Control of flow around a NACA 0012 airfoil with a micro-riblet film

The flow structure of the wake behind a NACA 0012 airfoil covered with a V-shaped micro-riblet film (hereafter, MRF) has been investigated experimentally. The results were compared with the corresponding results from an identical airfoil covered with a smooth polydimethylsiloxane (PDMS) film of the same thickness. The drag force acting on each airfoil, as well as the spatial distributions of turbulence statistics in the near wake behind each airfoil, were measured for Reynolds numbers (calculated based on the chord length, C = 75 mm) ranging from Re = 1.03 x 10(4) to 5.14 x 10(4). At Re = 1.54 x 10(4) (U-0 = 3 m/s), the drag force on the MRF-covered airfoil was about 6.6% lower than that on the smooth airfoil. In contrast, at the higher Reynolds number of Re = 4.62 x 104 (U-0 = 9 m/s), application of the MRF increased the drag force by about 9.8%. To determine the spatial distributions of turbulence intensity, including the mean velocity, turbulence intensity and turbulent kinetic energy, 500 instantaneous velocity fields of the wake behind each airfoil were measured using a 2-frame PIV technique and ensemble averaged. For the case of drag reduction (Re = 1.54 x 10(4)), the near wake behind the MRF-covered airfoil had a shorter vortex formation region and higher vertical velocity component compared with that behind the smooth airfoil. At the downstream end of vortex formation region, the Reynolds shear stress and turbulent kinetic energy for the MRF-covered airfoil were similar or slightly larger than for the smooth airfoil. Smoke-wire flow visualization showed that the presence of the MRF on the airfoil surface caused the smoke filaments to become thinner and to be separated by a smaller lateral spacing, indicating suppression of spanwise movement. For the drag-increasing case (Re = 4.62 x 10(4)), the presence of MRF grooves on the airfoil seemed to increase the vertical velocity component and decrease the height of the large-scale streamwise vortices, which interacted actively. This active interaction increased the turbulent kinetic energy and Reynolds shear stress in the near-wake region, increasing the drag force acting on the airfoil. (c) 2005 Elsevier Ltd. All rights reserved.