Terminal Velocity Determinations for Switchgrass Nodes and Internodes

Abstract

HighlightsA vertical wind tunnel was fabricated to aerodynamically suspend switchgrass particles to measure terminal velocity.Overall terminal velocity estimates for node and internode were 8.04 and 5.68 m s-1, respectively.Terminal velocity prediction equation for spherical particles (Mohsenin, 1970) produced values almost identical to experimental values unlike an equation for infinite-length cylindrical particles.Hollow switchgrass particle behavior was complex with particle bouncing, rolling, tumbling, and other dynamic behaviors during aerodynamic equilibrium. Abstract. Terminal velocity is a fundamental aerodynamic property often used to separate plant anatomical components. A vertical wind tunnel was fabricated to investigate whether terminal velocities of switchgrass stem internodes and nodes were distinctly different. Wind tunnel features included a clear observation section for suspended particles and fine adjustment of airflow. Each sample terminal velocity was measured with 45 anemometer readings from nine locations and five repeated measures. The experiment evaluated two sample moisture contents (52% and 15% wet basis, hereafter referred to as w.b.), three particle lengths (0.64, 1.27, and 2.54 cm), node versus internode, and 12 replications. The 6,480 anemometer readings provided significant differences between factors and interactions. The greatest magnitude in mean terminal velocity difference was 4.55 m s-1 (i.e. 9.62 to 5.07) that resulted in a velocity ratio of 190% [(9.62/5.07) x 100]. The least magnitude of mean terminal velocity difference, while significant, was 0.36 m s-1 (i.e. 6.59 to 6.23). This offered a limited difference and may indicate that separations may need to be confined within groups of moisture category and/or particle length. Terminal velocity measurements were most similar to those calculated using the Mohsenin (1970) spherical particle equation yielding a mean (6.86 vs. 6.91 m s-1), standard deviation (1.41 vs. 1.42 m s-1), minimum (4.65 vs. 4.69 m s-1), and maximum (9.95 vs. 10.03 m s-1) for measurement and spherical particle equation, respectively. Keywords: Anatomical component, Biomass property, Physical experiment, Separation, Sorting, Switchgrass, Vertical wind tunnel.