Drag Measurement of Barley (Hordeum vulgare L.) Heads With Varying Morphology

Abstract

The static aerodynamic loads on barley heads have an impact on stem susceptibility to lodging and head loss. This paper quantifies the drag coefficient values of barley heads and presents insights into the effect of head phenotypic variations on the drag coefficients and force moments on the stem adjoining the head, referred to as the peduncle. The measurements are performed against a range of Reynolds numbers for different head orientation angles relative to the incoming wind in the close-return wind tunnel. The heads with small awns angle relative to the rachis exhibit greater drag coefficients as compared to non-awned heads, particularly at lower Reynolds numbers, and would also have a relatively higher centre of pressure, resulting in greater force moment at the peduncle. The drag coefficient and moment arm decrease as the awn angle relative to the rachis increases. The lex-shaped and fan-shaped barley heads do not present a significant variation in drag coefficients. Moreover, the length and size of the head exhibit a minimal difference in drag coefficient values, but would result in higher bending moment values at the peduncle. The results also indicate that the heads with greater kernel angle relative to the rachis, keeping the grain number constant, have lower drag coefficients and moment arms, resulting in smaller bending moments at the peduncle. This work suggests that breeding programmes aimed at increasing barley yield by enhancing grain number, grain mass and plumpness may benefit from selecting for an increased kernel angle relative to the rachis, thereby limiting stem stress and the potential for mechanical breakage.