A publication of the Advanced Simulation & Computing Division, NA-121.2, NNSA Defense Programs
March 2009
NA-ASC-500-09—Issue 10
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Study of Turbulence Mixing Showcases Science and Computation
Some of the largest fully resolved simulations of turbulence mixing to date showcase the ASC Program’s scientific and computational science capabilities. New and unexpected physics have been revealed in the study of mixing driven by strong pressure gradients. Applications of interest include inertial confinement fusion targets, laser induced launching of a flyer plate, stellar pulsations, and supernova explosions.
The turbulence mixing problem, being a result of the processes of advection, stirring, and molecular mixing occurring at different scales—is unusually complex. In the flow visualization image shown, a large pressure gradient drives the interpenetration of two pure fluids of very different density.
A still image showing the visualization of two different density fluids. The heavy (blue) fluid has density three times larger than the light (red) fluid.
Unlike most mix processes studied, the turbulent mixing process between two very different density fluids is highly skewed: the light fluid mixes much more rapidly than the heavy fluid. Mathematically speaking, this is because the equations in the mixing of large density materials have cubic nonlinearities as well as the usual quadratic nonlinearities. In short, the pure heavy (blue) material lasts longer than pure light (red) material, and this effect increases with density ratio.
Experiments and numerical simulations such as these produce an overwhelming amount of data and complexity. Visualization is one of the best tools to deal with this data and complexity. Scientists Pat McCormick and Steve Martin at Los Alamos have developed a novel visualization code for heterogeneous architectures, such as Roadrunner’s new innovative architecture. Using Martin’s code, they have produced movies of a simplified model problem that isolates specific mixing physics. In the attached movie, the turbulent mixing problem starts from rest with two pure, different density fluids (blue heavy and red light) filling a closed box.
Los Alamos researchers Daniel Livescu, of the Computer, Computation, and Statistical Sciences Division, and J. Ray Ristorcelli and Rob Gore, of the Applied Physics Division, have published this work in journals,¹ diverse conference proceedings, and book chapters.
Movie obtained from the largest simulation to date of mixing driven by buoyancy. Initially the box is filled only by blue (heavy) and red (light) fluids. In time, fully mixed fluid (green) arises and eventually fills the whole box.
¹ J. Fluid Mech. 591, p. 43 (2007) and 605 p. 145 (2008), and J. Turbulence (to appear)
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