Numerical simulation of micro-bubble drag reduction of an axisymmetric body using OpenFOAM

The two-phase micro-bubble flow over an axisymmetric body is investigated using the OpenFOAM framework.The numerical model consists of an Eulerian-Eulerian two-fluid model with closure relationships for the interfacial momentum transfer to capture the multiphase flow,a standard A:-￡*model for the continuous phase and one turbulence model inside the OpenFOAM for the dispersed phase.The bubble sizes are calculated based on the solution of the transport equation of the interfacial area density.The simulations in this work are carried out with different air injection rates and different flow velocities.The effects of bubble size on drag reduction are analyzed.The numerical results are compared against some available experiments and other numerical simulations.The numerical results indicate that the airflow rate and air volume fraction within the boundary layer near the body play important roles in micro-bubble drag reduction.The frictional drag reduction effect by micro bubbles is larger for lower water speed,and the presence of the micro bubbles can increase the pressure resistance of the body.Drag reduction rates are generally higher when the bubble diameter is smaller.

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-e 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.