Particle Image Velocimetry in a Moonpool

Moonpools are used on many types of vessels. Waves and the corresponding motions of the vessel or simply forward speed in calm water can induce oscillations inside the moonpool. A very critical case of oscillation is observed in transit when the moonpool bottoms are left open. This can occur even on a perfect calm sea, as the excitation mechanism arises from the flow velocity in the vicinity of the opening. These oscillations during transit cause a tremendous increase in resistance.
Despite the research effort conducted on the subject, most of the solutions to dampen the oscillations in the moonpool are still found by trial and error. MARIN has launched a JIP to obtain more insight in the flow characteristics in and around the moonpool and the mechanism behind the excitation and develop a more adequate design methodology.
To gather information on the flow characteristics inside the moonpool, MARIN conducted a feasibility study and test campaign prior to the start of the JIP as well as a test campaign within the JIP. These tests were performed with a dedicated PIV set-up build in a vessel to measure inside a moonpool. For this purpose the DANTEC submerged PIV system was dismantled and used to make the set-up. The camera modules were mounted completely inside the vessel such that the remote control of the lenses and Scheimpflug adapter were still functioning.
The PIV technique appeared to be very well suited to capture the phenomena that occur in the moonpool. The result showed a vortex “travelling” through the moonpool. These results show that PIV is a suitable and valuable tool for the study on moonpool dynamics. This paper will discuss the set-ups used and presents the main outcome of the tests.