Aerodynamic properties of tef grain and straw material

Terminal velocities were measured for both tef (Eragrostis tef (Zucc.) Trotter) grains and straws using the suspension velocity method. The terminal velocity of tef grains increased linearly from 3.08 to 3.96 m s−1 with increase in moisture content from 6.5% to 30.1% wet basis (w.b.). Straws with different lengths, node-free, with a node in the middle, and with a node at the end were tested for their terminal velocities. Straws with nodes had higher terminal velocities compared with node free. Straws with a node at the end had the highest terminal velocity (5.4 m s−1 for 10-cm-long straws). Terminal velocity decreased from 3.69 to 2.13 and 3.08 to 1.70 m s−1 for middle-node and node-free straws, respectively, but for end-node straws the terminal velocity showed gradual increase from 3.32 to 5.40 m s−1 as the length of straw increased from 1 to 10 cm. The drag coefficients for grain and resistance coefficient (drag coefficient×frontal area) for straw were calculated from the experimentally obtained terminal velocities. Tef grains drag coefficient decreased from 0.83 to 0.65 with increase in moisture content from 6.5% to 30.1%. Node-free straws exhibited higher resistance coefficients and end-node straws the lowest.