Investigation of a grid induced turbulent environment for wind tunnel testing

In performing wind tunnel model tests of a two-dimensional nature, a grid constructed of 3.6 x 0.75 inch boards on 18 inch centers was used at the entrance of the tunnel. Important properties of the turbulent flow behind the uniform coarse grid were determined. The flow was found to be quite uniform at a distance of 38 bar widths downstream of the grid. Use of this flow to simulate atmospheric turbulent flows for testing sectional models of suspension bridges is discussed. The power spectra of the turbulence was found to agree quite well with the von Karman spectrum equation for atmospheric turbulence. The integral length scales were found to increase with increasing distance downstream of the grid. Integral length scales of the longitudinal turbulence component were found to increase in size as the mean velocity was increased. Measurements indicate that grid turbulence seems to model the atmospheric turbulence quite well for studies of suspension bridges and other elevated structures. Meaningful quantitative results may be obtained if geometric modeling of the integral length scales of the atmospheric turbulence is of minor importance for proper model response.