Early Stage Of Propagation Research Articles

数値モデルを用いたチベット高原付近の寒気の吹き出しの研究

Cold surges around the Tibetan Plateau in a numerical prediction model have been analysed.Cold surges trapped in the periphery of the mountain and in the lower troposphere are well simulated in the model. From the comparison of forecasts of models with different mountain heights, the enhancement of northerly wind along the eastern boundary of the mountain, which is similar in structure to that of coastal Kelvin waves, is demonstrated. The estimates of a width and a phase speed due to the linear theory are two or three times smaller than those due to the forecast results, which may indicate the limitation of the linear theory.To investigate the mechanism of cold surges, momentum budget analyses at the 850mb level have been performed on two cases showing enhancements of northerly winds. The first enhancement of northerly associated with the southward passage of a cold front is noticed, where the acceleration effect due to the ageostrophic component is dominent. However, this northerly has gradually weakened at the southeastern corner of the Tibetan Plateau. Main enhancement of the northerly occurs in the cold air behind the cold front and the northerly is further propagated southward. Although the effect due to the ageostrophic component plays a dominant role in the early stage of propagation, the effect due to the non-linear advection term becomes important in the late stage around 20°N. By this effect the northerly leaves the mountain and travels further southward. Although basic features are well simulated in the model, there remains some errors; One is the intrusion of the cold air into the Indo-China peninsula instead of the South China Sea. This error may be due to the insufficient treatment of the orography over the IndoChina peninsula. Slight improvement has been noticed in an experiment with the envelope mountain. Another error is the overprediction of cold surges around the mountain. This suggests the insufficient treatment of the effect of the mountain, probably the neglect of the effect due to the sub-grid scale undulations. Extra eddy fluxes due to mountain waves excited by the sub-grid scale undulations can be expected. In fact, an experiment including a parameterization of this effect showed a remarkable improvement of the forecast fields.

The effects of ion implantation on the initial stages of hot corrosion attack of cast Co22Cr11Al at 700 °C

The initiation and early propagation stages of low temperature hot corrosion (LTHC) attack of CoCrAlY-type overlay coatings is under study so that improved coatings can be developed through an understanding of the deterioration mechanisms. In an attempt to resolve the role played by the active element addition ( i.e. yttrium, hafnium etc.), the LTHC of both a cast Co22Cr11Al alloy and ion-implanted Co22Cr11Al alloys was studied. In this paper, results for yttrium-implanted and cobalt-implanted Co22Cr11Al are presented. Implanted and unimplanted alloy surfaces were pre-oxidized for 1 h at 700 °C before exposure to conditions that simulate the LTHC attack that is typically observed on hot-end components of gas turbines operated in a marine environment. Scanning electron microscopy and energy-dispersive X-ray analysis were used to characterize the oxide scales both prior to and after LTHC exposures. The LTHC attack was markedly more severe in the yttrium-implanted surfaces than in the unimplanted or the cobalt-implanted surfaces. Moreover, little difference was observed in the LTHC response between the cobalt-implanted and the unimplanted alloy. Both for yttrium-implanted and cobalt-implanted alloys and for the unimplanted alloy, the β-CoAl matrix phase was preferentially attacked whereas the α-Co solid solution phase showed little attack; the relative degree of LTHC attack on the alloy surfaces increased with exposure time. Mechanisms are proposed to explain the enhanced LTHC attack observed in the yttrium-implanted CoCrAl alloy.

Influence of thickness and temperature on stretched zone size inJ1Ctests

AbstractThe nature of the microscopic processes occurring at the crack tip during crack initiation and the early stages of propagation were investigated by detailed characterization of the stretched zone in J integral specimens of two thicknesses tested at two different temperatures. The J integral tests were conducted on HT–9, a martensitic stainless steel, using the single specimen compliance method. Tests were conducted on modified 0·5 in thick compact tension (CT) specimens at two thicknesses, 12·7 and 2·5 mm, fatigue precracked before the test and at temperatures of 25 and 200°C. The topography of the crack tip region was mapped by detailed stereographic measurements on scanning electron micrographs of the region containing the stretched zone. The measured J 1C values and the stretched zone dimensions for each test condition were then compared using the relationship of Knott: J 1C = mσfδc, where m is a constraint factor, σf is the flow stress, and δc is the crack opening displacement taken as twice t...

Determination of rate constants of initiation and propagation in the cationic polymerization of α‐methylstyrene with CF3SO3H and H2SO4

AbstractThe polymerization of α‐methylstyrene with CF3SO3H and H2SO4 as initiators was studied at 30°C in dichloroethane by the stopped‐flow/rapid‐scan spectroscopic technique. The propagating cation shows λmax at 336–340 nm. The initiation process and the early stage of propagation were analyzed kinetically on the basis of the cation formation and monomer consumption by taking into account the equilibrium monomer concentration. The rate of initiation with CF3SO3H was found to be proportional to the concentrations of CF3SO3H and monomer, but the initiation with H2SO4 is not proportional to the H2SO4 concentration. The rate constant of initiation was estimated to be (2 ± 1)·103 1.mol−1.s−1 with CF3SO3H, and similar values were found with H2SO4. The rate constant of propagation is 3.104 1.mol−1.s−1 with CF3SO3H as initiator and 106 − 107 1.mol−1.s−1 with H2SO4 as initiator. These kp values are close to that obtained previously in the radiation polymerization. Finally, λmax values of the propagating cation, obtained by the stopped‐flow technique, were collected.

