On the transition between different initiation structures of wedge-induced oblique detonations

Oblique detonation waves (ODWs) have been widely studied due to their application potential for airbreathing hypersonic propulsion. Moreover, various formation structures of wedge-induced oblique detonation waves have been revealed in recent numerical investigations. Given the inflow conditions, the wave configuration is dependent on the wedge angle. Hence, any wedge-angle change will induce a transient ODW evolution to transition from one configuration to another. In this study, the transient development created by instantaneously changing the wedge angle is investigated numerically, based on the unsteady two-dimensional Euler equations and one-step irreversible Arrhenius chemical kinetics. The evolution caused by the abrupt wedge-angle change from one smooth initiation structure to another, both with a curved oblique shock/detonation surface at high-Mach-number regime, is investigated. Two processes are analyzed; the first consists of the downstream transition of the ODW initiation region the by decreasing the angle, and the second is the upstream transition by increasing the angle. In the downstream transition, the overall structure moves globally and readjusts continuously, generating an intermediate kinklike initiation structure. In the upstream transition, a localized reaction region forms and induces a more complex process, mainly derived from the different responding speeds of the oblique shock and detonation waves. To avoid the generation of the new localized explosion region, which causes an abrupt change in the initiation position and potentially affects the ODWE’s stability and performance, it is suggested to vary the wedge angle in incremental steps within a certain time interval

Hypervelocity flow over spheres

The nature of the nonequilibrium flow of dissociating gases over spheres was investigated experimentally, numerically and theoretically. A series of experiments with three different gases, nitrogen, air and carbon dioxide, was performed in the shock tunnel T5 at GALCIT. Five spheres of different radii equipped with thermocouples for surface heat flux measurements were used. The state-of-the-art numerical method by Candler (1988) was used to conduct a parallel study which strongly complemented the experimental and theoretical efforts. Experimental heat flux measurements are presented. Good agreement was observed among the measured stagnation point heat transfer rates, computational results and Fay and Riddell’s theoretical predictions. For nitrogen and air, the measured heat flux distributions were also in good agreement with numerical computation results and Lees’ theory. For carbon dioxide, large deviations were observed. Early transition tripped by surface roughness is a possible cause for the deviation of heat flux distribution from the theory. The experimental differential interferograms were compared with the images constructed from computational flowfields. Good agreement of fringe pattern and shock shape was observed. An analytical solution is obtained for inviscid hypervelocity dissociating flow over spheres. The solution explains the correlation between the dimensionless stand-off distance and the dimensionless reaction rate parameter previously observed by Hornung (1972) for nitrogen. The physics of the correlation can be shown as the binary scaling. Based on the solution, a new dimensionless reaction rate parameter is defined to generalize Hornung’s correlation for more complex gases than nitrogen. Experimental and numerical results confirm the new correlation. The effect of nonequilibrium recombination downstream of a curved two-dimensional shock was also addressed. An analytical solution for an ideal dissociating gas was obtained, giving an expression for dissociation fraction as a function of temperature on a streamline. The solution agrees well with the numerical result and provides a rule of thumb to check the validity of binary scaling for the experimental conditions. The effects upon the binary scaling of the large difference in freestream temperature between flight and free-piston shock tunnel conditions are discussed

Effects of Person-job Fit and Person-organization Fit on Work Attitudes and Organizational Citizenship Behaviors of Foodservice Employees in the Continuing Care Retirement Communities

Employees who experience great level of person-job (P-J) fit and person-organization (P-O) fit may actively engage in their jobs and perform organizational citizenship behaviors at organization (OCB-O)and individual (OCB-I ) level. The purpose of this study is to investigate the effect of individual perceived P-J and P-O fit and work engagement (i.e., job satisfaction, job involvement and organizational commitment) on OCB-I and OCB-O. Two hundred and fifty responses from food-service employees in Continuing Care Retirement Communities (CCRCs) are targeted for final data collection. Descriptive statistics and structural equation modeling will be employed to report participants’ demographic characteristics and to examine the relationships among constructs. Findings will help managers recognize the importance of P-J and P-O fit and work attitude in the development of employees’ pro-socialization behaviors

Nonequilibrium recombination after a curved shock wave

The effect of nonequilibrium recombination after a curved two-dimensional shock wave in a hypervelocity dissociating flow of an inviscid Lighthill–Freeman gas is considered. An analytical solution is obtained with the effective shock values derived by Hornung (1976) [5] and the assumption that the flow is ‘quasi-frozen’ after a thin dissociating layer near the shock. The solution gives the expression of dissociation fraction as a function of temperature on a streamline. A rule of thumb can then be provided to check the validity of binary scaling for experimental conditions and a tool to determine the limiting streamline that delineates the validity zone of binary scaling. The effects on the nonequilibrium chemical reaction of the large difference in free stream temperature between free-piston shock tunnel and equivalent flight conditions are discussed. Numerical examples are presented and the results are compared with solutions obtained with two-dimensional Euler equations using the code of Candler (1988) [10]

Experiments on Hypervelocity Dissociating Flow over Spheres

This paper presents the experimental results of measurements of the stagnation point heat transfer rates experienced by spherical models of 1, 2, 3, 4 and 6 inch diameter in nitrogen, air and carbon dioxide at stagnation enthalpies ranging from 4 to 22 MJ/kg and stagnation pressures from 25 to 80 MPa. The experimental results are compared with existing results obtained in the ballistic range facility at NASA Ames and also with numerical calculations. The experimental results obtained by optical differential interferometry were compared with the images constructed from flowfields computed using the method of Candler (1988). Good agreement of fringe pattern and shock shape was observed. A novel flow visualization technique using sodium seeding to increase the sensitivity of conventional shadowgraphic techniques by resonant enhancement of the refractivity of the medium was also used. The resonantly-enhanced shadowgraph suggests a possible way of visualizing the vortical structure induced by the strong entropy layer

