THE EXPERIMENTAL METHOD AND CFD STUDY OFTHE BARE HULL FORM OF UNDERWATER VEHICLE
Thu Han Tun
Professor/Head, Marine Mechanical Department, Myanmar Maritime University
Thanlyin, Yangon Region, The Republic of the Union of Myanmar, e-mail: dr.thuhantun@gmail.com

Abstract
Hull form selection, resistance and powering are important in designing underwater vehicle. Anunderwater vehicle bare hull form is based on the five parameters due to the interaction
between the propeller and the hull. When they are running on the surface condition, there will
be problems likely as surface vessel, but the main hull of the underwater vehicle is below thewaterline with low freeboard. The underwater vehicles are operating with high speed at a highFroude Number. Therefore, the wave making component becomes important in surface
resistance. The flow around the ship’s hull is complicated, so that model experiments are still
the most reliable data source on ship resistance determination. The bare hull form of underwater
vehicle resistance is based on the model experiments and CFD results. The towing mechanism
arrangement should be considered at model. Therefore, t owing mechanism is designed formodel testing. This paper discusses the towing method and result between model test and CFD.
This paper also makes comparison of wave formation Towing tank test and CFD at variousspeeds. The model was tested with bare hull form of underwater vehicle in the Ship ModelTowing Tank at Marine Hydrodynamics Centre, Myanmar Maritime University. CFD analysis
is also carried out and the results are compared for surface condition. The software packages
FINEMarine and Solidworks are used for CFD simulations. The comparison of resultsshows that the coefficient differences are less staggered based on the speed.
Keywords: Underwater Vehicle, Model Experiment, Resistance, CFD, Towing Tank

NOMENCLATURE
LOA
LWL
LBLC
LCS𝛾
DHT
Δ
VRt
CT
CFCRReρS

Length overall (m)Length waterline (m)Bow length (m)Cylinder length (m)Conical Stern length (m)Conical Stern angle (degree)
Hull Diameter (m)Draft (m)Displacement (tons)
Speed of vessel (m/s)
Total resistance (N)Total resistance coefficient
Frictional resistance coefficient
Residual resistance coefficient
Reynolds number
Density of water (ton/m3)
Wetted surface of ship (m2)Introduction
Hull form selection, resistance and powering are important in designing underwater
vehicle. Model testing is carried out traditionally to predict the resistance and power at
the near surface condition. Model is bare hull form without any appendages.
Appendages resistance also should be considered for prediction of the resistance.
Nowaday CFD tools are widely used for prediction of ship’s resistance andpower. However the flow around the ship’s hull is complicated, so that modelexperiments are still the most reliable data source on ship resistance determination. The
model experiments will be carried out in the Ship Model Towing Tank at Marine
Hydrodynamics Centre, Myanmar Maritime University[1]. Towing tank size established2011 made in UK (CUSSON Technology) is 60m in length, 4m in breadth and 4m in
depth. Maximum carriage speed is 4 m/s. CFD codes are also used in design step,
validation of the results is carried out by comparing the model test results.

Objective
This paper focuses on towing mechanism which arranged for model testing with barehull form, comparing the total resistance of underwater vehicle model and to makeverification for model tests results with CFD results.

Study area
Underwater vehicles are designed to work under the water but they have to work at thewater surface for many reasons. So the resistance of the underwater vehicle operating at
the water surface is one of the major problems in designing that kind of vehicles. Their
resistance behavior at the water surface is similar to those of normal surface ships, but
underwater vehicles have low freeboard with majority of the displacement is below thewater surface. The operating speed of the underwater vessels is high. They are also
operating at high Froude Number, so the wave making component becomes dominant insurface resistance [2].
By towing scale models, the resistance can be measured and using appropriatescaling laws, the resistance of the full scale vessel can be predicted.
Model of 1.5 m in length is fabricated with wood and will be tested in modelbasin at Myanmar Maritime University. Total resistance of the model will be calculated
from the towing test results by using Froude’s Law of Comparison. In order to calculate
the underwater vehicle model total resistance non-dimensional coefficients will be used[3]. The residual resistance is found by the pressure distribution about the hull becauseof the waves and eddies generated by the vessel's motion. Therefore, the wave making
resistance of the underwater vehicle model at surface condition will be analysed byusing CFD tools. Friction component of resistance will be calculated by using ITTC’57
correlation line and coefficient of total resistance is counted by as below.

CT = CF + CR

𝐶 =(1).
()(2)

In general, the resistance of a vessel to motion through water can be calculated
in the following formulation:
𝑅 = 𝐶 𝜌𝑆𝑉

(3)

Methodology of Study
Determining the Main Dimensions
An underwater vehicle bare hull form is based on the following parameters due to theinteraction between the propeller and the hull –The fineness ratioPrismatic coefficientNose radius

Tail angle and

The position of the maximum section

Dimensions of underwater vehicle model in this paper are shown in Table 1with parallel middle body form. Relation L/D is equal to 8.3 because of limitation of
testing arrangement and model making by hand. Main hull is cylindrical shape and sternis conical as shown in Figure 1.

