Aerodynamics and Design of Spinning Samara Model

Abstract

AbstractA particular form of winged seed (samara) dispersal technique adopted by nature uses autorotative (unpowered rotation of the wing generating thrust force against gravity) descent; for e.g. in Maple and Mahagony trees. This technique provides the lowest descent velocities among various seed dispersal techniques found in nature ensuring the safety of the delicate seeds. Bio-mimicked solutions to important engineering problems in aerospace as well as in disaster management—air dropping of life-saving packages during floods can be inspired from the samara. The samara is a complex structure having a bluff root containing the seed attached to a three-dimensional wing. The dynamics of the samara from the instant of release is entirely unsteady, involving an initial transition phase where the samara tumbles until it achieves autorotation leading to a steady descent velocity. The distribution of mass and aerodynamic forces in this single structure ensures its stability during descent. Studies to comprehensively understand the physics of the samaras are limited. Recently, leading edge vortex (LEV) has been found to be responsible for the high thrust forces achieved during autorotation. The dependence of LEV on the morphology of the seed needs to be understood to design optimal devices for engineering applications. The principal aim of this study is to understand the effect of morphology on the aerodynamics of the samara with a particular focus on the characteristics of the LEV. The flow field around the autorotating samara is experimentally obtained using particle image velocimetry (PIV) in a specially designed vertical wind tunnel. However, natural samara has limited utility for parametric studies; therefore, 3D printed model is developed that closely mimic the functions of the natural samara. Drop tests of the natural samara and the 3D printed model show that the dynamics of the model and the samara are similar. For the first time, a complete characterization of the spanwise distribution of LEV has been carried out on the samara model which enables us to understand the inter-relationship between observed flow physics, morphology and performance parameters.KeywordsUnsteady aerodynamicsLeading edge vortexSpinning samarasPIV