Effect of Step Change in Surface Roughness on the Evolution of the Turbulent Thermal Boundary Layer

Surface roughness significantly impacts both thermal and momentum transport in turbulent flows, yet their coupled behavior—particularly under non-equilibrium conditions—remains less well understood. This experimental study investigates thermal transport mechanisms in a turbulent boundary layer subject to a step change in roughness, using a modular heated wall system in a low-speed wind tunnel. Surface temperature distributions are captured via high-resolution infrared thermography, revealing a persistent thermal boundary layer downstream of a mildly heated roughness patch. These temperature footprints indicate enhanced mixing and a deviation from the classical Reynolds analogy-type scaling. To probe the underlying mechanisms, cold-wire thermometry is employed to measure wall-normal temperature profiles, and hot-wire anemometry is applied to measure wall-normal velocities at multiple downstream locations. These measurements are correlated with the surface IR data to assess the thermal footprint of the boundary layer and the dissimilar growth of temperature and velocity profiles downstream of roughness, particularly within the roughness sublayer. The results will offer new insight into the interplay between scalar and momentum transport in rough-wall turbulent flows.