Generating atmospheric turbulence using passive grids in an expansion test section of a wind tunnel

Generating atmospheric turbulence in wind tunnels is an important issue in the study of wind turbine aerodynamics. A turbulent inlet is usually generated using passive grids. However, to obtain an atmospheric-like flow field relatively large length scales (L~30 cm) and high turbulence intensities (I~15 %) need to be reproduced. In this work, the passive grid technique has been used in combination with a downstream expansion test section in order to investigate the generation of atmospheric like turbulence, with the possibility of varying both the turbulence intensity and the integral length scale of the flow field independently. Four passive grids with different mesh and bar sizes were used with four wind velocities and five downstream measurement positions. It was found that the flow field is isotropic and homogeneous for distances less than what is recommended in literature (x/M~5). The effect of the expansion on the turbulence characteristics is also investigated in detail for the first time. The study confirms that by adding an expansion test section it is possible to increase both turbulence intensity and integral length scale downstream from the grid with limited impact on the overall flow quality in terms of anisotropy and energy spectra.