CHARACTERIZATION OF AGRICULTURAL SPRAY BY DIGITAL ANALYSIS OF SHADOWGRAPHY IMAGES

Agricultural sprays are among the most common two-phase flows studied
because their characteristics determine the efficiency of treatment as well as
environmental contamination. At present, the relevance of different
characterization techniques of the highly polydispersed sprays used in this
application remains controversial. Digital analysis of shadowgraphy images
presents an attractive option for the characterization of both velocity and
size of droplets present in the spray.
This study presents an overview of the contrast problems inherent in the use
of backlighted images and propose appropriate solution to ensure the quality
of measurements. Generally, background light presents heterogeneities
associated with light sources and optical arrangement. These can be solved
by substracting from each images a composite background. An other
particular focus is given to one major drawback of volumetric lighting, the
presence of out focus droplets. These droplets have to be removed during the
image analysis process because the measurement of their diameters can't be
done accuratly. The rejection of these out of focus objects is based on a
quantitative parameter which was calibrated with a obliquely shot
monodispersed spray.
The final step of the image processing is determining velocity of the droplet
by tracking a same droplet on two successive images. The tracking algorithm
is based on the size of the droplet, its more probable displacement and its
direction.