NCSA NEWS

Density currents

Story posted December 6, 2006

Avalanches, oil spills, thunderstorm fronts, and the dust cloud following a building collapse all generate heavier fluid intrusions into a lighter environment. Known as density currents, these flows are also responsible for transporting sediment into deep oceans. Over geological timescales, those deposits form offshore oil reservoirs.

Mathematical modeling and large scale simulations give engineers the means to study these three-dimensional flows, which are frequently immeasurable due to their destructive power. With those models, engineers come to understand the flow's dynamics and turbulent structures and how particles mix and move within it, among other features. Recent investigations by the University of Illinois at Urbana-Champaign's Marcelo Garcia and Mariano Cantero and the University of Florida's S. Balachandar are doing just that.

The team's June 2006 Journal of Applied Mechanics article, for example, shows the collapse of a heavy fluid in a lighter fluid in two configurations -- a horizontal plane and a cylinder. This paper marks the first time this sort of flow structure has been calculated for a 3D cylinder. The work models the sediment discharge of a river into the ocean and the initial stages of alluvial fan formation at the river's mouth. The flow shows the well-known pattern of lobes and cleft, which are related to regions of high-bottom friction and channel carving.

These models, along with others, were created using NCSA's Cobalt computing system. They typically produce more than one terabyte of raw data and 18 terabytes of processed data.

"This is a quantum leap," says Garcia, a professor of civil and environmental engineering. Smaller, two-dimensional models, "you can slice different ways, but you still only get a basic snapshot. Here, we see the whole evolution of the flow." With that sense of the evolution, engineers can "attack different problems. It's very fundamental."

Making the most of the results also requires the visualization and data analysis capabilities that NCSA offers. The team worked with NCSA's Dave Bock to visualize the models. (Several steps during the heavy fluid's release into a lighter environment are shown here.) They used specialized volume rendering software developed by Bock. Unlike off-the-shelf visualization products, the software can handle massive datasets such as those confronted by the team. Its ability to incorporate light and shadows gives a surface-like appearance to the flows without the computational expense of rendering the geometry of the entire surface.

"It's like doing a dissection instead of looking at a body from the outside," says Cantero, a PhD candidate at the U of I. The dissection of a body in motion, no less -- living, breathing, and changing over time.

This research is supported by the Office of Naval Research's Coastal Geosciences Program, 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.