Wind Tunnel Experiments on the Spatial Development of Fractal-Grid Turbulence (1st Report, Measurements of Fundamental Turbulence Properties by Using an I-Type Hot-Wire Probe)

Abstract

Spatial development of a turbulent field generated by a multiscale, fractal-grid is investigated in a wind tunnel. A square type (i.e., fractal elements with square shapes) fractal grid is placed at the inlet of the test section. The Reynolds numbers based on the length of the biggest grid bar are set to 5900 and 11400. Turbulent characteristics in the downstream region of the grid are measured by using a hot wire anemometer. The results on the centerline statistics are compared with those in the previous experiments at the same Reynolds numbers. The results on the centerline statistics generally agree with the previous measurements despite the difference in the size of the test sections used. The spatial developments of cross-sectional profiles are also measured. It is found that the non-uniformity in the mean velocity field almost disappear in the downstream direction, although the non-uniformities of higher-order statistics are still observed even in the far downstream region. In the entire cross section of the tunnel, the ratio of the integral length scale Lu to the Taylor microscale λ hardly changes in the decay region of the rms velocity. These results will provide useful information in clarifying the non Richardson-Kolmogorov cascade process in the fractal-generated turbulence found in the previous measurement (Mazellier and Vassilicos, Physics of Fluids, Vol. 22, 075101 2010).