Breakdown of the Reynolds analogy over drag-reducing riblets surface

An experiment was carried out in a low-speed wind tunnel to study the near-wall turbulence structure over a heated riblets surface. The results confirmed the main conclusion of previous study that riblets can enhance the convective heat-transfer rate by as much as 35 percent even within the drag- reducing region. The logarithmic profile of the mean temperature was shifted downwards, indicating that viscous-sublayer thickness of the thermal boundary layer was reduced over a riblets surface. The reason for the apparent breakdown of the Reynolds analogy seems to be related to the difference in the turbulence length scale between the thermal and momentum boundary layers in the near-wall region.