analysis of air injection system for drag reduction in
TRANSCRIPT
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:
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.
Int. J. Mar.Sci.Eng., 5(2), 65-75, Summer & Autumn 2015
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)
M. Soltaninejad et al.
70
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
Int. J. Mar.Sci.Eng., 5(2), 65-75, Summer & Autumn 2015
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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
Int. J. Mar.Sci.Eng., 5(2), 65-75, Summer & Autumn 2015
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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.
M. Soltaninejad et al.
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
Int. J. Mar.Sci.Eng., 5(2), 65-75, Summer & Autumn 2015
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.