CFD modelling of kiwifruit vines and leaves: a method of handling multiple thin surfaces

Abstract

Modelling air flow through vegetation is important in many areas of horticulture both for air quality monitoring and for calculating spray drift from agrichemical spraying operations. Modelling vegetation in computational fluid dynamics (CFD) has traditionally been carried out using the porous media properties of the leaves and branches. This entails an extra sink term in the momentum equations for the drag force of the leaves and branches; an extra source term for the turbulent kinetic energy generated by the vegetation; and the corresponding sink term for the turbulent dissipation. More complex models have been developed by Katholic University Leuven that have branching included into the CFD model with the leaves, small branches and petioles modelled using the porous media approach. The kiwifruit vine model developed here took this one step further to include details of the leaves but omitted the smaller branches and petioles, due to their small scales/sizes. The leaves were generated from the output of a model of a kiwifruit vine developed using the L-system-based plant modelling platform, L-studio. The output was converted into a suitable format for CFD using a conversion program, C4W. The leaf model developed did not include the leaf edges in the mesh as this would have created cells that were too small to run the model. The ANSYS workbench can include thin-surfaces in the model domain but it was impractical to handle that many leaves. The model was developed using the standard meshing in CFX version 12.1 to create a leaf with no edge. This was done by giving the leaf its normal width of about 200 μm and specifying a minimum cell size of 4.5 mm. The resultant leaf had a surface on either side, with about 35 faces each side to simulate the curved surface of the leaf. The present model was adequate only up to 200 leaves due to the large number of leaf surface boundaries that the CFD model cannot handle at present. Methods to reduce this are currently being investigated. The resultant CFD model was developed so it did not need sink/source terms for the vegetation and gave the main properties of the kiwifruit vine including the drag from the branches and leaves, as shown by the velocity field in Figure 1. This allows detailed modelling of droplet deposition on to the leaf, consideration of turbulence intensity and the inclusion of a droplet retention/deposition model. The problem of the change in drag created by movements or leaf fluttering is to be the subject of future research. The model will be validated using turbulence and wind data from a series of sonic anemometers sited within and above the plant canopy.