Nick Sirmas, Matei Radulescu
Videos of the molecular dynamics simulations of a colliding hard disk reactive medium. A simple event-driven algorithm is used to simulate these colliding hard disks allowing for the study of detonations at the microscopic level. This video was submitted as party of the Gallery of Fluid Motion for the American Physical Society 63rd Annual DFD Meeting.
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Liquid-fueled oblique detonation waves induced by reactive and non-reactive transverse liquid jets
本研究通过计算模拟揭示液体横向射流（反应性或非反应性）可诱导液体燃料斜爆震波，动量比决定机制：低比时靠雾化燃料促进低温反应，高比时靠物理阻塞产生强激波直接引爆。
Detonation propagation in a compressible medium wherein the energy release has been made spatially inhomogeneous is examined via numerical simulation. The inhomogeneity is introduced via step functions in the reaction progress variable, with the local value of energy release correspondingly increased so as to maintain the same average energy density in the medium, and thus a constant Chapman Jouguet (CJ) detonation velocity. A one-step Arrhenius rate governs the rate of energy release in the reactive zones. The resulting dynamics of a detonation propagating in such systems with one-dimensional…
The dynamics of frontal and transverse shocks in gaseous detonation waves is a complex phenomenon bringing many difficulties to both numerical and experimental research. Advanced laser-optical visualization of detonation structure may provide certain information of its reactive front, but the corresponding lead shock needs to be reconstructed building the complete flow field. Using the Multi-Layer Perceptron (MLP) approach, we propose in this study a shock front reconstruction method which can predict evolution of the lead shock wavefront from the state of the reactive front. The method is ver…
Multidimensional numerical simulations of a homogeneous, chemically reactive gas were used to study ignition, flame stability, and deflagration-to-detonation transition (DDT) in a supersonic combustor. The configuration studied was a rectangular channel with a supersonic inflow of stoichiometric ethylene-oxygen and a transimissive outflow boundary. The calculation is initialized with a velocity in the computational domain equal to that of the inflow, which is held constant for the duration of the calculation. The compressible reactive Navier-Stokes equations were solved by a high-order numeric…
Theory of weakly nonlinear self sustained detonations
基于反应性可压缩 Navier-Stokes 方程的渐近分析，提出弱非线性多维自持爆轰模型，并验证其可重现一维与多维爆轰的关键动力学特征。
The detonation wave stability is addressed using Fickett's equation, i.e., the reactive form of Burgers' equation. This serves as a simple analogue to the reactive Euler equations, permitting one to gain insight into the nonlinear dynamics of detonation waves. Chemical kinetics were modeled using a two-step reaction with distinct induction and reaction zones. An evolution equation for the detonation structure was derived using the method of matched asymptotics for large activation energy and slow rate of energy release. While the first order solution was found unconditionally unstable, the sec…
The present work revisits the problem of modelling the real gaseous detonation dynamics at the macro-scale by simple steady one-dimensional (1D) models. Experiments of detonations propagating in channels with exponentially expanding cross-sections were conducted in the H2/O2/Ar reactive system. Steady detonation waves were obtained at the macro-scale, with cellular structures characterized by reactive transverse waves. For all the mixtures studied, the dependence of the mean detonation speed was found to be in excellent agreement with first principles predictions of quasi-1D detonation dynamic…
Smoothed Dissipative Particle Dynamics (SDPD) is a mesoscopic method which allows to select the level of resolution at which a fluid is simulated. The aim of this work is to extend SDPD to chemically reactive systems.To this end, an additional progress variable is attached to each mesoparticle and evolves according to chemical kinetics. This reactive SDPD model is illustrated with numerical studies of the shock-to-detonation transition in nitromethane as well as the stationary behavior of the reactive wave.
This experimental study addresses the re-initiation mechanism of detonation waves following the Mach reflection of a shock-flame complex. The detonation diffraction around a cylinder is used to reproducibly generate the shock-flame complex of interest. The experiments are performed in methane-oxygen. We use a novel experimental technique of coupling a two-in-line-spark flash system with a double-frame camera in order to obtain microsecond time resolution permitting accurate schlieren velocimetry. The first series of experiments compares the non-reactive sequence of shock reflections with the r…
We use numerical simulations of the reactive Euler equations to analyze the nonlinear stability of steady-state one-dimensional solutions for gaseous detonations in the presence of both momentum and heat losses. Our results point to a possible stabilization mechanism for the low velocity detonations in such systems. The mechanism stems from the existence of a one-parameter family of solutions found in Semenko et al. (Shock Waves 26 (2), 141-160, 2016).
