Simulations of Vortex Ring Production in Sphagnu Moss

Mature capsules of Sphagnum moss dehydrate on a warm day and change from spherically to cylindrically shaped due to the collapse of cells in the capsule wall. This increases the pressure of the gas within the capsule until its lid is ripped free and the gas escapes The escaping gas creates a vortex ring, which travels at speeds exceeding 10 m/s and carries the dust-like spores within the capsule to a height of over 15 cm. Through this mechanism spores are carried through the laminar boundary layer near the earth's surface and into the turbulent boundary layer where they can mix into the atmosphere and be carried indefinitely. Since Sphagnum is a non-vascular plant and can't grow tall enough to reach the turbulent boundary layer this means of spore dispersal is crucial to its ability to colonize new habitats and might explain its abundance. While vortex rings are common in nature, Sphagnum is the only plant known to create them and is the only organism that uses a pressurized compressible fluid to generate them.



Here we present a finite element analysis of the formation of Sphagnum vortex rings using with ANSYS Fluent. Our model shows that the rapid ejection of gas from the capsule is too short to create slug flow, which is seen in most vortex ring models. We also observe that the lid of the capsule reduces the speed of the escaping gas while increasing the formation time of the vortex rings compared to models without a lid. Comparisons of the velocity profiles of different models reveal that the lid may help Sphagnum preferentially entrain fluid from within the capsule enabling the spores to be more effectively carried by the vortex ring.