Mechanisms of the destabilized Mach reflection of inviscid oblique detonation waves before an expansion corner

The stabilization of oblique detonation waves (ODWs) in an engine combustor is important for the successful applications of oblique detonation engines, and comprehensively understanding the effects of the inviscid reflection of ODWs on their stabilization and the relevant mechanisms is imperative to overall combustor design. In this study, the flow fields of ODW reflections in a space-confined combustor are numerically studied by solving the two-dimensional time-dependent multispecies Euler equations in combination with a detailed hydrogen combustion mechanism. The inviscid Mach reflections of ODWs before an expansion corner are emphasized with different flight Mach numbers, Ma, and different dimensionless reflection locations, zeta >= 0 (zeta = 0: the ODW reflects precisely at the expansion corner; zeta > 0: the ODW reflects off the wall before the expansion corner). Two kinds of destabilization phenomena of the inviscid Mach reflection of an ODW induced by different mechanisms are found, namely wave-induced destabilization at large zeta > 0 for moderate (not very low) Ma and inherent destabilization at any zeta > 0 for low Ma. Wave-induced destabilization is attributed to the incompatibility between the pressure ratio across the Mach stem and its relative propagation speed, which is triggered by the action of the secondary reflected shock wave or the transmitted Mach stem on the subsonic zone behind the Mach stem. Inherent destabilization is demonstrated through an in-depth theoretical analysis and is attributed to geometric choking of the flow behind the Mach stem

Influence of thermochemical nonequilibrium on expansion tube air test conditions: A numerical study

Using a Lagrangian solver, thermochemical nonequilibrium simulations are performed for the entire range of practical operating conditions of expansion tubes to isolate the influence of nonequilibrium and identify key features in large-scale facilities. Particular attention is given not only to the influence of the nonequilibrium unsteady expansion but also to the influences of the nonequilibrium region behind the primary shock and non-ideal secondary diaphragm rupture. The nonequilibrium unsteady expansion is found to be the most influential process in the test flow-it can significantly influence the flow properties and cause significant temporal variations in the properties during the test time. The nonequilibrium unsteady expansion is also found to accelerate the secondary shock and contact surface. The non-ideal secondary diaphragm rupture is found to increase the amount of nonequilibrium in the test flow due to the generation of a reflected shock. The nonequilibrium region behind the primary shock may be considered negligible in most conditions. Regarding the creation of thermochemical equilibrium test conditions, important factors for achieving this include having a high acceleration tube fill pressure, large-scale facility, and high total enthalpy. The combined effects of viscosity and nonequilibrium are postulated, and the results are supported by experimental works that report consistent findings. To provide an idea of the sensitivity of the numerical configuration, simulations of fixed-volume reactors at various de-excitation conditions are performed using different nonequilibrium models

Application of CE/SE method to gas-particle two-phase detonations under an Eulerian-Lagrangian framework

This study aims to extend the original Eulerian space-time conservation element and solution element (CE/SE) method to the Eulerian-Lagrangian framework to solve the gas-particle two-phase detonation problems. The gas-aluminum particle two-phase detonations are numerically investigated by the developed Eulerian-Lagrangian code, in which the gas-phase compressible Euler equations are solved by our in-house CE/SE scheme based on quadrilateral meshes. Additionally, the particle-phase Lagrangian equations, together with the stiff source terms of interphase interactions and chemical reactions, are explicitly integrated via the operator-splitting technique. A dynamic data structure is introduced to store particle information to overcome the tremendous communication costs when applying message passing interface parallel to the Eulerian-Lagrangian framework. The code is shown to be of better parallel efficiency in moderate-scale computations than that uses static arrays. Comparisons with previous one-dimensional and two-dimensional simulation results and experimental observations are conducted to demonstrate the accuracy and reliability of the developed Eulerian-Lagrangian CE/SE code in gas-particle two-phase detonation simulations. Moreover, the code is also applied to simulate polydisperse gas-particle detonations which is close to a realistic scenario, and significant differences in detonation characteristics are found when compared with the monodisperse counterparts. The great demands of using the Eulerian-Lagrangian method to obtain more physics-consistent gas-particle detonation results are addressed, which the traditional Eulerian-Eulerian simulations fail to observe. (C) 2019 Elsevier Inc. All rights reserved

A theoretical method for solving shock relations coupled with chemical equilibrium and its applications

In this study, a theoretical method is proposed to solve shock relations coupled with chemical equilibrium. Not only shock waves in dissociated flows but also detonation waves in com-bustive mixtures can be solved. The global iterative solving process is specially designed to mimic the physical and chemical process in reactive shock waves to ensure good stability and fast conver-gence in the proposed method. Within each global step, the single-variable equations of normal and oblique shock relations are derived and solved with the Newton iteration method to reduce the com-plexity of the problems, and the minimization of free energy method of NASA (National Aeronau-tics and Space Administration) is adopted to solve equilibrium compositions. It is demonstrated that the convergent process is stable and very close to the real chemical-kinetic process, and high accuracy is achieved in the solutions of normal and oblique reactive shock waves. Moreover, the proposed theoretical method has also been applied to many problems associated with reactive shocks, including the stability of oblique detonation wave, bow detonation over a sphere, and shock reflection in dissociated air. The great importance of using chemical equilibrium to theoretically pre-dict the theoretical range of the wedge angle for a standing oblique detonation wave (the standing window of the oblique detonation wave), the stand-off distance of bow detonation wave and the transition criterion of shock reflection in dissociated air with high accuracy have been addressed.(c) 2021 Chinese Society of Aeronautics and Astronautics. Production and hosting by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)