Development of turbulence behind the single square grid

In this paper, direct numerical simulations are carried out to study single-square grid-generated turbulence at a Reynolds number Re_L0 = 20 000 (based on the inlet velocity U_in and the length of grid bar L₀). Different from the regular grid and the multiscale/fractal grid, here only single large square grid is placed at the center near the inlet. First, we investigate the evolutions of turbulence characteristics (e.g., mean streamwise velocity, turbulence intensity, Taylor microscale, etc.) along the centerline. The common characteristics possessed by turbulent flows generated by the single square grid and by the fractal square grid are presented. We confirm the hypothesis proposed by Mazellier and Vassilicos ["Turbulence without Richardson-Kolmogorov cascade," Phys. Fluids 22, 075101 (2010)] that for the fractal square grid, the location of turbulence intensity peak along the centerline is mainly determined by large-scale wake interactions. Current numerical results show that in turbulence generated by the single square grid,wake interactions occur close to the grid and cause extreme/intense events. Then, the spatial development of invariants of the velocity gradient tensor is studied. For example, the (Q_W, -Q_S) maps are analyzed to show how turbulence generated by a single square grid obtains large scale vortices along the centerline.