The present experimental work investigates the evolution and transitional attributes of the geometry-induced separated shear layer (SSL) formed over an anisotropic or three-dimensional irregular rough surface – generated by sandblasting process. Emphasis is placed on the spanwise-vorticity field, thereby exploring the active region of SSL, referred to as the active shear layer, and correlating its topological changes with various stages of transition. In addition, the effect of surface roughness on the onset of shear layer roll-up and nonlinear breakdown to turbulence is examined. In this context, velocity measurements are carried out over the smooth ( Case-I ) and rough ( Case-II ) surfaces, using a single-wire hotwire anemometer. The freestream is set at a low Reynolds number, Re t = 30,000, based on model thickness and inlet velocity, with a turbulence intensity, Tu ≈ 0.8%. The rectification errors and scatter in the measured data are tackled using the modified tanh expression, tanh -NP. The modifications tend to reflect the nonparallel effects observed in the separated region. Comparative analysis of Case-I and Case-II results reveals that the deteriorated surface drags the mean trajectory of core vortices close to the wall. As a result, the wall-vortex interaction is enhanced and thereby, the two-dimensional linear disturbance growth is prematurely distorted in Case-II . Furthermore, the onset of shear layer roll-up and nonlinear breakdown to turbulence occur 20% and 14% earlier, respectively, compared to Case-I . For the present configurations, the laminar SSL is inviscidly unstable due to Kelvin-Helmholtz (KH) instability mechanism, despite the surface nature.
Thulasiraman et al. (2026) studied this question.