CFD Modelling of the Effect of Pear Tree Canopy on Air-Assisted Orchard Spraying

A computational fluid dynamics (CFD) model was used to study the effect of pear tree canopy on orchard spraying by integrating the 3-D architecture of trees planted in different years (1994, 2002 and 2004) into the model. Canopy architectural data (3-D internode coordinates and diameter) of pear trees (Pyrus communis) were collected in an experimental orchard (PCfruit, Sint-Truiden, Belgium) in spring 2007. The data were converted into 3-D virtual geometries with porous sub-domains created around each part of the canopy elements to represent the coverage of the leaves. A closure model used in the porous sub-domain considers momentum and kinetic energy losses that are functions of leaf area density (A) and drag coefficient (C-d). The model was applied to a cross flow orchard sprayer (BAB-Bamps, Sint-Truiden, Belgium). The flow of air (continuous phase) was solved using the Navier-Stokes equations and the k-epsilon turbulence model and the water droplets (discrete phase) in air were tracked using a Lagrangian particle tracking multiphase flow model. The model was implemented in ANSYS-CFX-11.00 (ANSYS, Inc., Canonsburg, Pennsylvania, USA) in steady state conditions. Simulations were made for A of 1, 3 and 5 (m(-1)) with C-d of 0.2 for all the trees and each of them were compared with simulations on bare trees. The jet velocity at the wake was well above 10 m/s for bare trees of all years within the sprayer height and this value decreased with increasing A. A similar behaviour was seen also in the normalized droplet concentration. At the wake more droplets were drifted to the ground on leafed trees than bare trees because considerable amount of droplets decay faster as they lose their velocity in the porous region. Particles drifting to the atmosphere were more on bare trees than leafed trees. The vertical profiles of the air jet velocity and droplet concentration results at the wake were consistent and meaningful from both physical and theoretical perspectives.