Figure 1 Model of underwater vehicle

Table 1: Main Particulars of Model at Design Condition
Main Particulars

Unit

Overall length, LOA (m)1.500

Hull Vertical Diameter, DVH (m)0.210

Hull Horizontal Diameter, DHH (m)0.180

Displacement, Δ (tons)

0.24

Bow length, LB (m)0.333

Cylinder length, LC (m)0.590

Conical Stern length, LCS (m)0.576

Draft, T (m)0.172

Conical Stern angle, γ (degree)20.2

Model Making and Towing Mechanism
Fabricated wooden model without appendages are shown in Figures 2 below.

Figure 2 Fabricated wooden model

When model without appendages is testing in towing tank at near surface
condition, even keel condition cannot be controlled. Therefore, the towing mechanism
arrangement should be considered at model as shown in Figure 3. Elements of towing
mechanism are listed in detail as the following Table 2 –Table 2: List of Towing Mechanism
SymbolItems

Load cell(sometimes called astrain gauge)

Function

To measure the downforce or lift force

Guide arm fit pins

To ensure the straight line position on flow
(2 numbers for bow and stern)

Load cell(sometimes called astrain gauge)

To measure the resistance of model(attached to model)

Vertical slipway or
towing post

To measure the heaving motion and tow themodel by carriage.

Figure 3 Towing mechanism arrangements

Experimental Approach
Two test case studies will be carried out for the speed range of 0.6 to 1.8 m/s. There areresults in Table 3 below for near surface condition.
Table 3: Model Resistance Test
Total Resistance of Model
Speed of Vessel V (m/s)
Rt(N)
0.6
0.800
0.8
2.187
1.0
4.522
1.2
7.937
1.4
7.457
1.6
12.4971.8
18.015

Down Force (N)
0.720
2.764
6.129
10.796
10.4468.794
7.363

Numerical Approach
The complexity of the flow around ship’s hull, model experiments are still the mostreliable data source on ship resistance determination; nevertheless, numerical methods
have strongly advanced in this field, so that a combined use of both model tests andCFD codes can be very useful for ship design and for understanding the shiphydrodynamics [4].
Numerical computations are carried out around vessel using the two software
packages FINEMarine with the turbulence model k-omega (SST-menter) andSolidworks 2020 Flow Simulation with turbulence only. The initial mesh is X axis 24,
Y axis 16, Z axis 12 and total cells 843146 presented in Figure 4(a and b).

Figure 4a Generated mesh of the whole modelFigure 4b Generated mesh of model
As the model test was carried out with fixed condition (locked for pitchingmotion), the CFD simulation was also carried out with fixed model to get the fixed trimand sinkage. CFD simulation is carried out for the model from the speed range of 0.6 to
1.8 m/s. Figure 5 (a to c) shows the comparison of wave formation of model test andCFD simulation at various speeds.

Figure 5(a) Towing tank test and CFD simulation with 0.8m/s

Figure 5(b) Towing tank test and CFD simulation with 1m/s

Figure 5(c) Towing tank test and CFD simulation with 1.8m/s
Figure 5 Comparison of wave formation Towing tank test and CFD at various speeds

Result Comparison
The resistance comparison of underwater vehicle is carried out for the speed of 0.2 to
1.8 m/s. At the Froude’s number of about 0.33, bow wave creates a trough at the sternand increases the stern wave system and also increase the wave making resistance andresistance hump will be result. The results are compared in Figure 6. According to theresults, the resistance behaviour for the two software packages CFD result are differing
at Fn=0.33.

Resistance Results
25

Resistance (N)CFD Fine Marine
Free motion

Resistance (N)20

Resistance (N)CFD SW
Fixed Trim15Resistance (N)CFD Fine Marine
Fixed Trim

105Resistance (N)Towing Test
Fixed Trim

000.2

0.4

0.6

0.8

11.2

1.4

1.6

1.8

2V(m/s)Figure 6 comparison of CFD and Towing test of ship resistance without appendages.

Conclusions
Towing test analysis was carried out for the model with fixed trim and pitch condition.
Then CFD analysis was also carried out with fixed model condition. The model test
results are compared with CFD results and the comparison shows that experimental
results and CFD results have good agreement for near surface condition. At the higher
speed, hydrodynamic forces acting on the vehicle will become larger and so, it’s stillneeded to consider the hydrodynamic effect of the model in freely moving condition
presented in Figure 7. After calculating the hydrodynamic effect of the model,hydroplane should be attached to ensure for fixed trim and pitch condition.

Figure 7 Hydrodynamic effect of the model (down force)

Acknowledgements
The authors would like to express their thanks to the staffs for all their support during
the experimental model tests.

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