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This paper presents a feature-based adaptive mesh refinement (AMR) method for Large Eddy Simulation of propagating deflagrations, using massive-scale parallel unstructured AMR libraries. The proposed method, named turbulent flame propagation-AMR (TFP-AMR), is able to track the transient dynamics of both the turbulent flame and the vortical structures in the flow. To handle the interaction of the turbulent flame brush with the vortical structures of the flow, a vortex selection criterion is derived from flame/vortex interaction theory. The method is built with the general intent to prioritise conservatively estimated parameters, rather than to rely on user-dependent parameters. In particular, a specific mesh adaptation triggering strategy is constructed, adapted to the strongly transient physics found in deflagrations, to guarantee that the physics of interest consistently reside within a region of high accuracy throughout the transient process. The methodology is applied and validated on several elementary cases representing fundamental bricks of the full problem: (1) a laminar flame propagation, (2) the advection of a pair of non-reacting vortices, (3) a flame/vortex interaction. The method is then applied to three different configurations of a three-dimensional complex explosion scenario in an obstructed chamber. All cases demonstrate the TFP-AMR capability to recover accurate results at reduced computational cost without requiring any ad hoc tuning of the AMR method or its parameters, thus demonstrating its genericity and robustness. [ABSTRACT FROM AUTHOR]

Exothermic autocatalytic reaction fronts propagating between two conductive walls shows an increase of speed and change in shape due to buoyancy driven convection. We modeled the system using reaction-diffusion-advection equations for chemical concentration and temperature. In these equations, a cubic autocatalytic exothermic reaction leads to a propagating front. The fluid flow is determined by the Stokes equation allowing for density changes due to thermal expansion. The front propagates in a liquid confined in a narrow rectangular domain resembling a two dimensional tube. Fluid motion enhances the front speed and modifies its curvature. In vertical domains, a transition to a nonaxisymmetric front takes place as the width increases. Heat conductivity across the walls delays the transition to larger critical widths. We find regions of bistability between nonaxisymmetric fronts and lower speed fronts at different values of conductivity. Heat losses diminish convection in horizontal tubes, resulting in a decrease of front speed. [ABSTRACT FROM AUTHOR]

This article introduces a novel, fully data-driven method for forming reduced order models (ROMs) in complex flow databases that consist of a large number of variables. The algorithm utilizes higher order dynamic mode decomposition (HODMD), a modal decomposition method, to identify the main frequencies and associated patterns that govern the flow dynamics. By incorporating various normalization techniques into an iterative process, clusters of variables with similar dynamics are identified, allowing the classification of different instabilities and patterns present in the flow. This method, known as hierarchical HODMD (h-HODMD), has been thoroughly tested in the development of ROMs using three different databases obtained from numerical simulations of a nonpremixed coflow methane flame. The effectiveness of h-HODMD has been demonstrated as it consistently outperforms HODMD in terms of modeling and reconstructing flow dynamics using a reduced number of modes. Additionally, the clusters of variables identified by h-HODMD reveal the algorithm's ability to group chemical species whose behavior is consistent from a kinetic perspective. h-HODMD allows for the construction of inexpensive reduced dynamical models that can predict flame liftoff, identify the occurrence of local extinction and blowout conditions, and facilitate control purposes. [ABSTRACT FROM AUTHOR]

In this study, shock stand-off distances for thermally and chemically non-equilibrium flows of nitrogen over wedges are computationally investigated via a hypersonic computational fluid dynamics solver, hyperReactingFoam by spanning a parameter space that consists of ranges of Mach number, 4–10, specific heat ratio, 1.40–1.61 and wedge angles, 60 ∘ –90 ∘ . Then, the space is reduced into the parameters of inverse density ratio across the shock and dimensionless wedge angle which will be used as variables for quadratic functions that represent shock stand-off distances. Besides the functions of shock stand-off distances, detached shock profiles of computationally modeled flows are represented by parabolic equations. The flows are observed to be chemically frozen for Mach number ranges of 4–5 regardless of the specific heat ratio value of the nitrogen mixture. Our results show that the shock stand-off distance decreases as Mach number is increased from 4 to 7, if the wedge angle and free-stream specific heat ratio are kept the same. On the other hand, if Mach number is increased beyond 7, the shock stand-off distance starts to extend due to the dissociation of nitrogen molecules behind the shock wave. At Mach 10, nitrogen completely dissociates over 90 ∘ wedge for all specific heat ratios considered in the present study. Increased leading edge angle of the wedge or specific heat ratio of free-stream yields longer shock stand-off distance. [ABSTRACT FROM AUTHOR]

