NAME OF THE CANDIDATE : JITHIN P N

AFFILIATION : DEPT. OF SHIP TECHNOLOGY, CUSAT

INTERNAL GUIDE : Dr. Dileep K Krishnan

Associate Professor

Dept. of Ship Technology

CUSAT

EXTERNAL GUIDE : Dr. Senthil Prakash M N

Associate Professor

Dept. of Mechanical Engineering

CUCEK

The oceanographic applications such as sea bed mapping and ocean

environment investigation and naval application including acoustic

detection of a submerged target and mine detection require an

underwater body to be moved in a stable condition in the ocean at a

pre-determined depth. This is usually facilitated by towed cable

array system. The towing systems possible are single part towing

system and two part towing system.

• The towed body rises as the towing vessel speed increases which

can only be adjusted by increasing the tow cable length

• Increasing the cable length increases the cable tension and drag

forces.

• Increased cable length requires massive array handling systems

• The Host Vessel manoeuvrability is restricted

• Ship motion induces instability to the Towed Body

• Gives a hydrodynamic depressive force (negative lift) for

the towed body

• Keeps the towed body at the required depth

• Decouples the towed body from wave induced ship motions

• Provides stability to the towed body at varying tow speeds

• Cable length can be reduced

• Weighted depressor (due to gravity forces)

• Hydrodynamic depressor (due to its fin shape)

• R. F. Becker (1950) – described the design, fabrication, model basin test and sea test of a

half scale and full scale model of a high speed, light weight depressor for towing sonar

array from ships. Results of the test program have verified the performance and

demonstrated the ease of handling a light weight depressor.

• Wilburn L. Moore (1962) – compared potential velocity distribution of some promising

bodies of revolution used for designing the body shape of depressors and towed bodies.

The most promising shape for use in boat nacelles is the DTMB series 58 model 4162 -

has the highest theoretical cavitation-inception speed with satisfactory drag characteristics.

The DTMB- EPH is the most likely second choice.

• Dessureault (1976) – developed a streamlined tow body “Batfish”, to house fast-

responding oceanographic sensors. It is towed behind a ship and its depth is controlled

from the vessel by a manually or automatically produced command signal.

• Chapman (1984) – developed a model to describe the dynamic behavior of an

underwater towed system. Ship induced pitching motion can be reduced by adjusting fin

size.

• David Hopkin (1993) – described the effectiveness of a two part tow for damping the

vertical heave motions at the tow-fish.

• Andrey N. Serebryany (1998) – demonstrated the effect of large amplitude internal

waves on a towed depressor using internal wave measurements near the Mascarene Ridge

in the Indian Ocean.

• Mehrdad Ghods (2001) – presented the results from the wind tunnel testing of a NACA

2415 wing and the analysis of this data.

• Roger E. Race- features and advantages of Type 1074 variable depth V- Fin depressor.

• C.A. Woolsey and A.E. Gargett (2002) – investigates the problem of stabilising the

longitudinal motion of a streamlined sensor platform, towed in a two stage arrangement,

using servo- actuated tail fins and an internal moving mass actuator.

• Steven D. Miller (2008) – has carried out wind tunnel test of NACA 0015 symmetrical

airfoil to determine the lift, drag and moment coefficients. As angle of attack is increased,

the flow will eventually separate from the upper surface of the airfoil resulting in a ‘stall’.

The angle of attack must be decreased below the separation angle of attack in order for the

flow to reattach.

• Carl Erik Wasberg and Bjorn Anders Pettersson Reif (2010) – described a

methodology for hydrodynamical simulation in FLUENT and is applied to CFD analysis

of two and three dimensional wings operating in air and water.

OBJECTIVE

• Modelling and numerical investigation of the hydrodynamic behaviour of depressor

for various arrangements under different towing speeds

SCOPE

• Modelling of the existing depressor in ANSYS design modeller

• A CFD analysis of the depressor for Drag and Lift Forces by using the software

package FLUENT from ANSYS Inc

• Verifying the results obtained from CFD analysis with the experimental data thereby

ensuring the reliability of the analysis

• Extending the numerical simulation of hydrodynamic depressor for various angles of

attacks under different towing speeds, and finding the maximum depressive force

generated by the system

• The depressor has been modelled in ANSYS Design Modeler

from ANSYS Inc

• For building the model a 2D sketch has been developed using

points, lines, arcs and curves

• The 3D model is developed from 2D sketch using loft and

revolve operation

• Airfoil geometry

• Wing area (A), Span (b) and Chord (c)

• Dihedral angle

Taper ratio ‘λ’, Sweep back angle ‘˄’ and Aspect ratio (AR)

