Aerodynamic properties of harvested perennial ryegrass: Effects of particle mass, size and moisture content.

Fibrous forage and biomass crops such as grass and maize are used at large scale as feed for ruminants and feedstock for biogas production. These crops are typically harvested by chopping them in small particles which are light and suitable for partial pneumatic conveying, where the particle trajectory is determined by their aerodynamic properties. While the aerodynamic properties of many grains have been studied, no reports were found for perennial ryegrass. Chopped ryegrass samples of leaf blades and stems were collected in the fields. After imaging the fresh harvested particles to quantify their size and surface area, their terminal velocity was measured with the fluidised-bed method. The relationship between the mass, projected area and moisture content of the stems and blades with their terminal velocities were modelled. In line with theory, mass and moisture content was positively correlated with terminal velocity, while projected area was negatively correlated with it. Smaller terminal velocities were observed for the blades (1.75 m s−1) compared to the stems (4.44 m s−1). The drag coefficients of grass particles were calculated, and for blades it was negatively correlated with the particle Reynold's number lying within Newton's regime (103 < Re < 3 × 105), whilst no significant correlation was found for stems. The average drag coefficient of blades (0.75) was larger than stems (0.54). No significant effect (p-value <0.01) of moisture content on the drag coefficient of grass particles was observed. This confirms that the drag coefficient is a shape parameter, related to formation of airstreams around the particle. • Aerodynamic properties of ryegrass were quantified in a wind tunnel. • The terminal velocity of grass blades was 61% smaller than that of stems. • Terminal velocities can be predicted from mass, area and moisture content (R2 = 0.77). • Grass particles' drag coefficient was calculated and compared to similar materials. • The drag coefficient of grass blades was 39% larger than that of stems. [ABSTRACT FROM AUTHOR]