Flow around an individual morphologically complex plant: investigating the role of plant aspect in the numerical prediction of complex river flow

Aquatic vegetation has a significant influence on the hydraulic functioning of river systems. Plant morphology has previously been shown to alter the mean and turbulent properties of flow, influenced by the spatial distribution of branches and foliage, and these effects can be further investigated through numerical models. We report on a novel method for the measurement and incorporation of complex plant morphologies into a computational fluid dynamics (CFD) model. The morphological complexity of Prunus laurocerasus is captured under foliated and defoliated states through terrestrial laser scanning (TLS). Point clouds are characterised by a voxelised representation and incorporated into a CFD scheme using a mass flux scaling algorithm, allowing the numerical prediction of flows around individual plants. Here we examine the sensitivity of plant aspect, i.e. the positioning of the plant relative to the primary flow direction, by rotating the voxelised plant representation through 15° increments (24 rotations) about the vertical axis. This enables the impact of plant aspect to be quantified upon the velocity and pressure fields, and in particular how this effects species-specific drag forces and drag coefficients. Plant aspect is shown to considerably influence the flow field response, producing spatially heterogeneous downstream velocity fields with both symmetric and asymmetric wake shapes, and point of reattachments that extend up to seven plant lengths downstream. For the same plant, changes in aspect are shown to account for a maximum variation in drag force of 168%, which equates to a 65% difference in the drag coefficient. An explicit consideration of plant aspect is therefore important in studies concerning flow-vegetation interactions, especially when reducing the uncertainty in parameterising the effect of vegetation in numerical models.