# HRT

## Seminar

### Mixing due to Rayleigh-Taylor instability

Seminar Room 1, Newton Institute

#### Abstract

Rayleigh-Taylor instability occurs when a dense fluid rests on top of a light fluid in a gravitational field. It also occurs in an equivalent situation, in the absence gravity, where there is a pressure gradient normal to a interface between fluids of different density such that the direction of acceleration is from the light to the heavy fluid. This situation occurs in Inertial Confinement Fusion Implosions (ICF), see for exapmle Amemdt et al [1].There have been a number of successful experiments on mixing due to Rayleigh-Taylor instability, for example Dimonte [2] and Dalziel [3]. However, it is impractical to perform the "perfect" experiment and experimental diagnostics are necessarily limited. High-resolution Large Eddy Simulation (LES) can now be used to greatly add to our understanding of the mixing processes and this is the subject of the talk. The numerical technique used,the TURMOIL code, was first used for Rayleigh-Taylor mixing by Youngs[4]. A MILES approach is used because of the need to treat discontinuities in the flow e.g. the initilal density discontinuity and shock waves (in some applications). The high Reynolds case is of most interest where it is assumed that the bulk properties of the turbulent zone are independent of the Reynolds number. It is argued that LES (rather that DNS) is then an appropriate technique. Mesh convergence, or near-mesh convergence, will be demonstrated for key statistical averages.

Results are discussed for a range of situations-(a) Rayleigh-Taylor mixing at a plane boundary, (b) three-layer Raleigh-Taylor mixing and (c) mixing in a spherical implosion (a simplified version of an ICF implosion).The three cases are illustrated in figs 1,2 and 3 in the attached file. Two main aspects of the mixing process will be discussed. Firstly the influence of initial conditions. It is argued that loss of memory of initial conditions is unlikely to occur in experimental situations. The initial conditions have a significant effect on the overall width of the mixing zone - an important issue for engineering models. It would very difficult to obtain corresponding results experimentally because of the lack of control and the difficulty in measuring initial conditions. Secondly, the internal structure of the turbulent mixing zone will also be discussed, in particular the dissipation of both turbulence kinetic energy and of density fluctuations. For the internal structure results are more universal and less dependent on the initial conditions.

1. P. Amendt et al., “Indirect-drive noncryogenic double-shell ignition targets for the National Ignition Facility: Design and analysis”, Physics of Plasmas, 9, p2221, (2002). 2. G Dimonte & M Schneider, “Density ratio dependence of Rayleigh-Taylor mixing for sustained and impulsive acceleration histories”, Physics of Fluids, 12, p304 (2000) 3. S.B.Dalziel, "Self-similarity and internal structure of turbulence induced by Rayleigh-Taylor instability", J. Fluid Mech. 399, p1,

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