Resistance estimation of 90T AHTS Vessel using resistance prediction method and CFD methods

Resistance estimation of 90T AHTS Vessel using resistance prediction

method and CFD methods and comparison with experimental results

A Project Report

Submitted by

FAHEEM K K

NA07B027

in partial fulfillment of the requirements

for the award of the Dual degree of

BACHELOR OF TECHNOLOGY

and

MASTER OF TECHNOLOGY

DEPARTMENT OF OCEAN ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY MADRAS

CHENNAI – 600036

MAY 2012

i

THESIS CERTIFICATE

This is to certify that the project titled Resistance estimation of 90T AHTS Vessel using

resistance prediction method and CFD methods and comparison with experimental

results, submitted by Faheem K K, to the Indian Institute of Technology Madras, for the award

of the dual degree of Bachelor of Technology and Master of Technology, is a bona fide record of

the project work done by him under my supervision. The contents of this report, in full or parts

have not been submitted to any other Institute or University for the award of any degree or

diploma.

Prof V Anantha Subramanian

Project Guide

Professor

Department of Ocean Engineering

Indian Institute of Technology Madras

Chennai - 600036

Place: Chennai

Date: May 25, 2012.

ii

ACKNOWLEDGEMENTS

First and foremost, I thank God for all the blessings brought into my life and for showing me the

light in difficult times. I express my profound gratitude to Prof V Anantha Subramanian for

giving me an opportunity to work on this project. His demand for perseverance and his

inspiration enabled me to complete the project in time. I am grateful to Prof C P Vendhan,

project coordinator for his support and guidance over the last one year.

I extend my sincere gratitude to all the faculty members of the department who guided me

throughout my academic career with their knowledge and wisdom.

I thank my friends at the department, Rakesh, Pritam, Shameem, Dinoj, Anitha, Neshal,

Anilsekhar, Rajitha and Sreedevi for their help in various aspects relating to courses and

project. I express my sincere gratitude for the creators of Google, Science direct and Wikipedia.

Finally, I would like to thank my parents who are my pillars of strength, my relatives who gave

the support that I needed, my wing mates and all my friends especially the friends’ circle which

we fondly call ‘madras mail’ who filled my college life with happiness and fun.

Faheem K K

iii

ABSTRACT

Hull resistance of 90T bollard pull AHTS Vessel is estimated using CFD analysis and Hotrop-

Mennen prediction method, and is compared with the experimental results from towing tank.

Even though model experiment method is the most widely used for resistance estimation, other

prediction methods are also devised for calculation of resistance when the complete ship form and

full description of ship are unavailable especially in the preliminary stage of ship design. Out of all

the available prediction methods, the Holtrop-Mennen method has proved to be highly effective at

the initial design stage for small vessels to establish the still water performance and for estimating

the required propulsive power.

Computational Fluid Dynamics (CFD) has made remarkable progress in the recent few decades,

including the developments in practical application of CFD to ship hydrodynamics. For this

particular ship, CFD simulations to obtain resistance characteristics are carried out in Star CCM+

solver. SHIPFLOW, a CFD tool developed especially for the marine industry, is also used to estimate

and compare the resistance values.

iv

Contents

THESIS CERTIFICATE ..................................................................................................................................... i

ACKNOWLEDGEMENTS ................................................................................................................................ii

ABSTRACT ....................................................................................................................................................... iii

CHAPTER 1 ....................................................................................................................................................... 1

INTRODUCTION AND LITERATURE REVIEW ........................................................................................ 1

1.1 General ....................................................................................................................................................................................... 1

1.2 Objective and scope of the Project ................................................................................................................................. 1

1.3 Organization of the project. .............................................................................................................................................. 2

1.4 Literature Review ................................................................................................................................................................. 2

CHAPTER 2 ....................................................................................................................................................... 4

ESTIMATION OF TOTAL CALM WATER RESISTANCE FROM TOWING TANK ............................. 4

2.1 Ship Particulars ...................................................................................................................................................................... 4

2.2 Extrapolation method for obtaining prototype resistance .................................................................................. 5

CHAPTER 3 ....................................................................................................................................................... 9

RESISTANCE PREDICTION – HOLTROP-MENNEN METHOD ............................................................. 9

3.1 Introduction ............................................................................................................................................................................ 9

3.2 Theory ........................................................................................................................................................................................ 9

3.3 Graphic User Interface for Holtrop Mennen method ........................................................................................... 14

3.4 Results and Discussions ................................................................................................................................................... 15

CHAPTER 4 .................................................................................................................................................... 16

RESITANCE CHARECTERISTICS FROM CFD SIMULATIONS ........................................................... 17

4.1 Introduction .......................................................................................................................................................................... 17

4.2 Geometric Modeling ........................................................................................................................................................... 17

4.3 Governing Equations and Solvers ................................................................................................................................ 18

4.4 Free Surface Treatment .................................................................................................................................................... 18

4.5 Geometry Handling and Meshing ................................................................................................................................. 18

4.5.1 Geometry Import and Domain Definition ......................................................................................................... 18

4.5.2 Meshing ........................................................................................................................................................................... 19

4.6 Physical Model ...................................................................................................................................................................... 21

4.7 Results...................................................................................................................................................................................... 22

v

CHAPTER 5 .................................................................................................................................................... 25

ESTIMATION OF RESISTANCE USING SHIPFLOW ............................................................................. 25

5.1 Introduction .......................................................................................................................................................................... 25

5.2 Modules and Applications. .............................................................................................................................................. 26

5.3 Governing equations and Methodology ..................................................................................................................... 27

5.3.1: Potential flow method: ............................................................................................................................................ 27

5.3.2: XVISC module: ............................................................................................................................................................. 30

5.3.3 Methodology ................................................................................................................................................................. 30

5.4 Results and Discussions. .................................................................................................................................................. 31

5.4.2 Resistance Estimation of 90T AHTS ........................................................................................................................ 32

5.4.2 Wave Cut Analysis Visualization ............................................................................................................................... 35

CHAPTER 6 .................................................................................................................................................... 38

CONCLUSIONS ............................................................................................................................................... 38

REFERENCES ................................................................................................................................................. 40

APPENDIX ...................................................................................................................................................... 41

1

CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

1.1 General

The anchor handling tug supply (AHTS) vessel is a multi-utility naval vessel that is concerned with the

objective of either tugging or towing an oil-rig or a ship. When it comes to oil rigs, these tugs form the

most important necessity as without their help, it would be impossible to place oil rigs in the required

sea and oceanic areas. Due to the complexity of the shape form with the presence of the appendages and

bulbous bow for efficiency, the effective hydrodynamic performance in still water is a fundamental

problem in design of an AHTS.

