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Asking "What If" Questions About Turbulence

Paul Woodward & David Porter, University of Minnesota

The work of Paul Woodward and colleagues at the University of Minnesota's Laboratory for Computational Science & Engineering (LCSE) has contributed significantly to improved understanding of turbulent convection in giant stars. Their current focus is the problem of simulating an entire giant star. To build the capability to simulate faithfully the dynamics of the entire star, they are developing the ability to do detailed computational modeling of small-scale phenomena, the results of which can be used as model parameters to advance flow in the large-scale model.

"This need to represent the large-scale effects of small-scale turbulence," says Woodward, "arises in many areas of science and engineering, from flow in pipes and combustion engines to meteorology." Using their PPM (piece-wise parabolic method) code, Woodward and LCSE colleague David Porter have developed a small-scale turbulence model, which they can use for quick exploratory runs on relatively small problems to gain rapid answers to "what if" questions that may affect large-scale model parameters.

While previous systems in general have been able to achieve their best performance on extremely large problems in batch mode, Woodward and Porter are collaborating with a TeraGrid resource provider, the Pittsburgh Supercomputing Center (PSC), and using the TeraGrid network to demonstrate that it is possible to carry out such exploratory runs. "We focused on the Cray XT3," says Woodward, "because it has the fastest processor interconnect in a machine containing thousands of CPUs, and we believe this feature is all important for enabling interactive steering of flow simulations."

In demonstrations at iGrid in San Diego and at SC05 in Seattle, Woodward and Porter used 512 processors of the TeraGrid XT3 at PSC (for a 512^3 grid) to simulate turbulent mixing and entrainment of one fluid by another moving past it, separated by a thin shear layer, flow that occurs in the inner convection zone of a giant star. They successfully demonstrated a prototype computational steering, visualization and data-analysis pipeline that produced volume-rendered images at a rate of up to a second per frame, with data streamed directly from the XT3 - compressing what would normally be a week of computing to an hour.

Along with the XT3's very fast interconnect, the demonstrations relied on software developed for TeraGrid by PSC staff, which can route simulation data from the XT3 in real time to remote users anywhere on the wide-area network. This software, called PDIO, assembles data fragments written by each processor into complete files at the receiving end. Real time response allowed Woodward and Porter to change the data displayed on the fly.

From simulations on Big Ben by Paul Woodward and David Porter, this image (produced by Porter, displayed courtesy of LCSE) shows the complex structure of vortex tubes that develop due to the instability of a thin shear layer separating to gases of different densities that are moving past each other. The flow is viewed from the side in a sequence of 3 images that together display the time development of this flow and its approach to turbulence.

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