An exponential horn geometry is introduced in order to establish cellular detonations with a constant mean lateral mass divergence, propagating at quasi-steady speeds below the Chapman-Jouguet value. Experiments and simulations demonstrated that such quasi-steady state detonations can be realized, hence permitting to obtain the relations between the detonation speed and mean lateral flow divergence for cellular detonations in an unambiguous manner. The experimentally obtained speed ($D$) dependencies on divergence ($K$) were compared with the predictions for steady detonations with lateral flo…
In this fluid dynamic video we present simulations of converging two-dimensional detonation in a radially expanding supersonic flow of ideal reactive gas. The detonation is found to be unstable and leads to formation of characteristic cellular patterns. Without any obstacles in the flow, the detonation keeps expanding radially. To retain the wave within a bounded region, we place a number of rigid obstacles in the flow so that the detonation shock stands some distance toward the center from the obstacles. This leads to generation of reflected shock waves from the obstacles which help the deton…
Detonation of a three-dimensional reactive non-isotropic molecular crystal is modeled using molecular dynamics simulations. The detonation process is initiated by an impulse, followed by the creation of a stable fast reactive shock wave. The terminal shock velocity is independent of the initiation conditions. Further analysis shows supersonic propagation decoupled from the dynamics of the decomposed material left behind the shock front. The dependence of the shock velocity on crystal nonlinear compressibility resembles solitary behavior. These properties categorize the phenomena as a weak deto…
Shock induced ignition and transition to detonation in the presence of mechanically induced non-linear acoustic forcing
该研究建立拉格朗日坐标解析模型，结合高保真数值模拟，揭示了振荡活塞机械强迫下反应介质中两种全新的激波诱导点火热点级联机制，阐明了不同强迫频率下的点火与爆燃转爆轰调控规律。
Weak Detonations Revisited: Uncovering Its General Nature Using Autoignitive Reaction Wave Concept
研究证明超音速自燃反应波与经典Rayleigh流数学等价，从而自然实现稳定的弱爆轰，并提出了无需特殊化学反应的普适实现条件，将其转化为实际可应用现象。
This study uses a simplified detonation model to investigate the behaviour of detonations with galloping-like pulsations. The reactive Burgers equation is used for the hydrodynamic equation, coupled to a pulsed source whereby all the shocked reactants are simultaneously consumed at fixed time intervals. The model mimics the short periodic amplifications of the shock front followed by relatively lengthy decays seen in galloping detonations. Numerical simulations reveal a saw tooth evolution of the front velocity with a period-averaged detonation speed equal to the Chapman-Jouguet velocity. The …
We introduce a new method to investigate linear stability of gaseous detonations that is based on an accurate shock-fitting numerical integration of the linearized reactive Euler equations with a subsequent analysis of the computed solution via the dynamic mode decomposition. The method is applied to the detonation models based on both the standard one-step Arrhenius kinetics and two-step exothermic-endothermic reaction kinetics. Stability spectra for all cases are computed and analyzed. The new approach is shown to be a viable alternative to the traditional normal-mode analysis used in detona…
The propagation of detonation waves in reactive media bounded by an inert, compressible layer is examined via computational simulations in two different geometries, axisymmetric cylinders and two dimensional, planar slabs. For simplicity, an ideal gas equation of state is used with a pressure-dependent reaction rate that results in a stable detonation wave structure. The detonation is initiated as an ideal Chapman-Jouguet (CJ) detonation with a one-dimensional structure, and then allowed to propagate into a finite diameter or thickness layer of explosive surrounded by an inert layer. The yield…
Detonation propagation in weakly confined gases
研究弱约束气体中爆轰在分层结构中的传播，通过CFD模拟分析声学阻抗比和层厚对爆轰行为的影响，提出解析模型解释不同驱动状态下的爆轰特性。
One of the most important and difficult parts of constructing a multidimensional numerical simulation of flame acceleration and deflagration-to-detonation transition (DDT) in a reacting flow is finding a reliable and affordable model of the chemical and diffusive properties. For simulations of realistic scenarios, full detailed chemical models are computationally prohibitive. In addition, they are usually inaccurate for high-temperature and high-pressure shock-laden flows. This paper presents a general approach for developing an automated procedure to determine the reaction parameters for a si…
