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The pressure rise caused by end-gas auto-ignition in spark-ignited engines is discussed using numerical simulation and theoretical approaches. The main objective of this study is to explain the mechanism by which the end-gas expansion during auto-ignition suppresses the pressure rise in spark-ignited engines, and theoretically to demonstrate using asymptotic analysis that the pressure rise depends on the Damköhler number. A one-dimensional direct numerical simulation (DNS) of end-gas auto-ignition is performed, and the modelling assumptions for it are discussed based on the DNS results. The Damköhler number, defined as the acoustic time scale and the characteristic time scale of the chemical reaction, is introduced in the modelling. The end-gas auto-ignition model is solved numerically, and it is shown that the pressure rise increases with the Damköhler number. Additionally, it is shown that the tendency of the pressure rise is due to the balance between the propagation rate of the expansion wave generated in the end gas and the reaction rate at auto-ignition, which varies with the Damköhler number. To derive the analytical solution of the relationship between the pressure rise and Damköhler number, the end-gas auto-ignition model is simplified based on the numerical results. The simplified model for end-gas auto-ignition is then solved using Newton's method, and the analytical solution of the pressure rise is derived. [ABSTRACT FROM AUTHOR]

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Two-dimensional oblique detonation waves (ODWs) induced by finite wedges in a stoichiometric hydrogen–air mixture have been investigated numerically based on reactive Euler equations with a detailed chemical reaction model. The main zone affected by the expansion wave emanating from the turning point of a wedge is the flowfield downstream of the intersection point of the oblique shock wave (OSW) and the expansion wave. The ODW would be reduced to Chapman–Jouguet (CJ) detonation or decoupled combustion downstream. Three combustion regimes, detonation, decoupled combustion, and no ignition, were observed successively, as the wedge length decreases. It is found that the location of the intersection point is a key parameter for the detonation initiation. When the intersection point is located upstream of the ODW transition point, the expansion wave may quench ODW. Then, the critical wedge length is obtained by theoretical analysis of wave structures and the initiation criterion of ODWs for finite wedges is proposed. When the wedge length is greater than the critical wedge length, ODWs can be initiated. On the contrary, the initiation of ODWs do not occur. For wedge lengths small enough, no ignition occurs. Previously proposed criteria that use the induction length are also examined and compared with the present critical wedge length criterion in this study, and the latter is proven to achieve better results. [ABSTRACT FROM AUTHOR]

This paper studies premixed flame dynamics in half-open tubes by solving the two-dimensional, fully compressible, reactive Navier-Stokes equations on a dynamically adapting mesh using a high-order algorithm. A simplified chemical-diffusive model was used to describe the chemical reaction and diffusive transports in a stoichiometric hydrogen-air mixture. The influence of the length scale was examined by considering four tube heights at a fixed aspect ratio α = 7. The numerical simulations show that the flame evolves into a tulip flame (TF) in all the tubes shortly after being ignited at the open end. Variation in tube size leads to differences in the evolution of TF and generation of expansion waves. In a sufficiently large tube (d > 0.5 cm), the TF further develops into a series of more unstable distorted tulip flames (DTFs). By contrast, in a small tube (d < 0.5 cm), the TF shape remains until the end of the combustion. In addition, both the flame and pressure oscillate significantly almost in the entire process of flame propagation in the large tubes, while the oscillating behaviour in flame or pressure is negligible in the small tube after TF forms. It was found that the TF formation mechanism is length-scale dependent even for the same type of geometry and condition. A detailed examination of the interactions between flame, boundary layer, and pressure waves showed two mechanisms of TF formation: (1) boundary layer effect for the larger tubes (d ≥ 0.5 cm), and (2) Rayleigh–Taylor instability driven by compression waves for the smallest tube (d = 0.25 cm). The DTF formation in the half-open tubes is closely related to the expansion waves generated by the collapse of the TF cusp. The expansion waves cause a reverse flow in the boundary layer ahead of the flame front and consequently initiate the DTF. [ABSTRACT FROM AUTHOR]

Two-dimensional oblique detonation waves (ODWs) induced by finite wedges in a stoichiometric hydrogen–air mixture have been investigated numerically based on reactive Euler equations with a detailed chemical reaction model. The main zone affected by the expansion wave emanating from the turning point of a wedge is the flowfield downstream of the intersection point of the oblique shock wave (OSW) and the expansion wave. The ODW would be reduced to Chapman–Jouguet (CJ) detonation or decoupled combustion downstream. Three combustion regimes, detonation, decoupled combustion, and no ignition, were observed successively, as the wedge length decreases. It is found that the location of the intersection point is a key parameter for the detonation initiation. When the intersection point is located upstream of the ODW transition point, the expansion wave may quench ODW. Then, the critical wedge length is obtained by theoretical analysis of wave structures and the initiation criterion of ODWs for finite wedges is proposed. When the wedge length is greater than the critical wedge length, ODWs can be initiated. On the contrary, the initiation of ODWs do not occur. For wedge lengths small enough, no ignition occurs. Previously proposed criteria that use the induction length are also examined and compared with the present critical wedge length criterion in this study, and the latter is proven to achieve better results.

