Effect of Regular Surface Perturbations on Flow Over an Airfoil

This paper presents an investigation on the effect of introducing large-scale roughness through static curvature modifications on the low speed flow over an airfoil. The surfaces of a standard Eppler 398 airfoil have been modified with regular perturbations or “bumps” of the order of 2%c for this purpose. While the actual E398 airfoil is not a suitable candidate for low Re cases due to extensive prevalence of boundary layer separation, it is expected that the bumps would exercise passive flow control by promoting early transition to turbulence, thereby reducing the extent of separation and improving the performance. The fluid dynamic mechanism behind this separation control methodology is not clearly understood, however. Smoke-wire flow visualization is performed for qualitative observation of the separation region in both the perturbed and unmodified airfoil geometry cases. At higher Re values, pressure probe measurements are made to quantify the wake momentum deficits. Unsteady 2D PIV measurements are employed to understand the near-wall flowfield behavior . The size and strength of vortical structures formed in the separating shear layer are examined, along with measurements on the laminar separation bubble. All the experiments are conducted for chord based Re values ranging from 25,000 to 500,000.