Skin Friction Variations over a Simulated NACA 0012 with Realistic Ice Accretion Roughness in Reynolds-Scaled Conditions

摘要

Prior investigations of the roughness convective enhancement models employed by LEWICE have demonstrated disagreements between predicted skin friction coefficients and measured skin friction coefficients. However, the prior investigations were performed using flow conditions with negligible acceleration. To improve the skin friction correlations employed in LEWICE and other ice accretion codes, skin friction measurements must be obtained in flows replicating acceleration conditions exhibited over airfoils. Skin friction measurements are also needed at different Reynolds numbers to expand the prediction validation range closer to flight conditions expected for future generations of aircraft. To acquire detailed skin friction measurements in accelerating flow, a new ceiling insert replicating the flow acceleration experienced on the surface of a NACA 0012 airfoil was created for the 24-in. by 24-in. subsonic wind tunnel at Baylor University. A flat plate with a removable panel system for changing surface roughness conditions was used to replicate realistic ice roughness conditions. The skin friction coefficient variations along the rough surfaces were characterized using hot-film anemometry measurements of Reynolds' shear stress when exposed to two different freestream flow conditions. The resulting skin-friction coefficient measurements generally follow the same trends as those exhibited for flow with negligible acceleration over the same surfaces. Finally, turbulence visualizations are presented and demonstrate that the integral length scale of the flow is essentially independent of the nominal freestream velocity over the region downstream of the maximum roughness location.