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Session 121 - Molecular Clouds/Star Formation.
Oral session, Saturday, January 10
Georgetown,
H_20 masers in star forming regions are ideal probes of the highly supersonic turbulence generated by outflows from the newly born stars in the surrounding gas. They demonstrate: strikingly similar dependence of the mean square velocity increment <(\delta v(l))^2> on point separation l to the classic Kolmogorov 2/3 law for incompressible turbulence and (2) a fractal pattern of the geometrical set on which turbulence dissipates, with a very low fractal dimension (D < 1). Yet, according to current understanding, both the energy dissipation in large-scale shock waves and the intermittency of dissipation (expressed by the fractal structure) should steepen the dependence of <(\delta v(l))^2> on l, the steepening growing with the decrease of fractal dimension. It has been argued before that energy dissipation in large-scale shocks may be hindered, if the vortical component of the flow is dominant due to appropriate boundary conditions. Here we address the second question: why does the low-fractal-dimension intermittency not steepen the velocity spectrum? The approach to incompressible turbulence proposed recently by Barenblatt amp; Chorin (Bamp;C) may lead to an answer. These authors argue that intermittency doesn't disturb, but in fact helps to establish the Kolmogorov law for the second and third order structure functions of turbulence, when the Reynolds number tends to infinity. We show that the Bamp;C approach is applicable to highly supersonic turbulent flows as well as to incompressible flows and that the Reynolds numbers of the supersonic turbulence probed by H_2O masers are very high indeed. Further development of the theory of supersonic turbulence requires elucidation of the role of a high Mach number and a study of the behavior of higher order structure functions.
