Experimental investigation of characteristics of turbulence induced by multiple oscillating grids

Experiments were conducted to systematically investigate the nearly homogeneous turbulence generated by multiple oscillating grids. Altogether 54 runs were completed by employing five different grids. For each run, four to nine layers of grids were installed, which were operated with different frequencies, strokes and grid spacings. The particle image velocimetry was used to obtain flow velocity characteristics. Instantaneous flow measurements reveal that the flow structure varies depending on the grid configuration. For regular grids, turbulent wakes of nearly uniform size were generally observed. In comparison, for double-scale grids, the size of wakes varied with the bar width. In the near-grid region, the horizontal distributions of Reynolds stresses are strongly influenced by the grid geometry, showing obvious inhomogeneity especially for wide bars. The inhomogeneity reduces as the distance from the grid increases. At the midplane between two neighboring grids, the flow characteristics appear similar for all grid configurations. The size of the region where the turbulence appears nearly homogeneous is affected by the grid spacing and stroke. Generally, the double-scale grid can produce a greater homogeneous region than the regular grid. This study shows that the decay law of turbulence intensity developed for a single oscillating grid can be successfully extended to multiple oscillating grids. In addition, a theoretical interpretation of the decay law is presented by considering the variation in the velocity and size of wakes when evolving away from the grid.