Characteristics Of The Turbulent Flow Within Short Canopy Gaps

The turbulent flow within short canopy gaps was experimentally investigated using planar particle image velocimetry. The gaps were situated in the fully developed region of dense homogeneous canopies in shallow submergence at a Reynolds number of 20 000, based on the flow depth and the free-stream velocity. The flow was studied for gaps with lengths L/h = 0.5, 1, 2, 3, and 4, where h denotes the canopy height. The results suggest that dense canopies behave as one unit despite the presence of a short gap following a long upstream patch; this is supported by the lack of upward flow deflection and the minimal disruption to in-plane vortical activity at the leading edge of the downstream patch. Across the five gaps, a recirculation zone, noted in the time-averaged flow field, forms past the upstream patch. Flow reattachment occurs at the trailing edge of the L/h = 4 gap and within the downstream patch for the other gaps. Building on previous studies, the results presented here reveal the importance of the length of the upstream patch in setting the flow regime within the gaps. A thorough examination of the time-averaged streamwise and wall-normal velocities, shear layer growth, in-plane turbulent kinetic energy, Reynolds shear stress, and two-point correlations suggests a classification of the flow into two regimes: a skimming flow regime (L/h = 0.5 and 1) and a shear layer growth regime (L I h = 2, 3, and 4). In contrast with the skimming flow regime, the shear layer and enhanced turbulence exhibit deep penetration within the gaps and promote mixing in the shear layer growth regime. It is important to note that the presented results and conclusions are based on planar measurements and, therefore, do not capture any variations in the spanwise direction. Volumetric measurements are suggested to capture such variations and to provide a more detailed description of the flow field.