Numerical study of air layer drag reduction of an axisymmetric body in oscillatory motions

The air layer drag reduction (ALDR) of an axisymmetric body in oscillatory motions is investigated in this paper with open source toolbox OpenFOAM. The unsteady Reynolds-averaged Navier-Stokes (URANS) equations are used to determine the viscous flow and the volume of fluid (VOF) model is adopted to capture the interface of the air-water two-phase flow. The k − ε turbulence model is adopted to simulate the turbulence. The dynamic mesh technique is applied to model the movement of the axisymmetric body. Firstly, the ALDR results are validated by the experimental data. Then, the effects of the movements of the body on the drag reduction during the ALDR state are investigated. Two representative kinds of movements are considered, namely, the pitch and the heave. The numerical results show that the drag reduction varies during the movements and the average drag reduction rates will be reduced. The variation of the drag reduction is related to the morphological change of the air layer. The heave motion is more likely to reduce the effects of the ALDR than the pitch motion. For both oscillatory motions, the large motion amplitude and the low motion period are not conductive to improving the effects of the ALDR. The effects of the oscillatory motion on the ALDR are more sensitive at high water speeds than at low water speeds. Besides, increasing the air flow ratio can be considered as one way to improve the effects of the ALDR.