Effects of block size and arrangement on downstream grid-generated turbulence structures in open-channel flow

Block arrays are widely used as laboratory roughness elements, yet how block size and arrangement affect downstream turbulence and momentum transport remains poorly understood. Particle image velocimetry measurements downstream of five beds in subcritical open-channel flow (Re=3.51×104 and Fr=0.08) compare a no-block (NB) reference with four near-cubic block arrays: small aligned (SA), small staggered (SS), large aligned (LA), and large staggered (LS). The approach flow is conditioned by a square-bar turbulence-generating grid, so the study characterizes how a finite block array reorganizes controlled, grid-generated turbulence. All arrays redistribute momentum vertically, producing near-block velocity deficits with faster flow above; block size dominates, larger blocks generating deeper, more intense deficits. Unlike NB, all rough cases sustain predominantly positive Reynolds shear stress across the domain, indicating continuous downward momentum transport attributed to block-generated shear layers and wakes. Turbulent kinetic energy production increases with block size, by 9.0% (SA), 28.6% (SS), 322.2% (LS), and 375.9% (LA) relative to NB. Integral length scales decrease in all rough cases, and quadrant decomposition confirms a shift toward ejection-sweep (Q2–Q4) dominance with geometry-dependent vertical penetration. Sweeps are more frequent near the blocks, but ejections are more energetic and dominate the Reynolds shear stress, even under hole filtering. Within the present conditioned-inflow configuration, block size governs the magnitude and vertical extent of roughness-induced turbulence, while arrangement modulates its spatial organization, guiding block-array design in laboratory open-channel flows.