Predicting the dynamic impact behaviour of spray droplets on flat plant surfaces.

The dynamic impact behaviour of water droplets on plant surfaces was investigated based on a multiphase computational fluid dynamics (CFD) model. The study was conducted using the Volume Of Fluid (VOF) approach. The static contact angle of water droplets on leaf surfaces of different plants (apple, pear, leek and cabbage) was measured and found to vary between 54.9 and 138.2°. Impact experiments were conducted by monitoring the flow and impact characteristics of water droplets on leaves in still air with a high speed camera. Droplets were generated by an agricultural flat fan spray nozzle moving across the leaf at constant speed. The nozzle produced droplets with diameters ranging from 20.6 up to 550.8 µm, and droplet velocity values near the impact between 0.03 and 13.2 m s(-1). The CFD model was capable of predicting the observed dynamic impact behaviour of droplets on the plant surfaces. The fate of the droplets after the impact process for adhesion, bouncing or splashing was accurately predicted for Weber numbers (We) in the range of 0.007 to 1096 and droplet Reynolds numbers (Re) between 5 to 8000. The process was highly dependent on the surface and droplet flow characteristics during the impact. Combinations of We, Re and Ohnesorge (Oh) numbers defined the droplet maximum spread factor, the number of secondary droplets generated as a result of the splashing process and the transition between the different impact outcomes. These criteria can then be used in field scale spray deposition and drift models to better understand agricultural spray operations.

Development of an imaging system for single droplet characterization using a droplet generator.

The spray droplets generated by agricultural nozzles play an important role in the application accuracy and efficiency of plant protection products. The limitations of the non-imaging techniques and the recent improvements in digital image acquisition and processing increased the interest in using high speed imaging techniques in pesticide spray characterisation. The goal of this study was to develop an imaging technique to evaluate the characteristics of a single spray droplet using a piezoelectric single droplet generator and a high speed imaging technique. Tests were done with different camera settings, lenses, diffusers and light sources. The experiments have shown the necessity for having a good image acquisition and processing system. Image analysis results contributed in selecting the optimal set-up for measuring droplet size and velocity which consisted of a high speed camera with a 6 micros exposure time, a microscope lens at a working distance of 43 cm resulting in a field of view of 1.0 cm x 0.8 cm and a Xenon light source without diffuser used as a backlight. For measuring macro-spray characteristics as the droplet trajectory, the spray angle and the spray shape, a Macro Video Zoom lens at a working distance of 14.3 cm with a bigger field of view of 7.5 cm x 9.5 cm in combination with a halogen spotlight with a diffuser and the high speed camera can be used.

Comparison of the performance between a spray gun and a spray boom in ornamentals.

Flemish greenhouse growers predominantly use handheld spray guns and spray lances for their crop protection purposes although these techniques are known for their heavy workload and their high operator exposure risks. Moreover, when these techniques are compared with spray boom equipment, they are often found to be less effective. On the other hand, handheld spraying techniques are less expensive and more flexible to use. Additionally, many Flemish growers are convinced that a high spray volume and spray pressure is needed to assure a good plant protection. The aim of this work was to evaluate and compare the spray deposition, penetration and uniformity between a manually pulled horizontal spray boom and a spray gun under controlled laboratory conditions. In total, six different spray application techniques were evaluated. In general, the total deposition results were comparable between the spray boom and the spray gun applications but the boom applications resulted in a more uniform spray distribution over the crop. On a plant level, the spray distribution was not uniform for the different techniques with highest deposits on the upper side of the top leaves. Using spray guns at a higher spray pressure did not improve spray penetration and deposition on the bottom side of the leaves. From the different nozzle types, the XR 80 03 gave the best results. Plant density clearly affected crop penetration and deposition on the bottom side of the leaves.

Optimization of the spraying equipment and technology used in ornamental crops.

In 2006, a research project concerning the optimization of the spraying equipment and technology used in ornamental crops was started. First, several greenhouse growers were surveyed on the spray equipment and technology they were using for their plant protection. Later on, different parts of their equipment were evaluated. It this way, we could advice the connected growers on how to improve their own techniques and equipment. Additionally, the survey showed that growers predominantly use knapsack sprayers and lances for crop protection purposes. These techniques are often proven to be less effective compared to spray booms, which could explain the high application rates (up to 6650 L ha(-1)) used by most growers. Since spray boom equipment could enhance spray distribution and minimize labour cost, operator exposure; the usability of this technique in ornamental crops was studied by means of laboratory tests, field trials and bio-efficacy experiments.

