Andrea Malagoli

Turbulent compressible convection

Numerical simulations with high spatial resolution (up to 96-cubed gridpoints) are used to study three-dimensional, compressible convection. A sequence of four models with decreasing viscous dissipation is considered in studying the changes in the flow structure and transport properties as the convection becomes turbulent. 39 refs.

On the nonlinear evolution of magnetohydrodynamic Kelvin-Helmholtz instabilities

We investigate the physical behavior in the nonlinear regime of Kelvin-Helmholtz (KH) instabilities in a simple conducting shear flow in the presence of magnetic fields, based upon the use of numerical simulations of the ideal magnetofluid equations of motion in two dimensions. The flow is characterized by three principal control parameters: the Mach number {ital M} of the shear flow, the ratio {alpha} of the Alfv{acute e}n speed to the sound speed, and the effective diffusivity; we investigate how these parameters affect the evolution and saturation of the instability. The key result of our study is that even relatively small magnetic fields (i.e., small compared to the equipartition intensity) affect the way the KH instability saturates with respect to the purely hydrodynamic case. If the magnetic field intensity is not sufficiently strong to suppress the KH instability entirely, then the field itself can still mediate the turbulent decay and diffusion of energy and mass across the layer. We present a detailed study of the various phases of this process for our simple shear layer configuration. {copyright} {ital 1996 The American Astronomical Society.}

On the thermal instability of galactic and cluster halos

The paper presents a detailed study of thermal instabilities in cooling flows associated with galaxies and clusters of galaxies. In the case of purely radiation-driven accretion onto a central object such as the cD galaxy M87, it is found that the gas is largely subject to overstability, rather than to monotonic instability. If thermal conductivity is taken into account, the flow is stabilized on scales of several kiloparsecs, even if the conductivity is appreciably reduced (e.g., about 1 percent) with respect to the Spitzer value. In no case are the globular perturbations (i.e., perturbations with comparable radial and azimuthal dimensions) found to be monotonically unstable. The paper presents numerical solutions of the local dispersion relation for the cooling flow in M87 and discusses the possible consequences of the results for a correct understanding of cooling flows. 24 references.

On the generation of sound by turbulent convection. I - A numerical experiment

Motivated by the problem of the origin of the solar p-modes, we study the generation of acoustic waves by turbulent convection. Our approach uses the results of high-resolution 3D simulations as the experimental basis for our investigation. The numerical experiment describes the evolution of a horizontally periodic layer of vigorously convecting fluid. The sound is measured by a procedure, based on a suitable linearization of the equations of compressible convection that allows the amplitude of the acoustic field to be determined. Through this procedure we identify unambiguously some 400 acoustic modes. The total energy of the acoustic field is found to be a fraction of a percent of the kinetic energy of the convection. The amplitudes of the observed modes depend weakly on (horizontal) wavenumber but strongly on frequency. The line widths of the observed modes typically exceed the natural linewidths of the modes as inferred from linear theory. This broadening appears to be related to the (stochastic) interaction between the modes and the underlying turbulence which causes abrupt, episodic events during which the phase coherence of the modes is lost.

Two-dimensional model of phase segregation in liquid binary mixtures with an initial concentration gradient

We simulate the phase segregation of a deeply quenched binary mixture with an initial concentration gradient. Our theoretical model follows the standard model H, where convection and di!usion are coupled via a body force, expressing the tendency of the demixing system to minimize its free energy. This driving force induces a material #ux much larger than that due to pure molecular di!usion, as in a typical case the Peclet number a, expressing here the ratio of thermal to viscous forces, is of the order of 105. Integrating the equations of motion in 2D, we show that the behavior of the system depends on the values of the Peclet number a and the non-dimensional initial concentration gradient c. In particular, the morphology of the system during the separation process re#ects the competition between the capillarity-induced drop migration along the concentration gradient and the random#uctuations generated by the interactions of the drops with the local environment. For large a, the nucleating drops grow with time, until they reach a maximum size, whose value decreases as the Peclet number and the initial concentration gradient increase. This behavior is due to the fact that the nucleating drops do not have the chance to grow further, as they tend to move towards the homogeneous regions where they are assimilated. ( 2000 Published by Elsevier Science Ltd. All rights reserved.

Turbulent supersonic convection in three dimensions

Previous numerical calculations of two-dimensional, compressible convection are extended to three dimensions, using a higher order Godunov scheme. The results show that the flow readily becomes supersonic in the upper boundary layer, where shock structures form intermittently in the vicinity of the strong downflow lanes. The convection as a whole is strongly time-dependent and evolves on a time scale comparable to the sound crossing time. The motions in the upper layers are characterized by the rapid expansion of the upward-moving fluid elements. In the interior, most of the heat is carried by a small fraction of the fluid residing in strong, highly coherent downflows. The remaining fluid is dominated by small-scale, disorganized turbulent motions.

3-D Simulations of Kelvin-Helmholtz Instabilities in Supersonic Jets

One of the key processes governing the structure and evolution of astrophysical jets is their interaction with the surrounding medium. A jet can deposit momentum and energy in the ambient medium, and entrain external material. The main physical process responsible for mixing between a jet flow and the ambient medium is the Kelvin-Helmholtz (KH) instability. We have previously analysed the 2D evolution of the axisymmetric modes of a cylindrical jet (Bodo et al 1994) and of the antisymmetric modes of a planar slab jet (Bodo et al 1995). These last are thought to give indications of the 3D evolution of the helical modes of a cylinder, since the linear behavior is very similar. In this contribution we present some preliminary results of fully 3D simulations comparing them with the mentioned 2D results.

The organization of turbulent convection

Highly resolved numerical simulations are used to study three-dimensional, compressible convection. The viscous dissipation is sufficiently low that the flow divides itself in depth into two distinct regions: (i) an upper thermal boundary layer containing a smooth flow with a granular appearance, and (ii) a turbulent interior pierced by the strongest downflows from the surface layer. Such downflows span the whole depth of the unstable layer, are temporally coherent, and are thermodynamically well correlated. A remarkable property of such convection, once it becomes turbulent, is that the enthalpy and kinetic fluxes carried by the strong downflows nearly cancel, for they are of opposite sense and nearly equal in amplitude. Thus, although the downflows serve to organize the convection and are the striking feature that emerges from effects of compressibility, it is the small-scale, disorganized turbulent motions (between the coherent downflow structures that serve as the principal carriers of net convected flux.