The resistance of a ship at a given speed is the force required to tow the ship at that speed in smooth

water, assuming no interference effects from the towing ship. The total resistance is mainly made up of

two components.

Frictional resistance, due to the motion of the hull through a viscous fluid.

Wave making resistance due to the energy that must be supplied continuously by the ship to the

wave system created on the surface of water.

Since up to the present time, predictions at full scale are based on model tests, the cost of procedure by

performing towing tank experiments is at times unaffordable. An alternative approach to face the

problem of resistance estimation is the application of CFD method. In addition, these tools can provide a

better understanding of the Reynolds scale effect, so that extrapolations at full scale may be improved. In

order to obtain accurate resistance predictions, any computational method has to deal with the free

surface around a ship unavoidably.

1.2 Objective and scope of the Project

The main aim of the project is to estimate the total calm water resistance characteristics of 90T AHTS

vessel hull. The resistance of the vessel is of paramount importance here. The calm water resistance is

attributed to two major components namely frictional resistance and wave making resistance. While the

Hotrop-Mennen method is used for predicting the approximate range of resistance values, the model

tests conducted in the towing tank gives more accurate results. The above results are validated against

the CFD simulations for an exact replica of the ship model under the same conditions.

2

1.3 Organization of the project .

Chapter 1 Introduction and literature review.

Chapter 2 deals with estimation of total calm water resistance from towing tank.

Chapter 3 involves the resistance prediction using Holtrop Mennen’s method.

Chapter 4 involves the resistance estimation by CFD simulations.

Chapter 5 deals with the computation of Total Calm Water Resistance using Computational Fluid

Dynamics package SHIPFLOW.

Chapter 6 involves Conclusions and suggestions for further work.

1.4 Literature Review

A brief note on the literature survey done on the resistance estimation of a ship hull using CFD,

Prediction and Experimental methods listed here.

VanMierlo (2006), talks about the estimation of wave resistance of ship hull forms using SHIPFLOW

.Tests were carried out on different hull forms and trend validation was done by comparing with the

experimental results. He concluded that a pure validation of the nonlinear free surface potential flow is

not possible due to uncertainties in both the towing tank results and the calculations. A trend validation

shows that the prediction of the wave resistance is poor for the low Froude numbers but improves when

the Froude number increases.

Holtrop, J. and Mennen, G.G.J.(1982), in their paper "An Approximate Power Prediction Method",

introduced a prediction method of ship resistance that was developed through a regression analysis of

random model experiments and full scale data.

M. Salas et al (2004) presented paper on ―Experimental and CFD resistance calculation of small

catamaran. In their study it has been found that in the low speed range CFD codes predicts higher CT

than the towing tank. In the paper, they concluded that the disagreement maybe due to the trim angle

being affected by pulling cable in the experimental tests. Between Fn 0.5 and Fn 0.9 Tdyn agrees closely

with experimental results as both total resistance curves are almost identical. Shipflow resistance

estimates are slightly higher than CFD Tdyn and Towing Tank resistance results in this speed range.

Calus Abt et al (2007) in their paper ‘A new approach to integration of CAD and CFD for Naval Architects’

describes the new integration environment FRIENDSHIP framework combining CAD and CFD allowing

rapid hydrodynamic evaluation of ship design.

3

Zhi-rong Zhang (2010) published a paper on ‘Verification and validation for RANS simulation of KCS

container ship without/with propeller’ in which he computes resistance and wave elevation from the

free surface simulation of a modern container ship KCS. Verification and validation of the results were

performed using recommended procedures proposed by ITTC.

Fred Stern et al (1995) in their publication, ‘Evaluation of Surface-Ship Resistance and Propulsion Model-

Scale Database for CFD Validation’, in which they conduct evaluation of databases for CFD validation

with regard to status and future uses and requirements.

4

CHAPTER 2

ESTIMATION OF TOTAL CALM WATER RESISTANCE FROM TOWING TANK

2.1 Ship Particulars

From hydrodynamic considerations, the model was built to scale 1:20 to suit the model testing facility.

The model was built in fiber glass to the above scale. The tank dimensions are 82.0m x 3.2m x 2.5m

(water depth).

Fig 2.1 Lines plan of the ship

Table 2.1 Ship Particulars.

Particulars Prototype Model

LOA 67 m 3.35 m

LBP 59.2 m 2.96 m

Breadth 16.8 m 0.84 m

Depth 6.8 m 0.34 m

Draft and corresponding

displacement 5.2 m, 4113 t 0.26 m, 500 kg

Design speed 13.5 knots 1.55 m/s

Scale of model 1 : 20

5

2.2 Extrapolation method for obtaining prototype resistance

The model on 1 : 20 scale was towed at different speeds on the basis of equivalent Froude numbers. The

resistance of model is scaled to prototype values on the basis of Froude’s method of extrapolation using

ITTC 78 prediction method. For this purpose, the flat plate frictional resistance was determined using

the ITTC 57 friction correlation line. The form factor was obtained by Prohaska’s method from low

speed towing tests. The formulation for calculation based on Froude’s method of extrapolation is given

here.

Symbols used are as follows:

= Geometric scale of model

RTS = Prototype resistance in Newton

RTM = Model resistance in Newton

VS = Prototype speed in m/s

VM = Model speed in m/s

SS = Wetted surface of prototype

SM = Wetted surface of model

SW = Density of sea water at prediction temperature of 25oC

FW = Density of fresh water at prediction temperature of 25oC

CTS = Coefficient of total resistance of prototype

CTM = Coefficient of total resistance of model

CRM = Residuary resistance coefficient of model

CRS = Residuary resistance coefficient of prototype

CFoM = Coefficient of equivalent flat plate resistance of model

CFoS = Coefficient of equivalent flat plate resistance of prototype

RnM = Reynolds’s number of model

RnS = Reynolds’s number of prototype

6

LM = Waterline length of model

LS = Waterline length of prototype

FW = Kinematic viscosity of fresh water for model

SW = Kinematic viscosity of sea water for prototype

R = (1+k) = Form factor

Physical data and constants:

Density of sea water at 250C, SW = 1025 kg/m3

Density of fresh water at 250C, FW = 997 kg/m3

Viscosity of sea water at 250C, SW = 0.9425 10-6 m2/s.