Effect of spray application technology on the biological control of aphids in Brussels sprouts.

A field trial was carried out to evaluate different application techniques for crop penetration and biological efficacy of aphid control in Brussels sprouts. Six different application techniques were tested at a pressure of 4.0 bar in a field trial in 3 parallels at the Provincial Vegetable Research Centre in Kruishoutem (PCG): a standard ISO 02 flat fan nozzle (at 200 l/ha), an ISO 04 twin air inclusion nozzle (at 800 l/ha), an ISO 03 drift reducing nozzle, an air injection nozzle (Airjet,) droplegs in combination with an ISO 03 drift reducing nozzle and an ISO 03 air inclusion nozzle (all at 400 l/ha). Best biological control of the aphids and spray distribution was found for the twin air inclusion nozzle, the air inclusion nozzle and the airjet-system. These are all drift reducing techniques because of their coarse droplet size spectrum or the effect of air support which makes the droplets faster. Both effects improve crop penetration. No added value was found for the droplegs for this type of spray treatments. Fine droplets, produced by a standard flat fan, did not give good results on biological control or penetration into the crop.

Drift-reducing nozzles and their biological efficacy.

In 2007 and 2008, field trials were carried out with different standard and drift-reducing nozzles in sugar beet, maize, chicory, Belgian endive (all herbicide applications), wheat (fungicide application) and potatoes (Haulm killing herbicide application). The effect of nozzle type (standard flat fan, low-drift flat fan, air injection), nozzle size (ISO 02, 03 and 04) and application volume on the biological efficacy was investigated. All applications were done using a plot sprayer with volume rates ranging from 160 to 320 l.ha(-1) at recommended dose rates with commonly used (mix of) plant protection products. For each crop, the experiments included four replicates in a randomized block design. Depending on the type of application, the efficacy was measured in terms of weed control, disease and yield level, percentage dead leaf and stem, etc. In a previous research, drift and droplet characteristics of the different techniques were measured. In general no important effect of application technique on biological efficacy was observed for the tested herbicide and fungicide applications within the interval of volume rates and droplet size tested. Drift-reducing nozzles performed similar as conventional nozzles under good spraying conditions and using a correct spray application technique.

Optimization of the spray application technology in bay laurel (Laurus nobilis).

Bay laurel is an evergreen, commercially grown and expensive ornamental pot plant, which is susceptible to different pests like aphids, scale and lerp insects, thrips, caterpillars of codling moth and sooty moulds. Recently, caterpillars of the Mediterranean carnation leafroller (Cacoecimorpha pronubana) cause more and more problems. These pests can lead to important financial losses for the growers. During summer the individual pot plants are placed on a field-container in a fairly dense configuration. Crop protection is traditionally done by moving with a spray lance between the rows of pot plants and treating each individual plant from bottom to top. Good penetration is clearly an important advantages of this spray technique but it is very time-consuming, unhealthy and laborious. Some other growers use a 'spray platform' on a high-clearance tractor. Plants sprayed from this platform are exclusively approached from above resulting in an inferior spray deposition on the lower parts of the plants. To overcome the disadvantages of both available techniques, the potential of an automated tunnel sprayer was investigated. Five different nozzle types were evaluated under laboratory conditions i.e. hollow cone, standard flat fan, air inclusion flat fan, deflector flat fan and twin air inclusion flat fan at spray pressures varying from 3.0 to 7.0 bar depending on the type of nozzle. For each nozzle type, three nozzle sizes were included in the experiments which resulted in 15 different spray application techniques. All experiments were done at a speed of 2.5 km x h(-1). This resulted in three different application volumes: 2450, 4900 and 7300 l x ha(-1). After optimizing the nozzle configuration (distance and orientation) using water-sensitive paper, deposition tests with five different mineral chelates as tracer elements were performed. Filter papers were used as collectors at 20 different positions to measure spray deposition, distribution and penetration in the canopy. For each application technique, four plants were selected as repetitions. Irrespective of the nozzle type and spray pressure, 4900 l x ha(-1) was found to be the optimal spray volume with deposition rates varying from about 50 to 70% depending on the nozzle type. The best results were found for the hollow cone, the standard flat fan and the air inclusion flat fan nozzles. Nozzle type and pressure and the corresponding droplet characteristics were closely related with the penetration and deposition results. With this automated tunnel system, it is possible to obtain a good spray result in combination with an increase in the productivity and a reduction in operator exposure.

Quantification of ventilation characteristics of a helmet.

