ABSTRACT: The reduction of the frictional Resistance by air injection made by micro bubbles is an important area of research regarding the Drag Reduction of Ships. Promising results have been obtained in Japan shipyards by placing micro bubble generators on the hulls; these systems are known as WAIP (Winged Air Induction Pipes) or MALS (Mitsubishi Technology bubble generators), which reduce the Ship Drag of 5% or more, but values of 10% or more are foreseen in future. The Micro-Bubble Drag Reduction (MBDR) due to the micro bubbles has been demonstrated by Mc Cormick and Bhattacharyya (Mc
Cormick et al., 1973). Later new tests and research have been made by Madavan, Deutsch and Merkle on flat plates tested in circulation canals (Madavan et al, 1985). More recently new tests have been made by Sanders et al. (Sanders et al., 2006) which confirmed and extended the previous tests at higher Reynolds Numbers, by using new plates. The flat plates have been tested in vertical and horizontal positions, by changing the water speed, the air flow and the bubble size. In this research similar testing conditions have been simulated by using CFD Techniques available on commercial programs (STAR-CCM+ by CD-ADAPCO) and similar results have been obtained, both by testing at the water velocities used in the experiments, by placing the flat plates in vertical and horizontal positions and by changing the size of the micro bubbles. A preliminary investigation has been made by comparing the numerical results and the experimental ones obtained by towing vertical flat plate in the towing tank of the University of Trieste, without using the micro bubbles. This preliminary investigation was aimed to define some preliminary parameters and models which have been used later in the investigation with the micro bubbles. The investigation has been made in a multi-phase domain, operating both by using a multi-phase Eulerian model and a Lagrangian model. The data presentation of the numerical results has been made by using dimensionless parameters, so as to give more generality to the results and to better compare the data by keeping a constant specific parameter, for example the micro bubble size. By using the dimensionless presentation, it has been possible to extend the investigation also to some parameters, which have not been investigated experimentally.
