Thermal stability effects on the separated flow over a steep 2-D hill

Turbulent transport of momentum and scalars in turbulent boundary-layer flow over complex topography has been of great interest in the atmospheric sciences and wind engineering communities. Applications include but are not limited to weather forecasting, air pollution dispersion, aviation safety control, and wind energy project planning. Linear models have been well accepted to predict boundary-layer flows over topography with gentle slope. However, once the slope of the topography is so steep that flow separation occurs, linear models are not applicable. Modeling the turbulent transport of momentum and scalars in such flows has to be achieved through non-linear models, such as Reynolds-averaged Navier-Stokes solvers and large-eddy simulations (LES). The dynamics of the separated boundary-layer flow over steep topography is affected by the shape and size of the topography, surface characteristics (e.g., roughness and temperature) and atmospheric thermal stability.

Most wind-tunnel experiments of boundary-layer flows over simplified topography (e.g. 2-D or 3-D hills, axisymmetric bumps) do not take thermal stability effects into account due to difficulty of physical simulation. Here we present an experimental investigation of stably- and unstably- stratified boundary layers over a steep 2-D hill at the Saint Anthony Falls Laboratory boundary-layer wind tunnel. The 2-D model hill has a steepest slope of 0.73 and its shape follows a cosine function. High-resolution Particle Image Velocimetry (PIV) provides dynamic information of the onset of flow separation, the recirculation zone and flow reattachment point. Turbulent momentum and scalar (heat) fluxes were characterized up to the top of the thermal boundary layer using a triple-wire (cross-wire and cold-wire) anemometer. Results indicate that promoted and suppressed turbulence related to the unstable and stable boundary layers substantially alters the topology of the circulation zone and the spatial distribution of turbulent momentum and heat fluxes around the steep hill. This work can improve our understanding of the effect of thermal stratification on topography under controlled conditions, and provide reliable data sets for development and validation of numerical models such as LES.