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Articles you may be interested inSolvent effects on the UV-visible absorption spectrum of benzophenone in water: A combined Monte Carlo quantum mechanics study including solute polarization Mesoscale structure and fractal nature of 1-propanol aqueous solution: A reverse Monte Carlo analysis of small angle neutron scattering intensity High-pressure neutron diffraction on fluid carbon tetrafluoride and interpretation by reverse Monte Carlo simulations Neutron diffraction measurements from 2.4 mole% solutions of tetrapropylammonium bromide ͑TPrABr͒, ͑C 3 H 7 ͒ 4 NBr, in water, using hydrogen/deuterium isotope substitution on the alkyl groups of the cation, is used to investigate the distribution of cations in the solution. The intermolecular partial pair correlation function for the alkyl group hydrogens is obtained, which shows a broad peak with a maximum at about 11 Å, with structure in the 2.5-4 Å region, suggesting that some of the alkyl chains of neighboring ions may not be separated by water molecules. The three-dimensional structure is modeled using the reverse Monte Carlo method to fit the HH partial structure factor. The simulation results show that at least half of the cations are not separated by water from one other cation and that a few clusters of three or four cations also occur. The close contact found between the TPrA ϩ ions is discussed in relation to previous results on the structure of water in TPrABr solution at the same concentration.

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The epithelium of the nasal airways is recognized as being at risk of developing cancer from inhalation exposure to toxic aerosols. Because groups that are revising or developing respiratory tract dosimetry models have pointed out the lack of information on the deposition of aerosols in human nasal airways, particularly as a function of the size and shape of these airways, we utilized magnetic resonance imaging to obtain in vivo measurements of the nasal airways of a human subject. A series of 40 contiguous 3-mm-thick coronal sections were obtained using a 1.5 Tesla proton imaging unit. These images were then digitized and perimeters and cross-sectional areas of the left and right airways were obtained. The results indicated that there were marked differences in areas between the left and right sides of this subject at any given time, and that the size of the airways were significantly smaller than areas that have been reported previously based on measurements obtained from cadavers. A separate set of images were also obtained after the subject had been administered a nasal decongestant. These airways dimensions were quantitatively more similar to the cadaver data. These preliminary data indicate that nasal airway dimensions are likely to be smaller than those previously reported. These differences may play a role in determining the patterns of aerosol deposition to be expected in nasal airways.It has become increasingly evident that the epithelium of the nasal airways is at risk of developing cancer from inhalation exposure to a variety of environmentally derived gases and particles (Bross et al., 1978; Roush 1979;Gerhardsson et al., 1985; Brinton et al., 1985; Olson and Asnaes 1986;Benjamin et al., 1979;Boecker et al., 1986). To establish appropriate exposure-dose-response relationships, particularly among different species, it is important to understand the factors that affect the deposition, retention and translocation of inhaled toxicants in the nasal airways. Several deposition studies of different sized aerosols in the human nasal airways have been reported, both in terms of total airway deposition (Heyder and Rudolf 1960;Hounam et al., 1971) and also deposition in gross subdivisions of the nose (Fry and Black, 1973). Those data, although useful, do not provide the

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Computational fluid dynamics (CFD) is a rapidly emerging field in wastewater treatment (WWT), with application to almost all unit processes. This paper provides an overview of CFD applied to a wide range of unit processes in water and WWT from hydraulic elements like flow splitting to physical, chemical and biological processes like suspended growth nutrient removal and anaerobic digestion. The paper's focus is on articulating the state of practice and research and development needs. The level of CFD's capability varies between different process units, with a high frequency of application in the areas of final sedimentation, activated sludge basin modelling and disinfection, and greater needs in primary sedimentation and anaerobic digestion. While approaches are comprehensive, generally capable of incorporating non-Newtonian fluids, multiphase systems and biokinetics, they are not broad, and further work should be done to address the diversity of process designs. Many units have not been addressed to date. Further needs are identified throughout, but common requirements include improved particle aggregation and breakup (flocculation), and improved coupling of biology and hydraulics.