Transport of momentum and scalars in turbulent boundary-layer flows over complex terrain has been of great interest in the atmospheric sciences and wind engineering communities. Applications include but are not limited to weather forecasting, air pollution dispersal, aviation safety control, and wind energy project planning. While linear models have been well accepted to predict boundary-layer flows over topography with gentle slope, modeling flow separation and recirculation induced by topography of sufficiently steep slope has to be achieved through non-linear models, such as Reynolds-averaged Navier-Stokes (RANS) solvers and Large-Eddy Simulations (LES). High-quality measurements in the field and laboratory settings are in high demand for development and validation of such numerical models. Dynamics of the separated boundary-layer flows over steep topography is affected by the shape and size of the topography, surface characteristics (e.g., roughness and temperature distribution) and atmospheric thermal stability. Majories of wind-tunnel experiments of boundary-layer flows over representative and idealized topography features (e.g. 2-D or 3-D hills, axisymmetric bumps) do not take thermal stability effects into account due to challenges in physical simulation. We conducted experimental investigation of stably-stratified boundary layers over a steep 2-D hill in the thermally-controlled boundary-layer wind tunnel. The 2-D model hill has a steepest slope of 0.73 and its shape follows a cosine square function. High-resolution Particle Image Velocimetry (PIV) provides dynamic information of the separated shear layer, the recirculation zone and flow reattachment. Mean surface shear stress and surface heat flux were directly measured in the wake. Results indicate that suppressed turbulence in the stable boundary layer noticeably alters the topology of the circulation zone. Surface shear stress and surface heat flux downwind of the 2D hill slowly approach the equilibrim values of the non-disburbed boundary layers. This work can improve our understanding of the effects of thermal stability on steep topography, and provide reliable datasets for development and validation of numerical models.
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