Hydrodynamic design for mitigation of bubble sweep down in sonar mounted research vessels

Article type: Research Article

Authors: Palaniappan, M.^{; *} | Subramanian, V. Anantha

Affiliations: Department of Ocean Engineering, Indian Institute of Technology, Madras, India

Correspondence: [*] Corresponding author: M. Palaniappan, Department of Ocean Engineering, Indian Institute of Technology, Madras, Chennai – 600 036, India. Tel.: +91 94449 36449; E-mail: [email protected].

Abstract: Oceanographic research vessels are fitted with acoustic sonar transducers at the bottom keel region. For a ship underway, atmospheric air gets mixed naturally with the surface water in the presence of wind and waves. Bubbles get entrapped in the region within the draft of the vessel and flow in the stream past the vessel. Literature records show that the bubbles are formed in the upper regions (made worse by the pitching motion) below the surface. When they flow immediately below and in the region of the sonar transducer resulting in bubble sweep-down phenomenon, they directly interfere with the acoustic transmission and deteriorate the functioning of the sonar transducer. Degradation of performance of the acoustic transducer seriously limits the mission capability of the vessel. This is a major concern and there is no complete remedy as of date, for the avoidance of the bubble formation in the flow stream. This paper describes a hydrodynamic re-design approach for the hull geometry in the forward region and the creation of an effective bubble diverter bow. A new modified bow form is investigated to help in deflecting the stream lines away from the location of the sonar transducer. The strategy in the approach here is to design the bow region to control hydrodynamic flow such that the bubbles-entrapped water of the upper surface layers is strategically diverted to flow side-ways of the hull or at the bottom side-ways well away from the location of the sonar transducer. Numerical flow simulations for the developed hull form using CFD tools demonstrate that the streamlines can be effectively thus diverted without degradation of the performance of the sonar transducer. The strategy for the hull form design is evolved by parametric variation of the side-shape using computer aided surface generation tools. The parameters influencing drag as well as diversion of the streamlines are the length parameter of the bubble diverter bow at side, the cross-sectional area parameter at a pre-defined section at the forward and the wetted surface parameter of the bubble diverter bow. The beneficial effect of the bubble diverter bow is to be weighed against increased hull resistance. The validation studies include simulation of the flow and drag assessment and comparison from towing tank tests as well. Three hull forms are created and the streamlines traces are studied in these cases respectively. The results demonstrate that a minor re-design of the forward sides of the hull form can drastically minimize the bubble streamline interference at the sonar transducer without penalty on the resistance. A major breakthrough is offered in the mitigation of bubble sweep down by the new design.

Keywords: Bubble sweep-down, oceanographic research vessel, bubble diverter bow, acoustic transducer, streamline trace

DOI: 10.3233/ISP-170139

Journal: International Shipbuilding Progress, vol. 64, no. 1-2, pp. 101-126, 2017