Showing papers on "Oblique shock published in 2017"

Initiation characteristics of wedge-induced oblique detonation waves in a stoichiometric hydrogen-air mixture

Abstract: The initiation features of two-dimensional, oblique detonations from a wedge in a stoichiometric hydrogen-air mixture are investigated via numerical simulations using the reactive Euler equations with de-tailed chemistry. A parametric study is performed to analyze the effect of inflow pressure P-0, and Mach number M-0 on the initiation structure and length. The present numerical results demonstrate that the two transition patterns, i.e., an abrupt transition from a multi-wave point connecting the oblique shock and the detonation surface and a smooth transition via a curved shock, depend strongly on the inflow Mach number, while the inflow pressure is found to have little effect on the oblique shock-to-detonation transition type. The present results also reveal a slightly more complex structure of abrupt transition type in the case of M-0 = 7.0, consisting of various chemical and gasdynamic processes in the shocked gas mixtures. The present results show quantitatively that the initiation length decreases with increasing M-0, primarily due to the in-crease of post-shock temperature. Furthermore, the effect of M-0 on initiation length is independent of P-0, but given the same M-0, the initiation length is found to be inversely proportional to P-0. Theoretical analysis based on the constant volume combustion (CVC) theory is also performed, and the results are close to the numerical simulations in the case of high M-0 regardless of P-0, demonstrating that the post-oblique-shock condition, i.e., post-shock temperature, is the key parameter affecting the initiation. At decreasing M-0, the CVC theory breaks down, suggesting a switch from chemical kinetics-controlled to a wave-controlled gasdynamic process. For high inflow pressure P-0 at decreasing M-0, the CVC theoretical estimations depart from numerical results faster than those of low P-0, due to the presence of the non-monotonic effects of chemical kinetic limits in hydrogen oxidation at high pressure. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.

Stability and modal analysis of shock/boundary layer interactions

Abstract: The dynamics of oblique shock wave/turbulent boundary layer interactions is analyzed by mining a large-eddy simulation (LES) database for various strengths of the incoming shock. The flow dynamics is first analyzed by means of dynamic mode decomposition (DMD), which highlights the simultaneous occurrence of two types of flow modes, namely a low-frequency type associated with breathing motion of the separation bubble, accompanied by flapping motion of the reflected shock, and a high-frequency type associated with the propagation of instability waves past the interaction zone. Global linear stability analysis performed on the mean LES flow fields yields a single unstable zero-frequency mode, plus a variety of marginally stable low-frequency modes whose stability margin decreases with the strength of the interaction. The least stable linear modes are grouped into two classes, one of which bears striking resemblance to the breathing mode recovered from DMD and another class associated with revolving motion within the separation bubble. The results of the modal and linear stability analysis support the notion that low-frequency dynamics is intrinsic to the interaction zone, but some continuous forcing from the upstream boundary layer may be required to keep the system near a limit cycle. This can be modeled as a weakly damped oscillator with forcing, as in the early empirical model by Plotkin (AIAA J 13:1036–1040, 1975).

Steady-State Analysis of Rotating Detonation Engine Flowfields with the Method of Characteristics

Abstract: A method for modeling the internal flowfield in a rotating detonation engine is developed using shock-expansion theory combined with the steady two-dimensional isentropic method of characteristics. An analytical model using the oblique shock relations, the Prandtl–Meyer function, and the detonation jump conditions is used to determine the basic shock structure. Once the structure is known, a shock-fitted method of characteristics solution is marched out to generate the rest of the flowfield. Reactant injection is handled analytically by solving the conservation equations for a flow undergoing a sudden expansion along with the method of characteristics compatibility relations to provide a new boundary condition. A new solution is then initialized using information from the previous solution to calculate the new shock structure. This process is repeated until the solutions converge. The converged solution is the ideal steady-state solution of a rotating detonation engine in the wave-fixed reference frame. T...

Prediction dynamic model of shock train with complex background waves

Abstract: Oblique shock waves are unavoidable in a rectangular hypersonic inlet, leading to a non-uniform flow field. While a significant body of the literature exists regarding the shock train modeling in a uniform incoming flow condition, few efforts have focused on the shock train behavior considering the influence of the shock wave boundary layer interactions. A low-order dynamic model of the shock train has been constructed with the help of the free interaction theory and a 1-D analysis approach. Experimental and numerical investigations have been carried out to evaluate the low-order model. The results show that the model has the capability of qualitatively analyzing the shock train behavior. In the cases with incident shocks, the rapid forward movement of the shock train has been observed by experiment. Besides this phenomenon was also modeled using the low-order model. Schlieren images show that when the shock train approaches the interaction zone, its behavior is characterized by oscillation and then follo...

