Spatio-temoral characterization of riblets-induced separation three-dimesionality on a curved surface

Separation in turbulent boundary layers subjected to adverse pressure gradients (APG) remains a key challenge in high-Reynolds-number flow control. Our recent experiments on converging–diverging (CD) riblet-modified surfaces have demonstrated that weak secondary flows, introduced upstream of a backward-facing smooth ramp, become significantly amplified under adverse pressure gradient conditions. This amplification leads to highly three-dimensional separation dynamics characterized by spanwise-varying bubble sizes, shear-layer instabilities, and turbulence intensities. In particular, low-momentum pathways (LMPs) exhibit larger, more persistent separation compared to high-momentum pathways (HMPs), where flow remains more attached. While time-averaged PIV has clarified mean and instantaneous structure of separation, the spatiotemporal evolution and three-dimensional nature of separation in these flows remains poorly resolved. The current work aims to extend this effort using high-speed planar PIV to capture time-resolved dynamics of shear-layer roll-up and vortex shedding, alongside volumetric (tomographic) PIV to resolve three-dimensional flow structures and vortex organization within the separated region. These measurements will enable a more comprehensive understanding of how riblet-induced secondary motions interact with separation and reattachment processes, ultimately guiding the development of passive control strategies for riblet-textured surfaces in high-Reynolds-number flows.