The Enigma of Claviaster libycus Survival Rate: Numerical Simulations Revealing its Superiority Post-Mass Extinction

Abstract

Previous research indicates that the survival rate of Claviaster libycus following mass extinction events surpasses that of regular echinoids. This investigation seeks to assess the flow dynamics and scour patterns resulting from the distinctive distorted morphology of Claviaster libycus using numerical modeling. In addition, this study examines the localized scour that occurs around tsunami stones. The numerical model employed in this investigation, Splash3D, has been adapted from the open-source code Truchas, originally developed by the National Laboratory of the United States. Splash3D is designed for solving the three-dimensional, incompressible Navier-Stokes equations. The Volume of Fluid Method (VOF) is utilized to characterize the kinematics of the water and sand surfaces. As Claviaster libycus partially submerges in the sand, the rheological behavior of the bottom sand is characterized using the Discontinuous Bi-Viscous Model (DBM), derived from the conventional Bingham Model (BM). Unlike the BM model, the DBM model employs the yield strain rate instead of the yield stress to differentiate the plug from the liquefied zone. In the Plug zone, high viscosity signifies solid characteristics, with the plug-zone viscosity significantly surpassing that of the liquified zone. The liquified zones represent sand disturbed by local currents around irregular echinoids, while the plug zones depict undisturbed sand. The yield strain rate dictates the stiffness of the bottom sand, and the DBM model is employed to describe local scour around obstacles. According to numerical simulations and experimental results, when the gonopore of Claviaster libycus is directed downstream, it can reduce the generation of horseshoe vortices. Therefore, compared to the gonopore pointing upstream, having the gonopore directed downstream can decrease the local scour around the sea urchin. Furthermore, in the event of a tsunami, intense local scouring occurs around large stones, leading to structural instability in the rock formations. Due to the tsunami's characteristic long wavelength, as the powerful water flow passes through, it eventually transports the large stones to the shoreline, forming what is known as tsunami stones. Detailed analysis results are presented at the conference.