Theoretical and visual flow analysis of Dipterocarpus zeylanicus (Hora) seed flight using airfoil theory and PIV method

Abstract

One of the most important evolutionary strategies for plant habitat expansion and successful reproduction is wind dispersal. Large-winged seeds produced by the endangered Sri Lankan species Dipterocarpus zeylanicus (Hora) descend by autorotation, a passive flight mechanism powered by gravity and asymmetric wing geometry. Impact damage is decreased, dispersal range is increased, and velocity is decreased with this controlled descent. The aerodynamic behavior of Hora seeds is still poorly understood, even though earlier research has empirically examined seeds such as maple and mahogany. By modelling lift and drag forces using Thin Airfoil Theory and Kutta–Joukowski circulation concepts, this study closes that gap. An inexpensive Particle Image Velocimetry (PIV) system was used to visualise the flow. A green laser and a rotating polygon mirror were used to create a laser sheet, and a basic wind tunnel was built. Tracer particles were supplied by an ultrasonic humidifier, and a variable-speed fan was used to adjust airflow to the seed’s natural terminal velocity. Two honeycomb structures made using PVC pipes were installed on both sides of the wind tunnel to stabilise the turbulence of the airflow mixed with water particles. There is an Anemometer used to measure the velocity of the vertical Wind flow. PIVlab, a MATLAB-based program, was used to process the airflow from high-speed video (240 fps) to produce velocity vectors and visualize the flow field. Assuming no horizontal wind, theoretical calculations showed a drag coefficient of 0.33 and a lift coefficient of 0.55. The upward lift force (0.074 N) nearly equals the seed’s weight (0.077 N). PIV analysis revealed symmetrical flow separation behind the seed, clear vortex generation, and upward airflow close to the leading edge. These patterns attest to efficient lift production and steady autorotation. The theoretically calculated relative velocities of the wind around the seed matched those calculated by the PIVlab application. Additionally, a centripetal force was noted to sustain wing angle, particularly at terminal velocity. The application of Thin Airfoil Theory to describe the aerodynamic behavior of Hora seeds is supported by the combined lift and drag forces. An evolved adaptation for passive flight is reflected in the effective stabilisation and deceleration mechanisms seen. These discoveries offer potential for bio-inspired applications in the design of low-speed aerial systems and aid in the ecological modelling of seed dispersal.