Re-initialization Process Research Articles

Behavior of detonation waves at low pressures

With respect to stability of gaseous detonations, unsteady behavior of galloping detonations and re-initiation process of hydrogen-oxygen mixtures are studied using a detonation tube of 14 m in length and 45 mm i.d. The arrival of the shock wave and the reaction front is detected individually by a double probe combining of a pressure and an ion probe. The experimental results show that there are two different types of the re-initiation mechanism. One is essentially the same as that of deflagration to detonation transition in the sense that a shock wave generated by flame acceleration causes a local explosion. From calculated values of ignition delay behind the shock wave decoupled from the reaction front, the other is found to be closely related with spontaneous ignition. In this case, the fundamental propagation mode shows a spinning detonation.

Detonation wave propagation through a single orifice plate in a circular tube

Detonation behavior associated with the propagation of a detonation wave through an orifice plate located within a circular tube is investigated. The tube and orifice diameter used in the study are 27.3 cm and 10 cm, respectively. The test gas used is hydrogen-air at 1 atmosphere and at various initial temperatures up to 650 K. Immediately after the orifice, the detonation wave decouples and either fails or reinitiates. The reinitiation process is characterized by either spontaneous initiation, initiation due to shock reflection, or deflagration-to-detonation transition (DDT). In the case of DDT, transition is preceded by the degeneration of the decoupled detonation wave to a velocity consistent with a CJ deflagration. Delineation between these various propagation regimes could not be correlated with the detonation cell size, λ, and orifice diameter, d . The data, although limited, demonstrate for the first time that the d c /λ =13 critical tube criterion obtained at room temperature may not apply at elevated temperature conditions. The evidence for this is data obtained at 500 K that shows no detonation transmission for 30% hydrogen in air that corresponds to d/λ =16.7. The tests also indicate that a simple d/λ correlation cannot be used to determine when reinitiation due to shock reflection is possible. For example, at 650 K detonation wave failure was observed for d/λ d/λ

Reinitiation of Protein Translocation across the Endoplasmic Reticulum Membrane for the Topogenesis of Multispanning Membrane Proteins

The reinitiation of the translocation of the growing nascent chain across the endoplasmic reticulum membrane is essential for the topogenesis of multispanning membrane proteins. We investigated the requirements for the reinitiation process using model proteins in which systematically designed sequences were inserted after two preceding topogenic sequences, namely the N-terminal signal sequence (S) and stop transfer sequence (St). The model proteins were translated in vitro in the presence of rough microsomes, and the final topology of the proteins in the microsomal membrane was examined by proteolytic digestion. The structural requirements for S and the reinitiation sequence (R) overlapped to some extent, but substantial differences were noticed. When St and R were separated by a short cytoplasmic segment (58 amino acids), the efficiency of the reinitiation was not affected by the concentration of the signal recognition particle (SRP) in the translation system, even though the sequence inserted as R was an SRP-dependent signal sequence. However, when the cytoplasmic segment was longer (100 amino acids), the reinitiation efficiency was reduced, and the SRP improved the overall efficiency as well as impaired the accessibility of the processing site after the R to the signal peptidase.

Facilitated Recycling Pathway for RNA Polymerase III

We show that the high in vitro transcription efficiency of yeast RNA pol III is mainly due to rapid recycling. Kinetic analysis shows that RNA polymerase recycling on preassembled tDNA•TFIIIC•TFIIIB complexes is much faster than the initial transcription cycle. High efficiency of RNA pol III recycling is favored at high UTP concentrations and requires termination at the natural termination signal. Runoff transcription does not allow efficient recycling. The reinitiation process shows increased resistance to heparin as compared with the primary initiation cycle, as if RNA polymerase was not released after termination. Indeed, template competition assays show that RNA pol III is committed to reinitiate on the same gene. A model is proposed where the polymerase molecule is directly transferred from the termination site to the promoter.

Fracture toughness of σ + x microstructures in the NbTiAl system

The NbTiAl system offers many intermetallics that can be potentially used for high temperature structural applications. The objective of this study has been to improve the room temperature toughness of the σ phase (Nb 2Al + Ti), which has a high melting point and good high temperature strength, by inclusion of semiductile phases. The incorporation of second-phase particles has been found to enhance the toughness of the σ phase considerably. Fractographic analysis has revealed that when the interface between the particles and the matrix is strong, the second-phase particles impede the crack front by trapping of the crack tip due to plasticity. The crack propagation continues by a reinitiation process ahead of the crack tip. Consequently, the crack is bridged by the second-phase particles. The cleavage of the second-phase particles results in the final separation of the crack faces. In the case of weakly bonded particles the level of internal stresses has been found to influence the crack path and therefore the toughness. In this paper the effects of the volume fraction, size and distribution of the particles as well as their nature on the fracture path and toughness are discussed.

