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The release of buoyant harmful gases within enclosed spaces, such as tunnels and corridors, may engender specific health, industrial and transportation risks. For safety, a simple ventilation strategy for these spaces is to impose a flow along the tunnel, whose velocity is defined as ‘critical’, that confines the front of harmful buoyant gases immediately downstream of the source of emission. Determining the critical velocity as a function of the geometrical and dynamical conditions at the source is a fundamental fluid mechanics problem which has yet to be elucidated; this problem concerns the dynamics of non-Boussinesq releases relating to large differences between the densities of the buoyant and the ambient fluids. We have investigated this problem theoretically, by means of a simplified model of a top-hat plume in a cross-flow, and in complementary experiments by means of tests in a reduced-scale ventilated tunnel, examining releases from circular sources. Experimental results reveal: (i) the existence of two flow regimes depending on the plume Richardson number at the source
, one for momentum-dominated releases,
, and a second for buoyancy-dominated releases,
, with a smooth transition between the two; and (ii) the presence of relevant non-Boussinesq effects only for momentum-dominated releases. All these features can be conveniently predicted by the plume-based model, whose validity is, strictly speaking, limited to releases issuing from ‘small’ sources in ‘weak’ ventilation flows. Analytical solutions of the model are generally in good agreement with the experimental data, even for values of the governing parameters that are beyond the range of validity for the model. The solutions aid to clarify the effect of the source radius, and reveal interesting behaviours in the limits
. These findings support the adoption of simplified models to simulate light gas releases in confined ventilated spaces.
The scaling of turbulent motions is investigated by considering the flow in the eigenframe of the local strain-rate tensor. The flow patterns in this frame of reference are evaluated using existing direct numerical simulations of homogeneous isotropic turbulence over a Reynolds number range from
up to 1131, and also with reference to data for inhomogeneous, anisotropic wall turbulence. The average flow in the eigenframe reveals a shear layer structure containing tube-like vortices and a dissipation sheet, whose dimensions scale with the Kolmogorov length scale,
. The vorticity stretching motions scale with the Taylor length scale,
, while the flow outside the shear layer scales with the integral length scale,
. Furthermore, the spatial organization of the vortices and the dissipation sheet defines a characteristic small-scale structure. The overall size of this characteristic small-scale structure is
in all directions based on the coherence length of the vorticity. This is considerably larger than the typical size of individual vortices, and reflects the importance of spatial organization at the small scales. Comparing the overall size of the characteristic small-scale structure with the largest flow scales and the vorticity stretching motions on the scale of
shows that transitions in flow structure occur where
and 250. Below these respective transitional Reynolds numbers, the small-scale motions and the vorticity stretching motions are progressively less well developed. Scale interactions are examined by decomposing the average shear layer into a local flow, which is induced by the shear layer vorticity, and a non-local flow, which represents the environment of the characteristic small-scale structure. The non-local strain is
in width and height, which is consistent with observations in high Reynolds number flow of a
wide instantaneous shear layer with many
-scale vortical structures inside (Ishihara et al., Flow Turbul. Combust., vol. 91, 2013, pp. 895–929). In the average shear layer, vorticity aligns with the intermediate principal strain at small scales, while it aligns with the most stretching principal strain at larger scales, consistent with instantaneous turbulence. The length scale at which the alignment changes depends on the Reynolds number. When conditioning the flow in the eigenframe on extreme dissipation, the velocity is strongly affected over large distances. Moreover, the associated peak velocity remains Reynolds number dependent when normalized by the Kolmogorov velocity scale. It signifies that extreme dissipation is not simply a small-scale property, but is associated with large scales at the same time.
During past three years from 1982 to 1984 we saw the further progress in the planets and satellites research by the space and ground-based technique, in the analysis and interpretation of the observational data. Inspite of some decrease of the activity in the planetary spacecrafts launches during this period (except of two Soviet missions to Venus) many important scientific results were obtained from the continued reduction and analysis of the measurements which were performed by Mariner 10 (Mercury), Pioneer Venus, Venera 13 and 14, Viking (Mars), Pioneer 10 and 11, and Voyager 1 and 2.
The Australia Telescope Compact Array has been used for observations, with arcsecond resolution, of the HII region N159 in the Large Magellanic Cloud. Images at 5 GHz reveal a second compact continuum component which has no obvious counterpart at optical and infrared wavelengths. Observations of HI 10α recombination-line emission, and HI and H2CO absorption, suggest that the object is a compact HII region embedded in a dense obscuring cloud on the edge of N159.