Detonation propagation in three-dimensional continuous curved ducts
本文通过三维反应欧拉方程数值模拟，研究了连续弯曲管道中爆轰波的传播，揭示了曲率、截面尺寸和活化能对波结构、模式转变及速度亏损的影响。
We extend the reactive Burgers equation presented in Kasimov et al. Phys. Rev. Lett., 110 (2013) and Faria et al. SIAM J. Appl. Maths, 74 (2014), to include multidimensional effects. Furthermore, we explain how the model can be rationally justified following the ideas of the asymptotic theory developed in Faria et al. JFM (2015). The proposed model is a forced version of the unsteady small disturbance transonic flow equations. We show that for physically reasonable choices of forcing functions, traveling wave solutions akin to detonation waves exist. It is demonstrated that multidimensional ef…
Simulations of a supersonic, premixed, reacting flow over a wedge were performed to investigate the effect of a boundary layer on the wedge surface on ignition and stability of oblique detonation waves (ODWs). Two computational domains were used: one containing a wedge of a single angle with a straight after-body, and the other containing a double-angle wedge geometry. Both domains were channels with a supersonic inflow of stoichiometric hydrogen-air and a nonreflecting outflow, and the wedge was modeled using an immersed boundary method. The compressible reactive Navier-Stokes equations were …
General concept for autoignitive reaction wave covering from subsonic to supersonic regimes
本研究提出“自点火反应波”概念，通过理论与数值模拟证明其在亚声速至超声速宽泛入口速度范围内均可维持稳态解，并发现一种无激波与胞结构的高稳定性超声速波，为新型燃烧器设计提供可能。
Toward Data-Driven Subgrid-Scale Modeling of the Zel'dovich Deflagration-To-Detonation Mechanism in Dense Stellar Plasmas
本研究针对热核超新星爆炸中的爆燃转爆轰机制，构建了基于直接数值模拟数据库的热点神经网络分类器，并将其集成入FLASH/Proteus代码作为亚格子模型，验证了其在真实条件下的准确性。
Space-time adaptive ADER-DG finite element method with LST-DG predictor and a posteriori sub-cell ADER-WENO finite-volume limiting for multidimensional detonation waves simulation
本文采用时空自适应ADER-DG-PN方法结合后验子单元限制器，通过局部时间步划分处理强反应性，实现了多维爆轰波的高效高精度模拟，正确再现了ZND结构并避免了非物理伪影。
The direct monitoring of a rotating detonation engine (RDE) combustion chamber has enabled the observation of combustion front dynamics that are composed of a number of co- and/or counter-rotating coherent traveling shock waves whose nonlinear mode-locking behavior exhibit bifurcations and instabilities which are not well understood. Computational fluid dynamics simulations are ubiquitous in characterizing the dynamics of RDE's reactive, compressible flow. Such simulations are prohibitively expensive when considering multiple engine geometries, different operating conditions, and the long-time…
The problem of shock induced ignition by a piston is addressed in the framework of Fickett's model for reactive compressible flows, i.e., the reactive form of Burgers' equation. An induction-reaction two-step chain-branching model is used to study the coupling between the energy release and the compressible hydrodynamics occurring during the shock ignition transient leading to a detonation. Owing to the model's simplicity, the ignition and acceleration mechanism is explained using the two families of characteristics admitted by the model. The energy release along the particle paths provides th…
We study the violent phase of the merger of massive binary white dwarf systems. Our aim is to characterize the conditions for explosive burning to occur, and identify a possible explosion mechanism of Type Ia supernovae. The primary components of our model systems are carbon-oxygen (C/O) white dwarfs, while the secondaries are made either of C/O or of pure helium. We account for tidal effects in the initial conditions in a self-consistent way, and consider initially well-separated systems with slow inspiral rates. We study the merger evolution using an adaptive mesh refinement, reactive, Euler…
Space-time adaptive ADER-DG finite element method with LST-DG predictor and a posteriori sub-cell WENO finite-volume limiting for simulation of non-stationary compressible multicomponent reactive flows
本研究提出带有限体积ADER-WENO限制器子单元后验校正的时空自适应ADER-DG有限元方法，通过拓展欧拉方程组描述多组分反应，采用自适应时间步长，可无需方向分裂与分数步方法模拟非定常可压缩多组分反应流，已验证ZND爆轰波经典问题。
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