Viscosity of fresh water at 250C, FW = 0.8929 10-6 m2/s.

RTS = CTS 0.5 SW VS2 SS

CTS = CFoS + CRS+ 0.0004 (correlation allowance)

CRS = CTM - CfoM

CTM = RTM / (0.5 FW VM2 SM)

CFoM = 0.075/(log RnM - 2)2

The addition of correlation allowance of 0.0004 is as per standard practice

Method of calculation of prototype resistance using model resistance data:

RTS = CTS 0.5S VS2 SS

VS = VM

CTS = CFoS +CRS

CRS = CRM

CRM = CTM – CFoM

CTM = RTM(N) / (0.5 FW VM2 SM)

7

CFoM = 0.075/(log RnM - 2)2

CFoS = 0.075/(log RnS - 2)2

RnS = VSLS/SW

RnM = VMLM/FW

Following the above scheme, it is possible to extrapolate the model resistance RTM (in Newton) at model

speed VM (in m/s) to prototype resistance RTS (in Newton) at corresponding ship speed VS.

Sl. no. Model speed (m/s) Model resistance

(N) Ship speed (knots)

Prototype

resistance (kN)

1 0.71 6.6 6.2 36.4

2 0.81 9.6 7.1 56.8

3 0.92 12.4 8.0 74.7

4 1.02 15.7 8.9 96.7

5 1.12 20.1 9.7 126.8

6 1.22 25.0 10.6 160.8

7 1.27 27.9 11.1 181.9

8 1.32 32.0 11.5 211.9

9 1.38 36.5 12.0 245.5

10 1.43 41.9 12.4 286.4

11 1.47 46.8 12.8 323.9

12 1.53 55.0 13.3 386.9

13 1.58 63.5 13.7 453.0

14 1.64 75.7 14.3 548.7

Table 2.2 Towing tank results.

8

0

10

20

30

40

50

60

70

80

0.5 0.7 0.9 1.1 1.3 1.5 1.7

Model Resistance

0

100

200

300

400

500

600

5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5

Ship Resistance

Velocity (m/s)

Fig 2.2 Resistance Vs Velocity graph for the model

Velocity (m/s)

Fig 2.3 Resistance Vs Velocity graph for the prototype

9

CHAPTER 3

RESISTANCE PREDICTION – HOLTROP-MENNEN METHOD

3.1 Introduction

During the initial phases of ship design, the resistance coefficient is estimated with approximate

methods based on systematic series or statistical regressions to experimental data. A systematic series is

a family of ship hulls obtained from a systematic variation of one or more shape parameters. Usually, the

changes are based on a parent form. The resistance of all the models that constitute a series is measured

experimentally.

Holtrop Mennen resistance prediction method is one of these techniques widely used for displacement

and semi-displacement vessels. Like all methods, however, this technique is limited to a suitable range of

hull form parameters. This algorithm is designed for predicting the resistance of tankers, general cargo

ships, fishing vessels, tugs, container ships and frigates.

The range of parameters for which the coefficients of the basic expressions are valid as following:

Ship Types Max.

Fn

CP L/B B/T

Min Max Min Max Min Max

Tankers, Bulk carriers 0.24 0.73 0.85 5.1 7.1 2.4 3.2

Trawlers, coasters, rugs 0.38 0.55 0.65 3.9 6.3 2.1 3.0

Container ships,

Destroyers

0.45 0.55 0.67 6.0 9.5 3.0 4.0

Cargo liners 0.30 0.56 0.75 5.3 8.0 2.4 4.0

RO-RO ships, Car ferries 0.35 0.55 0.67 5.3 8.0 3.2 4.0

Table 3.1 Range of Parameters for Holtrop Mennen

3.2 Theory

The procedures of calculating ship resistance using Holtrop and Mennen’s method (Holtrop and

Mennen, 1982) is as follows:

The total resistance of a ship has been subdivided into:

10

ATRBWAPPFTotal RRRRRkRR )1

1·(

FR , Frictional resistance of a ship according to the ITTC-1957 friction formula.

k11, Form factor describing the viscous resistance of the hull form in relation to FR .

APPR , Resistance of appendages.

wR, Wave-making and wave-breaking resistance.

BR , Additional pressure resistance of bulbous bow near the water surface.

TRR , Additional pressure resistance of immersed transom stern.

AR , Model-ship correlation resistance.

For the form factor of the hull the prediction formula:

6906.0521448.0

92497.0

12131 0225.0195.093.0·)1( lcbCCL

Bcck PP

R

Cp, Prismatic coefficient based on L;

Lcb, The longitudinal position of center of buoyancy forward of 0.5L as a percentage of L.

1·4

··06.01

P

PP

R

C

lcbCC

L

L

02.0479948.0

05.002.0479948.002.0·20.48

05.0

078.2

2228446.0

12

L

T

L

T

L

T

L

T

L

T

c

T, The average moulded draft

11

sternCc 003.0113

Cstern, Coefficient indicates the after body form.