A THERMAL SHOCK FATIGUE STUDY OF TYPE 304 AND 316 STAINLESS STEELS

Abstract—–Thermal fatigue crack initiation and propagation promoted by thermal quenches have been studied in AISI 316 and 304 stainless steels and correlated with isothermal strain cycling fatigue. Axially unconstrained specimens of rectangular section were held at bulk temperatures of 250°C to 500°C and symmetrically water‐quenched on the narrow faces to give equivalent surface strain ranges from 2.8 ×10−3 to 5.4 ×10−3. Crack initiation in smooth samples showed an apparent threshold at a surface strain range of 2.8×10−3 equivalent to a thermal amplitude of 150°C with no cracking being produced in 500,000 cycles. The crack growth in prenotched samples was evaluated by direct observation and by subsequent fractography and showed two modes of growth. The crack growth was strain controlled during the early stages of propagation where the crack tip was within the surface zone under conditions of fully plastic cyclic yield. At greater depths the propagation rates in the remaining elastically cycled material were found to correlate with calculated stress intensity values. In the chosen symmetrical quenched axially unconstrained configuration the crack growth rates decreased towards the centre of the specimen, indicating a crack arrest condition as expected from analysis. The results indicated a good correlation with the fracture behaviour observed from isothermal strain cycling fatigue behaviour in an air environment.

Field and glasshouse experiments on the control of potato mop‐top virus

SUMMARYField observations during 3 yr on a stock of potato cv. Red Craigs Royal partially infected with potato mop‐top virus (PMTV) confirmed that the virus was passed by an infected mother plant to only a proportion of its progeny tubers, and showed that in this cultivar symptomless plants gave rise only to symptomless progeny. The elimination of PMTV from stocks can therefore be greatly accelerated by removing symptom‐bearing plants. Infected potato tubers were not freed from PMTV by treating them at 37 °C for up to 8 wk. Treating ‘seed’ tubers bearing powdery scabs that contain PMTV‐carrying resting spores of Spongospora subterranea with formaldehyde or organo‐mercurial fungicide greatly decreased PMTV establishment when the tubers were planted in previously uninfective soil, but fumigation with 2‐aminobutane was ineffective.Decreasing the pH of infective soil to 5‐0 by applying sulphur greatly decreased the infection of potato cv. Arran Pilot with PMTV and S. subterranea in field experiments, but this treatment did not eliminate either; when the pH of treated soil was raised the transmission of PMTV resumed. Treating infective soil with a range of fungicides greatly decreased the infection of Nicotiana debneyi bait seedlings in glasshouse experiments but only calomel at 75 kg/ha controlled spread of PMTV and 5. subterranea to potato in field experiments. In other field experiments, applying zinc frit, zinc sulphate or zinc oxide to infective soil greatly decreased the spread of both to potato. The amount of zinc required increased with increase in clay content of the soil. However, treatment with zinc compounds did not eliminate PMTV‐carrying vectors from soil, and when treated soil was diluted with autoclaved soil many of the bait seedlings planted in the mixture became infected. The zinc frit was phytotoxic because of its boron content but zinc sulphate and zinc oxide caused little or no decrease in tuber yield. The zinc content of potato tubers was increased but not doubled in zinc‐treated plots, and during the first year after treatment the zinc content of topsoil decreased greatly. The zinc content of ryegrass grown after potatoes was greater than of potato tubers but did not reach a level considered dangerous to livestock. Treatment of soil with sulphur, zinc oxide or calomel may be useful for small plots used in the early stages of propagation of virus‐tested potato clones where there is risk of infection with PMTV.

Development of vibratory flame propagation in short closed tubes and vessels

The onset and the development of vibratory propagation of flames in propane-oxygennitrogen mixtures (initially at 0.2 to 12 atm and room temperature) contained in short closed vessels were studied. Observation of flame motion was made by means of high-speed schlieren cinematography, and simultaneous pressure measurements were obtained with a quartz piezoelectric transducer. It was found that the vibratory propagation triggered by the first contact of the flame with the side walls of the vessel is amplified by changes in flame structure (as in tubes open at the ignition, end) if the amplitude of the expansion waves, generated at the instant of contact, overtakes a threshold value (about 50 mbar for the mixtures experimented). Although this threshold is not reached in mixtures highly diluted by nitrogen, strong pressure oscillations have been found to appear, but only in the very last stage of the propagation and preferentially in mixtures of equivalence ratio near 1.3 (rich) at high initial pressure. The development of the vibratory phenomena was observed to be markedly influenced by the properties of the vessel walls (for instance, it is heavily damped by side walls coated with rough insulation, such as asbestos). This development is also counteracted in some mixtures, when secondary ignitions occur ahead of the main flame, along paths of heated solid particles pulled away from the wall in the early stage of propagation.

Combustion and Detonation in Gases

A photographic investigation of combustion in gases was undertaken to observe experimentally the mechanism of initiation and structure of detonation waves. Streak schlieren photographs of hydrogen-oxygen and carbon monoxide-oxygen explosions were made in a rectangular shock tube using precision optics and a rotating mirror camera. The photographs show that compression waves generated during the early stages of propagation of the explosion soon steepen into a shock wave which travels ahead of the combustion wave (zone of chemical reaction). This combination of shock and combustion wave is characteristic of a nonsteady detonation state in the moments before stable detonation is initiated. The distance between the two waves narrows as the explosion progresses down the tube due to the continued formation of new combustion centers ahead of the combustion wave by means of auto-ignitions. Eventually the combustion wave overtakes the shock wave and supplies it directly with energy from the chemical reaction; at this moment stable detonation is initiated. The internal structure of steady state detonations is found to be different from nonsteady state detonations; for example, whereas the distance between the shock and the zone of chemical reaction is of the order of several centimeters in the nonsteady state detonation, this distance in the steady state detonation is too small to be resolved by the optical system (i.e., less than 1 mm). Other structural features such as spinning detonation and a high-frequency oscillation of propagation velocity are also illustrated.