NOISE & VIBRATIONS Frans Hendrik Lafeber
NOISE & VIBRATIONS, URN Noise & vibrations are important factors when designing a ship: Structural Workability Regulations (hearing loss) Comfort! Underwater radiated noise (URN) as well: Research vessels / navy: self-noise and detection Effect on sea life: fish and marine mammals Future regulations to limit URN (EU, IMO, ) Image from: http://kilaueapoint.org/ 2
NOISE SOURCES Air-borne Structure-borne Water-borne exhaust HVAC auxiliary equipment flow noise propellers drive shaft main engine thrusters Image from: http://www.new-yacht-for-sale.com/ 3
NOISE SOURCES MARIN: hydrodynamic sources Propellers (cavitation!) R&D Thrusters (TT Joint Industry Project) Flow noise exhaust HVAC auxiliary equipment flow noise propellers drive shaft main engine thrusters Image from: http://www.new-yacht-for-sale.com/ 4
CAVITATION Pressure on the propeller becomes lower than the vapour pressure Volume accelerations of the cavity cause pressure fluctuations Effective at exciting the hull Broadband noise URN Resonance of ship s construction 5
EXPERIMENTAL APPROACH Depressurized Wave Basin (240 m x 18 m x 8 m) Minimum air pressure 30 mbar Cavitation tests with complete ship model Special purpose silent towing carriage 6
HULL-PRESSURE PULSE MEASUREMENTS Amplitude spectrum [Pa] Pressure [Pa] Measure the pressure on the hull: 21 pressure sensors Measure vibrations of the hull: Propeller-induced Drive train Determine the pressure amplitudes Correct the pressure amplitudes 140 300 200 100 0-100 -200-300 -400 0 45 90 135 180 225 270 315 360 Blade position [deg] 120 Integrate to obtain excitation forces Estimate vibration nuisance 100 80 60 40 20 0 0 38.7 77.5 116.2 154.9 193.6 232.4 271.1 frequency [Hz] 7
URN: BASIN-MOUNTED HYDROPHONES 8
UNDERWATER RADIATED NOISE Lp db Pa 2 /Hz ref 1 Pa Hydrophones in the basin measure up to 100 khz (Currently) only noise from cavitating propellers is determined 95 90 85 80 75 70 cavitating non-cavitating Needs a low background noise level Noise can also be measured on the hull for prediction of inboard noise 65 60 55 50 45 10 2 10 3 10 4 10 5 frequency [Hz] 9
COMPUTATIONAL APPROACH Wake field (RANS) Propeller cavitation (BEM) Radiated noise (ETV, Brown, Matusiak) 10
RESEARCH Currently ongoing research: Methods for computational prediction of URN URN measurements at low frequencies New criterion for judging the risk of vibrations Correlation model-scale & full-scale results MARIN ARD EU FP7 project SONIC (http://www.sonic-project.eu/) Collaboration project with several navies (mainly Dutch) Commercial projects TT JIP (tunnel thruster design) 11
SONIC: COMPUTATIONAL PROCEDURE ETV results: Maximum source level and corresponding frequency Low- and high-frequency slopes of spectrum Full scale URN data courtesy Kipple 12
RNL [db re 1 μpa 2 m 2 ] RESULTS ETV 190 185 cruise liner, 18 knots measured ETV - HF slope= -2.0 180 175 machinery noise cavitation noise ETV - HF slope= -1.75 170 165 160 155 150 145 140 10 100 1,000 10,000 100,000 frequency [Hz] 13
URN AT LOW FREQUENCIES Reverberation: can we distinguish the direct signal? What is the lowest frequencies for measurements? 14
RISK OF VIBRATIONS Current criterion to judge the risk of vibrations: vd Kooij criterion F c 0.75 75 / L Derived in the 1980 s Empirical constant c mostly determined for cargo ships Usually: c = 7 for VLCC s, container ships with bridge forward c = 5 for product tankers, container ships with bridge aft c = 3 for ferries, cruise ships (and yachts) Very simple model Zeq Needs to be checked and updated for modern ship designs and non-cargo ships 15
RISK OF VIBRATIONS Coupling with Finite Element Method (FEM) models Enter pressures directly into FEM? Compute integrated force and use that for FEM? Need to compute several ship types Construction of many ships similar due to class regulations FEM not fast enough Adjust criterion or derive new Image from: http://www.adhocmarinedesigns.co.uk/ Use our experience of high-frequency source levels for prediction of hull excitation and resulting inboard noise 16
CORRELATION MODEL-SCALE & FULL-SCALE Many results of model tests Also full-scale data from Trials & Monitoring department Check correlation for: Pressure amplitude Vibration amplitude (link with previous subject) Challenges: Sensors not in the same positions Different propeller loading Vibrations (influence measured pressure) If needed: Derive correlation factor for model test results Limited availability of full-scale URN results 17
MARKET How does this help you? We can indicate (with computations) in an early design stage whether URN is going to be an issue. We can perform model tests to confirm whether the requirements for URN and inboard noise and vibrations will be met. Predictions based on model tests give good results for full scale. If needed, the propeller (or hull or appendage) design can still be adjusted before the ship has been built. The end result: quieter and more comfortable ships 18