Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........28k&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF TEXAS AT AUSTIN, 1995.Source: Dissertation Abstracts International, Volume: 57-02, Section: B,
Other
Scientific paper
The process responsible for the supersonic internal motions observed in interstellar clouds is unknown. The present work investigates whether some part of these motions could be induced by vorticity generated as a shock passes over a cloud. Expressions are derived for the vorticity generated by a shock-density gradient interaction and the vorticity enhancement by a shock. Besides the kinematic vorticity generated at the shock, the dynamical post shock vorticity production rate is also derived, for the cases of a curved shock and the shock-density gradient interaction. The shock-density gradient interaction is investigated in detail for the case of clouds with radial density gradients and for a colored noise density distribution. Both analytical methods and numerical simulations are employed. The vorticity generated by the shock passage over smooth density gradients is found to be located primarily inside the cloud, in contrast to the sharp boundaried cloud, where the vorticity is located alone the edge of the cloud. It is found that the vortical speed is comparable to the compressional speed, and that the bulk motion imparted to the cloud is approximately the sum of the compressional and vortical components. The scaling of each of these speeds with cloud density is consistent with the observed linewidth-size scaling relation if the density scales inversely with size. The simulations indicate that the most important postshock effects for small density contrasts are dynamical effects due to induced velocity gradients and the baroclinic effect due to density gradients parallel to the propagation of the shock and pressure gradients along the shock. The pressure gradients along the shock are created when the shock first encounters the density stratified medium. For large density contrasts and isothermal clouds, curvature effects are probably important. In the case of the colored noise density distribution, the mean square vorticity and density gradient are calculated behind the shock. The vorticity is found to vary slightly with the spectral index, but the scale length, and hence vortical velocity are independent of spectral index. The vortical velocity is a significant fraction of the shock speed and is expected to generate further condensations in the density field.
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