| Over the last 60 years, there has been a great deal of research aimed at determining the impact of surface roughness on fluid flow and the mechanisms by which the resulting flow phenomena occur. The effects of surface roughness include increases in drag the likelihood of flow separation, and the ability to trigger premature transition to turbulent flow. The primary concern for aircraft performance is surface roughness on the leading edge of an airfoil, which can adversely affect the design characteristics. The research to date has examined many problems in this area and has continued to expand the knowledge base, but many gaps still exist for a full understanding of the roughness-fluid interaction.; In this study, a numerical investigation is conducted to examine the impact of surface roughness on external viscous flows. The focus of this study is on different types of airfoil leading-edge roughness and how these surface perturbations interact with the external flow field. The freestream conditions are varied to include both steady and unsteady flow at a constant angle-of-attack or in pitch-up. Several complex flow phenomena are examined in this work, including the laminar separation mechanism, leading-edge flow separation, stall characteristics, and vortex shedding.; Several key findings are observed for the impact of roughness on the flow field. The results show that small-scale surface roughness can significantly alter the characteristics of the laminar separation mechanism. In particular, surface roughness, fully contained within the boundary layer, can shift the laminar separation point upstream of the original, clean surface location. It is also observed that similar small-scale roughness affects the secondary separation mechanism in the dynamic stall process. With large-scale roughness, the inception time for the formation of the dynamic stall vortex is accelerated as compared to a clean airfoil. This study also shows that the current two-dimensional Navier-Stokes algorithm is capable of predicting regions of laminar and roughness-induced transitional flow for the roughness geometries considered in this work, provided the grid resolution is sufficient to capture the small-scale flow structures. |
