The effect of gaps on the turbulent flow within and above canopies resembling river vegetation

The turbulent flow within and above canopies resembling river vegetation was experimentally investigated using particle image velocimetry (PIV). Experimental tests were conducted in a water channel with a 0.2 × 0.2 × 2 m³ test section. The focus of the experiments was to characterize the flow surrounding gaps in canopies to better understand the turbulence and mixing levels that are produced. The experimental canopy model was composed of rectangular elements with height h arranged in a staggered pattern. The model spanned the entire width of the water channel and included a gap with a streamwise length L. The flow was measured in and surrounding gaps with  varying lengths (L/h = 0.5, 1, 2, 3 and 4). The center of the gap was located at 125 cm from the inlet, preceded by 80 cm of the canopy and followed by the 20 cm of canopy. Each gap length was also tested at different submergence depths H/h = 0.5, 1, 2, 3, and 4, where H is the flow depth. Incoming velocity was kept constant at U­­_∞ = 0.1 m/s for all cases, which yielded Reynolds numbers Re_H ≈ 5,700, 8,600, 11,500, and 14,300. The time-averaged streamwise velocity contours show a recirculation zone with low velocity at the bottom half of the gap. The recirculation zone grows in length as the size of the gap increases. However, the height of the recirculation zone decreases with increasing L. A region of high turbulence and Reynolds shear stress is present along the top of the gap and penetrates within it. Overall, the results indicate the effect of gap length on the mean flow field and turbulence quantities which govern the transport of scalars (e.g., nutrients) within canopies such as river vegetation.