IMPROVING ACOUSTIC STEALTH · IMPROVING ACOUSTIC STEALTH Analysis of the vibro-acoustic behavior of a submarine hull on a wide frequency range using experimental and numerical approaches
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POWER AT SEA
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IMPROVING ACOUSTIC STEALTH Analysis of the vibro-acoustic behavior of a submarine hull on a wide frequency range using experimental and numerical approaches
Valentin Meyer1, Laurent Maxit2
1 Nava l Group Research , Ol l iou les , F rance 2 Laborato i re V ibrat ions Acoust ique , IN SA Lyon, F rance
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CONTEXT
Hull acoustic performances Operational capability
Far-field radiated noise Acoustic stealth
Reflection/scattering Target strength
Self radiated noise Sonar performances
Submarine
Flank SONAR arrays
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Interest of studying the vibro-acoustic behavior of a submarine vehicle?
Frigate ASW (active and passive
detection)
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Predict the vibro-acoustic behavior of a submerged shell and understand the physical phenomena,
before its construction
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Development of numerical predictive tools
Experimental procedures
GOALS
Improve the acoustic stealth of the vessels
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• SONAR is able to detect a noise source from a few Hz to dozens of kHz Techniques to predict the vibro-acoustic response for a wide frequency range Simplified model of a cylindrical shell submerged in an infinite fluid medium
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Low frequency range: FEM/BEM
• Structural complexity
• Calculation cost depends on mesh size
High frequency range: SEA • Energy balance between
subsystems • Strong assumptions • Only global results
How to model on a wide frequency range?
CURRENT CHALLENGES IN NUMERICAL MODELING
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CHALLENGES IN MEASUREMENTS
Experiments at sea are: • costly and time-consuming • not ideal to understand the
physical phenomena • only when the submarine is built
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Sketch of the MARS500© hydrophone array
How can the vibro-acoustics of a stiffened shell be measured?
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Contents
1. The CTF method
2. Experimental work
3. Results and Discussion
4. Summary
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1. The CTF method
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A SUB-STRUCTURING APPROACH:
Non-axisymmetric internal structures
Submerged cylindrical shell
Stiffeners and bulkheads (axisymmetric)
*L. Maxit, J.-M. Ginoux, Prediction of the vibro-acoustic behavior of a submerged shell non periodically stiffened by internal frames, JASA 128(1):137-151, 2010.
**V. Meyer, L. Maxit, J.-L. Guyader, T. Leissing, Prediction of the vibroacoustic behavior of a submerged shell with non-axisymmetric internal substructures by a condensed transfer function method, JSV, 360:260-276, 2016.
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PRINCIPLE OF THE CONDENSED TRANSFER FUNCTION METHOD
• Extension of the admittance method for line coupled systems:
Solving the coupling forces between the subsystems:
𝐹𝑐 = 𝑌𝑖𝑗1 + 𝑌𝑖𝑗
2 + 𝑌𝑖𝑗3 −1
𝑈
F U
𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑎𝑑𝑚𝑖𝑡𝑡𝑎𝑛𝑐𝑒 𝑌 = 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑈
𝑓𝑜𝑟𝑐𝑒 𝐹
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• Requires only characteristics from the uncoupled subsystems • The admittances can be calculated by any method
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THE CTF METHOD APPLIED TO STIFFENED SUBMERGER CYLINDRICAL SHELLS
Fluid-loaded cylindrical shell Analytical solution
Stiffeners and bulkheads Finite Elements Method
CTF Method
Calculation of the admittances for each subsystem :
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Radial displacements Far-field radiated pressure
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ORCAA: tool developed at Naval Group for vibro-acoustics predictions Advantages of the hybrid method: • Low computation costs compared to FEM/BEM • Possibility to couple subsystems described by different approaches • No theoretical frequency limit for the CTF method • High versatility compared to analytical methods: different stiffeners spacing, various
internal structures
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THE CTF METHOD AS AN INDUSTRIAL TOOL
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2. Experimental work
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DESCRIPTION OF THE SUBSYSTEMS
Stiffened cylinder in steel Length: 1,5 m Radius: 100 mm Thickness: 1,5 mm Two end caps 3 different stiffeners spacing divided in 5 sections
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EXPERIMENTAL SETUP
• In air
• Semi-anechoic room
Rigid floor
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DEFINITION OF THE SCANNING GRID
• The maximum distance between two consecutive measurement to capture the physics is 15 mm
• It results in 101 points lengthwise • Measurement every 9° on half the cylindrical shell
(assumption of symmetrical system) • Microphone array to measure the pressure
around the cylindrical shell
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3. Results and discussion
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MAPS OF RADIAL VELOCITIES
Global mode (m,n)=(4,4) Local mode
Experimental Work
1980 Hz 3580 Hz
CTF method
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Global mode (m,n)=(4,4) Local mode
Experimental Work
1980 Hz 3580 Hz
CTF method
THE STATIONARY PHASE THEOREM TO CALCULATE THE RADIATED PRESSURE
𝑝 𝑟, 𝜉, 𝜃 = 2𝑗 𝜌0 𝜔
2
𝑟 𝑘0 𝑐𝑜𝑠 𝜉
𝑊 (−𝑘0 𝑠𝑖𝑛 𝜉, 𝑛)
𝐻𝑛2 ′(𝑅 𝑘0𝑐𝑜𝑠𝜉)
𝑒−𝑗𝑟𝑘0+𝑗𝑛(𝜃+𝜋2)
+∞
𝑛=−∞
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ACOUSTIC POWER RADIATED FROM THE SHELL
Power estimated by 3 means : • full experimental: summation over the microphone array • hybrid: experimental vibrations + stationnary phase theorem + integral over an
enclosing sphere • full numerical: CTF method + stat. phase th. + integral over an enclosing sphere
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4. Summary
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Summary
• A numerical method and an experimental procedure have been presented to study the response of a stiffened cylindrical shell
• The vibrations and radiated pressure of a scale model have been measured and calculated and some physical phenomena have been discussed
• Experimental validation of the numerical method
Perspectives:
• Optimization of the submarine and test new designs to improve acoustic stealth of submarines
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Thank you for your attention
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