Aerodynamics of Saccate Pollen and its Implications for Wind Pollination

Alex Colletti, Mechanical Engineering (on right in photo)

Faculty Mentor: Dr. Lisa Grega

              The pollen grains of conifers vary in many significant structural ways.  Three extant pollen grains, including Pinus, Falcatifolium and, Dacrydium, were studied to determine the effects of air-filled bladders on their settling velocities.  In previous research at Truman State University stroboscopic photography was used to create mathematical models of various grains from each of the three species and settling speeds were mathematically calculated based on prolate spheroids. 

              The current research goals are to determine, experimentally, the effects of the air-filled bladders and refine drag coefficient estimates.  Mathematical computer based solid models have been created of the pollen grains, with and without sacs, at 400 to 600 times the scale of the actual grains. The models were printed three dimensionally using stereo-lithography with their largest dimension reaching approximately 1 inch.  These models can be used to represent the actual pollen grain by dropping the models through a fluid more viscous (thicker) than air.  Settling velocities of the models are fine tuned to match Reynolds numbers (the ratio of inertial to viscous forces) with the actual pollen.  Then mass ratios, taken from measurements of the actual grains, of the pollen will be used to create a similar model without sacs.  Their settling speeds will be compared. 

              Before a direct comparison between sac and sac-less pollen can be made the effect of roughness factors on the pollen needs to be examined.  This was done experimentally by first comparing spherical models which are smooth to those with added roughness features.  Dropped at the same Reynolds numbers at which the pollen grains fall, these spheres represent a similar physical flow situation.  Preliminary results show that roughness features have little effect on the drag coefficient of spheres at low Reynolds numbers.  Results from experimentation will determine if the fine surface ornamentation on the actual pollen grains needs to be incorporated into the models. 

Personal Statement

              The summer undergraduate research program has introduced me to many aspects of experimental research.  As a potential mechanical engineering graduate student, the skills obtained over the summer should provide a solid foundation for future doctoral work.  Interdepartmental collaboration with the Math department at Truman State and the Biology department here at TCNJ, fine tuning control of independent variables in lab experiments, and the need to come up with creative solutions to problems as they arise are all skills which could not be developed as extensively in the classroom setting.  Additionally, much was learned though the experimentation process regarding making fine measurements of viscosity, density and volume.  These properties, which would normally be given in a textbook problem, proved difficult to obtain, with high precision, experimentally.  Workarounds and personally creating measurement equipment proved a valuable experience.