Parametrization of the Reproduction of a Representative Atmospheric Boundary Layer in the Wind Tunnel

Wind tunnels are commonly used to study applications where atmospheric flows are encountered, like wind effects on buildings, dispersion of pollutants, or wind resource assessment. To perform a proper physical modeling of an atmospheric flow in the wind tunnel, a crucial parameter is the reproduction of the wind inflow conditions. This reproduction requires a lot of attention and experience from the experimentalist to choose the appropriate boundary layer generators able to develop the proper velocity, turbulent profile and turbulent spectra that would best fit to the real wind conditions.

In the literature, lots of techniques are used to develop a representative boundary layer in the wind tunnel using grids, fences, spires, wings or ground roughness elements. The objective of this work is to perform a parametric study in order to determine the effect of individual elements on the boundary layer characteristics. Empirical and theoretical relationship linking boundary layer generation devices and boundary layer characteristics are proposed to help the experimentalist in choosing the most suitable configuration adapted to simulate the real wind profile.

The parametric study is carried out in two different wind tunnels of the von Karman Institute, VKI-L1, a close loop wind tunnel (3 m x 2 m x 15 m) and VKI-L2, a smaller wind tunnel suitable for parametric studies (0.35 m x 0.35 m x 2 m), both tunnels are equipped for atmospheric simulation. Measurements are taken with single and three-component hot-wire probes and they are compared with the literature and CFD simulation performed with FLUENT software.

The parametric study includes the investigation of the effect of the roughness element height and density, the fence height and number, and the presence of a grid or wings. Single and triple hot-wire probes are used in this study to characterize the simulated wind in all directions.

Results present the influence of each boundary layer generator device and their combination on the boundary layer characteristics: height, velocity and turbulence. From this, the experimentalist has tools to decide which device to use in the test section and how to modify it to fit the required profile. For instance, the fence height has a long distance influence and controls directly the boundary layer height but the number of fences has a very limited influence. The roughness element height is also directly linked to the boundary layer height but its influence is limited to a much lower level. It is then also possible to control the velocity and turbulence profile with the combination of devices. Results are discussed and compared with literature.