Despite the augmented safety offered by wearing a cyclist crash helmet, many cyclists still refuse to wear one because of the thermal discomfort that comes along with wearing it. In this paper, a method is described that quantifies the ventilation characteristics of a helmet using tracer gas experiments. A Data-Based Mechanistic model was applied to provide a physically meaningful description of the dominant internal dynamics of mass transfer in the imperfectly mixed fluid under the helmet. By using a physical mass balance, the local ventilation efficiency could be described by using a single input-single output system. Using this approach, ventilation efficiency ranging from 0.06 volume refreshments per second (s(-1)) at the side of the helmet to 0.22s(-1) at the rear ventilation opening were found on the investigated helmet. The zones at the side were poorly ventilated. The influence of the angle of inclination on ventilation efficiency was dependent on the position between head and helmet. General comfort of the helmet can be improved by increasing the ventilation efficiency of fresh air at the problem zones.

Direct and indirect drift assessment means. Part 1: PDPA laser based droplet characterisation.

The spray quality generated by agricultural nozzles is important considering the efficiency of the pesticide application process because it affects spray deposits, biological efficacy and driftability. That is why a measuring set-up for the characterisation of spray nozzles was developed. This set-up is composed of a controlled climate room, a spray unit, a three-dimensional automated positioning system and an Aerometrics PDPA laser system which measures droplet size and velocity characteristics based on light scattering principles. Using this set-up and a well defined measuring protocol, droplet size and velocity characteristics of 15 different nozzle-pressure combinations were measured. It was found that at a nozzle distance of 0.50 m, droplet sizes vary from a few up to some hundreds of micrometres and droplet velocities from about 0 m.s(-1) up to 16 m.s(-1). From the results, the importance of the nozzle type and size on the droplet size and velocity spectra is clear. Standard flat fan nozzles produced the finest droplet size spectrum followed by low-drift and the air injection nozzles which results in significant differences in the proportion of small droplets. The larger the ISO nozzle size, the coarser is the droplet size spectrum and the lower is the proportion of small droplets. This effect is most pronounced for the standard flat fan followed by the low-drift nozzles and is less important for the air inclusion nozzles. Comparing the PDPA measuring results with other studies confirmed the need for reference nozzles to classify sprays because of the considerable variation of absolute results depending on variations in reference sprays, measuring protocol, measuring equipment and settings. As described in part 4, these results will be linked with the drift potential of different nozzle-pressure combinations.

Direct and indirect drift assessment means. Part 2: wind tunnel experiments.

Wind tunnel measurements, performed in Silsoe Research Institute (SRI), were used to measure airborne and fallout spray volumes under directly comparable and repeatable conditions for single and static nozzles. Based on these measurements, drift potential reduction percentages (DPRP), expressing the percentage reduction of the drift potential compared with the reference spraying, were calculated following three approaches. The first approach was based on the calculation of the first moment of the airborne spray profile (DPRPv1). In the second and third approach, the surface under the measured airborne (DPRPv2) and fallout (DPRP(H)) deposit curve were used. These DPRP values express the percentage reduction of the drift potential compared with the reference spraying. Ten different spray nozzles were tested. The results showed the expected fallout profiles with the highest deposits closest to the nozzle and a systematic decrease with distance from the nozzle. For the airborne deposit profiles, the highest deposits were found at the Lowest collectors with an important systematic decrease with increasing heights. For the same nozzle size and spray pressure, DPRP values are generally higher for the air inclusion nozzles followed by the low-drift nozzles and the standard flat fan nozzles and the effect of nozzle type is most important for smaller nozzle sizes. In general, the bigger the ISO nozzle size, the higher the DPRP values. Comparing results from the three different approaches namely, DPRPv1, DPRPv2 and DPRP(H), some interesting conclusions can be drawn. For the standard flat fan nozzles, DPRPv1, values were the highest followed by DPRPv2 and DPRP(H) while for the low-drift nozzles opposite results were found. For the air inclusion nozzles, there was a relatively good agreement between DPRPv1, DPRPv1 and DPRP(H) values. All of this is important in the interpretation of wind tunnel data for different nozzle types and sampling methodologies.

Direct and indirect drift assessment means. Part 4: a comparative study.

