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Swirling strengths

Open the door of your house to go outside on a hot summer day and you’ll create a density, or gravity, current. The air outside is hot and therefore lighter than the colder and heavier air conditioned air inside. Two currents develop when the door is open: one of cold air from the house to the outside and one of hot air moving from the outside into the house. The cold current is in the bottom part of the door and the hot current is in the top part of the door. This is just a simple example of gravity currents, which can be found in many places, including oil spills, thunderstorm fronts, the dust cloud following a building collapse, and currents transporting sediment into deep oceans. Over geological time scales, those sediment deposits form offshore oil reservoirs.

Mariano Cantero and Marcelo Garcia at the University of Illinois at Urbana-Champaign, along with S. Balachandar from the University of Florida, conducted extensive research on gravity flows utilizing Cobalt, NCSA’s SGI Altix system, once NCSA’s Greg Bauer helped them solve memory issues (see Access, Summer 2006). Lately, the team’s been working with NCSA data visualization expert David Bock to study the flows with detail that is impossible to measure in experiments.

For example, using the velocity field data, the team was able to compute the “swirling strength,” an indication of how fast the fluid is rotating locally. The regions of the flow with a large value of swirling strength are vortices. Bock visualized the swirling strength using specialized volume rendering software he developed that can handle the 1 terabyte of raw data and the 18 terabytes of processed data. The results: vortices and their interaction clearly visualized in a flow field, which Cantero believes is a first. These visualizations helped the team to better understand the dynamics of the vortex interaction in gravity currents. The team’s work with gravity currents in a cylindrical setting, including the swirling strength visualizations, was recently accepted for publication in the Journal of Fluid Mechanics.

“Something very interesting is that with the great detail of the simulations we have been able to compute every term in the partial differential equation—without modeling turbulence, as is usually done—that governs vorticity dynamics and we have been able to explore the role and contribution of every term,” notes Cantero. “This together with David’s visualizations gives a complete description of the vortex dynamics in the flow. The vortical patterns of the flow have an influence in the shear stress pattern that controls erosion and deposition of sediment, which is related to the formation of [the sediment] deposits mentioned above.”

This research is supported by the Office of Naval Research’s Coastal Geosciences Program, the National Science Foundation, the Chicago District of the U.S. Army Corps of Engineers, the Metropolitan Reclamation District of Greater Chicago, and a graduate student fellowship from the University of Illinois at Urbana-Champaign’s Computational Science and Engineering Program.

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