Atmospheric stability effects on turbulent flow over a steep 2-D hill

Complex topography affects the distribution of turbulent fluxes of momentum and heat in thermally stratified boundary layers. Neutral boundary-layer flows over simplified topography (for example, 2-D or 3-D hills) have been extensively studied by wind-tunnel experiments and numerical simulation such as Large-Eddy Simulation (LES). Atmospheric stability, however, is seldom considered due to the difficulty of physical simulation in wind tunnels. Additionally, accurate prediction of separated flows induced by steep hills remains a challenge to LES modeling. In this study, systematic experimental investigation of various thermal stratification effects (stable and unstable) on the boundary-layer flows over a steep 2-D hill were conducted 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 for (H=7cm and L=14.5 cm). The ratio of the maximum height to the boundary layer thickness (H/δ) is about 0.12. High-resolution Particle Image Velocimetry (PIV) provided dynamic information of the onset of separation, the recirculation zone and flow reattachment. Turbulent momentum and heat fluxes were characterized using a triple-wire at selected stream-wise locations. Emphasis is made on the effect of thermal stratification on the dynamics of flow separation induced by the hill as well as the downwind flow recovery process. The present study aims to improve our understanding of thermally-stratified boundary layer behavior over a steep 2-D hill under controlled conditions, and provide reliable data sets for development and validation of LES models.