Flow patterns and drag coefficients of dandelion pappus models consisting of two oppositely oriented filament layers

Abstract

The architecture of real dandelion seeds is diverse. Previous studies have primarily focused on the flow patterns and drag coefficients of dandelion seeds with a pappus angle no greater than 180°, but have paid less attention to the cases of the pappus angle larger than 180°. This work therefore numerically investigated the flow patterns and drag coefficients of dandelion seeds with the pappus angle larger than 180° when the speed of incident flow is 0.2 m/s, using double-layered models consisting of two oppositely oriented filament layers. The simulated results were then compared to those obtained from single-layered models consisting of identical number of but leeward-oriented filaments (the pappus angle lower than 180°). It reveals that, either single-layered or double-layered models, the length (L) and width (W) of separated vortex rings increase, drag coefficients (CD) increase, but the shape index (L/W) and the relative location of separated vortex rings (zu) decrease, as the number of filaments increases. At a given filament number, L, W, and zu in double-layered models are smaller than those in single-layered models, but L/W and CD in double-layered models are larger than those in single-layered models, attributed to the windward-oriented filaments. In double-layered models, thanks to small difference in the drag force but significant difference in the projected area, CD is significantly higher when both windward-oriented and leeward-oriented filaments are installed at identical locations on the central disk's perimeter compared to cases where windward-oriented and leeward-oriented filaments are installed at different locations.