A program to observe millimetre-wave molecular transitions in a number of southern-sky molecular clouds is under way. Molecular clouds in both the Galaxy and the Magellanic Clouds are included in the sample. The aim of the program is to build a body of observational data which can be used to derive molecular abundances in southern-sky molecular clouds.
During 1990 we surveyed the southern sky using a multi-beam receiver at frequencies of 4850 and 843 MHz. The half-power beamwidths were 4 and 25 arcmin respectively. The finished surveys cover the declination range between +10 and −90 degrees declination, essentially complete in right ascension, an area of 7.30 steradians. Preliminary analysis of the 4850 MHz data indicates that we will achieve a five sigma flux density limit of about 30 mJy. We estimate that we will find between 80 000 and 90 000 new sources above this limit. This is a revised version of the paper presented at the Regional Meeting by the first four authors; the surveys now have been completed.
NGC 4945 is with D ~3-4 Mpc one of the nearest starburst galaxies known and a goldmine for molecular cloud research. A multi-line mm-wave study has been carried out towards its nuclear region with the Swedish-ESO Sub-millimetre Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules, including rare isotope-bearing species. Applying a Large Velocity Gradient (LVG) code to the data, H2 densities and column densities of 22 molecular species are calculated. Many of these species indicate the presence of a prominent high density interstellar gas component characterized by nH2 ~105cm-3. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an ‘overabundance’ of HNC and CN in the nuclear environment of NGC 4945. NGC 4945 is the second known starburst galaxy with an HNC/HCN abundance ratio ≥1. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. The data indicate that high 18O/17O, low 16O/18O and 14N/15N and perhaps also low 32S/34S ratios (6.4±0.3, 195±45, 105±25 and 13.5±2.5, respectively) are characteristic properties of a starburst environment in an advanced evolutionary stage.
Gamma-ray burst host galaxies are deficient in molecular gas, and show anomalous metal-poor regions close to GRB positions. Using recent Australia Telescope Compact Array (ATCA) Hi observations we show that they have substantial atomic gas reservoirs. This suggests that star formation in these galaxies may be fuelled by recent inflow of metal-poor atomic gas. While this process is debated, it can happen in low-metallicity gas near the onset of star formation because gas cooling (necessary for star formation) is faster than the Hi-to-H2 conversion.
This study documents the sediments, pollen, seeds, molluscs, vertebrates and mineral magnetism of deposits found in an abandoned water supply system at gasr Mm 10 in the Wadi Mimoun in the pre-desert of Tripolitania. These new data suggest (1) at or close to an episode of probable abandonment of that gasr, the growth of cereals (and perhaps general agricultural activity) still occurred on the wadi catchment upstream of the gasr, and (2) the vegetation of the adjacent wadi floor was richer in species (which included olive) than occurs today.
This article describes the interdisciplinary methods developed by the UNESCO Libyan Valleys Survey in connection with the study of ancient agriculture. These methods combine the techniques of settlement archaeology with those of the earth sciences and palaeoeconomics. The interactive nature of the enquiry does not resolve all the questions, but it can help to re-shape those questions and suggest new lines of enquiry. The example of the Wadi Mansur is used here to demonstrate the application of both conventional and new techniques and the contribution this can make to our appreciation of ancient land use in an arid zone. This provisional statement of results describes the geomorphology of the wadi, the settlement archaeology, ceramic dating evidence, palaeoeconomic data (including palynological evidence), wadi wall technology and typology.
In this chapter, we describe an effective procedure for working with MATLAB, and for preparing and presenting the results of a MATLAB session. In particular, we discuss some features of the MATLAB interface and the use of M-files. We describe how to write a simple loop as a precursor to a more robust discussion of MATLAB programming (in Chapter 6) that will enable you to make the most efficient use of M-files. We introduce the MATLAB publishing feature, which produces formatted output. It is the most effective method for presenting MATLAB results, although we also describe several alternate methods. Finally, we give some simple hints for debugging your M-files.
The MATLAB Interface
MATLAB's basic interactive interface, which launches when you start the program, is called the MATLAB Desktop. Embedded inside it is the Command Window that we described in Chapter 2.
By default, the MATLAB Desktop (Figure 1.1 in Chapter 1) contains four windows inside it, the Command Window in the center, the Current Folder Browser on the left, the Workspace Browser in the upper right, and the Command History Window in the lower right. Notice that if you minimize any of the latter three windows, then tabs appear that allow you to alternate between them.