Fig 3.1 Cstern Values

The wetted surface area of the hull can be approximated well by:

( )√ (

)

CM, Midship section coefficient,

CB, Block coefficient on the basis of waterline length L,

CWP, Water plane area coefficient,

ABT, Transverse sectional area of the bulb

The appendage resistance can be determined from

( )

, Water density,

V, Ship speed,

SAPP, Wetted area of the appendages,

(1+k2), Appendage resistance factor and

CF,The coefficient of frictional resistance of ship according to ITTC-1957 formula given by

( )

12

Approximate (1+k2) values

Rudder behind Skeg 1.5-2.0

Rudder behind stern 1.3-1.5

Twin screw balance rudders 2.8

Shaft brackets 3.0

Skeg 1.5-2.0

Strut bossings 3.0

Hull bossings 2.0

Shafts 2.0-4.0

Stabilizer fins 2.8

Dome 2.7

Bilge keels 1.4

Table 3.2 (1+k2) values

The equivalent (1+k2) for a combination of appendages is determined from:

( ) ∑( )

∑

The wave making resistance can be determined from

)··cos(··exp····· 2

21521

n

d

nW FmFmgcccR

37565.1

07961.1

78613.3

71 90···2223105

Ei

B

Tcc

25.00625.05.0

25.011.0

11.0229577.0

33333.0

7

L

B

B

L

L

B

L

B

L

B

L

B

c

( √ )

13

C2, Accounts for the reduction of the wave resistance due to action of the bulbous bow,

C5, Expresses the influence of transom stern on wave resistance,

AT, Immersed part of the transverse area of the transom at zero speed.

16302.0

3

34574.0

6367.030484.0

80856.0

·100···0225.01·1··exp891LB

LlcbCC

B

Li R

PWPE

The half angle of entrance iE is the angle of the waterline at the bow in degrees with reference to the

center plane neglecting the local shape at the stern.

The coefficient that determines the effect of the bulbous bow on the wave resistance is defined as

* ( √ )+

hB , Position of center of transverse area ABT above the keel line

TF, Forward draught of the ship.

1236.0·446.1

12·03.0·446.1

B

LC

B

L

B

LC

P

P

16

31

1 ·79323.4·75254.1·0140407.0 cL

B

LT

Lm

80.0·7067.073014.1

8.0·984388.6·8673.13·07981.8 32

16

PP

PPPP

CC

CCCCc

22

152 ·1.0·exp· nP FCcm

172751236.2

0.8

69385.1

17270.0

51269385.1

331

3

3

15

LL

L

L

c

9.0d

14

BFBT

BT

hTATB

Ac

·31.0··

·56.0 5.1

3

The additional resistance due to presence of the bulbous bow near the surface is determined from

( )

(

)

Where the coefficient PB is measure the emergence of the bow and Fni is the Froude number based on

immersion

√ ( )

√ ( √ )

The additional pressure resistance due to immersed transom is determined from

The coefficient c6 is has been related to the Froude number based on the transom immersion

( ) When or

When

√ ( )

CWP, Water plane area coefficient.

The model-ship correlation resistance is determined from

CA, Correlation allowance coefficient

3.3 Graphic User Interface for Holtrop Mennen method

Due to the complex nature of the formulas and variable dependencies in the calculation of resistance, a

simple GUI is created which require limited number of inputs making the process user friendly. Once all

the required inputs are given, the resistance characteristics are calculated and visualized in an output

window.

15

Fig 3.2. Input GUI for Holtrop Mennen’s method.

Fig 3.3. Output window for Holtrop Mennen’s method.

3.4 Results

Resistance prediction for the 90 T AHTS vessel was done by Holtrop Mennen method to get the

following results.

Draft (T) = 5.2 m

Case: 90 T AHTS without Nozzle

16

Velocity

(m/s)

Froude

Number

Towing Tank

(KN)

Holtrop

Prediction % Error

3.91 0.156 45.4 59.00 29.96

4.17 0.167 61.7 73.46 19.06

4.48 0.179 74.7 88.51 18.49

4.73 0.189 92.4 105.18 13.83

4.99 0.199 108.1 118.33 9.46

5.25 0.210 120.6 133.55 10.74

5.50 0.220 136.3 151.46 11.12

5.81 0.232 159.9 177.54 11.03

6.02 0.240 193.8 198.39 2.37

6.28 0.251 228.4 229.20 0.35

6.58 0.264 267 274.54 2.82

6.84 0.274 323.2 320.61 -0.80

7.05 0.282 397 363.75 -8.38

7.36 0.294 480.4 440.41 -8.32

Table 3.3 Comparison of results from Holtrop Mennen to experimental tests

Fig 3.4 Resistance Comparison EFD Vs Prediction

0

50

100

150

200

250

300

350

400

450

500

3.8 4.3 4.8 5.3 5.8 6.3 6.8 7.3 7.8

EFD PredictionVelocity (m/s)

17

CHAPTER 4

RESITANCE CHARECTERISTICS FROM CFD SIMULATIONS

4.1 Introduction

In the last few decades, CFD technology has been widely applied in the research of ship performance as a

result of rapid development of computer technology and computational fluid dynamics theory and

technology. In comparison with traditional tank towing methods and potential methods, the CFD

method displays an advantage of flow field simulation in the study of ship performance, and has been

accepted as an effective method to deal with study of ship resistance.

4.2 Geometric Modeling

A closed, high quality, three dimensional surface geometry of the vessel is required for ease of import.

The vessel geometry is typically modeled from the main deck to baseline, with the superstructure not

modeled for resistance studies. Superstructure geometry may need to be included for stack gas analysis

or wind resistance and moment studies relating to dynamic positioning predictions. Final CFD model is

created based on the existing model of the vessel used in towing tank tests. The vessel geometry is

modeled from the main deck to baseline.

Fig 4.1. Ship modeled in ANSYS ICEM

A commonly used NACA-020 foil is chosen for the rudder section. MARIN-19A Kort Nozzle is modeled

separately to simulate the flow conditions in the presence of propeller ducts.

18

Fig 4.2. Propeller nozzle modeled in ANSYS ICEM

4.3 Governing Equations and Solvers

The commercial CFD software Star CCM+ is used in the present study. It solves the RANS equations using

a cell-centered finite-volume method. The RANS equations can be written in the following form:

4.4 Free Surface Treatment

In order to obtain accurate resistance predictions, any computational method has to deal with the free

surface around a ship unavoidably. Here Volume of Fluid (VOF) method is applied to deal with water-air

two phase free surface interfaces. The VOF model is a fixed grid technique designed for two or more

immiscible fluids where the position of the interface between the fluids is part of the unknown to be

found through the solution procedure. In the VOF model, the fluids share a single set of momentum

equations, and the volume fraction of each of the fluids in each computational cell is tracked throughout

the domain.

4.5 Geometry Handling and Meshing

Once the geometry modeling is completed, it has to be brought to the CFD platform for conducting

simulation.

