<p>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 300000 to 600000 and inlet turbulence intensity is varied between 1&nbsp;%&nbsp;to 8.5&nbsp;%. Furthermore, different turbulence Reynolds numbers, i.e. turbulence length scales, are realised while the incident turbulence intensity is kept constant. Additionally, the turbulence intensity, Reynolds stress distributions and turbulence spectra in the free-stream along the flat plate are measured using x-wire probes. Results show a strong influence of roughness and turbulence intensity but also turbulence length scales on the onset of transition. The new data are compared with existing data from linear cascade measurements and correlations for the onset of transition. Finally, an improved correlation considering the roughness height and density as well as turbulence intensity and turbulence length scales is presented.</p>

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

Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2021/volume/84874

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