The Effect of a Small Patch of Realistic Surface Roughness on the Boundary Layer of an Accelerating Flow Over a Flat Plate Model With Elevated Freestream Turbulence Available to Purchase

Results are presented of extensive boundary layer measurements taken over a flat, smooth plate model representing the features of the front one-third of a turbine blade and over the model with a small embedded strip of realistic rough surface. The model included elevated freestream turbulence and an accelerating freestream in order to simulate conditions on the suction side of a rotor blade in the first stage of a high-pressure turbine; however, the study was limited by the relatively small size of the patch of realistic surface roughness. The realistic rough surface was developed by scaling actual turbine blade surface data provided by U. S. Air Force Research Laboratory. The results indicate that bypass transition occurred very early in the flow over the model and that the boundary layer remained unstable (transitional) throughout the entire length of the test plate. Results from the rough patch study indicate the boundary layer thickness and momentum thickness Reynolds numbers increased over the rough patch and the shape factor increased over the rough patch but then decreased downstream of the patch. It was also found that flow downstream of the patch experienced a gradual re-transition to laminar-like behavior but in less time and distance than in the smooth plate case. Additionally, the rough patch caused a significant increase in streamwise turbulence intensity and normal turbulence intensity over the rough patch and downstream of the patch. The skin friction coefficient over the rough patch increased by nearly 2.5 times the smooth plate value. Finally, the rough patch caused the Reynolds shear stresses to increase in the region close the plate surface.