The processes of interaction of an expansion wave with the flow in a laminar boundary layer are studied with the use of the theory of free interaction. Numerical solutions of a nonlinear problem are found for various values of the dimensionless velocity of wave motion. Distributions of pressure-induced friction stress on the surface are obtained. [ABSTRACT FROM AUTHOR]

The open circuit underwater breathing apparatus can be a one or two-stage regulator used in scuba diving or a two-stage regulator used in surface supplied installations. These installations are proper in underwater sites at small depth. The pneumatic circuit of a two-stage regulator is composed mainly of a first stage regulator mounted on the air cylinders and a second stage carried by the diver in his mouth. The two regulators are linked together by a medium pressure hose. The circuit opens when the depression created by the diver’s inhalation, in the second stage body, reaches a certain value. The second stage opening causes a transient movement, namely an expansion wave that propagates through the medium pressure hose to the first stage regulator. The first stage regulator opens and the air in the cylinders is allowed to flow to the diver. The longer the hose, the greater the duration of the expansion wave propagation. Investigations on the wave propagation offer data on the inspiration unsteady motion duration which influences the respiratory effort of the diver.

A novel needle‐free injection system by electrically induced microbubbles has been developed. First of all, we evaluated the depth of injected reagent for evaluating injection performance. Then, the cavitation‐induced shock wave has been focused to magnify the injection performance by semispherical reflector. We successfully visualized the shock wave by the schlieren method and succeeded in measuring the pressure of focused shock wave and expansion wave by hydrophone. [ABSTRACT FROM AUTHOR]

With the increasing demand of faster, safer, and more comfortable rail travel, the evacuated tube maglev train (ETMT) has gradually attracted hot concerns in recent years. According to Kantrowitz limit theory, ETMT may encounter chocked flows, leading to more complex generation and development of a series of waves as ETMT moves rapidly in the tube. To study the wave phenomena produced by ETMT running at the super high-speed, the computational domain geometry model was simplified and then 2-D axisymmetric compressible N-S equation was established in the paper. Dynamic mesh method and dynamic adaptive mesh method were used to simulate the real motion of ETMT and improve the capture accuracy of the waves respectively. Results show that the wave structure is mainly composed of expansion waves, reflected shock waves and normal shock waves as ETMT moves for enough time at the speed of 1250 km/h. The intensity of the normal shock wave in front of the head car experiences four states consisting of rapid increase, initial stability, sudden drop and final stability. In the wake, the flow velocity attenuates in a fluctuating way along the opposite motion direction of ETMT due to the complicated interaction between shock waves and expansion waves. • The 2-D axisymmetric model is established to simulate ETMT motion. • The combination methods of the layering mesh and adaptive mesh are used. • The different wave phenomena in the evacuated tube are revealed. • The propagation properties of various waves are observed. [ABSTRACT FROM AUTHOR]

Results of numerical and experimental investigations of a high-velocity flow in a plane channel with sudden expansion in the form of a backward-facing step, which is used for flame stabilization in a supersonic flow, are presented. The experiments are performed in the IT-302M high-enthalpy short-duration wind tunnel under the following test conditions: Mach number at the combustor entrance 2.8, Reynolds number 30 · 106 m−1, and total temperature T0 = 2000 K, i.e., close to flight conditions at M = 6. The numerical simulations are performed by solving full unsteady Reynolds-averaged Navier-Stokes equations supplemented with the k-ω SST turbulence model and a system of chemical kinetics including 38 forward and backward reactions of combustion of a hydrogen-air mixture. Three configurations of the backward-facing step are considered: straight step without preliminary actions on the flow, with preliminary compression, and with preliminary expansion of the flow. It is demonstrated that the backward-facing step configuration exerts a significant effect on the separation region size, pressure distribution, and temperature in the channel behind the step, which are the parameters determining self-ignition of the mixture. The computed results show that preliminary compression of the flow creates conditions for effective ignition of the mixture. As a result, it is possible to obtain ignition of a premixed hydrogen-air mixture and its stable combustion over the entire channel height. [ABSTRACT FROM AUTHOR]