The Initiation of Auxin Autonomy in Tissue from Tobacco Plants Carrying the Auxin Biosynthesizing Genes from the T-DNA of Agrobacterium tumefaciens

Tobacco (Nicotiana tabacum cv Havana 425) plants containing the indole-3-acetic acid biosynthesizing genes (1 and 2) from the T-DNA of Agrobacterium tumefaciens strain T37-ADH(2) (mutated at the cytokinin biosynthesis gene 4) were used to study the physiological basis of the suppression and reinitiation of the auxin autonomous phenotype. The plants, though normal in appearance and cross-fertile with nontransformed, wild type tobacco, are shown to contain multiple copies of genes 1 and 2. Plants carrying these genes respond to inoculation by Agrobacterium strains mutated at genes 1 and 2 in a virulent fashion. Despite the presence and potential in planta activity of these genes, pith explants from such plants require auxin or tryptophan for growth in vitro, as does wild type tobacco. In both cases the indole-3-acetic acid levels increase rapidly in pith explants cultured on tryptophan-containing medium. However, only the tissues containing genes 1 and 2 grow subsequently on auxin-free medium and accumulate indole-3-acetic acid to levels that support growth. The capacity of such tissues to utilize naphthalene acetamide as an auxin suggests that gene 2 is rapidly activated during the reinitiation process.

Chemical energy release in the cellular structure of gaseous detonation waves

The nonequilibrium Zel'dovich-von Neumann-Doring (ZND) model is applied to the three-dimensional cellular structure of ordinary and marginal gaseous detonation waves. Calculations of the nonequilibrium chemical energy release in the chain-propagating reactions of H 2 + Cl 2 at various shock temperatures show that relatively thick zones of highly vibrationally excited reaction products exist at varying distances behind all of the shock fronts in the cellular structure. Therefore, each of these shock fronts is supported to a different extent by the chemical reconstitution process. Calculations for ordinary detonations of H 2 + Cl 2 and 2H 2 + O 2 using a constant transverse wave strength and a simple rigid wall reflection for the transverse wave collision process yield good agreement with experimental cell parameters. The calculations indicate that approximately 1 4 of the energy required for self-sustaining propagation of the leading shock front is furnished by the transverse wave collision process. For a marginal detonation in 2H 2 + O 2 + 3Ar, calculations based on a decreasing transverse wave strength and the recent suggestion of Dormal et al. that the reinitiation process at the end of a cell includes a detonation in the precompressed explosive behind the incident wave agree with experimental measurements. This application of the nonequilibrium ZND model may explain the experimental observation of a cellular structure in an overdriven detonation whose heat of reaction is endothermic and suggest extensions of the Vasil'ev et al. model of a cell in a self-sustaining gaseous detonation wave.

Fatigue cracking characteristics of β-annealed large colony Ti-11 alloy

In order to better understand the large scatter in the fatigue results associated with β-annealed microstructures of α+ β titanium alloys, the fatigue crack initiation and propagation behavior of thin center notched Ti-11 specimens with a large colony α platelet microstructure was investigated. Colonies with a mean intercept diam of greater than 1 mm were grown in 2 mm thick specimens by means of a vacuum β-annealing process. This enabled crack path morphologies and crack propagation rates to be determined within single colonies by means of optical microscopy on the polished and etched surfaces. The results showed that the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload fracture. Intense shear bands were observed ahead of and in the same direction as the propagating cracks. The density of the shear bands increased with increasing stress intensity. Since the colonies are randomly oriented, the fatigue cracks propagated at various angles with respect to the tensile axis. The crack propagation rate across a single colony is no faster than the propagation rate in the equiaxed α+ β microstructure of the same material. However, cracks were halted at the colony boundaries and forced to reinitiate through a cycle consuming process into the next colony. It is mainly this reinitiation process and microstructurally dependent growth which are responsible for the slower crack growth rates and large scatter band obtained for the β-annealed microstructures when compared to the α+ β microstructures. It is suggested that by reducing the colony size the crack growth rates will be reduced and the fatigue scatter band will be narrowed.

Diminished Sensitivity of Re‐initiation of Translation to Inhibition by Cap Analogues in Reticulocyte Lysates

In a nuclease-treated reticuloyte lysate reinitiation of protein biosynthesis was less inhibited by 7-methylguanosine 5'-monophosphate (m7GMP), whereas primary-initiation could be totally blocked by cap analogues. We suggest that a cap binding factor binds to mRNA as a first step in initiation of protein synthesis and that this factor remains bound during the subsequent reinitiation cycles. Primarily because of the greater relative importance of the reinitiation process, globin synthesis in the untreated lysate was less inhibited by m7GMP.