Characterization of shock–turbulent structures within shock/cloud interaction in astrophysical galactic outflows

In this article, the numerical simulation of three-dimensional hydrodynamic (HD) shock/cloud interaction in galactic outflows has been carried out by solving compressible HD equations [Banda-Barragán et al., “Shock–multicloud interactions in galactic outflows—I. Cloud layers with lognormal density distributions,” Mon. Not. R. Astron. Soc. 499, 2173–2195 (2020)] in the computer code PLUTO [Mignone et al., “PLUTO: A numerical code for computational astrophysics,” Astrophys. J., Suppl. Ser. 170, 228 (2007)], using higher-order finite volume method. The detailed flow features involved in complex shock/cloud interaction have been discussed thoroughly in this present study. Furthermore, the invariants (P, Q, and R) of the velocity gradient tensor (∂ui∂xj) reveal the local flow topology [Chong et al., “A general classification of three-dimensional flow fields,” Phys. Fluids A 2, 765–777 (1990); Suman and Girimaji, “Velocity gradient invariants and local flow-field topology in compressible turbulence,” J. Turbul. 11, N2 (2010); Thaker et al., “Invariants of the velocity gradient tensor in a spatially developing compressible round jet,” J. Fluid Mech. 971, A18 (2023)] due to shock/turbulence interaction. The joint probability density function of Q–R obtained from numerical data depicts the presence of turbulent sheet-like unstable-focus-compressing (UFC) and turbulent tube-like stable-focus-compressing (SFC)-like structures during the shock/cloud interaction for both compressive and solenoidal clouds, respectively. Furthermore, the distinct behaviors of the identified turbulent structures (both UFC and SFC) have been examined in great detail, offering valuable insights into the mechanisms that govern star formation following shock/cloud interactions.