Journal of the American Mosquito Control Association, 12(2):372_379, 1996 Copyright O 1996 by the American Mosquito Control Association, Inc.

INFLUENCE OF AIRCRAFT VORTICESON SPRAY CLOUD BEHAVIOR

R. E. MICKLE

Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H5T4, Canada

ABSTRACT. For small droplet spraying, the spray cloud is initially entrained into the wingtip vortices so that the ultimate fate of the spray is conffolled by the motion of these vortices. In close to 10O aerial sprays, the emitted spray cloud has been mapped using a scanning laser system that displays diffusion and transport of the spray cloud. Results detailing the concentrations within the spray cloud in space and time are given for sprays in parallel and crosswinds. Wind direction is seen to potentially alter the motion and hence the fate of the spray cloud. In crosswind spraying, the vortex behavior associated with the 2 is found to differ, which leads to enhanced deposition from the upwind and enhanced drift from the downwind wins.

INTRODUCTION EXPERIMENTAL METHODS

The application of larvicides and adulticides The ARAL (AES Rapid Acquisition Lidar) by aircraft has been an effective mechanism for has been used in 2 experiments to map close to mosquito control. Howevet the benefits of 100 different spray scenarios in a variety of me- spraying are accompanied by the difficulty in teorological conditions (stable and unstable) en- targeting small droplets and by the potential en- compassing light to high winds at cross and par- vironmental impact of applying any chemical allel angles to the flight line. Aircraft used in into the environment in quantities that may be these studies have included a Cessna 188, TBM toxic to nontarget species. The success of spray- (Hoff et al. 1989, Mickle 1994), and a Bell 2o68 ing is driven by timing of the application and in Jetranger. Atomizers have included 11010 T-jets particular by the applicator, who ultimately is and wind-driven and high (14,000)-rpm electri- responsible for placing the pesticide into the tar- cally driven Micronair AU4OOO rotary atomiz- get area. Typically, aerial spraying for mosqui- ers. Simulants with physical properties similar toes incorporates an emission droplet size dis- to forestry insecticide formulations were used to tribution with a volume median diameter (D"o.r) produce representative emission droplet size dis- below 100 pm. Given the slow settling veloci- tributions. A computer-generated graphics pre- ties of these droplets, their initial motion is con- sentation of examples from both experiments trolled by the wingtip vortices of the spray air- can be obtained from the author. craft (Drummond 1987). Such small droplets The ARAL cloud mapper comprises a small have the disadvantage, however, that if they do Nd-Yag laser pulsing at 70 Hz and receiving op- not impact the target in a short time, they are tics to capture the returning signal. Under the highly susceptible to drift (Picot et al. 1986, control of a laptop computer, the laser beam is Crabbe and McCooeye 1989, Akesson et al. scanned through the spray cloud every 4 sec 1992, Crabbe et al. 1994). with a sweep taking slightly more than 2 sec. In order to diagnose the aerodynamics of the The time of flight of each laser pulse backscat- application technique, the Atmospheric Environ- tered from the droplets in the spray cloud gives ment Service (AES) of Environment Canada un- the range of the cloud from the ARAL, whereas dertook the construction of a lidar system (Hoff the intensity of the returning pulse is a measure et al. 1989, Mickle 1994) that was capable of of the droplet density within the reflecting vol- monitoring the spray drift cloud in real time. The ume of the cloud. Laser beam and receiving op- lidar system measures the intensity of the re- tics divergences permit volume sampling of I m3 turned laser pulse as a function of time (range) of the cloud at distances of I km from the thereby mapping the cloud density cross section ARAL system. The system is, therefore, capable as it moves away from the aircraft flight line. of clearly defining the spray cloud distribution From cross-sectional lidar profiles, it is possible within the wingtip vortices. to visualize the spray cloud application and, giv- Within the vortices (Fig. 1A), the cloud den- en the initial spray droplet size distribution and sity, as seen fTom a single laser pulse, was found a model for subsequent evaporation of the drop- to have 2 maxima of concentration, each asso- lets (Dumbauld et al. 1980, Picot et al. 1981, ciated with a center of the vortex pair. This dis- Mickle 1987, Teske et al. 1993), estimates of the tribution characterized the vast majority of the deposit and drift fractions can be calculated. operational atomizer configurations for ULV

