Bundeswehr Technical Center for Ships and Naval Weapons,

Maritime Technology and Research WTD 71

Turbulent flow noise generation under sea conditions

J.Abshagen, D.Küter, V.Nejedl

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• Noise sources of hydroacoustic sensors

• Underwater towed body measurements

• Turbulent wall pressure fluctuations

• Transfer behavior of plates excited by TBL

• Wavenumber-frequency analysis

Overview

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U

cavity

T

R

hull vibrations

structure-borne sound

wall pressure fluctuations

flow-induced noiseT

signal

ambient noise

signal/noise @sensor

3

Hydroacoustic sensors

1@),(

≥sensorambientflownoise

signal

Elboth, Lija, Reif, Andreassen

Geophysics 75(1) 2010

Example: Towed array

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Length : 5260 mm

Width (T.body) : 935 mm

Height (T.body) : 1353 mm

Width (total) : 1802 mm

Height (total) : 1715 mm

Weight (in air) : 2.8 t

Mass (in water): 3.5 t

Bouyancy: approx. 0.3 t

4

Towed body measurement system

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Towing experiments (Sognefjord, Norway)

FS Elisabeth Mann Borgese (IOW)

Towing vessel with towed body

Sound speed profiles

CTD probe

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course plot

Towing experiments (Sognefjord, Norway)

Towing depth: 90 - 180 m

Towing cable: 400 – 600 m

Towing speed: 4 - 12 kn

(approx. 2 – 7 m/s)

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Flat plate : fibre-reinforced plastic (2000 mm x 550 mm x 2 mm)

Baffle: steel (3 mm), damping material (10 mm)

Window: 575 mm x 300 mm

Hydrophones: TC4050 (fmh), 16x TC4013 (hyd.1-16, 11.5 mm)

U

184

1 . . . 8 9 . . . 16

3+102.0

64.521.5 43

5

fmh

hydrophone array

575

Measurement setup

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• Drift buoy

• Projector @100m depth

• SL = 180 dB(re1µPa@1m)

Calibration (TC4050)

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Configurations

Hydrophones embedded

Thick plate (frp+pu)

Thin plate (frp)

70

PU

420

21

2.03+10

baffle

frp

Abshagen, Nejedl, J.Acoust.Soc.Am. 135 (2014)

2.03+10

baffle

foam12.75frp

PU

Abshagen, Küter, Nejedl, Adv. Aircr. Spacecr. Sci., 3 (3) (2016), INTER-NOISE 2016, (2016)

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260 Hz

510 Hz

2110 Hz

Reson TC4050 (FEM)

Wall pressure fluctuations

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Wall pressure fluctuations

Potential flow velocity: ∞U

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)(),(),(ˆ fkfSkfS Φ=

Steel plate

Spatial coherence (d=21.5 mm)

Wall pressure fluctuations

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Interior Noise

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Interior Noise

Spatial coherence Identification of

underwater sound

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U

184

1 . . . 8 9 . . . 16

3+102.0

64.521.5 43

5

fmh

hydrophone array

575

Interior Noise

Spatial coherence

Φinc = {1 - γ2 (Hyd1, Hyd9) } Φ

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Reduction index : R = 10log10(1/τ)

τ = 1 / |1 + (Z´ ω/ (ρ0c) )2|

Z´frp = 6.2x104

Z´frp+pu = 4.3x104

Z´emb =1.6x104

Transmission loss

Interior Noise

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Wavenumber-frequency analysis

Towing speed U = 4.7 m/s

Towing speed U = 6.8 m/s

Underwater sound Flow-induced noise

non-acoustic

non-acoustic

Example (emb.):

• E=20 x109 Pa, µ =0.25,ρ = 1200 kg/m3

• Fluid loading: fair / 2.8

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Example (frp):

• E=10 x109 Pa, µ=0.25, ρ = 2000 kg/m3

• Fluid loading: fair / 2.8 Ch.Henke, Flow noise in sonar application, xxx.lanl.gov (2016)

Steel plate

Steel plate

with damping

Wavenumber-frequency analysis

Qualitative Comparison Experiment - Numerics

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Wavenumber-frequency analysis

Comparison: Experimental configurations

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Wavenumber-frequency analysis

Comparison: Non-acoustic wavenumber regime

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• Flow acoustic measurements under sea conditions

• Identification and separation of noise sources

• Transfer behaviour of plate excited by TBL

• Modal response to TBL excitation

Summary