Aerial Dispersal of Particles Emitted Inside Plant Canopies
Author | : Ying Pan |
Publisher | : |
Total Pages | : |
Release | : 2014 |
Genre | : |
ISBN | : |
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This work combines numerical, experimental, and theoretical methods to investigate the dispersion of particles inside and above plant canopies. The large-eddy simulation (LES) approach is used to reproduce turbulence statistics and three-dimensional (3-D) particle dispersion within the canopy roughness sublayer (the region of flow significantly modified by the presence of the canopy, extending from ground to about three canopy heights). The Eulerian description of conservation laws of fluid momentum and particle concentration implies that the continuous concentration field is advected by the continuous flow field. Within the canopy, modifications are required for the filtered momentum and concentration equations, because spatial filtering of flow variables and concentration field is inapplicable to a control volume consisting of both fluid and solid elements. In this work, the canopy region is viewed as a space occupied by air only. The sink of airflow momentum induced by forces acting on the surfaces of canopy elements is parameterized as a non-conservative virtual body force that dissipates the kinetic energy of the air. This virtual body force must reflect the characteristic of the surface forces exerted by canopy elements within the control volume, and is parameterized as a "drag force" following standard practice in LES studies. Specifically, the "drag force" is calculated as a product of a drag coefficient, the projected leaf area density, and the square of velocity. Using a constant drag coefficient, this model allows first-order accuracy in reproducing the vertically integrated sink of momentum within the canopy layer for airflows of high Reynolds number. The corresponding LES results of first- and second-order turbulence statistics are in good agreement with experimental data obtained in the field interior, within and just above mature maize canopies. However, the distribution of momentum sink among weak (low velocity) and strong (high velocity) events has not been well reproduced, inferred from the significant underestition of streamwise and vertical velocity skewness as well as the fractions of vertical momentum flux transported by strong events. Using a velocity-dependent drag coefficient that accounts for the effect of plant reconfiguration (bending of canopy elements due to the aerodynamic drag force), the "drag force" model leads to LES results of streamwise and vertical velocity skewness as well as the fractions of vertical momentum flux transported by strong events in better agreement with field experimental data. Specifically, modeling the impact of reconfiguration allows strong events to penetrate into deeper canopy regions, reducing the underprediction of streamwise and vertical velocity skewness as well as the vertical momentum flux transported by strong events from 60%, 60%, and 40% to 5%, 20%, and 5%, respectively. On the other hand, the vertically integrated sink of momentum within the canopy layer has been kept approximately the same, so do first- and second-order turbulence statistics.The link between plant reconfiguration and turbulence dynamics within the canopy roughness sublayer is further investigated. The "reconfiguration drag model" using velocity-dependent drag coefficient is revised to incorporate a theoretical model of the force balance on individual crosswind blades. In the LES, the dimension and degree of the reconfiguration of canopy elements affect the magnitude and position of peak streamwise velocity skewness within the canopy as well as the fractions of vertical momentum flux transported by strong events. The streamwise velocity skewness is shown to be related to the penetration of strong events into the canopy, which is associated with the passage of canopy-scale coherent eddies. With the profile of mean vertical momentum flux constrained by field experimental data, changing the model of drag coefficient induces negligible changes in the vertically integrated "drag force" within the canopy layer. Consequently, first- and second-order turbulence statistics remain approximately the same. However, enhancing the rate of decrease of drag coefficient with increasing velocity increases the streamwise and vertical velocity skewness, the fractions of vertical momentum flux transported by strong events, as well as the ratio between vertical momentum flux transported by relatively strong head-down "sweeps" and relatively weak head-up "ejections". Note that "sweeps" and "ejections" are defined based on streamwise and vertical velocity fluctuations, and are different from their classical definitions. These results confirmed the inadequacy of describing the effects of canopy-scale coherent structures using just first- and second-order turbulence statistics.The filtered concentration equation is applied to the dispersion of particles within the canopy roughness sublayer, assuming that a virtual continuous concentration field is advected by a virtual continuous velocity field. A canopy deposition model is used to model the sink of particle concentration associated with the impaction, sedimentation, retention, and re-entrainment of particles on the surfaces of canopy elements. LES results of mean particle concentration field and mean ground deposition rate were evaluated against data obtained during an artificial continuous point-source release experiment. Accounting for the effect of reconfiguration by using a velocity dependent drag coefficient leads to better agreement between LES results and field experimental data of the mean particle concentration field, suggesting the importance of reproducing the distribution of momentum sink among weak and strong events for reproducing the dispersion of particles. LES results obtained using a velocity-dependent drag coefficient are analyzed to estimate essential properties for the occurrence of plant disease epidemics, i.e., the fraction of particles that escape the canopy (escape fraction) and the growth of the particle plume in the vertical direction. The most interesting finding is that an existing analytical function can be used to model the crosswind-integrated mean concentration field above the canopy normalized by the escape fraction for particles released from the field interior.Our LES results suggest that the escape fractions of particles released close to the canopy leading edge are greater than those released in the field interior, especially for particles released in the bottom half of the canopy. Effects of the canopy leading edge on the escape fraction can be tracked to the effects on the fractions of particles removed by deposition on modeled "canopy elements" and on the ground. The rate of deposition on canopy elements can be suppressed by enhanced modeled retention and re-entrainment of particles in the region of strong mean wind, while the rate of deposition on the ground can be suppressed by non-negligible mean vertical advection with respect to vertical turbulent transport. Away from the source, the vertical growth of the plume above the canopy-leading-edge area is slower than that above the field interior, due to greater shear of mean streamwise velocity in the internal boundary layer (IBL) than that in the fully-developed canopy roughness sublayer above the canopy.Spore dispersal downwind from the source field is investigated by representing the infected field as a prescribed constant mean concentration at a reference height near the canopy top. This "source-in-the-mean" model neglects the spatial heterogeneity of infections, release rates, and escape fractions, allowing a first-order accuracy in reproducing the effective source strength of a severely infected field. For dispersion of particles emitted from finite area sources in the atmospheric boundary layer (ABL), pre-existing theoretical models proposed for neutral conditions are extended to unstable conditions. The major effects of buoyancy are accounted for by modifying the profile of vertical velocity variance and considering the ratio between friction and convection velocities. Theoretical predictions of mean concentration profile, plume height, and horizontal transport above the source as well as ground deposition rate downstream from the source are in good agreement with LES results for the plume within the atmospheric surface layer.