Investigation of Three-Dimensional Shock Wave/Turbulent-Boundary-Layer Interaction Initiated by a Single Fin

Abstract: Three-dimensional shock wave/turbulent-boundary-layer interaction of a hypersonic flow passing a single fin mounted on a flat plate at a Mach number of five and unit Reynolds number 3.7×10^7 was conducted by a large-eddy simulation approach. The performed large-eddy simulation has demonstrated good agreement with experimental data in terms of mean flowfield structures, surface pressure distribution, and surface flow pattern. Furthermore, the shock wave system, flow separation structure, and turbulence characteristics were all investigated by analyzing the obtained large-eddy simulation dataset. It was found that, for this kind of three-dimensional shock wave/turbulent-boundary-layer interaction problem, the flow characteristics in different regions have been dominated by respective wall turbulence, free shear layer turbulence, and corner vortex motions in different regions. In the reverse flow region, near-wall quasi-streamwise streaky structures were observed just beneath the main separation vortex, indicating that the transition of the pathway of the separation flow to turbulence may occur within a short distance from the reattachment location. The obtained large-eddy simulation results have provided a clear and direct evidence of the primary reverse flow and the secondary separation flow being essentially turbulent.

Shock Train Unsteadiness Characteristics, Oblique-to-Normal Transition, and Three-Dimensional Leading Shock Structure

Abstract: The structure and unsteadiness characteristics of a shock train in a constant-area ducted flow are studied experimentally as a function of duct back pressure, which is mechanically controlled by a ...

Flow structure oscillations and tone production in underexpanded impinging round jets

Abstract: Flow structure oscillations and tone generation mechanisms in an underexpanded round jet impinging on a flat plate normally have been investigated using compressible large-eddy simulations. At the exit of a pipe nozzle of diameter D, the jet is characterized by a nozzle pressure ratio of 4.03, an exit Mach number of 1, a fully expanded Mach number of 1.56, and a Reynolds number of 6×104. Four distances between the nozzle and the plate of 2.08D, 2.80D, 3.65D, and 4.66D are considered. Snapshots of vorticity, density, pressure, and mean velocity flowfields are first presented. The latter results compare well with data of the literature. In three cases, in particular, a Mach disk appears to form just upstream from the plate. The convection velocity of flow structures between the nozzle and the plate, and its dependence on the nozzle-to-plate distance, are then examined. The properties of the jet near pressure fields are subsequently described using Fourier analysis. Tones emerge in the spectra at frequencies...

A theoretical approximation of the shock standoff distance for supersonic flows around a circular cylinder

Abstract: Many previous studies have addressed the problem of theoretically approximating the shock standoff distance; however, limitations to these methods fail to produce excellent results across the entire range of Mach numbers. This paper proposes an alternative approach for approximating the shock standoff distance for supersonic flows around a circular cylinder. It follows the philosophy that the “modified Newtonian impact theory” can be used to calculate the size of the sonic zone bounded between the bow shock and the fore part of the body and that the variation of the said zone is related to the standoff distance as a function of the upstream Mach number. Consequently, a reduction rate parameter for the after-shock subsonic region and a reduction rate parameter for the shock standoff distance are introduced to formulate such a relation, yielding a new expression for the shock standoff distance given in Equation (32). It is directly determined by the upstream Mach number and the location of the sonic point a...

Initiation structure of oblique detonation waves behind conical shocks

Abstract: The understanding of oblique detonation dynamics has both inherent basic research value for high-speed compressible reacting flow and propulsion application in hypersonic aerospace systems. In this study, the oblique detonation structures formed by semi-infinite cones are investigated numerically by solving the unsteady, two-dimensional axisymmetric Euler equations with a one-step irreversible Arrhenius reaction model. The present simulation results show that a novel wave structure, featured by two distinct points where there is close-coupling between the shock and combustion front, is depicted when either the cone angle or incident Mach number is reduced. This structure is analyzed by examining the variation of the reaction length scale and comparing the flow field with that of planar, wedge-induced oblique detonations. Further simulations are performed to study the effects of chemical length scale and activation energy, which are both found to influence the formation of this novel structure. The initiat...

