Air lubrication
MARIN’s involvement in research on drag reduction by air lubrication dates back to the early nineties, when model experiments were conducted for an inland waterway vessel. This vessel was to be mounted with air chambers or cavities designed in the bottom of theh ship. Although initial tests showed indeed a significant drag reduction, a later imposed constraint that the air chambers should be designed as “bolt on” chambers rendered the results not very profitable.

In 2000, at the time that Japanese researchers reported significant gains at model scale (up to 90% reduction of frictional drag of a flat plate in a laboratorium set-up in specific cases), a more encompassing project was started at MARIN; The PELS 1 project, where a first attempt was made to better understand the mechanisms for air lubrication and drag reduction. To this end, a flat plate test set-up was prepared for mounting in MARIN’s large cavitation tunnel, where water velocities of up to some 8 m/s could be adjusted. Attempts to measure the change in boundary layer velocity profile in the inner region of the boundary layer (laminar sublayer and buffer layer) where a unique relation between shear stress and velocity profile exists failed, because of a lack of resolution in the velocity measurements close to the wall. These measurements were made with an LDV technique. This problem is symptomatic for the difficulty of measuring detailed stresses in air lubricated boundary layers.

The same project also provided for tests with a large scale, full block inland waterway vessel, to measure the integral drag forces. For this ship, three different mechanisms for ventilation were applied: Air chambers, air bubble ventilation and air film ventilation in combination with a so called super repellent coating. This type of coating was purchased from the Japanese industry who had also provided this coating to a Japanese Research Group that claimed up to 90% frictional drag reduction on a flat plate. Of these techniques, the air chamber and the air film with water repellent coating were the most successful, showing reductions in overall power in excess of 5%.

Extrapolation of model scale results to full scale was however a highly uncertain process, as the exact physical mechanism was and is still not yet known. To get a better appreciation of the interpretation of these model scale results, another two Joint Industry Projects were initiated in 2005 and 2006 (PELS2 and the EU project SMOOTH). The PELS2 project not only aimed at an improved ship design with model testing, but buildinng and testing a prototype inland waterway vessel was also included. Due to unexpected design issues with regard to the air lubrication however, the project has suffered from severe delays, and the full scale trials are yet to be made.

In parallel, a research project was initiated by the hydromechanics groups of Delft and Twente University, applying 3 PhD students to get more grip on the fundamental mechanisms of drag reduction by both air bubble and air film lubrication. This project is started in 2007 and is expected to finish in 2012. Close relations are maintained in this project with the most active Japanese Research Group of Kodama and the US research group of Ceccio, who is engaged in an ONR sponsored research project where use is being made of the Large Cavitation Channel in Memphis.

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