Unsteady flow structure of an airfoil in ground effect

Particle image velocimetry (PIV) experimental results of wake flow structure of a NACA0012 airfoil with small attack angle mounted above water surface are introduced. The experiment was carried out in a small-scale wind-wave tunnel. The diameter of wind-wave tunnel test section is 1.7 m (long) × 0.4 m (width) × 0.4 m (height). The flow fields around the airfoil were measured under four different conditions by varying the distance between the airfoil and the water surface. The attack angle of the airfoil was kept 10° during the experiment. For each experimental condition, the time series of particle images was captured to calculate continuous evolution of the velocity fields. The velocity fields were ensemble averaged to get the statistic parameters such as mean velocity and vorticity. Typical instantaneous velocity fields for each case are introduced to show the basic flow structure of wind surface flow separation. The aerodynamic loads acting on the airfoil are analyzed qualitatively according to the mean vorticity distribution in the flow field based on the theory of vorticity aerodynamics. The results indicate that the flow structures and drag/lift force of the airfoil alter remarkably with the changing distance between the airfoil and water surface.