Bubble-Sweep down Study and Mitigation for Improved ADCP Data Quality

1. Bubble-Sweep down Study and Mitigation for Improved ADCP Data Quality

2. Credits go to: • Mr. Bob Fratantonio - Department of Ocean Engineering University of Rhode Island • Dr. Thomas Rossby • Graduate School of OceanographyUniversity of Rhode Island • Dr. Charles Flagg • School of Marine and Atmospheric Sciences Stony Brook University • Dr. Stephan Grilli • Department of Ocean Engineering University of Rhode Island • National Science Foundation (NSF) • Smyril Line

3. The M/F Norröna • Build year/Shipyeard: 2003 / Flendern Werft AG, Lübeck • Ship contract price: € 93,4 mill. • Length over all: 165,74m • Breath: 30,00m • Draft: 6,30 m • Dwt: 6.350 • GT: 35.966 • NT: 15.922 • Cabins: 318 (1012 beds) • Passenger capacity: 1482 • Crew: 118 • Cars: 800 or Trailers: 130 • Lane m.: 1830 • Cargo capacity: 3.250 tonnes • Service speed: 21 knots • Main engines: 30.000 BHP • Bow Thrusters: 4.755 BHP • Helicopter pad: On top deck at the ferries stern • Stabilizers: 1 pair of stabilizers

4. ADCP • 75 kHz RD Instruments Ocean Surveyor • Installed in a 1-week dry dock period in January 2006 in Hamburg, Germany • Cable runs 8 decks to the DAQ system • ADCP is mounted 60 m from bow

5. Dry Dock

6. The Bubble Fairing

7. Previous Results using the Bubble Fairing

8. The Problem • An ADCP system was installed on the M/F Norröna in January 2006 in Hamburg, Germany • Instrument was functioning properly, but the data was spotty and poor • Data improved as M/F Norröna passed through fjords towards Bergen, Norway • As the ferry entered open seas, the acoustic backscatter amplitude became erratic and of poor quality

9. Candidates for Source of Problem • Internal machinery-generated vibration • Propeller noise • Electronic interference due to the long length of cable that necessarily ran along-side some of the ship's power cables • Bubble Sweepdown • Breaching of the bow-thruster openings?

10. CritterCam • Greg Marshall at the National Geographic Society loaned us the CritterCam • Features • Autonomous • Records Internally • Diver Deployable • Records 1 minute of video every 4 hours • Permanent magnets attach camera to the hull

11. CritterCam Results • Best results come from videos taken during daylight hours • Bubble clouds are produced in the turbulent bow wave as the ferry pitches up and down • Clouds approach lens at fairly regular intervals • Using the height of the fairing (21 cm) as reference, one can estimate the thickness of the clouds seen in the video as roughly 30 cm thick

12. CritterCam Results If the video clip does not play automatically, it can be accessed by clicking the following link: http://www.unols.org/meetings/2009/200903fic/bubblesweep.AVI Windows users may need to download the free divx codec to view the video clip. The download is available at: http://www.divx.com/en/products/software/windows/divx

13. Cosmos Floworks • Computational Fluid Dynamics were performed to address the following questions… • Can the shape of the fairing be improved to reduce the stagnation pressure at the leading edge of the fairing? • Can the addition of rails placed ahead of the fairing produce significant upwelling to bring bubble-free waters from depth up to the face of the transducer? • Used Cosmos Floworks CFD package • Fully embedded in Solidworks • Easy to use • Computations were performed on a Dell Optiplex 755 running Windows XP Professional • 8 GB of RAM • Intel® Core™ 2 Duo CPU E6850 @ 3.00 GHz

14. Rails • The next step was to investigate the influence of rails upstream of the fairing • Rails were modeled after a hyperbolic tangent function y = A * tanh(x) + b • A systematic approach was taken to optimize the parameters of the rails • Once the rails were optimized, the rail-fairing interaction could be simulated and studied

15. Varying Opening Width • The first parameter to change was the opening between the two rails. • The slope of the rails remained constant and only the opening was changed • Equation • y= A*tanh(x)+b • Varying b changes the width of the opening between rails

16. Some Results

17. Vortices Generated by Rails Very encouraging! The rails do appear to generate upwelling * Note this figure is upside down

18. Varying Slope • The next parameter to change was the slope • The opening between rails remained constant and only the slope was changed • Equation • y= A*tanh(x)+b • Varying A and offsetting b the same amount changes the width of the opening between rails

19. Final Rail Profile • The rails were shortened from their original 4 meters of length (in the x-dir) to 2 meters • The opening was optimized as the same width as the fairing, ~0.5 meters • The height of the rails matched that of the fairing, ~20cm

20. Rail – Fairing InteractionPlanview of Z-Velocity Rails are set 10 meters upstream of the fairing

21. Chines • Can we simplify the rails even more? • Straight rails (chines) were of interest due to their simplicity • Easy and less expensive to manufacture and install • But do they perform as well as the rails? • Use approximately same slope as the hyperbolic tangent rails

22. Chines vs. Rails Chines Rails

23. Particle Trajectories - Chines Water particles released downstream 0.5 meters below the hull starting from the centerline and spanning 1 meter starboard

24. Particle Trajectories - Rails

25. Particle Displacement Profile (Y-Z) The rails and chines create a similar swath

26. Sketch of New Fairing/Rails Position • The fairing was moved closer to the centerline of the ship with the hyperbolic tangent rails ~10 meters upstream

27. The Rails Photos of the rails just before the ship was refloated, courtesy of Eike Bayer, the Blohm and Voss project director.

28. Plans for the Future • Still having difficulty collecting good ADCP data • Not entirely sure why • Lack of Zooplankton for acoustic backscatter? • Would like to use the camera to get visual evidence of whether the rails are successfully creating local upwelling