Wind-tunnel experiments of stably-stratified boundary layers over a steep 2-D hill

Complex topography is known to significantly affect the distribution of turbulent fluxes of momentum and heat in stably-stratified boundary layers. Boundary-layer flows over simplified topography, e.g.2-D or 3-D hills, have been extensively studied by wind-tunnel experiments and numerical simulation such as Large-Eddy Simulation (LES). Thermal stratification, however, is seldom considered due to difficulty of physical simulation in wind-tunnel experiments. Additionally, accurate prediction of separated flow induced by a steep hill remains a challenge to LES modeling. This work presents an experimental investigation of stably-stratified boundary layers over a 2-D hill with both rough and smooth surfaces at the Saint Anthony Falls Laboratory atmospheric boundary-layer wind tunnel. The 2-D model hill has a steepest slope of 0.73 and its shape follows h=Hcos(πx/L) for -L/2≤x≤L/2 (the maximum height is H=7cm and the total width is L=15 cm). High-resolution Particle Image Velocimetry (PIV) provides dynamic information of the onset of separation, the recirculation zone and flow reattachment. Turbulent momentum and heat fluxes were characterized up to the top of the thermal boundary layer using a triple-wire at selected stream-wise locations. Results reveal remarkable effect of the roughness on the dynamics of flow separation induced by the hill compared to that over the smooth surface. These results will be further used to compare with neutral and unstable boundary layers over the same 2-D model hill in order to address thermal stratification effects. The current study aims to improve our understanding of the stably-stratified boundary layer behavior over a simplified topography under controlled conditions, and provide reliable data sets for development and validation of LES models.