Numerical investigations on drag reduction by micro-bubbles of KVLCC2 ship model by means of OpenFOAM

To explore the performance of drag reduction and flow field characteristics of microbubble drag reduction for low-speed ships, the Eulerian-Eulerian two-fluid approach has been utilized using the software OpenFOAM to investigate the bubble drag reduction on the standard ship KVLCC2 model. The interphase forces and bubble coalescence and breakup have been considered. Effects of ship velocity, air injection rate and initial bubble size on different drag components of the ship are analyzed. Distributions of air void fraction, pressure, velocity and bubble diameter, and their influence mechanisms on drag reduction are examined. The results demonstrated that bubbles can simultaneously lower the frictional resistance and viscous pressure resistance. Bubble diameter can affect the resistance reduction performance by modifying the spatial uniformity of air distribution and coverage area. The mechanisms of bubbles to reduce viscous pressure resistance are associated with the modulation of the velocity field in the stern region by bubbles. Bubbles can make the velocity field more homogeneous, thus suppressing the flow separation. Bubbles in the flow field downstream of the ship will become larger on account of bubble coalescence. Smaller bubbles can produce the stronger bubble coalescence effects. Higher flow velocity will obtain the greater drag reduction of the ship. • A numerical model of microbubble drag reduction applicable to the standard ship model is established and validated. • The drag reduction characteristics for different drag components of the standard ship model are investigated. • Bubble size can affect the drag reduction through the uniformity of gas distribution and coverage area. • Reduction of viscous pressure drag of ships is related to the modulation of velocity field in the ship's aft region. • Bubbles in the flow field downstream of the ship will become larger. Smaller bubbles have stronger coalescence effects.