In present work, an acoustic characterisation of the University of Genoa cavitation tunnel is presented, with the aim to obtain suitable transfer functions in order to take into account (at least partially) the effects of the confined environment, particularly important for small-scale facility like the one considered. The acoustic characterisation is performed considering two hydrophones in significantly different positions (inside the tunnel and in an external tank), typical, for different reasons, of noise measurements in cavitation tunnel; the procedure is based on the logarithmic sine sweep signal. The obtained transfer functions have been applied to a series of measurements of model propellers in correspondence to considerably different functioning conditions, in terms of cavitation number and propeller loading, showing the applicability of the procedure and the improvement of the measurement quality, in view of predictions in full scale.
Energy saving within shipping is gaining more attention due to environmental awareness, financial incentives, and, most importantly, new regional and international rules, which limit the acceptable emission from the ships considerably. One of the measures is installation of energy saving devices (ESD). One type of such a device, known as pre-swirl stator (PSS), consists of a number (usually 3 to 5) of fins, which are mounted right in front of the propeller. By modifying the inflow and swirl into the propeller, the fins of a PSS have the possibility to increase the total propulsion efficiency. However, at the same time, they may introduce additional resistance either due to changes in pressure distribution over the aft ship or due to its own resistance of fins. In this paper, the authors present experimental and numerical investigation of a PSS for a chemical tanker. Numerical analysis of the vessel with and without PSS is performed in the model and full scale. Model testing is performed with and without PSS to verify the power savings predicted numerically. Among other quantities, 3D wake field behind the hull is densely measured at different planes, starting from the PSS plane to the rudder stock plane. 3D wake measurements are also conducted with a running propeller. The measurements show considerable improvement in the performance of the vessel fitted with PSS. On the numerical side, analyses show that scale effect plays an important role in the ESD performance. Investigation of the scale effect on the vessel equipped with an ESD provides new insight for the community, which is investing more into the development of energy saving devices, and it offers valuable information for the elaboration of scaling procedures for such vessels.
This paper presents experimental results in damping the pitch motion on a Surface Effect Ship (SES) in high vessel speeds in presence of regular wave. The SES is a marine craft supported by pressurized air cushion. It consists of two side-hulls, a flexible rubber bow and reinforced stern seal. The air cushion is pressurized thanks to the lift fans blowing air into the cushion. Modern SES are equipped with controllable vent valves that allows adjusting the pressure of the air cushion. Air Split Cushion SES is one of the new design by Umoe Mandal AS where the main air cushion is divided to four smaller air chambers, each one equipped with a lift fan and controllable vent-valve. Using a derivative controller, this paper presents an active pitch control in waves. Controlling the pressure in each air cushion through adjusting the air leakage on the vent-valve, provides an effective pitch regulator that demonstrates, through simulations and experiments, reduction of up to 60 % of the pitch motion in waves and high speeds. The effectiveness of the proposed system is examined through experimental model testing of the vehicle in SINTEF Ocean's towing tank.
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