NEW METHODS FOR FLUID DYNAMIC TESTS OF AMMONOID SHELL HYDRODYNAMICS

We present a novel case study of Ammonoid Hydrodynamics that leverages the power of both 3D modeling and 3D computational Fluid Dynamics. Fluid dynamics data provide valuable insights into the potential biomechanics of swimming organisms like ammonoids. Fluid dynamic experiments, however, are difficult to tune and scale. We show how modern 3D-centric methods circumvent some of these difficulties, allowing nuanced assessments across a wider variety of shell morphologies.

We first design shell models in a custom application, which is based on the Unreal gaming engine and allows the user to specify ammonoid shell shape variables. We use the software to replicate fossil geometries, express idealized geometries, and to hybridize fossil morphologies with idealized alterations. This allows us to test hydrodynamic consequences of specific traits, and suites of traits. Next, we simulate fluid flow across shells at a range of speeds from 1-50 cm/s, in both steady state and time dependent flow analyses. We calibrate the fluid flow methods with sensitivity analyses and comparisons to experimental data. The results confirm previous results specific to ammonoid shells: drag increases with overall inflation and, to a lesser degree, with umbilical exposure. More importantly, we identify the nuanced impact of both size and speed on the hydrodynamic efficiency of distinct shell shapes. Umbilical exposure and whorl expansion rate have subtle impacts on drag, but these increase with greater size and/or speed. Finally, we discuss the importance of acceleration in time-dependent analyses. Overall, these methods are useful for a variety of fossil and ecology questions.