372 373 JUNE1996 Syuposruu: Asnosot- Ct-ouo DvNeutcs

20 z E h15 t coREs tr i-"",:: oio J ul ts-5 J ul tr 0 1000 1100 {200 450 550 650 DISTANGEFROM ARAL (m) DISTANCEFROM ARAL (m)

Fig. 1. Backscatter return from a single lidar pulse. A. TBM with 1O wind-driven AU4000 at lO,00O rpm. B. Bell 2O68 with 2 electrically driven AU4OOO at 14,000 rpm.

spraying regardless of atomizer configuration or the differences between spraying in parallel and type. The example in Fig. 1A is for a TBM con- crosswinds. Selected cross sections of the spray figured with 5 AU40O0 atomizers mounted ap- cloud for parallel and crosswind cases are com- proximately I m below mid-chord of each wing. pared in Fig. 2 for various times after the aircraft The outermost atomizer was located at 567o of had passed a vertical plane swept by the laser the wing span. Despite the low mounting below beam. In both cases, the aircraft is flying away the wing, clearly the material has been moved from the observer (i.e., into the page) at a dis- spanwise towards the vortex core as the vortices tance of 1-2 km from the ARAL located on the were formed. No atomizers were mounted at the port side of the aircraft. Areas of equal droplet position of the vortex cores. The minimum be- density (concentration) are mapped with colors tween the peaks is associated with the lack of ranging from black (high concentration) to light atomizers below the center of the aircraft. It is gray (less than O.5Voof the high concentration). interesting that the shape of the droplet density A more detailed analysis of this case can be within the cloud is characteristic ofthe expected found in Mickle (1994). Figure 3 presents the deposit profile if an aircraft was flying close to time-integrated concentration (dosage) for these the ground. The only exception (Fig. 18) to this 2 spray cases at different heights and distances general shape was observed for a specialized relative to the flight line. Figure 4 presents the mounting of 2 AU40O0 atomizers near the cen- breathing height concentration of the spray with ter of a pallet slung approximately 3 m below a time, and Fig. 5 gives the accumulated deposit Bell 2068 Jetranger. In this case, the spray from from the loss of material in the surface layer. each atomizer formed an annulus around the Crosswind spray: In this example, a TBM vortex center at a distance characteristic of its with 5 Micronair AU4000 rotary atomizers lateral position from the aircraft center line. Al- (flow rate = 2 liters/min/head; D"o, : 4O-50 though material was found towards the center of pm) mounted under each wing was flown at a the vortex, clearly the contribution to spray height of 25 m above a clear-cut area. Winds at within the vortex core from atomizers at these aircraft height were light (1.8 m/sec) at 7O" to in-board locations is limited compared to the the flight line. during this early more outboard locations as shown in the previ- morning stable spray run (0650 h AST) were ous example. near 9"C with relative near 9O7o.With- in 9 sec after aircraft passage, the ULV spray CASE STUDIES had been swept into the wingtip vortices (Fig. Of the many flights that have been mapped, 2A) with the highest concentration (black) found the following 2 examples tend to characterize at the vortex centers. Within these 9 sec, the 374 JounNlr- or rtlg Avenlcat Mosquno CoNrnol AssocrerroN Vol. 12,No.2

TBM WITH AU4OOO TBM WITH 11010TJETS 50 z 100 o F E+o 9 SEC f, 980 3 SEC F z F30 F60 (,u z o 920 o40 o Irl U tv A =20 B F .! I 0 U 0 E

z 50 o 100 F t g80 €oo 27 SEC F 27 SEC z E F30 o F60 -_-&- z I (920 (,o o40 q ITI =10 F =20 I U 0 c 0