Centre line

Leading edge, LE

Trailing edge, TE Ct

Cr

Ct /4

Ct /4

Wing span, b Semi span, s Semi span,s

˄, sweep angle

taper ratio λ= Ct/Cr

AR= span/ mean chord

= b/C

= b2/ A

C = (Ct +Cr)/2

• NACA airfoil profiles

• developed by the National Advisory Committee for Aeronautics

(NACA)

• NACA airfoils are described using a series of digits following the

word “NACA”

The NACA four-digit wing sections define the profile by:

• One digit describing maximum camber as percentage of the chord

• One digit describing the distance of maximum camber from the airfoil leading

edge in terms of percentage of the chord

• Two digits describing maximum thickness of the airfoil as percent of the chord

For example, the NACA 2412 airfoil has a maximum camber of 2% located 40%

(0.4 chords) from the leading edge with a maximum thickness of 12% of the chord.

General geometric specifications of the depressor

Body Shape DTMB EPH

Reference length 35" (889 mm)

Body max diameter 10" (254 mm)

Length to tail trailing edge 30.5" (775 mm)

Wing span 45" (1143 mm)

Overall height 16.4" (417 mm)

Tow point 13.2" (335 mm) aft of nose

Geometric details of the main wing

Airfoil section NACA 0015

Aerodynamic center 13.5" (343 mm) aft of nose (25% chord)

Wing span 45" (1143 mm)

Area - total with included body 4 ft2 (0.372 m2 )

Mean chord 12" (305 mm)

Root chord 15" (381 mm)

Tip chord 9" (229 mm)

Taper ratio 0.6

Aspect ratio 3.5

Incidence 4.50, leading edge down

Geometric details of the tail wing

V configuration trailing edges 450 from vertical

Airfoil section NACA 0015

Span 12" (305 mm)

Total Area 1 ft 2 (0.093 m2 )

Mean chord 6" (152 mm)

Root chord 8.5" (216 mm)

Tip chord 3.5" (89 mm)

Taper ratio 0.41

Aspect ratio 2.0

Incidence 00

NACA 0015 FIN

The general equation of a NACA four- digit airfoil is given by:

c is the chord length

x is the position along the chord from 0 to c

y is the half thickness at a given value of x (centre line to surface)

t =0.15 (for NACA 0015), is the maximum thickness as a fraction of the chord

(so 100 t gives the last two digits in the NACA 4-digit denomination)

Coordinates for the section at the mean chord of the main wings

(mean chord= 305mm)

x(mm) 0 30 60 90 120 150 180 210 240 270 300 305

+y(mm) 0 17.75 21.80 22.88 22.21 20.38 17.70 14.39 10.55 6.21 1.34 0.48

-y(mm) 0 17.75 21.80 22.88 22.21 20.38 17.70 14.39 10.55 6.21 1.34 0.48

Coordinates for the section at the mean chord of the tail wings

(mean chord= 152mm)

x(mm) 0 10 20 30 40 50 60 70 90 100 110 120 130 140 150 152

+y(mm) 0 7.57 9.76 10.87 11.34 11.37 11.06 10.49 8.79 7.71 6.52 5.20 3.78 2.25 0.59 0.24

-y(mm) 0 7.57 9.76 10.87 11.34 11.37 11.06 10.49 8.79 7.71 6.52 5.20 3.78 2.25 0.59 0.24

Coordinates for the depressor body

x/L 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Y/Ymax 0 0.637 0.841 0.950 0.997 0.989 0.929 0.817 0.652 0.434 0.000

x (mm) 0 88.90 177.8 266.7 355.6 444.5 533.4 622.3 711.2 800.1 889.0

+y(mm) 0 80.92 106.8 120.7 126.6 125.6 118.0 103.8 82.80 55.13 0.000

-y(mm) 0 80.92 106.8 120.7 126.6 125.6 118.0 103.8 82.80 55.13 0.000

Reference length, L=889mm; Max radius, Ymax=127mm

Main wings

body

Tails

inlet

outlet

The computational fluid dynamics simulations have to be conducted by using the

software package FLUENT from ANSYS Inc.

