Wind-tunnel study of the aerodynamic characteristics and mechanical response of the leaves of Betula platyphylla Sukaczev

Abstract

It is necessary to examine the aerodynamic behaviour of leaves in wind in order to understand wind-induced tree disasters. The effect of the wind deflection angle on the aerodynamic characteristics of leaves remains relatively unexplored. Thus, the reconfiguration, vibration and wake characteristics of the Betula platyphylla Sukaczev leaves at various wind deflection angles and vegetative Cauchy numbers were investigated in a wind tunnel. Wind deflection angle (θ) is defined as the angle between the wind direction and the normal vector of the lamina, and a vegetative Cauchy number characterises the reconfiguration of an elastic structure under flow and is defined as the ratio of the product of the dynamic pressure and the third power of the slenderness ratio to the elasticity modulus. The results showed that at the different cases with an increase of Cauchy number, all the leaves mainly experience three states, a static bending of the laminas, and an opening and closing vibration of the laminas on left and right sides and a reconfiguration of the leaf. Except for the leaves with θ = 0, the leaves with θ = π/6, π/3 and π/2 also experience torsional motion. Four critical Cauchy numbers were defined corresponding to the four reconfigurations and vibration states. The mean critical Cauchy numbers for the torsional motion of the leaves were at θ = π/6, π/3 and π/2 this decreased with the increasing θ. The reconfigured mean Cauchy numbers of leaves for θ = π/6, π/3 and π/2 were significantly larger than those in θ = 0. Furthermore, compared with the case for θ = 0, the cases with θ = π/6, π/3 and π/2 proved to be disadvantageous for the reduction of vortex-induced vibrations through reconfiguration.