On the effect of height heterogeneity on canopy turbulence

The flow development above and within homogeneous and heterogeneous canopies was studied using PIV in a refractive-index-matching open channel. The homogeneous model (base case) is constituted of elements of height h arranged in staggered configuration; whereas the heterogeneous canopy consisted of elements of two heights h1 = h + 1/3 h and h2 = h - 1/3 h alternated every two rows. Both canopies had the same roughness density, element geometry, and mean height. The flow was studied under three submergences H/h = 2, 3, 4, where H denotes the flow depth. High spatial-resolution planar and high-temporal resolution stereo-PIV measurements were performed at Reynolds number ReH = 6500, 11300, and 12300. The planar measurements are aimed at characterizing the flow along the entire length of the canopies, while the stereo measurements focused on quantifying the dispersive stresses at various vertical zones of the flow and the structure of the velocity fluctuations within the developed region. Turbulence statistics complemented with quadrant analysis and proper orthogonal decomposition reveal richer flow dynamics induced by height heterogeneity. Topography-induced spatially-periodic mean flows are observed for the heterogeneous canopy. In contrast to the homogeneous case, non-vanishing vertical velocity is maintained across the entire length of the heterogeneous canopy with increased levels at lower submergence depths. The results indicate that heterogeneous canopy exhibits enhanced vertical turbulent exchange at the canopy interface, suggesting a potential for greater scalar exchange and greater impact on channel hydraulic resistance.

The flow development above and within homogeneous and heterogeneous canopies was studied using PIV in a refractive-index-matching open channel. The homogeneous model (base case) is constituted of elements of height h arranged in staggered configuration; whereas the heterogeneous canopy consisted of elements of two heights h1 = h + 1/3 h and h2 = h - 1/3 h alternated every two rows. Both canopies had the same roughness density, element geometry, and mean height. The flow was studied under three submergences H/h = 2, 3, 4, where H denotes the flow depth. High spatial-resolution planar and high-temporal resolution stereo-PIV measurements were performed at Reynolds number ReH = 6500, 11300, and 12300. The planar measurements are aimed at characterizing the flow along the entire length of the canopies, while the stereo measurements focused on quantifying the dispersive stresses at various vertical zones of the flow and the structure of the velocity fluctuations within the developed region. Turbulence statistics complemented with quadrant analysis and proper orthogonal decomposition reveal richer flow dynamics induced by height heterogeneity. Topography-induced spatially-periodic mean flows are observed for the heterogeneous canopy. In contrast to the homogeneous case, non-vanishing vertical velocity is maintained across the entire length of the heterogeneous canopy with increased levels at lower submergence depths. The results indicate that heterogeneous canopy exhibits enhanced vertical turbulent exchange at the canopy interface, suggesting a potential for greater scalar exchange and greater impact on channel hydraulic resistance.