Simulation of Internal Wave Wakes and Comparison with Observations J.K.E. Tunaley London Research...

Simulation of Internal Wave Wakes and Comparison with Observations

J.K.E. TunaleyLondon Research and Development Corporation,

114 Margaret Anne Drive, Ottawa, Ontario K0A 1L0, Tel: 1-613-839-7943

http://www.London-Research-and-Development.com/

Outline

• Objectives• Modelling• Loch Linnhe Trials• Hull Designs• Simulations• Discussion

Objectives

• Towards an evaluation of use of internal wave wakes in wide area maritime surveillance

• Towards understanding their generation from surface ships– Start with simplest scenario– Surface ship with stationary wake (in ship frame)

• The effect of hull form on the wake

Georgia Strait: ERS1

Modelling

• Layer models– Discrete (e.g. loch, fjord)– Diffuse

• Internal wave wake model– Linearized– Far wake

Loch Linnhe Trials• Trials from 1989 to 1994 in

Scotland• Ship displacements from 100 to

30,000 tonnes• Shallow layer• Ship speeds typically 2 to 4 m/s• Wake angles 10 to 20º• Airborne synthetic aperture radars 20

18

16

14

12

10

8

6

4

2

0

0 0.05 0.1 0.15

N (rad/s)

Dep

th (m

)From Watson et al, 1992

Wigley Hull• Canoe shaped: Parabolic in 2-D, constant draft• Useful theoretical model but block coefficient is 4/9

Wigley Offsets

Practical Hulls

• Taylor Standard Series– Twin screw cruiser

• David Taylor Model Basin Series 60– Single screw merchant

• National Physical Laboratory– Round bilge, high speed displacement hulls

• Maritime Administration (MARAD) Series– Single screw merchant, shallow water

• British Ship Research Association Series– Single screw merchant

DTMB Offsets CB = 0.60

Taylor OffsetsStern Bow

Sir Tristram, 2m/s

From Watson, Chapman and Apel, 1992

Sir Tristram Parameters

Ship Length, L (m) 136

Ship Beam, B (m) 17

Ship Draft, T (m) 3.9

Estimated Block Coefficient, CB 0.59

Ship Speed, U (m/s) 2.0

Layer Depth, h (m) 3.0

Layer Strength, δ 0.004

Pixel size (m2) 4x4

Simulated Wake

Observed Surface Velocity

From Watson et al, 1992

Simulated Surface Velocity

Wigley: h=5.0 m, δ=0.0024 Wigley: h=3.0 m, δ=0.004)

Simulated Surface Velocity

Taylor CB=0.48 DTMB CB=0.6

Taylor CB=0.7 DTMB CB=0.8

Effect of Hull Model

• In this application:– Minor changes to velocity profile as a function of

hull model– Minor changes to velocity profile as a function of

CB

– Shifts shoulder downwards in plots as CB increases

Olmeda (cf Stapleton, 1997)

Length = 180 mBeam = 26 mDraft = 9.2 m

Speed = 2.2 m/sWake Angle 18º

Layer: h = 3 m, δ = 0.004

Taylor CB=0.7

Conclusions• Simulations are reasonably consistent with

observations• Sir Tristram observed maximum water velocity

at sensor is about 3 cm/s; same as simulations• Olmeda observed maximum velocity at sensor

is about 5 cm/s; same as simulations• Wake determined mainly by block coefficient• Structure in first cycle appears to be similar in

observations and simulations