Turbulent flow over homogeneous canopies with gaps of various lengths

The fluid flow within a canopy of uniform, densely packed elements containing a gap of various lengths was measured using planar particle image velocimetry. The model was submerged within a water channel in an open channel configuration with a flow depth three times the height of the canopy. The canopy included a gap which measured 0.5, 1, 2, 3, or 4 times the canopy height. The Reynolds number based on the submergence depth of the flow was found to be 12,600. Based on mean flow quantifications and turbulence statistics, the flow within the gap was found to fall into two regimes based on the ratio of the gap length to gap height. For the short gap regime, the shear layer at the top of the gap does not experience vertical growth and therefore the turbulence does not penetrate into the gap; however, for the long gap regime, the shear layer experiences significant vertical growth and enhances mixing within the gap. When compared to a solid cavity, significant differences are found in the behavior of the shear layer that develops at the top of the gap. Finally, a fluctuating velocity cross-correlation was performed to analyze the structure of vortices across the top of the canopy and over the canopy gap and additional differences were noted in the development of vortical structures over the gap when compared to the solid cavity.