Sam A. E. G. Falle

Hubble Space Telescope Observations of Jets from Young Stars

We report on Hubble Space Telescope (HST) emission line ([S II]λλ6716,6731, Hα, and [O I]λ6300) and nearby continuum imaging of the HL Tau and HH 30 jets from our own HST program as well as a study of HST Archive [S II]λλ6716,6731 images of the HH 1 and HH 34 jets. It is found in all cases that these jets are well resolved in the lateral direction (with FWHM diameters 02) as far as we can follow them to their source which, in the case of HH 30, is as close as 025 (35 AU). Assuming the jet has essentially zero angular width at its origin, one can deduce a lower bound for the initial opening angle, and the values obtained are very large indeed (e.g., 60° for the HH 30 jet and counterjet). Our data are shown to support models in which the jet is initially poorly focused before being asymptotically collimated into a column with a diameter of order a few tens of AU. As regards the origin of the knots seen in these jets, it is found that many of the knots in the HH 1 and HH 34 jets resemble internal bow shocks, at least far away from their driving source (5 and 10 in the case of HH 1 and HH 34 jets, respectively). This is consistent with models in which the knots are attributed to internal working surfaces caused by temporal variations in the outflow from the source. It is found in the case of the HH 30 jet, however, that its knots, at least close to the source, might have another origin.

One-dimensional nonlinear stability of pathological detonations

�������������������������������� ���������������������������������������������� ����������� �������������������������������������������� ��������������������������������� � �������� !�����∀�#�∃�����%����∀�!�&�∋�#��(�)))∗����+�,��������� �����������−����.��� ����������������� ��������������������� ������������������������������ In this paper we perform high-resolution one-dimensional time-dependent numerical simulations of detonations for which the underlying steady planar waves are of the pathological type. Pathological detonations are possible when there are endothermic or dissipative effects in the system. We consider a system with two consecutive irreversible reactions A→B→C, with an Arrhenius form of the reaction rates and the second reaction endothermic. The self-sustaining steady planar detonation then travels at the minimum speed, which is faster than the Chapman–Jouguet speed, and has an internal frozen sonic point at which the thermicity vanishes. The flow downstream of this sonic point is supersonic if the detonation is unsupported or subsonic if the detonation is supported, the two cases having very different detonation wave structures. We compare and contrast the long-time nonlinear behaviour of the unsupported and supported pathological detonations. We show that the stability of the supported and unsupported steady waves can be quite different, even near the stability boundary. Indeed, the unsupported detonation can easily fail while the supported wave propagates as a pulsating detonation. We also consider overdriven detonations for the system. We show that, in agreement with a linear stability analysis, the stability of the steady wave is very sensitive to the degree of overdrive near the pathological detonation speed, and that increasing the overdrive can destabilize the wave, in contrast to systems where the self-sustaining wave is the Chapman–Jouguet detonation.

A numerical study of the vorticity field generated by the baroclinic effect due to the propagation of a planar pressure wave through a cylindrical premixed laminar flame

The importance of vorticity production in combustion systems has been highlighted previously by several authors (Markstein 1964; Picone et al. 1984). The consequent distortion and enlargement of flame surfaces can lead to substantial enhancement of the burning rate which may be beneficial or disastrous depending on the physical context. We describe the results of numerical simulations of an experimental configuration similar to that described by Scarinci & Thomas (1992), who examined the effect of initially planar pressure signals on two-dimensional flame balls. The flame ball is here set-up from ignition using a code, based on the second-order Godunov scheme described by Falle (1991). A simple Arrhenius reaction scheme is adopted in modelling a unimolecular decomposition. As in previous papers (Batley et al. 1993 a, b ) the thermal conductivity is assumed to vary linearly with temperature, and the Lewis and Prandtl numbers are taken as unity. A short time after ignition, when the flame ball has reached a radius of approximately 2 cm, a very short-lengthscale pressure step disturbance is introduced, propagating towards the combustion region. As the signal crosses the flame, the interaction of the sharp, misaligned pressure and density gradients, creates a strong vorticity field. The resulting roll-up of the flame eventually divides it into two smaller rotating reacting regions. In order to gauge the effect of the chemical reaction and in particular the viscous diffusion on the evolution of the vorticity field, the results are compared with analogous solutions of the Euler equations.

The Fanno model for turbulent compressible flow

The paper considers the derivation and properties of the Fanno model for nearly unidirectional turbulent flow of gas in a tube. The model is relevant to many industrial processes. Approximate solutions are derived and numerically validated for evolving flows of initially small amplitude, and these solutions reveal the prevalence of localized large-time behaviour, which is in contrast to inviscid acoustic theory. The properties of large-amplitude travelling waves are summarized, which are also surprising when compared to those of inviscid theory.

The generation of optical emission-line filaments in galaxy clusters

Recent data support the idea that the filaments observed in Ha emission near the centres of some galaxy clusters were shaped by bulk flows within their intracluster medium (ICM). We present numerical simulations of evaporated clump material interacting with impinging winds to investigate this possibility. In each simulation, a clump falls due to gravity while the drag of a wind retards the fall of evaporated material leading to elongation of the tail. However, we find that long filaments can only form if the outflowing wind velocity is sufficiently large, ∼10 8 cm s -1 . Otherwise, the tail material sinks almost as quickly as the cloud. For reasonable values of parameters, the morphological structure of a tail is qualitatively similar to those observed in clusters. Under certain conditions, the kinematics of the tail resemble those reported in Hatch et al. A comparison of the observations with the numerical results indicates that the filaments are likely to be a few tens of Myr old. We also present arguments which suggest that the momentum transfer, from an outflowing wind, in the formation of these filaments is probably significant. As a result, tail formation could play a role in dissipating some of the energy injected by a central active galactic nuclei (AGN) close to the cluster centre where it is needed most. The trapping of energy by the cold gas may provide an additional feedback mechanism that helps to regulate the heating of the central regions of galaxy clusters and couple the AGN to the ICM.

STEADY NON‐IDEAL DETONATION

Highly non‐ideal explosives, such as commercial ammonium nitrate based explosives used in mining and blasting, have critical charge diameters of several centimetres and relatively low detonation speeds. Shock polar match analyses between these explosives and confining inert materials give two main types of interactions. For the first type (denoted here by case I), the detonation drives an oblique shock into the confiner. For the second type (case II), a wave propagates in the confiner ahead of the detonation wave in the explosive. In case I, numerical simulations show that for a given explosive model there is a unique relationship (valid for all charge diameters and confinements) between the velocity of detonation (VoD) and the curvature of the detonation shock at the charge axis. This relationship is shown to be well predicted by a quasi‐one‐dimensional analysis. A simple detonation shock dynamics method which uses this relationship predicts the VoD provided the explosive is sufficiently confined (usuall...

MAGNETIC FIELDS AND GALACTIC STAR FORMATION RATES

The regulation of galactic-scale star formation rates (SFRs) is a basic problem for theories of galaxy formation and evolution: which processes are responsible for making observed star formation rates so inefficient compared to maximal rates of gas content divided by dynamical timescale? Here we study the effect of magnetic fields of different strengths on the evolution of giant molecular clouds (GMCs) within a kiloparsec patch of a disk galaxy and resolving scales down to

Instability in shocked granular gases

simeq0.5:{rm{pc}}

MAXIMUM ENTROPY THEORY OF NON‐IDEAL DETONATION

. Including an empirically motivated prescription for star formation from dense gas (

Influence of turbulent fluctuations on detonation propagation

n_{rm{H}}>10^5:{rm{cm}^{-3}}