The Influence of Deterministic Surface Roughness and Free-Stream Turbulence on Transitional Boundary Layers: Heat Transfer Distributions and a New Transition Onset Correlation

Abstract

Abstract Heat transfer measurements in transitional flat plate boundary layers subjected to surface roughness, strong pressure gradients and free stream turbulence are presented. The surfaces considered, consist of a smooth reference and twenty six deterministic surface topographies that vary in roughness element aspect ratio, height and density. They are designed to cover the full range of roughness regimes from smooth over transitionally rough to fully rough. For each surface, two pressure distributions, characteristic for a suction and a pressure side turbine vane, are investigated. Inlet Reynolds numbers range from 3.0 · 105 to 6.0 · 105 and inlet turbulence intensity is varied between 1% to 8%. Furthermore, different turbulence Reynolds numbers, i.e. turbulence length scales, are realized while the incident turbulence intensity is kept constant. Additionally, the turbulence intensity and Reynolds stress distributions in the free-stream along the flat plate are measured using x-wire probes. Results show a strong influence of roughness and turbulence intensity on the onset of transition. The new data set is used to develop an improved correlation considering the roughness height, density and shape as well as the turbulence intensity and turbulent length scales.