Three contrasting drift risk assessment means were evaluated when predicting absolute losses of sedimenting pesticide drift from field crop sprayers namely PDPA laser measurements, wind tunnel measurements (both indirect drift risk assessment means) and field drift experiments (direct drift risk assessment means). In total, 90 PDPA laser measurements, 45 wind tunnel experiments and 61 field drift experiments were performed with 10 different spray nozzles at a pressure of 3.0 bar. The effect of nozzle size (ISO 02, 03 04 and 06) and nozzle type (standard flat fan, low-drift flat fan, air inclusion) on the amount of near-field sedimenting spray drift was studied. The reference spray application was defined as a Hardi ISO F 110 03 standard flat fan nozzle at a pressure of 3.0 bar with a nozzle or boom height of 0.50 m and a driving speed of 8 km.h(-1) for the field measurements; conditions that were always used for the comparative assessment of the different investigated nozzle-pressure combinations. A comparison is made between the results obtained with the indirect drift assessment means and the direct drift assessment method to evaluate the potential of these three different drift assessment means. Droplet size as well as droplet velocity characteristics are related with DRPt (field experiments) and DPRP (wind tunnel experiments). Because of the strong intercorrelation between droplet size and velocity characteristics for the nozzle-pressure combinations investigated in this study, simple first-order linear regressions with one of the droplet characteristics as a predictor variable, were the best choice to predict DRPt and DPRP. Results showed that with the indirect risk assessment means (wind tunnel and PDPA laser measurement), driftability experiments can be made with different spraying systems under directly comparable and repeatable conditions and both methods are suited to permit relative studies of drift risk. Moreover, based on these indirect drift measurements and a statistical drift prediction equation for the reference spraying, it is possible to come to a realistic estimate of field drift data at a driving speed of 8 km.h(-1) and a boom height of 0.50 m.

Direct and indirect drift assessment means. Part 3: field drift experiments.

A whole series of field drift experiments were performed to investigate the effect of nozzle type (flat fan, low-drift, air inclusion) and size (ISO 02, 03, 04 and 06) on sedimenting spray drift. Sedimenting spray drift was determined by sampling in a downwind area at 24 different positions using horizontal drift collectors in combination with a fluorescent tracer with measurements up to 20 m from the directly sprayed zone. Meteorological conditions were continuously monitored. Based on 27 drift experiments with the reference spraying at various environmental conditions, the important effect of atmospheric conditions on the amount of near-field sedimenting spray drift was demonstrated and quantified. A non-linear drift prediction equation was set up and validated. This equation was used to compare the drift results of the different spraying techniques under various weather conditions with the reference spraying by calculating their total drift reduction potential (DRPt). Air inclusion nozzles have the highest drift reduction potential followed by the low-drift nozzles and the standard flat fan nozzles and the effect on drift deposits is high with DRPt values varying from -136.5 up to 89.8%. The effect of nozzle type is most important for smaller nozzle sizes A large database with (absolute) near-field drift results is made available to enlarge the international drift database with information about the effect of climatological conditions and spray application technology. The results are generally in good agreement with the results from different other studies although drift studies are difficult to compare due to differences in weather conditions, spray application techniques, methodologies and crop conditions.

Interception of spray drift by border structures. Part 1: wind tunnel experiments.

This research investigated the drift-intercepting potential of structures surrounding the field borders, like artificial screens and crops, which are not yet a part of the drift mitigation measures for field crop sprayers in Belgium. Drift-interception experiments were performed in the wind tunnel of the International Centre for Eremology (Ghent University, Belgium) with various interception structures: Artificial screens with heights of 0.5, 0.75 and 1 m and screen open areas of 16, 36 and 63%; a row of plastic Christmas trees with heights of 0.5 and 0.75 m; and a potato canopy. The interception structure was positioned at 1 m from the field border. From the results it was found that type of border structure has a pronounced effect on the drift interception, while the height of the border structure had no significant effect.

Interception of spray drift by border structures. Part 2: field experiments.

This research studied the effect of drift-intercepting structures surrounding the field borders, like artificial screens and natural hedges, which are not yet a part of the drift mitigation measures for field crop sprayers in Belgium. Drift-interception experiments were performed in a grassland (Lolium perenne) with various interception structures: Artificial screens with heights of 1, 1.5 and 2 m and screen open areas of 16, 36 and 63% and a row of Fagus sylvatica trees with a height of 1.5 m and an average leaf area index of 1.12 m2/m2. Experiments were performed according to the international standard ISO 22866. The interception structure was positioned at 1 m from the field border. From the results it was found that type of border structure as well as screen open area and screen height, have an important effect on the amount of spray drift. Highest drift reduction was found with a 1.5 m artificial screen with a 16% open area.

Evaporation drift of pesticides active ingredients.