50 100 €oo 69 SEC g80 ..1, 69 SEC F30 :s-'@'ttt F60 I __+-_ 'rS: I +1.;ls j o20 940 qMqF' 10 i{:,, ':,f' ul IIU - 20 0+ ,:{g:' 0 r050 1100 1150 1200 1250 {750 1800 1850 1900 1950 DISTANCEFROM LIDAR (m) DISTANCE FROM LIDAR (m) Fig. 2. Spray cloud concentration cross sections.A. Crosswind spray. B. Parallel wind spray. Contours representa 2-fold change in concentration. spray cloud had descended nearly lO m, which drift downwind, stretching longitudinally due to is characteristic of the vortex descent rate for the wind shear and with a vertical extent compara- TBM. Over the next 20-30 sec, the upwind vor- ble to the aircraft height. Concentrations within tex maintained its shape, carrying the spray from the cloud were typically less than lOVo of the the port wing as it impacted the ground approx- maximum concentration at 9 sec. Whereas the imately 25 sec after release and 2O-3O m down- spray from the upwind wing quickly impacted wind from the flight line. In this particular case, the surface, spray from the downwind wing the starboard vortex diffused quickly so that at tended to remain airborne longer as it diffused. 27 sec, its vertical motion had arrested, leaving The total dosage cross section (Fig. 34) clearly the spray from the starboard wing drifting away shows the bulk of the spray cloud impacting the from the flight line. The early erosion ofthe star- surface at 1,150 m, the location of the peak de- board vortex was probably due to combined ef- posit (Fig. 5A). Beyond the location of initial fects of prop wash and crosswind shear. The net descent of the vortices (i.e., 1,150 m), the re- effect of this early vortex destruction was to maining cloud begins to diffuse vertically above leave the spray from the downwind wing air- the height of the aircraft. The observed down- borne to be transported away from the flight line wind distance of drift is limited by the 2-min as a drifting cloud with concentrations diluted to tracking duration. The concentration ofthe spray 2O-4OVa of the maximum concentration. The at breathing height (2 m) (Fig. 4,{) shows that potential for a high drift fraction during stable the maximum concentration occurred 25-30 sec conditions has been observed elsewhere during after the aircraft had passed and was associated forestry trials to relate atmospheric stability to with the vortex from the port wing impacting the wind drift (Crabbe and McCooeye 1989, Crabbe ground. Breathing-height concentrations in the et al. 1994). By 69 sec, the cloud continued to remainder of the spray cloud decreased as the Svuposruu: AsnosoL Cr-ouo DYNevrcs

IJJ (9 tr @ -? 60 o F o I u.l 40 F iE J t tu t

1100 1150 '1200 1250 DISTANCEFROM ARAL (m) 100

80 UJ (' e o =60 o F o I IJJ Ei 40 F I I lrJ t

0 1750 1800 1850 1900 {950 2000 2050 DISTANCEFROM ARAL (m) Fig.3. Total dosage cross sections. A. Crosswind spray. Contours represent a2-fold change in dosage. B. Parallel wind spray. Contours represent a 5-fold change in dosage. 376 JounNal oF THE AMERTcIN Mosquno CoNrnol Assocnrrou

z =<9P o try lrJ a frnr- =/ 60 rno lrJ E '30t-

Fig. 4. Temporal and spatial variation in breathing-height concentrations. A. Crosswind spray. B. Parallel wind spray. Contours represent a 2-fold change in concentration.

cloud diffused further downwind. Beyond 6O from Fig. 4,{ indicates a nearly constant deposit sec, the concentration is uniform within the sur- with time beyond 60 sec at a rate that is 4OVoof face layer with a secondary concentration max- the deposit rate associated with the upwind vor- imum (gray) lingering near 1,200 m. The result tex. of this is reflected in a small secondary maxi- Breathing-height concentrations close to the mum in the total deposit profile (Fig. 5,A) at spray line have been plotted as a function of 1,20O m. A calculation of the rate of deposit time in Fig. 6. At 35 m downwind of the spray Jrxr 1996 SyMposIUM: Agnosol Ct-ouo Dvttlulcs 3'7'7

E25 o Ozo o- H15 lrJ' F 5s uJ tr0 1050 1150 1250 1350 1450 1700 1800 1900 2000 2100 DISTANCEFROM ARAL (m) DISTANGE FROM ARAL (m)

Fig. 5. Total deposit. A. Crosswind spray. B. Parallel wind spray.

