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排序方式: 共有268条查询结果,搜索用时 15 毫秒
1.
2.
The interaction between multiple incompressible air jets has been studied numerically and experimentally. The numerical predictions have been first validated using experimental data for a single jet configuration. The spreading features of five unequal jets in the configuration of one larger central jet surrounded by four smaller equi‐distant jets, have been studied, for different lateral spacing ratios of 1.5, 2.0 and 2.5 and a central jet Reynolds number of 1.24×105 (corresponding to a Mach number of 0.16). Flow of five equal jets has also been simulated, for the sake of comparison. The jet interactions commence at an axial distance of about 3–4 diameters and complete by an axial distance of about 10 diameters for the lowest spacing ratio of 1.5. For larger spacing ratios, the length required for the start and completion of jet interaction increase. Peripheral jets bend more towards the central jet and merge at a smaller distance, when their sizes are smaller than that of the central jet. The entrainment ratio for multiple jets is higher than that for a single jet. Excellent agreement is observed between the experimental data and theoretical predictions for both mean flow field and turbulent quantities, at regions away from the jet inlet. The potential core length and initial jet development, however, are not predicted very accurately due to differences in the assumed and actual velocity profiles at the jet inlet. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
Govindarajan Sankar A. Sultan Nasar 《Journal of polymer science. Part A, Polymer chemistry》2007,45(8):1557-1570
A series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, 1H NMR, and 13C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature 1H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007 相似文献
4.
G. Date T. R. Govindarajan P. Sankaran R. Shankar 《Communications in Mathematical Physics》1990,132(2):293-313
We compute the homotopy groups
0 and
1 of the classical configuration space of anO(3) invariant field theory on
×, where
is a compact two dimensional manifold for arbitrary genusg and- denotes the time coordinate. We also present the finite dimensional, unitary, irreducible, inequivalent representations of the appropriate fundamental groups and comment on some of their implications. 相似文献
5.
Summary It has recently been reported (6, 7) that poly(methyl methacrylate) particles that are sterically stabilised by polydimethylsiloxane flocculate on cooling when dispersed in short chain n-alkanes. Since the LCFT was reported to be essentially independent of the nature of the dispersion medium, it was postulated (2) that incipient flocculation of these particles was caused by crystallisation of the polydimethysiloxane chains, which would have allowed the attractive van der Waals forces to become operative. This hypothesis has now been confirmed by low temperature X-ray studies.
With 1 figure 相似文献
Zusammenfassung Von (6, 7) wurde kürzlich mitgeteilt, daß Poly(methylmethacrylat)-Teilchen, die mit Polydimethylsiloxan sterisch stabilisiert sind, in kurzkettigenn-Alkanen als Dispersionsmittel ausflocken. Da die LCFT im wesentlichen unabhängig von der Natur des Dispersionsmediums ist, wurde von (2) angenommen, daß die beginnende Ausflockung dieser Teilchen auf eine Kristallisation der Polydimethylsiloxan-Ketten zurückzuführen ist, bedingt durch die wirkenden van der Waals-Anziehungskräfte. Diese Hypothese wurde nun durch Röntgenuntersuchungen bei tiefer Temperatur bestätigt.
With 1 figure 相似文献
6.
Kavitha V Sundararajan K Viswanathan KS 《The journal of physical chemistry. A》2005,109(41):9259-9264
Conformations of trimethoxymethylsilane were studied using matrix isolation infrared spectroscopy and ab initio computations. Trimethoxymethylsilane was trapped in both argon and nitrogen matrixes using heated nozzle effusive sources and a supersonic jet source, in an effort to alter the conformational population in the matrix. Ab initio calculations were carried out at the HF and B3LYP level using 6-31++G basis set to support our experimental observations. The frequencies computed at the B3LYP level was found to fit well with our experimental data. A conformer with a C1(g(+/-)g(+/-)t) structure was predicted by our computations to be the ground state conformer. 相似文献
7.
