Experimental investigation on the degradation of turbulent friction drag reduction over semi-circular riblets

Experimental investigations of turbulent boundary-layer flows over a semi-circular riblet surface were conducted to study the mechanisms of friction drag reduction. Particle-image velocimetry (PIV) was performed to analyze the statistics of the turbulent boundary layer and the structural properties of the large-scale motions and hotwire anemometry (HWA) was conducted to determine the energy spectra of the near-wall turbulent structures. The skin friction coefficients varied from drag reduction of −5.75% via 0.3% to 10.7% drag increase for riblet spacings in inner units of 17, 31.3, and 50.4 at three freestream velocities. The analyses of the near-wall flow structures indicate that the streamwise coherence is enhanced in the drag reduction regime, while the spatial correlation of the near-wall turbulent flow structures is reduced in the drag neutral and increase regimes. The streamwise energy spectra indicate that in the near-wall region, the energy content is increased by coherent structures at wavelength in inner units in the range of 100–300 for the drag increase regime.