Forces acting on the depressor

• Weight of the depressor

• Buoyant force

• Lift and drag forces on the body and the fins

• Force from the towing cable

LIFT AND DRAG FORCES ON THE AIRFOIL

Lift Force, Fl =1/2 ×ρ×A×V2×Cl

Drag Force, Fd = 1/2 ×ρ×A×V2×Cd

• Branch of fluid mechanics that uses the numerical methods and algorithm to

solve and analyze problem that involve fluid flow

Governing equations of CFD

Continuity equation:

ρ = density, is the velocity component in the ith direction i=1, 2, 3 and

In case of incompressible flows continuity equation becomes:

Momentum equation

Ʈij is the Reynolds stress tensor =

p = static pressure,

gi = gravitational acceleration in the ith direction ,

ij is the Kroneker delta and is equal to unity when i= j; and zero when i j

i

j

ij

i

ji

j

i gxx

puu

xu

t

)()(

]3

2)]([ ij

l

l

i

j

j

i

x

u

x

u

x

u

The Reynolds-Averaged form of the above momentum equation including

the turbulent shear stresses is given by:

Where, R ij = is called the Reynolds stress.

is the instantaneous velocity component i = 1, 2, 3

The Reynolds stresses are additional unknowns introduced by averaging

procedure. They must be modeled in order to close the equation.

''

jiuu

''

3

2)()( ji

jil

l

i

j

j

i

j

ji

j

i uuxx

p

x

u

x

u

x

u

xUU

xU

t

'

ju

Steps involved in CFD

1. During pre-processing

• Creating the geometry

• The volume occupied by the fluid is divided into discrete cells (the

mesh). The mesh may be uniform or non uniform

• The physics of the model is defined . For example, the equations of

motions + enthalpy + radiation

• Boundary conditions are defined

2. The simulations are started and the equations are solved iteratively as a steady

state or transient condition

3. Postprocessor is used for the analysis and visualization of the results

• FLUENT solvers are based on the finite volume

method(FVM)

• Domain is discretized onto a finite set of control

volumes (or cells).

• General conservation (transport) equations for

mass, momentum, energy, etc. are solved on this

set of control volumes.

• Partial differential equations are discretized into a

system of algebraic equations.

• All algebraic equations are then solved numerically

to render the solution field.

Fluid regions of pipe flow

discretized into finite set of

control volumes

2D Pipe

•Wing section

• A study on single part and two part towing systems were done

• Basic theories and geometric definitions for depressors were studied

• An extensive literature survey has been carried out

• The 3D model of the depressor is developed in ANSYS Design modeller

• Simple 2D and 3D meshing problems have been done as a part of familiarisation

of the CFD pre-processing tool ICEM CFD

• Meshing and modification of the flow domain

• Setting up of the problem in Fluent

• Conduct of domain independence and grid independence study

• Conduct of simulation of flow through depressor for its hydrodynamic

parameters and comparison with that from experimental data

• Modelling of a depressor with a different arrangement

• Meshing and modification of the flow domain

• Setting up of the problem in Fluent

• Conduct of simulation in Fluent for the depressor and analyzing its

hydrodynamic performance

• Conclusion

[1] R.F. Becker, “High speed sonar array depressor program final report”, prepared for

Office of Naval Research, Virginia, 1981.

[2] Wilburn L. Moore, “Bodies of revolution with high cavitation-inception speeds- for

application to the design of hydrofoil-boat nacelles” , 1962 .

[3] David Hopkin, Jon M. Preston, Sonia Latchman, “Effectiveness of a two-part tow

for decoupling ship motions”, Defence Research Establishment Pacific, IEE, pages

1359-1364, 1993.

[4] Carl Erik Wasberg and Bjorn Anders Pettersson Reif, “Hydrodynamical simulations

in FLUENT”, Norwegian Defence Research Establishment, 2010.

[5] Andrey N. Serebryany, “Effect of large-amplitude internal waves on a towed

depressor”, N.N. Andreyev Acoustics Institute, Moscow, 1998.

[6] Steven D. Miller, “Lift, drag and moment of a NACA 0015 airfoil”

[7] Roger E. Race, “The variable depth V-Fin depressor” Endeco INC pages 1359-

1364.

[8] D.A. Chapman, “A study of the ship induced roll motion of a heavy towed fish”,

Ocean Engineering, Volume 11, Issue 6, pages 627-654, 1984.

[9] C.A. Woolsey, A.E. Gargett, “passive and active attitude stabilization for a tow-

fish”. Proceedings of the 41st IEEE conference on Decision and Control, Las Vegas,

Nevada USA, 2002.

[10] Dessureault “Bat fish a depth controllable towed body for collecting

oceanographic data”.

[11] Mehrdad Ghods,“Theory of wings and wind tunnel testing of a NACA 2415 airfoil”

[12] E.L. Houghton, P.W. Carpenter, “Aerodynamics for engineering students”.

[13] Anderson J.D “Computational fluid dynamics”.

[14] Anderson J.D “Fundamentals of aerodynamics