4.5.1 Geometry Import and Domain Definition

After the modeling of the ship hull in ANSYS ICEM, the geometry in STereoLithography (.stl) format is

imported to Star CCM+ for simulation. Below dimensions are followed in creating the bounding box for

simulation:

Upstream = 1.5L ; Downstream = 1.5L L – Ship Length

Above = 1L ; Below = 2L

19

Fig 4.3. Bounding domain showing the types of boundaries

Once the domain is defined, the boundary conditions need to be given specifying the following :

Wall – With Slip and No-Slip Conditions: While the surfaces of bounding box which are perpendicular to

the flow are considered are wall with slip condition, the hull of the ship is specified as wall with No-slip

condition.

Velocity Inlet – The face of the bounding domain through which the flow enters is given as inlet.

Pressure outlet – The face of the bounding domain through which the flow exits.

4.5.2 Meshing

The partial differential equations that govern the fluid flow around the ship are not usually amenable to

analytical solutions, except for very simple cases. Therefore, in order to analyze fluid flows, flow

domains are split into smaller subdomains (made up of geometric primitives like hexahedra and

tatrahedra in 3D, and quadrilaterals and triangles in 2D) and discretized governing equations are solved

inside each of these portions of the domain. Typically, one of three methods is used to solve the

approximate version of the system of equations: finite volumes, finite elements, or finite differences.

In star CCM+, following tools are available for starting the surface geometry meshing.

Surface Remesher

Surface wrapper

Hole filler, including non-simple holes

20

Edge zipper Automatic and

Hand based feature curve extraction and editing

Three types of volume meshes are available in this CFD platform,

Tetrahedral - tetrahedral cell shape based core mesh

Polyhedral - arbitrary polyhedral cell shape based core mesh

Trimmed - trimmed hexahedral cell shape based core mesh

For the particular model, the following configurations are used to generate the required mesh. This

includes Custom mesh size for different parts of the hull for a much refined grid generation.

Meshing models:

o Prism Layer Mesher, Surface Wrapper, Surface Remesher, Trimmer.

Mesh Properties :

o Base Size = 0.3m

o Number of Prism Layer = 6

o Surface growth rate = 1.3

Customized mesh sizes

o Hull – Minimum Size = .006m ; Target Size = .018m

o Rudder – Minimum Size = .0024m ; Target Size = .0024m

o Skeg – Minimum Size = .0024m ; Target Size = .0024m

o Deck – Minimum Size = .018m ; Target Size = .03m

o Shaft – Minimum Size = .0024m ; Target Size = .0024m

o Nozzle – Minimum Size = .0015m ; Target Size = .0030m

21

Fig 4.4 Meshed geometry

4.6 Physical Model

Physical Model is defined to create the exact environment as the test conditions required to get the

appropriate results. Following physical models are used in this particular case.

K-epsilon Turbulence Model -The K-epsilon model is one of the most common turbulence

models, although it just doesn't perform well in cases of large adverse pressure gradients. It is a

two equation model that means it includes two extra transport equations to represent the

turbulent properties of the flow.. The first transported variable is turbulent kinetic energy, k. The

second transported variable in this case is the turbulent dissipation, epsilon. It is the variable

that determines the scale of the turbulence, whereas the first variable, k, determines the energy

in the turbulence.

Implicit Unsteady & Three dimensional

Multiphase mixture

Volume of Fluid (VOF) – This is a numerical technique for tracking and locating the free surface

(or fluid-fluid interface). It belongs to the class of Eulerian methods which are characterized by a

mesh that is either stationary or is moving in a certain prescribed manner to accommodate the

evolving shape of the interface.

22

Two layer all Y+ treatment - The Two-Layer All y+ Wall Treatment is a hybrid approach that

seeks to recover the behaviors of the other two wall treatments in the limit of very fine or very

coarse meshes. It contains a wall boundary condition for epsilon, which is consistent with the

two-layer formulation.

Segregated flow - The Segregated Flow model solves the flow equations (one for each

component of velocity, and one for pressure) in a segregated, or uncoupled, manner. The linkage

between the momentum and continuity equations is achieved with a predictor-corrector

approach.

4.7 Results

Various aspects of the fluid flow were analyzed from the CFD simulation of the fluid flow around the

ship. The modeling of virtual towing tank in Star CCM+ made it possible to obtain the forces acting on

the ship hull when the fluid flows past the ship which is the total ship resistance.

Total force variation is monitored over the time plotting the results. Following is the total force variation

at 1.55m/s.

Fig 4.5 Force Vs Time graph for the model

23

Total Resistance values calculated from the CFD Results are compared with that of experimental results

obtained from the towing tank. Change in the values of towing tank (EFD) resistance values and CFD

Simulations results are depicted in the plot below.

Table 4.2 Comparison of CFD results to EFD results

Fig 4.6 Resistance Vs Velocity graph for the model (CFD & EFD)

Velocity

(m/s)

Towing Tank

(Newton)

CFD

(Newton)

% Error

in CFD

0.71 6.6 6.98 5.7

0.81 9.6 10.10 5.2

0.92 12.4 13.69 10.4

1.02 15.7 16.92 7.8

1.12 20.1 21.21 5.5

1.22 25 26.61 6.45

1.27 27.9 29.71 6.5

1.32 32 33.24 3.88

1.38 36.5 36.96 1.26

1.43 41.9 43.01 2.66

1.47 46.8 49.92 6.67

1.53 55 61.35 11.54

1.58 63.5 68.01 7.11

1.64 75.7 80.99 6.99

0

10

20

30

40

50

60

70

80

90

0.7 0.9 1.1 1.3 1.5 1.7

Res

ista

nce

(N

)

Velocity (m/s)

Model Resistance Comparison - With Nozzle

EFD

CFD

24

Steady state visualizations at 1.55m/s :

Fig 4.7 Free surface visualization - I Fig 4.8 Free surface visualization - II

Fig 4.9 Velocity Variation (TOP) Fig 4.10 Pressure Variation

Fig 4.11 Velocity Variation (Profile)

25

CHAPTER 5

ESTIMATION OF RESISTANCE USING SHIPFLOW

5.1 Introduction

From the mathematical point of view, the equations governing the fluid motion around a vessel were

known since early 19th century. Although the equations have practical applications they cannot be

solved analytically without further simplification. In the recent years Navier-Stokes equations have been

solved using numerical algorithms. Following this, many commercial packages were introduced. They

could compute the Total Calm Water Resistance of vessels using the codes developed by the researchers

in the early 1980s. Fully nonlinear packages like SHIPFLOW, SHALLO, RAPID, SWIFT and

FSWAVE/VSAERO were introduced.

