, Los Alamos National Laboratory

Pages: pp. 16-17

In this second of two issues devoted to the frontiers of simulation, we feature four articles that illustrate the diversity of computational applications of complex physical phenomena. A major challenge for computational simulations is how to accurately calculate the effects of interacting phenomena, especially when such phenomena evolve with different time and distance scales and have very different properties.

When time scales for coupling different effects are long—compared with those that determine each effect's evolution separately—then the system is "loosely coupled." It is then possible to couple several existing calculations together through an interface and obtain accurate answers.

Two of the articles—"Virtual Watersheds: Modeling Regional Water Balances," by Winter et al., and "Large-Scale Fluid-Structure Interaction Simulations," by Löhner et al.—discuss how to do this for specific loosely coupled systems and give example codes and results. A third article, "Simulation of Swimming Organisms: Coupling Internal Mechanics with External Fluid Dynamics," by Cortez et al., describes methods for calculating how deformable animals ranging in size from microbes to large vertebrates swim through fluids. The fourth article, "Two- and Three-Dimensional Asteroid Impact Simulations," by Gisler et al., describes a closely coupled calculation of hydrodynamics and radiation transport for asteroids striking the Earth. The coupling time for the radiation and material is much shorter than the time step, so the radiation transport and hydrodynamics motion must be solved simultaneously.

Linking together existing modules has tremendous advantages compared to developing new ones with a similar capability. If the modules already exist, the time between defining the problem and solving it can be much shorter. Second, the modules have already been tested, and thus have a lot of verification and validation. Third, code developers and users already have experience with how to use the modules correctly. The largest remaining issue is how to pass data among modules and how to handle different types of adjacent meshes. The calculation in Winter et al.'s article employs a generalized software infrastructure that connects separate parallel applications and couples three existing software packages. This method appears to be particularly powerful for calculating fluid flows through a fixed geometry. Löhner et al. discuss their solutions for how to enforce accurate coupling between packages with very different mesh types and geometries. Their simulations include deformation of a solid object due to force loading from the fluid. Cortez et al. examine how to treat the interaction of highly deformable objects (such as bacteria and nematodes) within the fluids through which they move via an immersed boundary framework. This powerful technique helps calculate self-consistent solutions for the force balance between the swimming organism and the fluid through which it moves.

Obviously, the coupling between the constituent parts of asteroid impacts—matter and radiation—occurs on a time scale much shorter than practical time steps. Gisler et al. calculate the radiation-matter interaction implicitly. The material and radiation both move through the same fixed Cartesian mesh. Although the common mesh simplifies the treatments of different phenomena, it does so at a potential cost of numerical diffusion if the resolution is inadequate. They achieve additional resolution by adaptive mesh refinement (AMR)—that is, by increasing the number of mesh cells locally wherever increased accuracy is needed.

Douglass Post is an associate editor in chief of *CiSE* magazine. He has 30 years of experience with computational science in controlled magnetic and inertial fusion. His research interests center on methodologies for the development of large-scale scientific simulations for the US Department of Defense and for the controlled-fusion program. Contact him at post@lanl.gov.