Abstract A new Ludwieg tube for transonic testing has been put into operation. For this, an existing shock tube has been modified to a Ludwieg tube. The ideal operating principle is described by theory and corresponding theoretical values are compared to experimental ones. Pressure histories as well as test section flow visualization demonstrate a better performance concerning flow quality and testing time compared to the former shock tube. Static as well as total pressure measurements have been performed within the test section. Pitot rake measurements show the homogeneity of the test section flow field. Additional boundary layer profile measurements give evidence of the temporal growth of the test section wall boundary layer. These results are compared with theoretical ones deduced from a theoretical approach for a boundary layer behind an expansion wave. Both, boundary layer and Pitot rake measurements allow to discuss the influence of a turbulent boundary layer on these measurements. Cookie cutters at the entrance to the test section act as boundary layer bleed which partially avoid the entering of the tube wall boundary layer into the test section. Their influence on the test section flow is discussed as well. [ABSTRACT FROM AUTHOR]

This paper reports an experimental result of expansion wave and cavitation bubble generation by underwater shock wave reflection at the interface for understanding of shock wave interaction phenomena related to the mechanism of shock wave human tissue damage. Underwater shock wave was generated by detonating a micro-explosive (silver azide pellet). The process of underwater shock wave reflection at the interface, expansion wave and cavitation bubble generation were visualized by shadowgraph method and recorded by an ultra-high-speed framing camera at high spatiotemporal resolution. The pressure time history near the interface in water was measured simultaneously by a needle hydrophone. High-speed shadowgraph sequential images show that a compression wave (shock wave) was reflected from a thin interface plate (Polystyrene, acrylic, Teflon, thickness of t=0.5 mm), and expansion wave was then generated in water by reflection from the air. In addition, a cavitation bubble was created behind the expansion wave. The simultaneously measured pressure history also shows that an expansion wave propagated behind shock wave. The negative peak pressure of expansion wave in the case of a thin plate shock wave interaction was larger than that in water surface interaction.

Предложена модель, в которой горный удар (техногенное землетрясение) – представлен как выход ударной волны на внутреннюю стенку шахты. При этом возникает волна разгрузки, растяжения, двигающаяся в обратном (вглубь стенки) направлении. Ударная волна сопровождается движением среды в направлении распространения ударной волны со скоростью значительно меньшей, чем у волны. Совокупность явлений, происходящих на внутренней поверхности шахты, приводит к выбросу угольной пыли и растворенного в угле метана, что является причиной взрыва.

The prompt oblique detonation wave (PODW) sustained by a finite-length wedge is investigated by numerical simulation. The numerical results show that it is possible to stabilize a PODW on a finite-length wedge shorter than the induction length of the mixture behind the inert shock by numerically imposing a premature initiation of combustion in the initial flow field. The fully coupled and the partially coupled PODWs are observed in the numerical results. For the fully coupled PODW, the upstream facing transverse waves (UF TW) are swept downstream and consequently a fully coupled PODW can persist. For the partially coupled PODW, the UF TWs propagate upstream and the downstream facing transverse waves are weakened by the expansion wave emanating from the corner. As a result, a partially coupled PODW forms. Further, it is found that the stability of the partially coupled PODW is weak. The configuration of the partially coupled PODW can be altered by local explosions occurring downstream. [ABSTRACT FROM AUTHOR]

In the recent decades southern European cities experienced morphological changes and a demographic transition towards zero (or negative) growth and aging. Population dynamics shifted from the impressive growth of the post-industrial period into recent de-concentration processes determining the spillover of commercial and residential settlements across the surrounding rural areas. Based on long-term demographic data, the present study hypothesizes that urban expansion did not follow a one-way linear path from compactness to dispersion while reflecting differentiated growth patterns based on the specific socioeconomic context at the local scale. Along a sufficiently long time period non-linear expansion waves alternating settlement densification and scattering are expected to be found especially in complex urban contexts such as those developed in the Mediterranean region. This hypothesis was tested for a paradigmatic case study in southern Europe (Athens, Greece) using demographic data covering 160 years (1848-2011). Urban growth in Athens was assessed through the analysis of long-term census data made available on a district level. These data provide information on the spatial distribution of resident population and characterize distinct expansion waves according to the dominant socioeconomic context. Results of the study were discussed in the light of the debate on future development of the Mediterranean cities and the (changing) economic relations with the surrounding region. [ABSTRACT FROM AUTHOR]