To investigate the flow around a model, SHIPFLOW splits up the flow into three regions as shown in the

Figure 5.1.The region of potential flow, which neglects all viscous effects and is associated with the

wave making pattern, the region of boundary layer flow and the region where complete Navier-Stokes

equations are solved.

Fig 5.1 Flow regions in SHIPFLOW (Van Mierlo, 2006)

In Zone 1 the flow is assumed to be potential where viscous effects are zero. It is a decent

approximation considering the fact that there is not much of turbulence here. By assuming non viscous

and irrotational flow the governing equations produced are linear partial differential Laplace equations

based on mass continuity. The nonlinear free surface boundary conditions are linearized and solved by

using an iterative process until satisfactory convergence is reached.

26

In Zone 2 the development of the boundary layer is investigated using momentum integral equations for

the thin viscous layer along the hull. By ignoring cross flow in the boundary layer, which is created due

to a pressure gradient in the vertical direction of the ship hull the results are ordinary differential

equations that are solved by Range-Kutta techniques. This prediction cannot be used at the stern of the

ship where a thick viscous region occurs due to convergence of the streamlines.

Towards the stern of the vessel, Reynolds averaged Navier Stokes (RANS) equations along with mass

continuity equations describe the flow in the Zone 3. The solution to the complex Navier Stokes

equation requires a lot of computational time and is therefore restricted to the stern of the vessel only,

where a dense panel is created. The unsteadiness of the turbulence region is averaged out and

instantaneous values of pressure and velocity are separated into a mean with fluctuations by the

introduction of Reynolds stresses.

The software uses Zonal approach to solve the complex problems in short time. The programming is

split into six modules and each module is taken at a time. The method is unidirectional, in other words

the results of the last module do not affect, for example the second module. These six modules along

with their capabilities are listed below.

5.2 Modules and Applications.

XFLOW - The module defines the general physical attributes of the vessel surroundings, for

example the fluid characteristics, initial ship position, ship speed etc.

XMESH - This is the panel generator for the potential flow module XPAN. XMESH can be

executed as a separate program to check the panelisation of the body and free surface .The

module is also executed when sinkage/trim or nonlinear iterations are performed and the

panelisation is updated in each iteration. XMESH also generates panels used for a sink disk

representation of the propeller in the potential flow. Off body points can also be generated using

this module. The points are used in the potential flow method when the results are to be

displayed at points in the flow field outside the hull surface.

XPAN- XPAN computes the potential flow around the model (Zone 1) and free surface, which are

made up of quadrilateral panels. XPAN can operate under linear or nonlinear free surface

boundary conditions. Results obtained from the XPAN are displayed by the post processor and

listed in output files. The capabilities of XPAN include wave resistance, wave profile, lift, induced

resistance, sinkage and trim and pressure distribution.

27

XBOUND-This is a module for thin turbulent boundary layer computations. The momentum

integral equations for boundary layers are solved along streamlines traced from a potential flow

computation. XBOUND is also capable of computing laminar boundary layer and the transition to

the turbulent layer for simpler cases with a well-defined stagnation point. XBOUND can compute

the following- Boundary layer thickness, Momentum thickness, shape factor, skin friction

coefficient, transition between laminar and turbulent flow, limiting streamlines.

XGRID-Similar to XMESH, this generates the grid used for viscous computations in Zone 3 (by

using XVISC and XCHAP) where the Navier –Stokes equations describe the fluid flow. It

generates grid around any ship part with the exception of appendages and bulbous bows. The

grid is done quite well in the stern region where it is needed the most.

XVISC-XVISC is a finite difference code which uses the standard two equation turbulence model

( ) to solve the Reynolds averaged Navier Stokes equation. XVISC provides the viscous

pressure resistance coefficient (CVP) and therefore the total resistance CT can be estimated. XVISC

can also be used to investigate the wake and to estimate axial, radial and tangential velocities at

various planes towards the stern.

XCHAP-This uses a finite volume code to solve the Reynolds Averaged Navier Stokes Equation. It

computes the velocity field, pressure, turbulence kinetic energy, local skin friction coefficient,

frictional and pressure resistance coefficients for the hull part covered by the grid.

5.3 Governing equations and Methodology

This section gives a brief introduction about the Potential flow. Turbulence models to solve RANSE are

also discussed.

The Potential flow module XPAN and the viscous flow module XVISC have been used in the present

work. XPAN computes wave making resistance alone. The XVISC module gives the viscous pressure

resistance coefficient (CVP).It uses the XPAN results of wave making resistance to obtain the Total Calm

Water Resistance coefficient.

5.3.1: Potential flow method:

Potential flow is inviscid, incompressible, irrotational and steady. These conditions are imparted on

Navier stokes equation (5.1) to obtain the relation (5.2)

(

) (5.1)

28

(5.2)

The continuity equation remains the same.

(5.3)

The velocity vector can thus be written as a gradient of a scalar.

(5.4)

This is substituted in the Bernoulli and continuity equations.

( ) (5.5)

(5.6)

The boundary conditions form the next part of the solution system. The assumptions to get the

boundary conditions are as follows.

Flow is irrotational & inviscid (potential flow).

Pressure on free surface is uniform and constant.

Seabed is horizontal, fixed and impermeable.

Wave is two dimensional and its form is invariant in time and space.

.

Fig 5. 2 Wave profile

The bottom boundary condition is given by the below relation

(5.7)

x

z

H η

d 𝑣

29

Free surface boundary conditions can be expressed as follows.

( ) ( ) (5.8)

Kinematic free surface boundary condition states that the fluid particle continues to be on the free

surface at all times. In other words fluid moves only tangentially on the free surface.

(5.9)

The expressions relating to the boundary condition are listed below.

(5.10)

(5.11)

(5.12)

or,

(5.13)

For a two dimensional wave,

(5.14)

(5.15)

The dynamic free surface Boundary Condition states that the pressure in water at the free surface is

equal to the atmospheric pressure.

