Dispersion of particles released at the leading edge of a crop canopy

• Flow adjustment at leading edge of a canopy was studied via large-eddy simulation.
• Division of canopy edge into six regions with different flow features was proposed.
• Escape of particles from the canopy depends on flow features near the source.
• Close to source particle plume geometry is sensitive to source location in canopy.
• Away from the source plume geometry is insensitive to source location.

A large-eddy simulation (LES) approach was used to investigate the flow characteristics at a canopy leading edge and their impact on the dispersion of particles released from point sources inside the canopy. Comparison of results from these LES simulations with those for a canopy that is infinite and uniform in both streamwise and spanwise directions reveals important insights about the adjustment lengths for mean flow, turbulent kinetic energy (TKE), and canopy-shear-layer vortices. Two critical locations were identified in the flow adjustment at the leading edge: (1) the location at which canopy-shear-layer vortices begin to develop and (2) the location at which the flow is fully developed. Simulations were conducted for particles released from continuous point sources at four streamwise locations downwind from the leading edge and three heights within the canopy. The four streamwise source locations corresponded to the canopy leading edge, the location at which canopy-shear-layer vortices began to develop, the transition region, and the fully developed region. The adjustment of flow near the leading edge has a profound impact on the dispersion of particles close to the source, which is where most particle escape from the canopy takes place. Particles released close to the canopy leading edge have much higher maximum escape fractions than particles released in the fully developed region. The adjustment length for particle escape is greater than that for the flow. Away from the source (approximately sixteen canopy heights for the present dense canopy), the geometries of the mean plume become similar for particles released from different regions. Within a few tens of canopy heights from the leading edge, the growth rates of converged mean plume height and depth are lower than those for the case of an infinite canopy.