Losses of pesticide active ingredients (a.i.) into the atmosphere can occur through several pathways. A main pathway is evaporation drift. The evaporation process of pesticide a.i., after application, is affected by three main factors: Physicochemical properties of the pesticide a.i., weather conditions and crop structure. The main physicochemical parameters are the Henry coefficient, which is a measure for the volatilization tendency of the pesticide a.i. from a dilute aqueous solution, and the vapour pressure, which is a measure for the volatilization tendency of the pesticide a.i. from the solid phase. Five pesticide a.i., with various Henry coefficients and various vapour pressures, were selected to conduct laboratory experiments: metalaxyl-m, dichlorovos, diazinon, Lindane and trifluralin. Evaporation experiments were conducted in a volatilization chamber. It was found that the evaporation tendencies significantly differed according to the physicochemical properties of the a.i.

Occurrence of spray drift for different crop types: cereal, cereal stubble and grassland.

Pesticide spray drift is affected by 4 main factors: weather conditions, spray application technique, physicochemical properties of the spray Liquid and surrounding characteristics. This research studied the importance of crop type being sprayed for drift occurrence. Drift experiments were performed over cereals, cereal stubbles and grassland according to the international standard ISO 22866. From the results it was found that drift occurrence in cereals and cereal stubbles was lower than drift occurrence in grassland. The differences between cereals and cereal stubbles were significant only at low wind speed.

The effect of air support on droplet characteristics and spray drift.

Air assistance on field sprayers creates a forced airstream under the spray boom which blows the spray droplets into the crop. The advantages of this relative new technique are less drift of spray droplets and the possibility to reduce the amount of pesticides and spray Liquid. The purpose of this work was to investigate the effect of air assistance on the characteristics of spray droplets and their driftability. Based on air velocity measurements on an air assisted field sprayer, a system of air assistance was developed in addition to a laser-based measuring set-up for the characterisation of spray droplets. With this set-up, the effect of air support on the droplet characteristics was investigated for different settings of the air assistance. The effect on spray drift was quantified based on field drift measurements. A reducing effect on the total amount of spray drift was demonstrated for the Hardi ISO F 110 02, F 110 03 and LD 110 02 nozzles with drift reduction factors a(d) of, respectively, 2.08, 1.77 and 1.53. The use of air support had no significant effect for the LD 110 03 nozzles on the total amount of spray drift. Comparing droplet size and drift results, it was found that air support has the highest impact on the amount of spray drift for the finer sprays by increasing droplet velocities. The effect of air support on droplet sizes is rather limited.

Classification of spray nozzles based on droplet size distributions and wind tunnel tests.

Droplet size distribution of a pesticide spray is recognised as a main factor affecting spray drift. As a first approximation, nozzles can be classified based on their droplet size spectrum. However, the risk of drift for a given droplet size distribution is also a function of spray structure, droplet velocities and entrained air conditions. Wind tunnel tests to determine actual drift potentials of the different nozzles have been proposed as a method of adding an indication of the risk of spray drift to the existing classification based on droplet size distributions (Miller et al, 1995). In this research wind tunnel tests were performed in the wind tunnel of the International Centre for Eremology (I.C.E.), Ghent University, to determine the drift potential of different types and sizes of nozzles at various spray pressures. Flat Fan (F) nozzles Hardi ISO 110 02, 110 03, 110 04, 110 06; Low-Drift (LD) nozzles Hardi ISO 110 02, 110 03, 110 04 and Injet Air Inclusion (AI) nozzles Hardi ISO 110 02, 110 03, 110 04 were tested at a spray pressures of 2, 3 and 4 bar. The droplet size spectra of the F and the LD nozzles were measured with a Malvern Mastersizer at spray pressures 2 bar, 3 bar and 4 bar. The Malvern spectra were used to calculate the Volume Median Diameters (VMD) of the sprays.

Spray drift as affected by meteorological conditions.

Spray drift can be defined as the quantity of plant protection product that is carried out of the sprayed (treated) area by the action of air currents during the application process. This continues to be a major problem in applying agricultural pesticides. The purpose of this research is to measure and compare the amount of drift for different climatological conditions under field conditions. Spray drift was determined by sampling in a defined downwind area at different positions in a flat meadow using horizontal drift collectors (sedimenting spray drift) and pipe cleaners (airborne spray drift) for a reference spraying. Meteorological conditions were monitored during each experiment. A drift prediction equation for the reference spraying was set up to predict the expected magnitude of sedimenting drift at various drift distances and atmospheric conditions (wind speed and temperature). This equation can be used to compare measurements using other spraying techniques under different weather conditions to the reference spraying. In 2005, more measurements will be performed to validate the statements and the model reflected in this paper.