( ) (5.16)

(5.17)

The above equation is Dynamic Free surface boundary condition. Upon linearization and using the

kinematic free surface boundary condition, we have (3.18) as the combined equation.

(5.18)

30

5.3.2: XVISC module:

XVISC is a finite difference code which uses the standard two equation turbulence model (k-ε) to solve

the Reynolds Averaged Navier-Stokes Equations. The RANS Equations and the turbulence model of

solving are described below.

Continuity Equation :

=0 (5.19)

Momentum Equation :

(

)

(5.20)

The RANSE require turbulence model that couples Reynolds stresses to the average velocities. All

turbulence models used for ship flows are semi empirical. They use some theories about the physics of

turbulence and the missing terms are supplied as empirical constants. SHIPFLOW uses the k-ε model for

computations. k is the kinetic energy of turbulence and ε is the dissipation rate of k. This model

expresses the eddy viscosity as a simple function of k and ε.The k-ε model appears suitable for flows

with a predominant boundary-layer character. Problems with defining a reference length, as in many

algebraic models, are avoided and at least the important physical aspect of turbulence transport is

explicitly reflected in the model. (Practical Ship Hydrodynamics, Bertram).

k and ε are expressed as follows.

,.

/

- (5.21)

,.

/

- (5.22)

5.3.3 Methodology

The potential cannot be solved directly. So it’s linearized by splitting into base flow and perturbation

flow. The linear free surface potential flow computation starts with the computation of the base flow.

The slow ship approximation is used which means no free surface waves are present. To determine the

perturbation both hull and free surface are meshed. The problem with the linear free surface potential

flow is that it doesn’t take into account the hull surface above the still water line.

31

The nonlinear case is an extension to the linear case. It uses the result from the linear case. The hull and

the free surface panels are moved and perturbation is calculated again. The steps are repeated till

convergence is reached.

The first advantage of the nonlinear case is that it gives a solution of system of equations and is no

longer an approximation. The second advantage of the system is that when the panels are moved they

are adjusted to fit the new intersection between the hull and the free surface. This way the shape of the

hull above the free surface is taken into consideration.

Coefficient of wave making resistance is estimated using following techniques.

Pressure Integration is one of the two methods It determines the wave making resistance by

integrating the pressure on the hull panels. The pressure on the hull is hydrostatic and

hydrodynamic pressure. For the linear case only the latter is left as the hydrostatic part sums up

to zero. For the nonlinear case both the pressures are taken into account .The magnitude of

hydrostatic pressure is quite large and this can create some problems in the accuracy of the

results. This can be rectified by using sufficient number of panels.

The Wave cut Analysis is the second method. It determines the wave resistance by analyzing the

wave pattern. Longitudinal or transverse wave cuts can be used but the transverse wave cut is

used because it puts less demand on the size of the free surface. This method basically

determines wave elevation in a number of transverse wave cuts behind the ship.

5.4 Results and Discussions.

The total calm water resistance (RT) can be obtained by estimating the coefficient of wave making

resistance using SHIPFLOW and then by using the following formula.

CT = (1+k)*CF+CW

CF - Frictional Resistance Coefficient

CW - Wave Resistance Coefficient

1+k – Form Factor

CT – Total Resistance Coefficient

CF is calculated using the ITTC Formula,

32

( )

Rn – Reynold’s Number.

Cw values are obtained from the XPAN module iterations of the Shipflow simulation

5.4.1 Resistance Estimation of 90T AHTS

V (m/s) Cw

3.65 0.00223

3.91 0.00268

4.17 0.00367

4.48 0.00413

4.73 0.00463

4.99 0.00504

5.25 0.00529

5.50 0.00541

5.81 0.00556

6.02 0.00607

6.28 0.00693

6.58 0.00736

6.84 0.00860

7.05 0.00933

7.36 0.00990

Table 5.1 Wave Resistance coefficient results (SHIPFLOW)

The trends of both experimental and numerical results are matching quite accurately. However, the

SHIPFLOW results are seen to be much higher than the experimental values. Similar observations were

made by M. Salas et al in their paper titled ‘Experimental and CFD Resistance Calculation of a Small Fast

Catamaran’.

Following graph captures the deviation of Cw values from the ones obtained from the towing tank

experiments.

33

Fig 5.3 Coefficient of Wave Resistance value comparison (EFD Vs. Shipflow)

Fig 5.4 Total resistance variation of 90 T AHTS (SHIPFLOW)

0.00000

0.00200

0.00400

0.00600

0.00800

0.01000

0.01200

3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00

Cw - EFD

Cw - Shipflow

Velocity (m/s)

0

50

100

150

200

250

300

350

400

3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00

Resistance (Rt) in KN

Velocity (m/s)

34

Fig 5.5 Panel Generation for 90T AHTS in SHIPFLOW

Fig 5.6 Total Pressure coefficient for the hull at 8.1 knots

35

5.4.2 Wave Cut Analysis Visualization

Fig 5.7 Wave cut profile at Centerline and y/Lpp = 0.133 for V = 7.1 knots


36




37


From Figure 5.7 to 5.12 it can be observed that at low Froude numbers the wave profile is

irregular and is associated with many crests and troughs.

As the Froude number increases, the wave heights are higher as it can be seen in figures.

The effect of wave interference is prominent in this region and hence we have higher

amplitudes.

Further observation shows that wave trough created by aft hull constantly shifts further away as the

Froude number increases. The shift of the wave trough can also be observed when we compare a

mono hull and catamaran at same Froude number.

38

CHAPTER 6

CONCLUSIONS

The resistance values of the AHTS vessel were estimated successfully using nonlinear potential

flow CFD code, SHIPFLOW and were compared with available experimental values.

o The nonlinear free surface potential flow has problems to predict the same trend as the

towing tank results for the low Froude numbers. For the higher Froude numbers the

prediction improves but still shows differences compared to the towing tank results.

o Part of the difference can be explained by the fact that the residuary resistance

determined in the towing tank is very sensitive to measurement errors at low Froude

numbers but most of the difference is caused by the modeling error in the nonlinear free

surface potential flow.

o The towing tank results contain both wave resistance and viscous pressure resistance.

