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Fractal grids generate multi-scale turbulent flows which interact with each other and form new multi-scales with different properties compared with those generated by classical grids. The goal of this work was to study the influence of geometry multiscale fractal on the generation and decay of turbulence by comparing the turbulence produced by a traditional fractal square grid FSG for short to that produced by a spaced fractal square grid SFSG for short with similar physical properties. The velocity measurements were performed in an open circuit suction type wind tunnel using a constant temperature hot wire anemometer at various positions in the x direction along the tunnel’s center line and for three different Reynolds numbers ReL0 based on the inlet velocity U? and the length of the largest grid bar L0. It was found that the turbulence intensity decays with distance from the grid for SFSG. Whereas, for FSG at low Reynolds numbers the turbulence intensity shows the same behaviour as for SFSG but for higher Reynolds numbers the turbulence intensity increases to reach a peak at a distance xpeak from the grid and then decays. In the inhomogeneous region, for both types of grids, a scaling range spans over a decade with an exponent near, but not exactly, k?5/3. In contrast, the length of the scaling range larger for FSG than for SFSG.

The Decay of Turbulence Generated by Square Grid and Spaced Square Grid