Sprayer speed a turbulent topic

As sprayers go faster, aerodynamics start to come into play. A PAMI project is evaluating if they affect spray deposition

On the surface, spraying seems straightforward. But a closer look at the multitude of factors that can influence how best to get product from the spray tank to the plant surface reveals something akin to a massive puzzle with a couple of pieces missing.

One of the people trying to make that picture complete is Hubert Landry, research scientist (engineering) with PAMI in Humboldt, Sask. With funding from the Western Grains Research Foundation (WGRF), and assistance from Tom Wolf of Agrimetrix Research and Training in Saskatoon, Landry is halfway through a study looking at how travel speed and sprayer configuration can affect air turbulence around sprayer nozzles and how, in turn, that turbulence might affect spray deposition.

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First step? Quantify airflow and air turbulence.

“Sprayers are getting larger and longer and faster,” says Landry. “So we have to look at the airflow patterns to see how they could affect spray deposition. Does air turbulence matter? We don’t know yet, but it should be quantified so that we can find out.”

Measuring thin air

Clearly, any solid body moving through space will cause air turbulence in its wake. But think about a sprayer with its boxy tractor body, spray tank, long skinny boom, bulky little nozzles, big fat tires — every one of those components displaces air differently. Add travel speed to the mix and you get even greater potential for air currents and eddies powerful enough to interfere with spray deposition.

Landry and his research team conducted field trials in October and December of 2015, using a Spra-Coupe 4640. Three ultrasonic anemometers (wind speed measurement tools) were placed at key positions around the sprayer — one in front and two behind, with the ability to move the rear anemometers to different positions to gather a more complete picture of air turbulence at specific locations along the boom itself, as well as a short distance behind it.

A weather station was set up in the field to measure ambient wind speed and direction.

“As much as possible, we wanted to keep wind out as one of the variables,” says Landry, adding that both test days experienced only mild wind.

Air turbulence was measured as TKE, or turbulence kinetic energy, which indicates the intensity of air movement, and measurements were taken at two travel speeds: two and eight metres per second (7.2 km/h and 28.8 km/h).

A total of 45 trials were carried out and, crunching the data, Landry has already spotted some trends, one of which concerns tires.

“We observed that, fairly consistently, the area behind the rear tire had a higher TKE,” he says. “It was more erratic. And the higher the velocity, the more turbulence there was.”

Indeed, compared to TKE behind other parts of the boom and sprayer, the space behind the tire is a veritable tornado. Computer modelling also suggests that the bigger the tire, the bigger the TKE, as well — which has implications for larger machines.

But it wasn’t just the tires.

“We think the machine itself, its geometry and size, has an impact on air flow,” says Landry, adding that this year, his team will be conducting further field experiments with a new 120-foot boom John Deere. This should give them some good data not only about how air turbulence differs behind components of a larger machine, but how the sheer size of that machine has an impact on air movement all around it.

But does it make a difference?

If you’re wondering if you have to add air turbulence to the long list of factors you need to consider when you spray, hold on for now.

Remember, this first year of field experiments have measured and quantified air turbulence at various speeds — that’s it. Nozzles were turned on only to see if the presence of spray would affect airflow, but no information on spray deposition was collected.

“At this point in the research, we can see three possible outcomes,” says Landry. “First, we may find that air turbulence doesn’t matter at all to spray deposition. Second, farmers may need to watch speed more closely — there may be a combination of ambient conditions and speed that will affect deposition. And third, there may be a need to design sprayers differently in terms of the geometry of the boom.”

Further study will tell the tale. Landry and Wolf have secured funding to take the research to its next logical step. “We will look at how turbulence plus speed will affect deposition,” he says. “We will collect data that may or may not confirm the trends we saw in the first year.”

It could be one more piece in the complex puzzle of accurate spraying and, says Landry, a very practical one at that. “We all want to use as few inputs as possible, so it would be good to understand what’s happening behind the sprayer in terms of airflow.”

WGRF is a farmer-funded and -directed non-profit organization investing in agricultural research that benefits western Canadian producers. For 35 years the WGRF board has given producers a voice in agricultural research funding decisions. WGRF manages an Endowment Fund and the wheat and barley variety development check-off funds, investing over $19 million annually into variety development and field crop research. WGRF is the largest producer funder of research in Canada.

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