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This study presents experimental results on the effects of riblets on the coherent structures of turbulence within a turbulent spot. The riblet spacings of the study correspond to 0.5 and 1.5 times the natural spacing of the low-speed streak. The cross-sectional dimensions of the riblets were chosen to control the spatial distribution of wave packets consisting of streamwise-aligned hairpin vortices. Both riblet spacings demonstrated effective control on the spanwise positioning of the wave packets. The wider-spaced riblets reduced spanwise mutual interaction between wave packets. The closer-spaced riblets promoted this interaction via spanwise-oriented vortical structures which produced stronger turbulent fluctuations.

This study presents experimental results on the effects of streamwise-oriented riblets on the coherent structures of turbulence. Hotwire measurements were performed in artificially created turbulent spots. The riblet spacings of the study correspond to 0.5 and 1.5 times the natural spacing of the low-speed streaks and fall into the category identified as wider-spaced in published literature. The cross-sectional dimensions of the riblets were chosen to promote more effective control on the development and spatial distribution of wave packets consisting of streamwise-aligned hairpin vortices.The riblets proved to be highly effective in controlling the spatial positioning of wave packets. Of the two riblet spacings considered, the wider spacing increased the spanwise spacing of the low-speed streaks beyond their natural spacing and stabilized the wave packets over the riblet tips, enabling a reduction in their mutual interaction and realizing a reduction in their spanwise density compared to the conditions on a smooth surface. This effect may be optimized to achieve skin-friction drag and aerodynamic noise reduction. The closerspaced riblets were observed to have even more control on the spanwise positioning of the wave packets, and produced notably stronger sweep and ejection events by reducing the spanwise spacing between wave packets and promoting mutual interaction of hairpin vortices via spanwise-oriented vortical structures created by a Kelvin-Helmholtz instability mechanism.This effect may be used to achieve increased convection heat transfer in various applications without significant penalties in pressure loss.iii Acknowledgments This work would not have been possible without the support and mentorship of my supervisor, Professor Metin I. Yaras. I would like to thank him for his patience and his ability as an educator to bring out the best in his students while always keeping their development in mind. This manuscript represents only a small portion of the knowledge I have gained working with him.