Although a large part of the global domestic dog population is free-ranging and free-breeding, knowledge of genetic diversity in these free-breeding dogs (FBDs) and their ancestry relations to pure-breed dogs is limited, and the indigenous status of FBDs in Asia is still uncertain. We analyse genome-wide SNP variability of FBDs across Eurasia, and show that they display weak genetic structure and are genetically distinct from pure-breed dogs rather than constituting an ADMIXTURE of breeds. Our results suggest that modern European breeds originated locally from European FBDs. East Asian and Arctic breeds show closest affinity to East Asian FBDs, and they both represent the earliest branching lineages in the phylogeny of extant Eurasian dogs. Our biogeographic reconstruction of ancestral distributions indicates a gradual westward expansion of East Asian indigenous dogs to the Middle East and Europe through Central and West Asia, providing evidence for a major expansion that shaped the patterns of genetic differentiation in modern dogs. This expansion was probably secondary and could have led to the replacement of earlier resident populations in Western Eurasia. This could explain why earlier studies based on modern DNA suggest East Asia as the region of dog origin, while ancient DNA and archaeological data point to Western Eurasia. [ABSTRACT FROM AUTHOR]

The velocity by which the disturbance travels through the medium is the wave velocity. Pulse wave velocity is one of the main parameters in hemodynamics. The study of wave propagation through the fluid-fill elastic tube is of great importance for the proper biophysical understanding of the nature of blood flow through of cardiovascular system. The effect of viscosity on the pulse wave velocity is generally ignored. In this paper we present the results of experimental measurements of pulse wave velocity (PWV) of compression and expansion waves in elastic tube. The solutions with different density and viscosity were used in the experiment. Biophysical model of the circulatory flow is designed to perform measurements. Experimental results show that the PWV of the expansion waves is higher than the compression waves during the same experimental conditions. It was found that the change in viscosity causes a change of PWV for both waves. We found a relationship between PWV, fluid density and viscosity. [ABSTRACT FROM AUTHOR]

• Phase transition during flashing is modeled by a six-equation formulation. • Capability of a simple HLLC type Riemann solver in simulating flashing with secondary nucleation is established. • Effect of secondary nucleation from a single site on the propagation of the evaporating front is investigated. Rapid depressurization of a stagnant liquid below its prevailing saturation brings it to a metastable superheated state. Thermodynamic equilibrium is regained through a rapid evaporation or flashing which starts in a thin layer at the free surface and continues to propagate downward by nucleation and evaporation in the subsequent layers of superheated liquid below it. Secondary nucleation (from spurious locations) in the bulk of the superheated liquid or from the wall of the container uniquely affects the propagation of the free surface evaporating front. We computationally simulate the propagation of evaporating front in the presence of secondary nucleations, adopting a one dimensional, compressible two-phase flow model with diffuse interface. For simulating secondary nucleation at pre-determined sites, vapour density is assumed at the location and active interface is imposed locally. Secondary nucleation is noted to slow down the propagation of the evaporating front while the effect is more prominent with the proximity of the site. The secondary phase change interface is observed to grow towards the primary one and has a higher velocity as has been reported in different experiments. Secondary nucleation also hinders the propagation of the wave reflected back from the end of the domain. We also discuss some limitations of the model. [ABSTRACT FROM AUTHOR]

Aerothermal effects generated by the evacuated tube maglev train (ETMT) cannot be ignored as it moves at the super high-speed. The study on the aerothermal mechanism induced by ETMT is of great significance to the design and application of heatproof materials on the skins of tube wall and vehicle. Based on the combination methods of dynamic layering mesh and dynamic adaptive mesh, the high-speed movement process of ETMT was reproduced by numerical simulation. The results show that the main frequency and amplitude of the ETMT skin temperature fluctuation in the "steady" stage have certain stochastic characteristics in spatial distribution. Separation and reattachment of the boundary layer contribute to decrease and increase of the ETMT skin temperature respectively. Besides, both expansion wave and shock wave near tail car increase the ETMT skin temperature. The normal shock wave in front of ETMT makes the tube wall temperature reach the maximum, while the normal shock wave behind ETMT makes the tube wall temperature reach relatively low due to the complicated interactions between reflected shocks wave and expansion shocks wave in wake. In wake, the tube wall temperature caused by the shock wave is higher than that caused by the expansion wave at any time. • The combination methods of the motion mesh and adaptive mesh are used to simulate ETMT motion. • Frequency domain characteristics of ETMT skin temperature are studied. • Typical spatial distributions of ETMT skin temperature are observed. • Aerothermal effects of various waves on the tube skin are considered. [ABSTRACT FROM AUTHOR]