Regimes describing shock boundary layer interaction and ignition in shock tubes

Abstract: Regimes of shock boundary layer interaction are proposed in consideration of shock tube kinetic experiments. For this, we examine three ways that the reflected shock wave interacts with the boundary layer: incipient separation occurs when the shock is just strong enough to subject the flow to an adverse pressure gradient leading to flow reversal; shear layer instabilities manifest after a certain length of time and can cause inhomogeneities in the test gas; and shock bifurcation occurs when the back pressure of the test gas is sufficient to contain the boundary layer fluid within a stagnation bubble causing severe inhomogeneities in the test gas. Theory delineating these regimes is developed, and these delineations are compared to simulations of shock tube experiments as well as experimental data, where reasonable agreement is found. Through the theory applied to the incipient separation regime, it is determined that boundary layer separation likely occurs in most shock tube experiments; however, separation is unlikely to affect a chemical kinetic experiment except at long test times. To quantify the effect of the boundary layer, a bifurcation Damkohler number is introduced, which is found to perform sensibly well at classifying strong and weak ignition in shock tubes, implying that these combustion phenomena are determined by a competition of physical and chemical timescales. Finally, simulations suggest that tailoring the incident shock Mach number for a given experiment could provide opportunities for mitigating inhomogeneities in the test gas.

Reformation of rippled quasi-parallel shocks: 2-D hybrid simulations

Abstract: One-dimensional (1-D) hybrid simulations have demonstrated that a quasi-parallel shock is non-stationary and undergoes a reformation process. Recently, two-dimensional (2-D) hybrid simulations have revealed that ripples along the shock front is an inherent property of a quasi-parallel shock. In this paper, we investigate reformation process of a rippled quasi-parallel shock with a 2-D hybrid simulation model. The simulation results show that at a rippled shock, incident particles behave differently and just can be partially reflected at some specific locations along the rippled shock front, and the reflected particles will form an ion beam that moves back to the upstream along the magnetic field. Then, the beam locally interacts with upstream waves, and the waves are enhanced and finally steepen into a new shock front. As the upstream incident plasma moves to the shock front, the new shock front will approach and merge with the old shock front. Such a process occurs only before these locations along the shock front, and after the merging of the new shock front and old shock front is finished, a relatively plane shock front is formed. Subsequently, a new rippled shock front is again generated due to its interaction with the upstream waves, and it will repeat the previous process. In this pattern, the shock reforms itself quasi-periodically, at the same time, ripples can shift along the shock front. The simulations present a more complete view of reformation for quasi-parallel shocks.

Mathematical Model of Shock-Train Path with Complex Background Waves

Abstract: The existence of the complex compression and expansion waves in an isolator induces a special motion path of the shock train. Numerical simulations with two different inlet models are conducted. The results indicate that the local parameters govern the shock train’s motion, and the pressure gradient along the surface plays an extremely important role. The shock train’s location is determined by the entrance condition and backpressure, whereas it is the parameter along the surface that determines its path. The start and end positions of the rapid forward motion are located where the pressure gradient approaches zero. Streamwise parameters and the surface pressure gradient are introduced in Waltrup and Billig’s (“Structure of ShockWaves in Cylindrical Ducts,” AIAA Journal, Vol. 11, No. 10, 1973, pp. 1404–1408) empirical correlation to characterize the rapid forward motion. Then, a mathematical model is established based on Billig’s correlation and surface pressure datasets in the current paper.

Improved Flamelet Modeling of Supersonic Combustion

Abstract: This paper has the objective of addressing a few basic issues pertaining to the use of the laminar flamelet method to model turbulence-combustion interactions in supersonic combustion. Specifically...

Reprint of: A supersonic jet target for the cross section measurement of the 12 C(α, γ) 16 O reaction with the recoil mass separator ERNA

Abstract: 12C(α, γ)16O cross section plays a key-role in the stellar evolution and nucleosynthesis of massive stars. Hence, it must be determined with the precision of about 10% at the relevant Gamow energy of 300 keV. The ERNA (European Recoil mass separator for Nuclear Astrophysics) collaboration measured, for the first time, the total cross section of 12C(α, γ)16O by means of the direct detection of the 16O ions produced in the reaction down to an energy of Ecm = 1.9 MeV. To extend the measurement at lower energy, it is necessary to limit the extension of the He gas target. This can be achieved using a supersonic jet, where the oblique shock waves and expansion fans formed at its boundaries confine the gas, which can be efficiently collected using a catcher. A test version of such a system has been designed, constructed and experimentally characterized as a bench mark for a full numerical simulation using FV (Finite Volume) methods. The results of the commissioning of the jet test version and the design of the new system that will be used in combination with ERNA are presented and discussed.