Experimental characterization of the flow and turbulence generated by fractal oscillating grids

Inspired from the existing literature on fractal grids in channels and as an extension to classical oscillating grid experiments with simple Cartesian grids, an original investigation of fractal oscillating grid turbulence is here reported. The flows generated by a simple Cartesian grid, a fractal Cartesian grid, a fractal square grid, and a fractal I-shaped grid are studied using particle image velocimetry. Three oscillation frequencies (0.5, 1, and 1.5 Hz) and three stroke amplitudes (0.02, 0.035, and 0.05 m) are considered. The flows are broken down into mean (time averaged), oscillatory (phase dependent), and turbulent contributions using the triple Reynolds decomposition. The oscillation frequency is found to linearly impact the intensity of the mean and the oscillatory flows and the root mean square values of the turbulent fluctuations. In turn, an increase in the stroke amplitude tends to change the topology of the mean and the oscillatory flows. The turbulence intensity is increased by the fractal nature of the grids and is impacted by the mean flow topology, especially for the fractal I-shaped grid for which turbulence is transported away from the grid wake region. The study of the turbulence length scales and spectra reveals that the scales of turbulence mainly depend on the stroke amplitude and the grid geometry. We thus show how fractal oscillating grids can be used to generate turbulence with tailored properties for fundamental studies and practical applications.