Numerical investigation on the flow characteristics of model dandelion seeds with angles of attitude

Abstract

A stable and axisymmetric "separated vortex ring"formed above the pappus can greatly improve the aerodynamic efficiency of dandelion seeds and reduce the material requirement for flight. Since the dandelion seed is not always in a "regular"state (i.e., the pappus plane parallel to the horizontal plane) when flying, this paper uses a simplified rigid pappus model to simulate and analyze the influence of flight attitude on the flow patterns and the resulting changes in aerodynamic characteristics under different porosities. The numerical results are obtained by solving the three-dimensional incompressible steady RANS equations with the Spalart-Allmaras turbulence model in ANSYS Fluent software package. The results show that when there is an angle of attitude, the separated vortex ring will break with one of the vortices, losing its original recirculating structure, which leads to a considerable loss of the drag performance. In addition, the high-pressure region below the pappus plane shifting to one side will produce a horizontal force and a recovery moment against the rolling direction. The horizontal force component generated by a large-porosity pappus disk is much larger than that of a solid disk, which shows the good flight efficiency of dandelion seeds. At the same time, the inclined pappus will produce a recovery moment to keep the seed from rolling away from the horizontal plane. These conclusions show that the pappus structure has the potential to adjust the flight attitude and aerodynamic characteristics, which provides a preliminary understanding for further study on the flight dynamics of dandelion seeds.