Characterization of vertical momentum transport of turbulent wind flow over complex terrain: Ridge vs hill

The momentum transport of turbulent winds over complex terrain poses critical challenges for assessing wind loads on civil infrastructure, optimizing wind resources for energy planning, and enhancing surface pollutant removal efficiency. However, the spatiotemporal characteristics of turbulent momentum transport within the topography-modulated boundary layer remain poorly understood. This study investigates the modulation effects of terrain topology for idealized three-dimensional (3D) hill and two-dimensional (2D) ridge cases. High-resolution large-eddy simulations, validated against wind tunnel measurements, are employed to quantify Reynolds shear stress and vertical momentum transport using a quadrant analysis. Results reveal that the ejection (Q2)-sweep (Q4) cycle dominates Reynolds stress production, particularly within shear layers, while terrain topology strongly modulates the relative contributions of Q2 and Q4 events. The 2D ridge and 3D hill demonstrate distinct transport processes. The former exhibits minor spanwise variation due to homogeneous boundary conditions, whereas the latter shows weakened vertical transport and pronounced lateral variability driven by free-side shear flows. The Q2 and Q4 events exhibit a double-Gaussian shaped spanwise distribution, which evolves downstream into a typical Gaussian profile for sweeps, while ejections maintain the double-Gaussian form. Temporal analyses further indicate probabilistic behavior of event duration and interval, with Q4 events occurring regularly and Q2 events marked by relatively shorter persistence and longer recurrence gaps. These findings highlight the critical role of terrain-induced shear layers in shaping turbulence transport mechanisms and provide new insights into the spatial-temporal organization of coherent motions over complex terrain.