Low molecular weight organogels based on long-chain carbamates 总被引:2,自引:0,他引:2
Moniruzzaman M Sundararajan PR 《Langmuir : the ACS journal of surfaces and colloids》2005,21(9):3802-3807
Thermoreversible organogels were prepared from carbamates with alkyl side chains of different lengths. Gelation was possible only up to an alkyl side chain length of 12 carbons, beyond which crystallization occurs, due to the dominant van der Waals interaction between the alkyl chains. This is in contrast to other alkane-based organogels, in which gelating efficiency increased with the length of the alkane chain (see Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237). The critical concentration for gelation decreases drastically with an increase in the side chain length. Xerogels of these show birefringent fibers with uniform cross section and unlimited growth in one direction. The extent of this unlimited growth is affected by the length of the alkyl side chain in the carbamate, which finally ceases the gel formation ability of the carbamate. Cryogenic scanning electron microscopy images of the gels are similar to those of xerogels. From X-ray diffraction of the fibers, we propose that the growth direction is along the plane of hydrogen bonds between the carbamate molecules. The thickness of the fibers depends on the length of the alkyl side chain. Morphological differences are seen between gels prepared by slow cooling and quenching of the solution. Thus, the morphology of the fibrous xerogels of the carbamates can be tailored for specific applications, by the choice of the alkyl side chain length and the rate of cooling the solution. 相似文献
8.
9.
Vaduganathan Yasodha Subbaiah Govindarajan John N. Low Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(5):m207-m215
The structures of five metal complexes containing the 4‐oxo‐4H‐pyran‐2,6‐dicarboxylate dianion illustrate the remarkable coordinating versatility of this ligand and the great structural diversity of its complexes. In tetraaquaberyllium 4‐oxo‐4H‐pyran‐2,6‐dicarboxylate, [Be(H2O)4](C7H2O6), (I), the ions are linked by eight independent O—H...O hydrogen bonds to form a three‐dimensional hydrogen‐bonded framework structure. Each of the ions in hydrazinium(2+) diaqua(4‐oxo‐4H‐pyran‐2,6‐dicarboxylato)calcate, (N2H6)[Ca(C7H2O6)2(H2O)2], (II), lies on a twofold rotation axis in the space group P2/c; the anions form hydrogen‐bonded sheets which are linked into a three‐dimensional framework by the cations. In bis(μ‐4‐oxo‐4H‐pyran‐2,6‐dicarboxylato)bis[tetraaquamanganese(II)] tetrahydrate, [Mn2(C7H2O6)2(H2O)8]·4H2O, (III), the metal ions and the organic ligands form a cyclic centrosymmetric Mn2(C7H2O6)2 unit, and these units are linked into a complex three‐dimensional framework structure containing 12 independent O—H...O hydrogen bonds. There are two independent CuII ions in tetraaqua(4‐oxo‐4H‐pyran‐2,6‐dicarboxylato)copper(II), [Cu(C7H2O6)(H2O)4], (IV), and both lie on centres of inversion in the space group P; the metal ions and the organic ligands form a one‐dimensional coordination polymer, and the polymer chains are linked into a three‐dimensional framework containing eight independent O—H...O hydrogen bonds. Diaqua(4‐oxo‐4H‐pyran‐2,6‐dicarboxylato)cadmium monohydrate, [Cd(C7H2O6)(H2O)2]·H2O, (V), forms a three‐dimensional coordination polymer in which the organic ligand is coordinated to four different Cd sites, and this polymer is interwoven with a complex three‐dimensional framework built from O—H...O hydrogen bonds. 相似文献
10.
New divalent transition metal 3,5-pyrazoledicarboxylate
hydrates of empirical formula Mpz(COO)2(H2O)2,
where M=Mn, Co, Ni, Cu, Zn and Cd (pz(COO)2=3,5-pyrazoledicarboxylate),
metal hydrazine complexes of the type Mpz(COO)2N2H4
where M=Co, Zn or Cd and Mpz(COO)2nN2H4·H2O,
where n=1 for M=Ni
and n=0.5 for M=Cu
have been prepared and characterized by physico-chemical methods. Electronic
spectroscopic data suggest that Co and Ni complexes adopt an octahedral geometry.
The IR spectra confirm the presence of unidentate carboxylate anion (Δν=νasy(COO–)–νsym(COO–)>215
cm–1) in all the complexes and bidentate
bridging hydrazine (νN–N=985–950 cm–1)
in the metal hydrazine complexes. Both metal carboxylate and metal hydrazine
carboxylate complexes undergo endothermic dehydration and/or dehydrazination
followed by exothermic decomposition of organic moiety to give the respective
metal oxides as the end products except manganese pyrazoledicarboxylate hydrate,
which leaves manganese carbonate. X-ray powder diffraction patterns reveal
that the metal carboxylate hydrates are isomorphous as are those of metal
hydrazine complexes of cobalt, zinc and cadmium. 相似文献