Milton E. Teske

AGDISP: The Aircraft Spray Dispersion Model, Code Development and Experimental Validation

ABSTRACT OVER the last 4 years the USDA Forest Service and the DOD Atmospheric Sciences Laboratory have continued the development of the AGDISP (AGricultural DISPersal) computer code that predicts the deposition of material released from helicopters or fixed-wing aircraft. The features of the code are reviewed and the predictive capability of the code is assessed by comparison with recent test data. Code applications and limitations are also discussed.

Vortex interactions and decay in aircraft wakes

The dynamic interaction of aircraft wake vortices was investigated using both inviscid and viscous models. For the viscous model, a computer code was developed using a second-order closure model of turbulent transport. The phenomenon of vortex merging which results in the rapid aging of a vortex wake was examined in detail. It was shown that the redistribution of vorticity during merging results from both convective and diffusive mechanisms.

Computation of wake/exhaust mixing downstream of advanced transport aircraft

The mixing of engine exhaust with the vortical wake of high speed aircraft operating in the stratosphere can play an important role in the formation of chemical products that deplete atmospheric ozone. An accurate analysis of this type of interaction is therefore necessary as a part of the assessment of the impact of proposed High Speed Civil Transport (HSCT) designs on atmospheric chemistry. This paper describes modifications to the parabolic Navier-Stokes flow field analysis in the UNIWAKE unified aircraft wake model to accommodate the computation of wake/exhaust mixing and the simulation of reacting flow. The present implementation uses a passive chemistry model in which the reacting species are convected and diffused by the fluid dynamic solution but in which the evolution of the species does not affect the flow field. The resulting analysis, UNIWAKE/PCHEM (Passive CHEMistry) has been applied to the analysis of wake/exhaust flows downstream of representative HSCT configurations. The major elements of the flow field model are described, as are the results of sample calculations illustrating the behavior of the thermal exhaust plume and the production of species important to the modeling of condensation in the wake. Appropriate steps for further development of the UNIWAKE/PCHEM model are also outlined.

Initial Development and Validation of a Mechanistic Spray Drift Model for Ground Boom Sprayers

This article summarizes the initial development of a mechanistic spray drift model for ground boom sprayers, built upon the same analytical Lagrangian approach used in the aerial spray model AGDISP. For ground boom sprayers the wake and turbulence of the aircraft are removed and more detailed modeling of the nozzle jets (and jet entrainment) is included. The model predicts the spray deposition downwind from the application area for any set of initial conditions, and recovers results compared with Spray Drift Task Force and Canadian field datasets for two ground sprayer nozzle types.

Rotary atomizer drop size distribution database

Wind tunnel measurements of drop Size distributions from Micronair A U4000 and A U5000 rotary atomizers were collected to develop a database for model use. The measurements varied tank mix, flow rate, air speed, and blade angle conditions, which were correlated by multiple regressions (average R-2 = 0.995 for A U4000 and 0.988 for AU5000). This database replaces an outdated set of rotary atomizer data measured in the 1980s by the USDA Forest Service and fills in a gap in data measured in the 1990s by the Spray Drift Task Force. Since current USDA Forest Service spray projects rely on rotary atomizers, the creation of the database (and its multiple regression interpolation) satisfies a need seen for ten years.

Building canopy retention into agdisp: Preliminary models and results

AGDISP uses canopy structure and collection efficiency to model canopy interception but assumes droplets that intercept a leaf surface are retained at that point and do not bounce or shatter. This behavior is not the case for many plant species, particularly species that are moderately to very difficult to wet, for which canopy deposition would be overestimated with this assumption. This article summarizes the initial implementation of process-based models for spray droplet bounce and shatter within an experimental build of AGDISP in order to predict spray retention within plant canopies. Spray simulations were run for formulations ranging in surface tension, applied to three species (wheat, canola, and capsicum) with different wettabilities and leaf orientations, and using the AGDISP ground model with and without the retention model, to evaluate the impact of the retention model on canopy deposition within AGDISP. The model outputs were also compared to previously determined tracksprayer results. The current AGDISP canopy deposition (interception) model was unable to fully account for differences in retention due to the spray formulation used or species studied. Incorporation of the process-driven models for bounce and shatter allowed these differences to be predicted. Over the three species and four formulations studied, there was good agreement (Pearson’s correlation coefficient = 0.9308, p = 0.0000, indicating an almost certain correlation) between predicted and experimentally determined retention. Considerable work is still required to make this approach practical. However, this article illustrates that the ability to model retention is important for many crops (and weeds) and that the approach outlined herein is an effective adjunct to the current AGDISP interception model.

Comparison of FSCBG Model Predictions with Heather Seed Orchard Deposition Data

Abstract Aerial applications of esfenvalerate in a Douglas-fir seed orchard in the Oakridge Ranger District of the Willamette National Forest are used to evaluate the effectiveness of insecticide treatments to control seed and cone insects of Douglas fir. Data from Kromekote spray deposition cards are compared with canopy penetration and deposition predictions from FSCBG, a Forest Service computer model that predicts the behavior of pesticide sprays released from aircraft. Predictions of canopy penetration and ground deposition am within an average of 7.8% of the overall field dataset. These results demonstrate the usefulness of the simulation model for selecting spray application parameters and describing the environmental distribution of pesticides.

A simulation of boom length effects for drift minimization

The near-wake portion of the USDA Forest Service aerial application prediction model FSCBG is applied to a sensitivity study of the length of the spray boom relative to the length of the aircraft wingspan or helicopter rotor diameter. Building on extensive previous work, this study examines the predictions by the near-wake Lagrangian trajectory model of swath width, mean deposition within the spray block, and drift fraction downwind of the edge of the field. Ten aircraft and four BCPC droplet size distributions are used to simulate a typical agricultural application scenario. Findings from this study demonstrate the effects of varying boom length on downwind drift, the reduction of downwind drift with larger droplets, and the inherent ability of certain aircraft type to reduce downwind drift more easily than others. Model results indicate that a broad range of boom length (between 60 and 100% of aircraft wingspan or helicopter rotor diameter) recovers approximately the same levels of downwind drift, decreasing levels of mean deposition within the spray block, and increasing swath width between flight lines. The suggestion that boom length should be less than 75% of wingspan or rotor diameter is perhaps based more on the anticipated position of the rolled-up vortices than on solid experimental evidence.

Modification of Droplet Evaporation in the Simulation of Fine Droplet Motion Using AGDISP

The current version of AGDISP incorporates a well-established representation of the aircraft vortices, wind speed, evaporation, canopy, and spray physical properties. However, several modeling areas have not been re-examined in several years, and it seems appropriate to investigate the algorithms involved with parsing the drop size distribution, the impact of the evaporation rate on droplet size, the behavior of the integration time step, and the approximate effect of spray cloud relative humidity.

Development and Validation of a Mechanistic Ground Sprayer Model

This paper summarizes the development of a mechanistic ground sprayer model, built upon the same analytical Lagrangian approach used in the aerial spray model AGDISP. For the ground sprayer model the wake and turbulence of the aircraft are removed and more detailed modeling of the nozzle jets (and jet entrainment) are included. The ground model predicts the spray deposition downwind from the application area for any set of initial conditions, and recovers results consistent with Spray Drift Task Force field data.