Radiated Noise of Research Vessels A multidisciplinary Acoustics and Vibration problem CAV Workshop 15 May 2012 Christopher Barber Applied Research Laboratory Penn State University
Ship Radiated Noise What makes noise? Propulsion Machinery Hydrodynamic sources, transient sources and transducers How can you build and operate a quiet ship? Propulsor and hull design Noise control technologies Operational awareness Why care? Environmental Impact Shipboard Habitability ICES Impact on Shipboard Mission Systems (self-noise) How to measure it? Acoustic ranges, portable systems Shallow water measurements
Radiated Noise Sources Sources Propulsor Noise Motor and Aux Machinery Noise Sea connected systems (pumps) Transient sources incl. active acoustic transponders Hydrodynamic sources Generator Rotational Paths Direct acoustic propagation Shaft line propagation Sound/structure interaction Diffracted paths Tanks 2X - Rotor Mechanical Bearing Cap Vertical - 3600 RPM
Figure courtesy of Noise Control Engineering 4
Machinery Sources 25 MW Alstom Generator 2E 1R 2X Measurements taken 30 Sept 1998 SHAFT ROTATING 1R AND 2R CORE MAGNETOSTRICTION 2E 2E - Full load 2E - No load with excitation Frequency, Hz 5 to 15 Knots Low Speed Limits Generator Rotational 2X - Rotor Mechanical Stator Core Radial Bearing Cap Vertical - 3600 RPM
Paths for Machinery Noise Airborne First Structureborne Secondary Structureborne U/W Radiated Noise Figure courtesy of Noise Control Engineering 6
Sea Connected Systems Fluid-coupled paths Pump generated fluidborne acoustic energy travels via piping systems. Figure courtesy of Noise Control Engineering 7
Propeller Noise Cavitation typical dominates broadband ship signature Mitigation: Design prop for maximum cavitation inception speed Restrict noise-sensitive operations to speeds less than cavitation inception 140 135 130 FRV-40 Propeller Estimated Broadband Noise Envelope FRV-40 Goal 11 kts with Tip Vortex Cavitation and Suction Side Leading Edge Cavitation Inception at 10.5 knots SPL 125 11 kts Noncavitating (design) 120 115 110 10 1 10 2 10 3 10 4 Frequency (Hz) Printed 10 Feb 1999 13:14:03
Non-propulsion flow-related noise Hull and appendage cavitation Rudders, Struts Fairings, Bilge Keels Bow wave transients Acoustic source Bubble sweepdown Mitigation: good hydrodynamic design
Sonar Self-Noise Sources Hull-mounted sonars Bow-area flow noise Bow wave transient Flow-induced structural excitation Installation details window material and attachment mechanism fairings Propagation of external ship sources into sonar machinery / prop noise via hull grazing path Bottom reflected path Source Level Transmit/Receive Directivity Receive Reverb Ambient + Self-Noise SNR = [SL-2TL + 20logH T H R +TS]-{NR+(NL 0 -DI R )} Transmission Target Strength Directivity Index (Propagation) Loss
Impact - Environmental Noise Studies ongoing to assess impact of anthropogenic noise on marine mammals general shipping noise Local radiated noise Science mission sources Table from Hildebrand, Sources of Anthropogenic Sound in the Marine Environment
ICES Criteria for Fisheries RV s Impact of research vessel noise on fish surveys Based on estimates of fish hearing for various species Impact to both acoustic and catch surveys From Mitson, UNDERWATER NOISE OF RESEARCH VESSELS, 1995
Radiated noise measurements in a harbor environment using a vertical array of omnidirectional hydrophones Brian Fowler Graduate Program in Acoustics Dr. D. Christopher Barber The Applied Research Laboratory The Pennsylvania State University Research Sponsored by ONR 331 CAV Workshop, May 14-15, 2012 1
Overview Research Field Test Omni-directional measurements Beamforming Far-field Theory Measurements Near-field 2
Research Testing and Objectives Acoustic Research Detachment at Lake Pend Oreille in Bayview, Idaho Summer 2010 and 2011 SEAJET In conjunction with near-field acoustic holography (NAH) testing Validate NAH estimates 3
Problems with a shallow water harbor environment Multipath Environment Surface reflections Bottom reflections Reflections off other underwater interfaces Near-field Environment Proximity from hydrophone to source may prohibit far-field plane wave assumption 4
Hydrophone Array 16 14 Element Hydrophone Array 5
Top-view Geometry S e a J e t A Few Array Locations (suspended by crane from barge) Barge Sources (varying depth) 6
14 Element Array Side-view Geometry SeaJet Barge J9 Reference Hydrophone Range ITC 1001 7
Omni-directional measurements Reference Phone Ambient Run Hydrophone 1 9
Far-field beamforming theory p(r,θ, t) = Far field assumptions: r 1 r i 1 r N i=1 A r i ej(ωt kr ) N 1 d so all r i are approximately parallel for all i where r is the distance from array center to source r = r 1 1 N 1 r where r = d sin θ, 2 so r i = r 1 (i 1) r Kinsler, Frey, Coppens, Sanders. (2000) Line Array [Figure]. In Fundamentals of Acoustics, 4 th Ed. pg 195. Hamilton Press 10
Far-field beamforming theory p(r,θ, t, θ 0 ) = A N 1 r ej(ωt kr) j e 2 k r N e j[φ i+ i 1 k r] i=1 where φ i = i 2π λ d sin θ N multiplying a e jφ i i=1 to the unsteered FFT gives us the beamsteered data in the frequency domain 11
Calculated broadside f = 1860 Hz beams f = 3720 Hz Changing the steered frequency affects the main lobe width as well as the number and size of the side lobes. 12
Calculated steered beams f = 1860 Hz, steered to 10 degrees Steering the beam affects the direction in which the main lobe points and also affects the size and direction of the side lobes 13
Calculated steered beams 14 Element Array steered in the direction of an oncoming plane wave 14
Far-field beamforming results 15
Far-field beamforming results 16
Near-field beamforming theory and application Measure from geometric center of array Remove far-field assumptions and recalculate This will negate the plane wave assumption and account for spherical spreading from the source Vary ranges to find accurate range 17
References Burdic, William S. Underwater Acoustic System Analysis. Englewood Cliffs, NJ: Prentice Hall, 1991. Print. Kinsler, Lawrence E., Austin R. Frey, Alan B. Coppens, and James V. Sanders. Fundamentals of Acoustics. New York: J. Wiley, 2000. Print. Acknowledgements Carlos Uribe, Wyatt Tyahla, and David Van Hoof (PSU) Earl Williams and Nicolas Valdivia (NRL) 18