Hydrodynamic drag reduction in turbulent boundary layer flow using riblets

Abstract

Reducing the hydrodynamic drag of marine vessels is a key approach to decrease energy consumption. In addition to saving fuel costs, this also decreases greenhouse gas emissions. One possible method to reduce turbulent drag for a ship is coating its hull with riblets. Riblets are micro-fabricated structures whose design is based on shark skin. Earlier riblet studies have reported maximum drag reduction of 10%. The goal of the present work is to accurately measure the drag of 3 different riblet coatings in the cavitation tunnel. This setup, located at the Ship Hydromechanics group of the TU Delft generates a turbulent boundary layer for freestream velocities between 0.3 and 3.5 [m s ]. Two steps were taken to achieve this goal. Firstly an accurate measurement procedure was developed, which allows 1% accurate drag reduction studies in the cavitation tunnel. This procedure is then sub-sequently used to study the 3 different riblet coatings. These were provided by Fraunhofer IFAM and Océ, who used a mould pressing and elevated printing technology respectively. Cavitation tunnel measurements were analysed using the propagation of uncertainty and Particle Imaging Velocimetry[PIV]. The former determines the contribution of individual measuring sensors to the uncertainty of the measured drag reduction. It was found that the velocity and drag force require the longest measuring time to guarantee statistically converged results. PIV studies were used to determine the correct magnitude of the freestream velocity, as the pressure sensor of the cavitation tunnel systematically overestimated the freestream velocity. Measurements for the supplied coatings showed drag reduction for all plates. For Fraunhofer IFAM maximum drag reduction amounted to 7%, while for Océ this value is 5%. Comparing with values from literature shows slight under performance. This is presumably caused by small manufacturing effects on the riblet surface. To improve current work, the velocity region where accurately can be measured must be enlarged. This is because the expected optimal performance for the Océ and Fraunhofer IFAM coatings are located before and beyond this region respectively. For the low limit velocities, small ranged force and velocity sensors should provide more accurate results. For high velocities entrained air must be removed from the setup. This can be done using a vacuum pump, although more analyses on this system is required as the measured drag is affected by this device.