Large scale structures modification of a spatially evolving turbulent boundary layer grazing over circular cavities

Two experimental investigations were conducted to characterise the impact of circular cavities on the streamwise development of a turbulent boundary layer. Hot-wire anemometry was employed to measure the boundary layer at different streamwise locations along a perforated surface. While the modification of the boundary layer thickness was negligible, the shape factor increased and the friction coefficient decreased with respect to the smooth baseline case. The skin friction downstream of the perforations was investigated by oil droplet interferometry, confirming the skin friction reduction reported in previous investigations. The contour of the premultiplied energy spectrogram showed a “two-peaks” behaviour, where the inner peak shifted towards lower wavelengths, λX+, and upwards, suggesting that the streaks were shortened and lifted up by the cavities in the perforated region. The outer peak, can be associated with the enhancement of turbulent mixing and inner/outer layer interaction. Particle image velocimetry measurements, conducted in a streamwise plane downstream of the cavities, showed an increased Reynolds shear stress associated with a significant enhancement of the ejections contribution. A negligible variation in the number of Uniform Momentum Zones in the presence of cavities was observed, but a reduction in the modal velocity associated with the logarithmic region would confirm the formation of low momentum pathways. These outer structures play a significant role in the turbulent mixing enhancement that could be exploited for heat transfer purposes.