Abstract Details
SUBSONIC AND SUPERSONIC SHEAR FLOWS IN LASER DRIVEN HIGH-ENERGY-DENSITY PLASMAS
Author: Eric C. Harding
Requested Type: Oral Only
Submitted: 2009-04-21 22:29:42
Co-authors: R.P. Drake, O.A. Hurricane, J.F. Hansen, Y. Aglitskiy, T. Plewa, B.A. Remington, H.F. Robey, J.L. Weaver, A.L. Velikovich, R.S. Gillespie, M.J. Bono, M.J. Grosskopf, C.C. Kuranz, A. Visco
Contact Info:
University of Michigan
2455 Hayward St.
Ann Arbor, MI 48109
USA
Abstract Text:
Shear flows appear in many high-energy-density (HED) and astrophysical systems, yet few laboratory experiments have been carried out to study their evolution in these extreme environments. The distinction between subsonic and supersonic shear flows is important since the compressibility of the flow can influence the development of the shear layer. Most shear flows containing steep velocity gradients are Kelvin-Helmholtz (KH) unstable if the wavelength of the interface perturbation is much greater than the scale length of the velocity gradient. Until now the KH instability, in HED experiments, has been primarily observed in Rayleigh-Taylor unstable systems as a secondary instability. The shear layer generated between the high-density spikes and low-density bubbles causes the spike tips to grow into mushroom shaped structures. In this way the KH instability generates smaller length scales that allow the flow to dissipate energy and transition to a turbulent state. Understanding this transition is important since mixing rates and diffusivities can increase by several orders of magnitude in the turbulent regime.
We present two dedicated shear flow experiments that produced subsonic and supersonic shear layers in HED plasmas. In the subsonic case the Omega laser was used to drive a shock wave along a rippled plastic interface (single mode with λ = 400 μm and amplitude = 30 μm), which subsequently rolled-upped into large KH vortices that were accompanied by bubble-like structures. This was the first HED experiment to generate well-resolved KH vortices and observe their evolution. Interestingly the origins of bubble-like structures, which appeared as distinct, bright regions in the x-ray radiographs, are unknown. In a separate experiment, the Nike laser was used to drive a supersonic flow of Al plasma (Mach ~ 2-3) along a low-density foam surface seeded with a ripple (single mode with λ = 300 μm and amplitude = 15 μm). Unlike the subsonic case, detached shocks developed around the ripples in response to the supersonic Al flow. Both the Omega and Nike experiments have implemented novel target geometries that can be used for future investigations of the transition to turbulence in HED systems.
This research was sponsored by the Naval Research Laboratory through contract NRL N00173-06-1-G906, the NNSA Stewardship Sciences Academic Alliances through DOE Research Grant DE-FG52-04NA00064, the DOE NNSA under the Predictive Science Academic Alliance Program by grant DE-FC52-08NA28616, and under the National Laser User Facility by grant DE-FG03-00SF22021.
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