Simulating approach flows similar to the atmospheric boundary layer in a newly built atmospheric boundary layer wind tunnel

At Eindhoven University of Technology a new large-scale Atmospheric Boundary Layer Wind Tunnel (ABLWT) was inaugurated in December 2017. The closed-loop subsonic WT can be operated in open and closed test section (Fig. 1a). When operated in closed test section, the test section is 27 m long and has a cross-section of 3 m width and 2 m height. The long test section allows the development of approach flows using a relatively long fetch of artificial roughness. In an initial attempt to simulate reduced-scale approach flows with inherent characteristics similar to those found in ABL flows, a parametric study was conducted using several passive devices. The aim of this study was fourfold: (i) to investigate differences in the approach flow when using differently shaped roughness elements (i.e., cubes and L-shapes), (ii) to analyze the effect of uniform / non-uniform roughness element heights on the approach flow (Fig. 1b), (iii) to assess differences in the approach flow due to different packing densities of roughness elements, (iv) to discuss the effect of differently shaped base extensions for vortex generators (i.e. type of spires proposed by Irwin, 1981) on the approach flow. First results suggest that the use of non-uniform roughness element heights is essential for the simulation of reduced-scale boundary layers that show characteristics similar to those found in the atmospheric Prandtl layer (corresponding to approximately the lowest 100 meters of the ABL). The effect of differently shaped base extensions for vortex generators, on the other hand, showed little effect with respect to flow characteristics of the approaching vertical profile but had a limiting effect on the span wise homogeneity measured at the end of the fetch of roughness.