Dynamical topology of the near wake behind a slender circular cylinder with variable surface roughness

We present a controlled experimental dataset of the near wake behind a long, smooth, and rough-surface circular cylinder, acquired with time-resolved classical and stereoscopic PIV in a low-turbulence free stream. A high-aspect-ratio model is used to approximate the infinite-span limit, and measurements are taken in the central span to minimize end effects across a subcritical Re of about 5k. We report robust, facility-independent results: instantaneous and mean velocity fields in the streamwise and crossflow planes of the cylinder wake. Emphasis is placed on measurement fidelity and reproducibility: calibration and self-calibration procedures, mapping accuracy, timing, correlation peak metrics, and ensemble convergence are documented to enable error estimation. Spanwise homogeneity checks substantiate the “long-cylinder” approximation in the measurement region and delineate its limits. The effect of cylinder surface roughness is studied by presenting results for both a smooth cylinder, R0, and a cylinder with defined sand roughness, R2. The resulting dataset provides a clean baseline for CFD/LES validation and a reference against which future studies can superimpose controlled roughness, imposed inflow turbulence, or finite-length/end-plate effects. Below are examples of typical time-mean flow fields for both the smooth and rough cylinders.