Résumé des résultats (Abstract)
(Anglais)
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The aim of the European project Seabus-Hydaer is the development of a new generation of Fast Ferries (>100 knots) with the help of specific hydrofoils for stabilisation and aerodynamic wings, using surface effect, for lifting purposes and drag reduction.
The main objective of the present work is the validation of the stabilising capabilities of Air fed profiles through an experimental investigation of a set of hydrofoils at reduced scale. Two control foils, a strut and take-oft foils have been tested. Air injection is provided on the suction and pressure sides as well as in the leading and trailing edges. Tests are carried out in the LMH high-speed cavitation tunnel. A measurement of hydrodynamic forces (lift and drag) as well as flow visualisations are performed for different flow conditions and air feed configurations. Besides these experiments, numerical simulations of the air injection effect using a 2-phases Navier-Stockes code (Fluent) have been also achieved.
Four different control and take-off Airfed profiles have been successfully tested in the LMH high-speed cavitation tunnel of the Swiss Federal Institute of Technology. Different configurations of air injection have been investigated for different flow conditions. Lift and drag coefficients as well as flow visualisations have been carried out to study the effect of injected air on hydrodynamic and cavitation performances of these hydrofoils. In general, for air and/or vapour cavities extending beyond the trailing edge, injecting air in the suction side leads to a decrease of the lift coefficient while air injection in the pressure side is responsible of an increase of the lift coefficient.
Flow analysis, using experimental and computation results, shows that air injection causes a deep change in the pressure field over both pressure and suction sides. The pressure upstream to the air feed slot increases due to a stagnation point while the pressure on the pressure side decreases in a uniform way along the chord length.
In order to improve the mixing layer between air, vapour and water, a modified geometry of the NACA control foil has been tested. A significant improvement has been obtained by enlarging the injection slot. This improvement, which has been reached at low velocities, should be more pronounced for full-scale conditions. Furthermore, the relationship between flow-rates of injected air and the lift coefficient was found similar to the results already reported in 1998 with the original geometry.
Unlike the take off foil, flow velocities that could be tested for control foils (Naca and Panda) were far below the full scale velocity (25 m/s instead of 45 m/s). This limitation is due to an excessive profile deformation at higher velocities and incidence angles.
Tests of the Strut foil have been carried out at velocities below 20 m/s for safety reasons. In fact, a strong interaction of the Kàrman vortices, shed from the trailing edge, with the cavitation tunnel pipe led to intense flow oscillations and vibration. This interaction was found much reduced when air is injected at the trailing edge.
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