Evolution of turbulent diffusivity in heterogeneous surface boundary layers

This investigation presents a systematic characterization of boundary layer dynamics and turbulent diffusion properties across heterogeneous terrain surfaces through wind tunnel experiments. Five representative surface topologies were engineered to mimic natural terrain features, including smooth plate, sand surface, gravel surface with graded particle sizes (1–4 mm and 4–8 mm diameter ranges), and grass surface. Boundary layer evolution within the test section was quantitatively mapped using Pitot tube measurements, while hot-wire anemometer captured turbulent transport phenomena at five distinct Reynolds numbers (Re = 2.05 × 105–4.93 × 105) within the fully developed flow region. Comparative analysis of streamwise and vertical diffusion coefficient distributions under different surface roughness and Re conditions revealed nonlinear vertical evolution patterns of turbulent diffusivity, with particular emphasis on mechanistic interpretations. The results exhibit significant differences in diffusion coefficients across various underlying surfaces. The increase in diffusion coefficients along the streamwise and vertical direction within the near-surface region is primarily attributed to the development of integral length scale, while the subsequent decline following the peak value is closely associated with energy variations within the low-frequency range. For streamwise diffusion coefficients, all surface types demonstrate similar unimodal distribution patterns, with peak values increasing significantly with surface roughness. Grass surface exhibits a distinct unimodal distribution in vertical diffusion coefficients, with identical peak positions to streamwise diffusion coefficients.