A three-dimensional twisted hydrofoil with an attached cavitaty closely related to propellers was observed with a high-speed camera at the University of Delft Cavitation Tunnel. Reentrant flow coming from the sides of the cavity aimed at the center plane-termed side-entrant flow-collided in the closure region of the cavity, pinching off a part of the sheet resulting in a periodic shedding. The collapse of the remainder of the sheet appears to be a mixing layer at the location of the colliding reentrant flows. Collision of sideentrant jets in the closure region of a cavity is identified as a second shedding mechanism, in addition to reentrant flow impinging the sheet interface at the leading edge. Fig. 7 A close-up of Fig. 6 "marked A… shows the formation of a large spanwise vortex at 2 kHz. As the main sheet collapses, a trail of very small spanwise vortices is created, merging in several distinct larger structures.
The reduction of resistance and the increase of propulsive efficiency are major drivers for ship designers both for economic reasons and increasingly for reducing the ship’s environmental footprint. Reducing the frictional resistance by air injection below the ship in combination with special coatings is an active area of research; anecdotally, performance gains are usually large. The paper gives an overview of some model scale and full scale measurements results of ships with one type of air lubrication—air bubble lubrication—performed by MARIN. The experiments were performed under the SMOOTH project. The first series of experiments focused on an inland shipping vessel that was tested both on model scale and on full scale, with and without air lubrication. A second series of tests consisted of maneuvering and seakeeping tests with a model painted with different coatings and with and without air lubrication. No appreciable effects of air bubble lubrication were found during the resistance and propulsion tests at either model or full scale and no significant effects of air bubble lubrication on maneuvering and seakeeping model tests could be determined.
In two exploratory setups, a high-frequency pressure transducer has been used to determine both the flow and the structure borne noise above 200 kHz. In the first set of tests the impact noise due to a single bubble is investigated in order to gain insight in the acoustic signals emitted by an imploding bubble. A quantitative analysis of the signals indicates a short and clear acoustic signal in the fluid and a long chiming signal in the structure. In the second set of tests the noise signal emitted by sheet cavitation implosion on a hydrofoil is acquired. The convoluted signals of individual bubbles can be identified both in the fluid and in the structure. Analyses of the signals by examining the peak distribution for sheet cavitation indicates a relation with the cavitation index and suggest that fluid and structure borne noise are not per se linked. Acoustic signals correlate well with visual observation.
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