How turbulence develops from a randomly spinning active grid

We present results from time-resolved PIV measurements taken directly downstream of a turbulence-generating active grid installed in a water channel with Re_λ ≈ 10³. Grid-generated turbulence is typically studied at locations downstream of the grid, where it is developed and homogeneous, isotropic turbulence. The turbulence at these locations finds its origin farther upstream, in the region of inhomogeneous turbulence directly downstream of the active grid, that has remained relatively unexplored. To unravel the mystery of the interaction between the active grid and the flow passing through, we compare three fully-random active grid operation modes of different mean wing-rotation frequencies, and a static grid reference case of minimum blockage, at locations between 2 and 4 mesh sizes (0.1 and 0.2 channel-widths) downstream of the grid. With increasing wing rotation frequencies, we find a decrease in the mean turbulence intensity. This indicates that the rotation of the wings does not introduce vorticity to the flow directly, as this would show the opposite relation, but that perhaps another production mechanism is involved. We explore this idea and compare results obtained close to the grid to positions farther downstream to gain insight in the evolution of grid-generated turbulence.