UC Riverside

Turbulence simulations of a hydrodynamic fluid are performed to explore various nonlinear aspects of a nearly incompressible (NI) fluid in a regime where temperature fluctuations dominate. The NI model was developed primarily to understand weak compressive effects in interplanetary and interstellar media. Nonlinear structures generated by turbulent relaxation are shown to exist in our two-dimensional fluid simulations. Dynamically weak compressive effects are associated with passively convected thermal fluctuations which enhance the rate of selective decay in decaying NI turbulence. Turbulent relaxation leads to self-organization in thermally dominated NI velocity fluctuations and predicts the formation of large-scale steady-state coherent structures via an inverse cascade mechanism. In agreement with theoretical predictions, density fluctuations are slaved to the incompressible velocity fluctuations and exhibit a Kolmogorov-type power law. Thermal and density fluctuations are found to be anticorrelated in an adiabatic fluid. This suggests that a large fraction of the high plasma-beta fluid departs from a thermal equilibrium. Furthermore, compressional effects in nearly incompressible turbulence enhance decay rates significantly and lead to the formation of coherent vortices on much faster time scales when compared with incompressible turbulence. (C) 2006 American Institute of Physics.