VERY LOW REYNOLDS NUMBER FLOW OVER NON-SPHERICAL PARTICLES: APPLICATIONS TO POLLEN AERODYNAMICS

Flows at the very low Reynolds number regime (<10) typically have application to the understanding of the dispersion of micro-scale particulates in the atmosphere, as well as microorganisms. Most particulates are non-spherical, requiring specialized studies in order to determine their settling velocities, drag forces, or interactions. The present study considers flow over saccate pollen with the goal of better understanding their flight dynamics. The pollen grains of several gymnosperm groups consist of a main body and one to three air-filled bladders, or sacci, forming ellipsoidal lobes. Previous studies have demonstrated that sacci increased the resistance coefficient of the grain compared to one without sacci, thereby improving its aerodynamic efficiency by increasing dispersal distance. In order to better quantify the effect of sacci position, size, and orientation on pollen dispersion, scaled-up physical models were created based on electron microscopy images. Furthermore, surface ornamentation or texture could be added to the models, adding a higher degree of realism. The models were suspended inside of a glycerin-filled tank capable of translating at very low speeds, producing Reynolds numbers as low as 0.05. Particle Image Velocimetry (PIV) was used to measure velocity fields in the wake of the pollen models. This experimental arrangement facilitated the ability to produce both steady and unsteady (i.e. accelerating or decelerating) flows. Differences among the wake flowfields were related to previously made measurements of pollen shape factors. These studies suggested that both sacci size, orientation or relative position on the main body, as well as surface texture affected this shape factor. The PIV measurements are capable of resolving wake details which demonstrate a wide stagnant flow region behind the main body and between the sacci. This is in contrast to a typical spherical or ellipsoidal geometry, which would be characterized by a single stagnation streamline at the aft, with the flow remaining attached and no wake present when Re<1. At these low Reynolds numbers, there does not appear to be evidence of flow reversal between the sacci; however rapid flow deceleration was capable of producing such reversals.