area by

using the air layer injection technique, the

proportion of frictional drag reduction increased

and in this way, substantially a reduction in fuel

consumption and thrust force was obtained.

CONCLUSION

As was observed, a new method of drag

reduction for floats was introduced. Since all

ships annually consume more than 2.7 billion

barrels of fuel (8.6% of the world oil resources),

a slight decrease in fuel consumption creates a

massive economy in annual fuel consumption.

On the other hand, ships generate one million

tone CO2 (2% of world CO2), 16% of world SOX

and 14% of world NOX, that reduction in fuel

consumption, effects valuable influences on the

environment.

The total drag of a ship directly is proportional

to the frictional drag. If frictional drag could be

decreased, the total drag can be significantly

reduced.

Reduction of frictional drag can be through the

air layers between the ship's hull and the

surrounding flow. This theory under the heading

of air lubrication that was the dream of 19th

century scientists has been discussed. Creation

of a cavity and cross channel of air can be easily

just in order to reduce hull drag. While the air

from a hole is injected, drag reduction up to 20

percent can be achieved. Experiments indicate

that the pattern of wave model drag reduces due

to the air injection and change in pressure fields.

Reduction in wave drag in comparison with the

viscosity drag is small.

According to the results that were obtained from

simulation software ANSYS-Fluid Flow, one

could say are liable way to reduce the model

manufacturing cost and save the testing time in

towing tank is attained.

REFERENCES

Arndt, R. E. A; Hambleton, W.T.; Kawakami,

E.; Amromin E.L. (2009). Creation and

Maintenance of Cavities under Horizontal


75

Surfaces in Steady and Gust Flows. Journal

of Fluids Engineering, Vol. 131.

Ceccio, S.L., (2010-a). Friction Drag Reduction

of External Flows with Bubble and Gas

Injection. Annual Review of Fluid

Mechanics, Vol. 42, pp. 183-203.

Ceccio, S.L.; Perlin, M.; Elbing, B.R., (2010-

b).A cost-benefit analysis for air layer drag

reduction. Proc. Int. Conf. On Ship Drag

Reduction- SMOOTH-SHIPS, Istanbul,

Turkey.

Davenport, J.; Hughes, R. N.; Shorten, M.;

Larsen, P. S., (2011). Drag reduction by air

release promotes fast ascent in jumping

emperor penguins a novel hypothesis. Marine

Ecology- Progress Series, Vol. 430, pp. 171-

182.

Matveev, K. I., (2003). Air Cavity Ships Are

Ready for a Wider Market. Speed at Sea, Feb.

2003, pp. 13-16.

Harris, J. C.; Grilli, S. T., (2007).Computation of

the wave making resistance of a Harley

surface effect ship. Proceedings of

Seventeenth International Offshore and Polar

Engineering Conference.

How to cite this article: (Harvard style)

Soltaninejad, M.; Azarsina, F.; Javid, A. H., (2015). Analysis of air injection system for drag

reduction in high speed vessels using numerical simulation software ANSYS-Fluid Flow. Int. J. Mar.

Sci. Eng., 5 (2), 65-75.

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Analysis of air injection system for drag reduction in