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.
