High-Reynolds-number turbulent pipe flow over anisotropic roughness: response and recovery downstream of a smooth-rough-smooth step change

Surface roughness is an intrinsic characteristic of almost every surface we encounter when designing, measuring or modelling a fluid dynamic system, and is a persistent source of uncertainty in the description of wall turbulence. Real textures are rarely homogeneous or isotropic: they are multiscale and directionally anisotropic, making it difficult to represent their aerodynamic effect using a single geometric descriptor. This difficulty is particularly relevant in wall-bounded turbulent flows over realistic rough surfaces, where roughness morphology can alter wall-turbulence dynamics and tangential stress, with direct consequences for near-wall momentum transfer and drag. This thesis analyses the response of a turbulent flow in a circular duct at high Reynolds number to real roughness, applied as a wall insert of finite length and with partial circumferential coverage, followed by a sudden restoration of the original smooth wall, so as to achieve a controlled transition from rough to smooth (R–S). The experiments are conducted in the CICLoPE facility (University of Bologna), and the measurements are performed using single-wire hot-wire anemometry (CTA), acquiring profiles normal to the wall at multiple axial positions relative to the R–S edge. The data set shows that the recovery of the flow downstream of the rough-smooth transition cannot be described as a single monotonic relaxation process towards the smooth reference. On the contrary, the region near the wall and the outer region can evolve according to distinct trajectories, in accordance with the physical framework of the internal boundary layer and with the scale-dependent recovery trends reported in the literature. In particular, the partial rough coverage and the morphology of the real surface make it problematic to use a single “equivalent” roughness parameter.