| Roughness can have a significant impact on turbine performance through increased profile losses from aerodynamic drag and increased internal blade cooling requirements resulting from external heat transfer enhancement. Transitional flow boundary layers have been shown to exist over turbine blades for various operating conditions. Although the turbine blade roughness is nonuniform and three-dimensional, not much work has been done to determine the multi-scale effect of variations in roughness on boundary layer transition.;An experimental study was conducted using flow over a heated flat plate with two different levels (or scales) of roughness placed on the surface. Since turbine blade measurements indicate that the leading-edge is the roughest area, the first roughness scale was larger than the second. To test geometric dependency, the leading-edge roughness took the form of a sandpaper strip, a single cylinder, or a rectangular strip, and the downstream surface was either smooth or covered with sandpaper. Under conditions of low free-stream turbulence (0.5--0.9%) and zero pressure-gradient, boundary layer and surface heat transfer measurements were made to determine the structure of the transition process, and spectral and wavelet analyses of the data were conducted.;Roughness levels as small as k+ < 0.6, although considered aerodynamically smooth, were shown to significantly hasten the onset of transition. The intermittency function obtained from the concentrated breakdown model followed the rough-wall intermittency data reasonably well. The growth of disturbances during transition over rough surfaces was observed to be spectrally broader than the Tolhnien-Schlichting frequency range. The step-change between the two roughness scales was discovered to be the dominant contributor to transition onset as a result of step-induced separation and corresponding vortex dynamics, which were verified through spectral analysis. During transition, distributed roughness was shown to reduce the growth of velocity fluctuation through faster development of turbulent dissipation, and wallward transport of momentum was significantly enhanced without a corresponding increase in thermal transport. |