The viscous pressure resistance and the influence of the viscous effects on the trim are

not accounted for in the nonlinear potential flow. The influence of these effects on the

residuary resistance at low speeds is significant but decreases at the higher speeds.

Holtrop-Mennen method was used to predict the resistance values of the AHTS vessel. Though

Holtrop and Mennen’s method is supposed to give accurate results to that of towing tank, the

errors that exist in the final result can be accounted for by taking into consideration the

following parameters.

o Increasing in Froude number which will create a greater residuary resistance (wave

making resistance, eddy resistance, breaking waves and shoulder wave) is a common

phenomenon in small ships. As a result, errors in total resistance increase.

o Small vessels are easily influenced by environmental condition such as wind and current

during operational.

o The Holtrop method uses data for which there may be no standard quantifiable

definition of measurement, such as half-angle of entrance and stern coefficient.

o This method is also limited to hull form resembling the average ship described by the

main dimension and form coefficients used in the method.

39

o The methodology uses wetted surface to describe the size of the vessel. The true dynamic

wetted surface would be the most precise approach (as is used in the planning analysis,

for example), but the measurement of the wetted surface on a moving model is not easy,

so the at-rest wetted surface is typically used as the datum value. This can lead to a

somewhat incorrect contribution of the various applied resistance components and in

turn to inaccurate extrapolation of the model results to full scale, particularly at higher

speeds.

CFD simulations were also run using Star CCM+ to get the resistance characteristics of the 90 T

AHTS vessel. The results seem to be fairly in agreement with the towing tank test results

obtained.

Analyzing the results of all the above mentioned methods, we can conclude that, each method of

finding resistance is suited for different situations. For example, when we don’t have the

complete geometry details, we can get an estimate of resistance with prediction methods and if

we do have the offset files of many ships which are to be analyzed, SHIPFLOW will be a good

option in that case for accurate results.

40

REFERENCES

1. Bertram,V. Practical Ship Hydrodynamics, Butterworth-Heinemann Linacre House,

Jordan Hill, Oxford,2000.

2. Bhattacharyya,R. Dynamics of Marine Vehicles, A Wiley-InterScience Publication, New

York, 1978

3. Robert.D.Moody, “Preliminary Power prediction during early design stages of a ship”.

4. Doctors, L.J. and Day, A.H. (1997). Resistance prediction for transom stern vessels. Fifth

Intl.Conference on Fast Sea Transportatio, FAST ’97.

5. Holtrop.J, Mennen.G.G.J,”An Approximate Power Prediction method”, International

Shipbuilding progress, vol.29, July 1982.

6. Lewis, E.V. “Principles of Naval Architecture”, Vol II, SNAME Publication, NewYork

(1988).

7. Perret, C. (2005). Design Optimization of a 50’ sailing Catamaran. Department of

Shipping & Marine Technology,Chamlers University of Technology,Chamlers

8. Pham,X.P., Kantimahanthi,K. and Sahoo,P.K.(2001). Wave Resistance Prediction of

Hard Chine Catamarans using Regression Analysis.Intl.Symposium on Ship Propulsion, St

Petersburg,Russia,

9. Sahoo, P.K. and Doctors, L.J.(2004) Theoretical and experimental study of motion

characteristics of high-speed catamaran hull forms. In Proc. Ninth Symposium on

Practical Design of Ships and Other Floating Structures, Schiffbautechnische Gesellschaft,

Lubeck-Travem unde, Germany, September, pp. 665–671.

10. Sahoo, P.K., Browne, N.A. and Salas, M. (2003). Experimental and CFD Study of Wave

Resistance of High-Speed Round Bilge Catamaran Hull Forms. Proceedings of Fast Sea

Transportation 99, pp 803-814, Seattle, USA.

11. Users Manual, SHIPFLOWR; Flowtech International, Edition 1, December 2003.

12. Van Mierlo, K.J.(2006). Trend Validation of SHIPFLOW based on the bare hull upright

resistance of the Delft Series. Faculty of Aerospace Engineering, Delft University of

Technology.

41

APPENDIX

SHIPFLOW code for the estimation of total calm water resistance.

Velocity = 13.3 Knots

xflow

titl ( titl="AHTS" )

prog ( xmesh, xpan, xbound )

hull ( mono, h1gr="main", ogrp="stern", fbgr="bulb",

fsflow,coarse)

offs ( file="offset", xaxdir=-1.0, ysign=1.0,

xori=63.6, zori=5.2, lpp=59.2 )

ipos( trim = 0 )

vship ( vknot = [13.3],number =1, reflen=63.6)

end

xmesh

body ( grno = 1, station = 61, point = 16,

str2 = 5, df2 = 0.005, dl2 = 0.0075 )

free ( grno = 4, xdow = 2, y4side =-0.6,

point = 16, str1 = 1, df1 = 0.02,

stau = 31, stru = 1, dlu = 0.012,

stam = 86,

stad = 41, strd = 1, dfd = 0.012 )

end

xpan

cont ( free, nonlin)

para ( nthr = 4 )

end

42


xflow




fsflow,coarse)


xori=63.6, zori=5.2, lpp=59.2 )

ipos( trim = 0 )


end

xmesh


str2 = 5, df2 = 0.005, dl2 = 0.0075 )

free ( grno = 4, xdow = 1.6, y4side =-0.6,

point = 16, str1 = 1, df1 = 0.02,

stau = 31, stru = 1, dlu = 0.012,

stam = 86,

stad = 41, strd = 1, dfd = 0.012 )

end

xpan


para ( nthr = 4 )

end

43


xflow




fsflow,coarse)


xori=63.6, zori=5.2, lpp=59.2 )

ipos( trim = 0 )


end

xmesh


str2 = 5, df2 = 0.005, dl2 = 0.0075 )

free ( grno = 4, xdow = 1.3, y4side =-0.6,

point = 16, str1 = 1, df1 = 0.02,

stau = 31, stru = 1, dlu = 0.012,

stam = 86,

stad = 41, strd = 1, dfd = 0.012 )

end

xpan


para ( nthr = 4 )

end

Resistance estimation of 90T AHTS Vessel using resistance prediction method and CFD methods

Documents

Transcript of Resistance estimation of 90T AHTS Vessel using resistance prediction method and CFD methods