Dynamic stall and cavitation of stabiliser fins and their influence on the ship behaviour
Author Gaillarde, G
Title Dynamic stall and cavitation of stabiliser fins and their influence on the ship behaviour
Conference/Journal FAST 2003
Year 2003

Abstract
The lifting characteristics of stabiliser fins and their efficiency are evaluated most of the time with a static and steady approach. Usually the same is applied in numerical simulations, both in the frequency or time domain. Lift degradation due to stall is sometimes used in numerical models.
Although this approach can be assumed to be correct when the fins are working at low angle of attack in mild sea conditions, strong non-linearity appears when reaching their limits in rougher sea conditions. When working in a real environment and especially at high speed, the lift and stall of the fins is subject to dynamic effects. When stall occurs while sailing, the degradation in roll behaviour of the vessel is such that the fins can hardly recover their lift until lower waves are encountered. Hysteresis loops are then observed in the lift slope curve. This effect was shown previously in a number of experimental studies, mainly conducted for aircraft or helicopter applications, see references (1)-(7). This effect was also measured on stabiliser fins during seakeeping tests conducted with a free running model sailing in stern-quartering seas (8).
At large fin angle of attack and ship speed above 25 knots, cavitation will also occur. Non-linear effect due to cavitation, resulting in lift degradation, was investigated on a T-foil in a cavitation tunnel. The set-up allowed the fin to oscillate at different frequencies and amplitudes in order to simulate unsteady flow conditions. The results were presented and discussed in (9). Also, scale effects were investigated and presented in (10). This study showed that for hydrofoils with laminar section types and relatively high aspect ratio, scale effects on lift in an unsteady flow are rather small, while in steady flow condition scale effect can be relatively larger.

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