Numerical simulations of compressively driven interstellar turbulence: I. Isothermal gas - AstrophysicsReport as inadecuate

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Abstract: We performed numerical simulations of supersonic isothermal turbulence drivenby mostly compressive large-scale forcing, using both a static grid andadaptive mesh refinement with an effective resolution N=768^3. After atransient phase dominated by shocks, turbulence evolves into a steady statewith an RMS Mach number about 2.5, in which cloud-like structures of over-densegas are surrounded by highly rarefied gas. The index of the turbulence energyspectrum function beta = 2.0 in the shock-dominated phase. As the flowapproaches statistical equilibrium, the spectrum flattens, with beta = 1.9. Forthe scaling exponent of the root mean square velocity fluctuation, we obtaingamma = 0.43 from the velocity structure functions of second order. Theseresults are well within the range of observed scaling properties for thevelocity dispersion in molecular clouds. Calculating structure functions oforder p=1,

.,5, we find for all scaling exponents significant deviations fromthe Kolmogorov-Burgers model proposed by Boldyrev. Our results are very welldescribed by a general log-Poisson model with a higher degree of intermittency,which implies an influence of the forcing on the scaling properties. Contraryto previous numerical results for isothermal turbulence, we obtain a skewedprobability density function of the mass density fluctuations that is notconsistent with log-normal statistics and entails a substantially higherfraction of mass in the density peaks than implied by the Padoan-Nordlundrelation between the variance of the density fluctuations and the Mach number.In conclusion, it seems necessary to account for the production mechanism ofturbulence in the ISM.

Author: Wolfram Schmidt, Christoph Federrath, Markus Hupp, Sebastian Kern, Jens C. Niemeyer


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