Direct numerical simulation of the interaction of a laser-induced plasma with isotropic turbulence.

摘要

Numerical simulations have been used to study laser-induced breakdown in air and interaction of a laser-induced plasma with isotropic turbulence. A parallel compressible Navier-Stokes solver has been developed for the purpose. A number of numerical issues have been addressed. Three different models for air with increasing levels of physical complexity are used in the simulations.;Spherical energy deposition is studied as a model problem to understand some aspects of laser-induced breakdown, evaluate the numerical method used and compare results to theory (Taylor 1950, Sedov 1959) and past numerical simulations. Laser-induced breakdown in air is studied using all three simulation models. Evolution of the resulting flow field is classified into formation of a shock wave, its propagation into the background and subsequent collapse of the plasma core. Each stage is studied in detail. All three models used in the simulations predict rolling up of the core. Vorticity is observed to be generated in the flow at short times due to baroclinic production and at longer times due to rolling up of the core. Effects of deposited laser energy and Reynolds number on the resulting flow field are discussed. Jumps across the shock front are found to scale with the amount of energy deposited but the overall flow field evolves in qualitatively similar manner. The plasma core does not roll up at very low Re.;The effect of laser energy deposition on isotropic turbulence is studied. Simulations are conducted for Relambda = 30 and Mt = 0.001 and 0.3. For both cases, a shock wave is observed to propagate into the background compressing the turbulence. Significant expansion is observed in the core. Turbulence intensities are observed to get amplified due to compression near the shock wave and get suppressed in the core due to expansion there. This behavior is spatially inhomogeneous and non-stationary in time. Effect on the turbulence is studied by computing statistics for different flow variables. The effect of turbulence on the mean flow is also studied in detail. Turbulent vorticity amplification is also observed and explained. Budgets are computed for the turbulent kinetic energy equation to understand the mechanism underlying transfer of energy from the mean flow to the turbulence. The production, pressure dilatation and diffusion terms are found to play dominant roles.