In this study, a series of experimental investigations has been made of drag reducing efficiency and drag reduction mechanism of the outer-layer vertical blades first devised by Hutchins and Choi. From the previous result of Hutchins(2003), the reduction efficiency of local skin friction by the vertical blades was announced to reach as large as 30% calculated with the comparisons of the local skin-friction coefficients. because of the less information of side effect such as flow disturbances and the increase of wetted surface area by the presence of the blades, there has existed the limitation of engineering application.
In the experimental investigation, A POD analysis based on time-resolved PIV and Streo PIV measurements in a circulating water channel has been conducted to identify the skin friction reduction mechanism of outer-layer vertical blades. the influence of vertical blades array upon the characteristics of the turbulent coherent structures was analyzed by the POD method. It is observed that the vortical structures are cut and deformed, as well as the turbulent intensity and the Reynolds stress were weakened by the blades. These phenomena strongly associate the skin-friction drag reduction mechanism in the turbulent boundary layer flow.
Lastly, with a view to assessing the effect in the flow around a ship, the arrays of outer-layer vertical blades have been installed onto the side bottom and flat bottom of a 300k KVLCC model. A series of towing tank test has been carried out to investigate resistance (CTM) reduction efficiency and improvement of stern wake distribution with varying geometric parameters of the blades array. The installation of vertical blades led to the CTM reduction of 2.16∼2.76% near the service speed. The nominal wake fraction was affected marginally by the blades array and the axial velocity distribution tended to be more uniform by the blades array.