line (i.e., I,150 m from the ARAL), air concen- placed at 3 downwind locations from a single trations peak27 sec after the aircraft has passed. spray line. The measurements were taken during This maximum in concentration is associated an aerial application to a forest during very sta- with the spray entrained in the upwind vortex ble meteorological conditions. Figure 7 presents with the material in the downwind vortex having the temporal variation in concentration after 3 passed overhead. Concentrations decrease over spray passes on the same spray line. At the 20O- the subsequent 40 sec as the spray cloud trans- m station, variations on the order of a factor of lates further downwind. At the 85-m (i.e., 1,200 2 are observed in the peak tracer concentrations m from the ARAL) station, peak concentrations associated with each spray pass. By 400 m, peak are about one-third of those at the closer site. concentrations were diminished by a factor of 4 With lateral diffusion of the cloud, the lifetime from those at 20O m, whereas by 1,200 m, peak of the cloud at this location is in excess of 80 concentrations were about 5Vo of those at 2OO sec. m. Whereas each spray cloud took about 100 sec Similar results were presented in Crabbe et al. to pass the 2OO-m station, by I,2OO m, it appears (1984) and are reproduced in Fig. 7. These data were taken using chemical sequential samplers

I I I 200 m STATION

200 35 m STATION z tr O4 ul F J tr b roo zF g uo z 4O0 m STATION z 50 o I I I I *9 o8 E ul F 86 m STATION z F ul I 220 ut4 o v. o 1.200 m STAnON 10

r"r t,'t I 0 30 60 90 120 T|ME FROM BEGtNNtNc OF SPRAY (SEC) 200 /+00 600 800 1000 T]ME FROM BEGINNING OF SPRAY (SEC) Fig. 6. Temporal variation in breathing-heightcon- centrations at 2 locations downwind of the spray line Fig. 7. Tracer concentration at tree height (from for crosswind spray, Crabbe et al. 1984). 378 JounNar or rgp Atlgntcen Mosquno CoNrnol Assocrarrou Vor-.12, No. 2 that the clouds from the separate sprays had movement of the small drops tends to redistrib- merged, taking close to 500 sec to pass. ute the small drops towards the vortex core. Parallel wind spray: In this case, the TBM Although only 2 studies have been given, they aircraft with 12 I l0l0 T-jets per wing (flow rate tend to represent the majority of cases that were 3.8 liters/min/head; = D,0., 60-70 p,m) was mapped. Consistently, sprays that were made in flown along a line 1,820 m from the ARAL and parallel winds from high-flying aircraft resulted at a height of 34 m above ground level over a in the potential for drifting clouds that remained canopy during midmorning neutral conditions. airborne for longer periods of time. Deposition Average canopy height was l0 m. The tail winds was slow compared to the crosswind cases. were moderate (4 m/sec) at l5o to the flight line. Faced with targeted spraying of small areas, the and relative humidity were 13oC influence of wind direction on vortex behavior, and 8OVo,respectively. As in the previous case, especially when flying high above ground, at 9 sec, the bulk of the material was wrapped should be considered. For ULV spraying of into the vortices, which had tended to rotate small blocks, it is recommended that applicators around each other (Fig. 2B). However, unlike should attempt to spray in crosswinds in order the crosswind case, the downwind vertical mo- to enhance rapid transport of the pesticide into tion of the vortices was quickly arrested. By 27 the target area. For large block drift spraying, sec, the bulk of the spray had risen to a height spraying in parallel winds appears to reduce the above the original release height. In this parallel rate of deposit at ground level. spray, each cross section is for a new volume of The crosswind example shown here has also spray that has been transported down the spray indicated that vortex behavior can significantly line and into the field of view of the laser. By influence spray fate during conditions where 69 sec, the cloud extended to 80 m, 2 times the vortex lifetimes are long. Preferential deposition original spray height. The total dosage profile from the upwind wing could be utilized to better (Fig. 38) shows the dosage evenly distributed target deposit into a selected area. Quantification around the spray height with the majority of the of preferential deposit from the upwind wing has cloud remaining airborne. Unlike the crosswind been observed by McCooeye et al. (1993). On case, the high concentration of material (dark the other hand, spraying from the downwind colors) remained aloft for up to 8O sec with only wing appears to increase the potential for drift a low concentration (light shade) actually reach- and hence could be utilized where drift spraying ing the target. The majority of the spray cloud is desired. was skewed to the port side of the aircraft, re- flecting the slight off-axis direction of the wind. REFERENCES CITED Deposition (Fig. 48) to the surface only began Akesson, N. B., R. K. Hitch and A. W. Jones. 1992. to occur about 60 sec after the aircraft had An analysis of airbome transport of pesticides from passed and was associated with the spray emit- large scale aerial applications.NAAA-ASAE Paper ted nearly 200 m upwind along the spray line. AA 92001. American Society of Agricultural En- Despite an application rate of nearly 4.5 times gineers,St. Joseph,MI. greater than the previous case, the total deposit Crabbe,R. S. and M. McCooeye. 1989. Kapuskasing (Fig. 58) over the first 80 sec after the aircraft field study relating atmospheric stability to wind had passed was substantially less than in the drift from aerial forest spray operations. NRC Tech- nical Report LTR-UA-99. National ResearchCoun- crosswind case. For spray control strategies em- cil, Ottawa, Ontario, Canada. ploying drift techniques, this spray strategy re- Crabbe,R. S., M. McCooeye and L. Elias- 1984. Ef- sulted in a cloud that diffused extensively and fect of atmospheric stability on wind drift in aerial was slow to impact the ground. forest spray trials. Neutral to stableconditions. NRC Technical Report LTR-UA-73. National Research Council, Ottawa, Ontario, Canada. CONCLUSIONS AND Crabbe,R. S., M. McCooeye and R. E. Mickle. 1994. RECOMMENDATIONS The influence of atmosphericstability on wind drift from ultra-low-volume aerial forest spray applica- Two case studies of parallel and crosswind tions. J. Appl. Meteorol. 33:5OO-507. spraying have been presented to highlight the Drummond, A. M. 1987. Aircraft vortex effects on role that wind direction plays on the drift and droplet motion and vortex stability, pp. 51.-68. In: (Ed.). deposit from ULV spraying. Lidar mapping of G. W. Green Proceedingsof symposium on the aerial application of pesticidesin forestry. NRC the ULV spray cloud shortly after emission has Report 29197. National ResearchCouncil, Ottawa, clearly shown that the ULV spray is quickly en- Ontario, Canada. trained and redistributed within the vortices. For Dumbauld, R. K., J. R. Bjorkland and S. J. Saterlie. ULV sprays, the concentration within the cloud 1980. Computer models for predicting aircraft dis- is controlled by the vortex roll up. Spanwise persion and deposition above and within forest can- Jwe 1996 Syrr,rpostuu: Clouo DvNeuIcs 379

