Molecular clouds turbulence

Two mechanisms have been considered as candidates for the additional structural support for molecular clouds turbulence and magnetic fields. The theoretical and observational investigations into the importance of each of... 

A deflagration can best be described as a combustion mode in which the propagation rate is dominated by both molecular and turbulent transport processes. In the absence of turbulence (i.e., under laminar or near-laminar conditions), flame speeds for normal hydrocarbons are in the order of 5 to 30 meters per second. Such speeds are too low to produce any significant blast overpressure. Thus, under near-laminar-flow conditions, the vapor cloud will merely bum, and the event would simply be described as a large fiash fire. Therefore, turbulence is always present in vapor cloud explosions. Research tests have shown that turbulence will significantly enhance the combustion rate in defiagrations. 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

However, as a result of the relatively large beamwidth of radiotelescopes the apparent turbulent velocities of molecular clouds are partly the result of large-scale systematic motions, such as rotation. 

The location of an observed molecular radio transition in its energy level scheme and its measured interstellar intensity contain important information concerning the physical state of the molecular cloud in which the transition is observed. It will therefore be an important task for future interstellar molecular research to observe and measure as many transitions of any one molecule in any one particular cloud. Doppler shifts, i.e. the difference in frequency between the rest frequency (known from laboratory measurements) and the observed interstellar line frequencies, provide information on the large scale motion of the molecular clouds while the linewidths indicate the turbulence within the clouds. 

The study of the chemical and physical processes occurring in T Tauri outflows is of great importance as these stars have been proposed as the energy source of Herbig-Haro objects (Schwartz 1978 Canto 1978) and turbulence in molecular clouds (Norman and Silk 1980, Franco 1983). In this work, the physical model of the outflow is taken from Hartmann, Edwards and Avrett (1982) in which the primary stellar wind (i.e. wind that has not interacted with its environment) is ionized and heated by Alfven waves in the star s convection zone to reach a terminal velocity (of about 230 kms ) and a maximum temperature (of 20,000 K, cf. Hartmann et al. model no. 2) at z = 3 to 5, where z is the radial ordinate in units of stellar radii (r t 2 x 10 cm). Thereafter the wind expands and cools radlatively and adiabatically. Other parameters for the model are the initial wind density at z = 1 (oq lO - cm ), the density at z = 5 (n < 5 X 10 to 10 cm ) and the stellar photospheric temperature 4000 E). The cooling rate of the wind is obviously dependent on the physical conditions within the ejecta and in any case is by no means certain. Hartmann et al. suggest a... 

Molecular observations are almost always concerned with specific discrete transitions. These are generally observed at millimeter or centimeter wavelengths. The intensity of a source is determined by the rate of collisional versus radiative transitions between levels. Because of the extremely low densities usually associated with molecular environments, whether in a circumstellar envelope or a molecular cloud, pressure broadening is unimportant. Instead, the molecule radiates at its local velocity into the line of sight. This dispersion of velocity may be due strictly to the thermal motions of the particles, or it may be due to the presence of turbulence or large-scale chaotic motions within the medium. Either way, the local profile, < (v) is a Gaussian with a finite width in frequency. 

The study of molecular cloud stability and evolution leads naturally to studies of the physical and chemical evolution of the star formation process. Fundamental to this study of the star formation process is the characterization of the physical conditions in the gas and dust comprising these regions. For the gas, volume density n (cm ), kinetic temperature Tk (K), chemical composition X, turbulent motion Ai (km sec ), and magnetic field strength B (Gauss) are fundamental physical quantities. For the dust, the dust temperature (K), dust volume density... 

The physical nature of the turbulent component of a spectral line in a molecular cloud is currently a source of considerable debate. Physical processes that have been suggested as sources of the turbulence in molecular clouds are expanding HII regions, supernova remnants, cloud-cloud collisions, galactic differential rotation, and stellar winds. Unfortunately, for all of these processes there are theoretical problems with couphng the energy produced into turbulence. 

The distribution of tracer molecule residence times in the reactor is the result of molecular diffusion and turbulent mixing if tlie Reynolds number exceeds a critical value. Additionally, a non-uniform velocity profile causes different portions of the tracer to move at different rates, and this results in a spreading of the measured response at the reactor outlet. The dispersion coefficient D (m /sec) represents this result in the tracer cloud. Therefore, a large D indicates a rapid spreading of the tracer curve, a small D indicates slow spreading, and D = 0 means no spreading (hence, plug flow). 

Turbulent eddies larger than the cloud size, as such, tend to move the cloud as a whole and do not influence the internal concentration distribution. The mean concentration distribution is largely determined by turbulent motion of a scale comparable to the cloud size. These eddies tend to break up the cloud into smaller and smaller parts, so as to render turbulent motion on smaller and smaller scales effective in generating fluctuations of ever smaller scales, and so on. On the small-scale side of the spectrum, concentration fluctuations are homogenized by molecular diffusion. 

In a deflagration the flammable mixture burns relatively slowly. Flame propagation is mainly determined by molecular diffusion and turbulent transport processes. Mixtures of hydrocarbons and air burn in the absence of turbulence, i.e. under laminar or almost laminar conditions, with flame speeds of the order of 5-30 m/s. If there is no confinement this is too slow to produce tangible overpressures and only a flash fire is produced. That is why there is always turbulence involved in a vapour cloud explosion (turbulent flame speeds 100-300 m/s), which increases the rate of combustion and hence the overpressure [12]. 

It requires a finite time for the effects of k to be appreciable so that the moment Eqs. (25.18) to (25.22) provide a small-time approximation, in the case of a cloud (or near-source in the case of a steady release), to the true k 0 result. The time period required for molecular diffusive effects to reduce concentration is generally large with respect to the time periods over which large-scale turbulent convective motions provide significant displacements of contaminant fluid. Thus, many qualitative features of the moment Eqs. (25.18) to (25.22) are observed in experimental flows. 

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