Effect of porosity on separated vortex rings of dandelion seeds

Research on unstable airflow in nature has long been an important subject in work on aerodynamics, and most principles of flight have been found in nature. The design of an aircraft requires considering not only the lift required to balance gravity but also the stability of the state of balance during the cruise. Dandelion seeds happen to have a similar flow stability during flight. These seeds have a pore-like disk structure such that when air flows through them, a separated vortex ring is generated above them. Dandelions stabilize the airflow by changing the tip structure of the fluff. This study uses computational fluid dynamics to replace the fluff structure of dandelion seeds with a rigid porous disk structure and numerically simulates the resulting flow model to examine the flight of dandelion seeds. The results show that when the porosity of the disk structure exceeds a threshold, an axisymmetric and stable separated vortex ring is generated. A comparison of the simulation with observations from a wind tunnel experiment showed that the two yielded similar results, which confirms the principle of the model of flight of dandelion seeds. The authors then explore the condition for the separation of the vortex ring and conclude that the change in porosity affects the characteristics of the vortex ring.