opies. Forest Pest Management Report 80-11. Mickle, R. E. 1994. Utilizing vortex behaviour to USDA Forest Service, Methods Application Group, minimize drift. J. Environ. Sci. Health B 29:621- Davis, CA. 645. Hoff. R. M.. R. E. Mickle and F. A. Froude. 1989. A Picot, J. J. C., B. Chitrangad and G. Henderson. 1981. rapid acquisition lidar for aerial spray diagnostics. Evaporation rate correlation for atomized droplets. Trans. Am. Soc. Agric. Eng.32:1523-1528. Tlans. Am. Soc. Agric. Eng.24:552-554. McCooeye, M. A., R. S. Crabbe, R. E. Mickle, A. Picot, J. J. C., D. D. Kristmanson and N. Basak- Robinson, E. B. Stimson, J. A. Arnold and D. G. Brown. 1986. Canopy deposit and off-target drift Alward. 1993. Strategy for reducing drift of aerially in forestry aerial spraying: the effects of operational applied pesticides. NRC Report IEC-93. National p:uirmeters. Tlans. Am. Soc. Agric. Eng.29:9O-96. Research Council, Ottawa, Canada. Teske, M. E., J. F Bowers, J, E. Rafferty and J. W. Mickle, R. E. 1987. A review of models for ULV Barry. 1993. FSCBG: an aerial spray dispersion spraying scenarios, pp. 179-188. 12.' G. W. Green model for predicting the fate of released material (Ed.). Proceedings of symposium on the aerial ap- behind aircraft. Environ. Toxicol. Chem. 12:453- plication of pesticides in forestry. NRC Report 4&. 29197. National Research Council, Ottawa, Ontario, Canada.