Localization of unresolved regions in the selective large-eddy simulation of hypersonic jets

A method for the localization of the regions where the turbulent fluctuations are unresolved is applied to the selective large-eddy simulation (LES) of a compressible turbulent jet of Mach number equal to 5. This method is based on the introduction of a scalar probe function f which represents the magnitude of the twisting-stretching term normalized with the enstrophy [1]. The statistical analysis shows that, for a fully developed turbulent field of fluctuations, the probability that f is larger than 2 is zero, while, for an unresolved field, is finite. By computing f in each instantaneous realization of the simulation it is possible to locate the regions where the magnitude of the normalized stretching-twisting is anomalously high. This allows the identification of the regions where the subgrid model should be introduced into the governing equations (selective filtering). The results of the selective LES are compared with those of a standard LES, where the subgrid terms are used in the whole domain. The comparison is carried out by assuming as high order reference field a higher resolution Euler simulation of the compressible jet. It is shown that the selective LES modifies the dynamic properties of the flow to a lesser extent with respect to the classical LE

Numerical Simulations of Torsional Alfv\'en Waves in Axisymmetric Solar Magnetic Flux Tubes

We investigate numerically Alfv\'en waves propagating along an axisymmetric and non-isothermal solar flux tube embedded in the solar atmosphere. The tube magnetic field is current-free and diverges with height, and the waves are excited by a periodic driver along the tube magnetic field lines. The main results are that the two wave variables, the velocity and magnetic field perturbations in the azimuthal direction, behave differently as a result of gradients of physical parameters along the tube. To explain these differences in the wave behavior, the time evolution of the wave variables and the resulting cutoff period for each wave variable are calculated, and used to determine regions in the solar chromosphere where strong wave reflection may occur.Comment: Submitted to Solar Physics (accepted

Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields

This paper presents an application of the recent relativistic HLLC approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing normal component of the magnetic field. The numerical scheme is validated in two dimensions by investigating the propagation of axisymmetric jets with toroidal magnetic fields. The selected jet models show that the HLLC solver yields sharper resolution of contact and shear waves and better convergence properties over the traditional HLL approach.Comment: 12 pages, 5 figure

An Exact Integration Scheme for Radiative Cooling in Hydrodynamical Simulations

A new scheme for incorporating radiative cooling in hydrodynamical codes is presented, centered around exact integration of the governing semi-discrete cooling equation. Using benchmark calculations based on the cooling downstream of a radiative shock, I demonstrate that the new scheme outperforms traditional explicit and implicit approaches in terms of accuracy, while remaining competitive in terms of execution speed.Comment: 7 pages, accepted by ApJS. Revision 2, with error in eqn. 13 fixe

The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution

The stability properties of one-dimensional radiative shocks with a power-law cooling function of the form $\Lambda \propto \rho^2T^\alpha$ are the main subject of this work. The linear analysis originally presented by Chevalier & Imamura, is thoroughfully reviewed for several values of the cooling index $\alpha$ and higher overtone modes. Consistently with previous results, it is shown that the spectrum of the linear operator consists in a series of modes with increasing oscillation frequency. For each mode a critical value of the cooling index, $\alpha_\textrm{c}$, can be defined so that modes with $\alpha < \alpha_\textrm{c}$ are unstable, while modes with $\alpha > \alpha_\textrm{c}$ are stable. The perturbative analysis is complemented by several numerical simulations to follow the time-dependent evolution of the system for different values of $\alpha$. Particular attention is given to the comparison between numerical and analytical results (during the early phases of the evolution) and to the role played by different boundary conditions. It is shown that an appropriate treatment of the lower boundary yields results that closely follow the predicted linear behavior. During the nonlinear regime, the shock oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle. The modes of oscillations during this phase do not necessarily coincide with those predicted by linear theory, but may be accounted for by mode-mode coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical Journa

Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane

At least 5 per cent of the massive stars are moving supersonically through the interstellar medium (ISM) and are expected to produce a stellar wind bow shock. We explore how the mass loss and space velocity of massive runaway stars affect the morphology of their bow shocks. We run two-dimensional axisymmetric hydrodynamical simulations following the evolution of the circumstellar medium of these stars in the Galactic plane from the main sequence to the red supergiant phase. We find that thermal conduction is an important process governing the shape, size and structure of the bow shocks around hot stars, and that they have an optical luminosity mainly produced by forbidden lines, e.g. [OIII]. The Ha emission of the bow shocks around hot stars originates from near their contact discontinuity. The H$\alpha$ emission of bow shocks around cool stars originates from their forward shock, and is too faint to be observed for the bow shocks that we simulate. The emission of optically-thin radiation mainly comes from the shocked ISM material. All bow shock models are brighter in the infrared, i.e. the infrared is the most appropriate waveband to search for bow shocks. Our study suggests that the infrared emission comes from near the contact discontinuity for bow shocks of hot stars and from the inner region of shocked wind for bow shocks around cool stars. We predict that, in the Galactic plane, the brightest, i.e. the most easily detectable bow shocks are produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure