Predicting Rough Wall Heat Transfer and Skin Friction in Transitional Boundary Layers: A New Correlation for Bypass Transition Onset

Abstract

This paper summarizes the experimental results of a comprehensive study on heat transfer and aerodynamic losses of a highly loaded turbine blade with surface roughness. A few hundred test cases conducted at several Reynolds numbers, freestream turbulence levels, and different deterministic roughness geometry have been examined. Some of these results have been published in two former papers showing a strong effect of roughness on laminar-turbulent bypass transition on the airfoil suction side. Beside roughness height, roughness anisotropy has turned out to be one of the major influencing factors. The airfoil heat transfer distribution of these measurements is used for detecting transition onset. Additionally, further transition onset data from literature is reevaluated. Thus, important roughness (geometry) parameters are identified and a new correlation for transition onset is deduced including roughness parameters as well as freestream turbulence. Moreover, a method to extract the relevant roughness parameters from real surface roughness is presented. Additional heat transfer and aerodynamic measurements are conducted for two different real surface roughness types. Calculations with a 2D-boundary layer code on these surfaces are presented in order to validate the new model.