Numerical study on the aerodynamics of an airfoil moving close to an air-water interface

Abstract

This study investigates the aerodynamics of an airfoil which passes close to an air-water interface with heaving motions by means of numerical simulation. Simulations are performed by solving the Navier-Stokes equation on a curvilinear coordinate system whose axes fit the moving airfoil surface in contrast to the conventional approaches which neglect the air viscosity. The air-water interface is captured by a CIP method. We show the crucial importance of taking the air viscosity into account for the successful prediction of the airfoil aerodynamics in heaving motions by demonstrating the observation of vortex separations near the leading edge of a NACA0012 airfoil at the Reynolds numbers of 50000 and 66138 based on the viscosity and the density of air and the translational velocity of the airfoil. It is found in this study that the existence of the water surface has a significant impact on the lift force on the heaving NACA0012 airfoil and the deviation of the lift coefficient from the one measured in the simulation conducted in the open space depends not only on the clearance between the airfoil and the water surface but on the mean angle of attack and the phase of the heaving. When the airfoil moves close to the surface, the surface effect on the airfoil-aerodynamics is emphasized due to the squeezed-film damping and the interaction of the separated vortices and the air-water interface. The importance of analyzing the fluid motion of both air and water phases is illustrated by showing that the kinetic energy of the moving airfoil transferred through the the air-water interface is rather significant.