A data-based reduced-order model (ROM) is developed to accelerate the time integration of stiff chemically reacting systems by effectively removing the stiffness arising from a wide spectrum of chemical time scales. Specifically, the objective of this work is to develop a ROM that acts as a non-stiff surrogate model for the time evolution of the thermochemical state vector (temperature and species mass fractions) during an otherwise highly stiff and nonlinear ignition process. The model follows an encode-forecast-decode strategy that combines a nonlinear autoencoder (AE) for dimensionality reduction (encode and decode steps) with a neural ordinary differential equation (NODE) for modeling the dynamical system in the AE-provided latent space (forecasting step). By means of detailed timescale analysis by leveraging the dynamical system Jacobians, this work shows how data-based projection operators provided by autoencoders can inherently construct the latent spaces by removing unnecessary fast timescales, even more effectively than physics-based counterparts based on an eigenvalue analysis. A key finding is that the most significant degree of stiffness reduction is achieved through an end-to-end training strategy, where both AE and neural ODE parameters are optimized simultaneously, allowing the discovered latent space to be dynamics-informed. In addition to end-to-end training, this work highlights the vital contribution of AE nonlinearity in the stiffness reduction task. For the prediction of homogeneous ignition phenomena for H2-air and C2H4-air mixtures, the proposed ROM achieves several orders-of-magnitude increase in the integration time step size when compared to (a) a baseline CVODE solver for the full-chemical system, (b) statistical technique – principal component analysis (PCA), and (c) computational singular perturbation (CSP), a vetted physics-based stiffness-reducing modeling framework.

A new definition of entropy fluctuation is provided in this paper, that allows properly separating entropy and mixture composition fluctuations. This decomposition into linearly independent variables prevents from overestimating compositional noise in indirect noise prediction. When considering quasi one-dimensional flow in nozzles, a new resulting system of linearized Euler equations is obtained. Two analytical solutions of this system of equations are investigated in this study, one dedicated to low-frequency perturbations and another one for all frequency perturbations. This new theory is then validated by comparing the model predictions with direct numerical simulation of nozzle flows in which composition fluctuations are pulsed. To do so, new Navier–Stokes characteristics boundary conditions to account for the new properly defined entropy wave are provided. Furthermore, non-reflecting inlet boundary conditions have been newly derived, relaxing on the axial mass-flow rate, static temperature and species mass fractions. Finally, a parametric study based on an air-kerosene (equivalent C 10 H 20 ) mixture and an ideal framework shows that composition noise can reach a maximum of 10% of entropy noise for lean combustion and choked nozzle while for rich combustion, composition noise and entropy noise show comparable levels. [Display omitted] • New entropy noise decomposition that prevents overestimation of the compositional part of indirect combustion noise. • New system of linearized equations that characterize indirect combustion noise. • New non-reflecting Navier–Stokes boundary conditions for pulsing species mass factions fluctuations. [ABSTRACT FROM AUTHOR]

The interplay between chemical reaction and substrate deformation is discussed by adapting Ranz's formulation for scalar mixing to the case of a reactive mixture between segregated reactants, initially separated by an interface whose thickness may not be vanishingly small. Experiments in a simple shear flow demonstrate the existence of three regimes depending on the Damköhler number Da = ts/tc where ts is the mixing time of the interface width and tc is the chemical time. Instead of treating explicitly the chemical cross-term, we rationalize these different regimes by globalizing it as a production term involving a flux which depends on the rate at which the reaction zone is fed by the reactants, a formulation valid for Da > 1. For Da < 1, the reactants interpenetrate before they react, giving rise to a 'diffusio-chemical' regime where chemical production occurs within a substrate whose width is controlled by molecular diffusion. [ABSTRACT FROM AUTHOR]

One-dimensional turbulence (ODT) is an accurate and computationally efficient model for simulating turbulent flows. ODT has been applied to a wide range of flow problems including reaction, multiphase, differential diffusion, heat release, buoyancy, and wall flows. Applications include use as a standalone model and as a closure for large-eddy simulation (LES). Its strength lies in the ability to capture a full range of turbulent length and time scales. The ODT model is strongly coupled with its implementation, complicating its formulations. We present a modern, open-source, object-oriented C++ implementation of ODT. The code described here and made available online can be used as a starting point to understand, apply, and extend the ODT model, enabling its further application to turbulent flow research.

Recent advances and findings reported in the literature show that the lattice Boltzmann method can be a viable and rather efficient alternative to classical numerical methods in modeling multi-species flows. Based on the kinetic theory of multicomponent gases, multi-fluid approaches are derived. Each species evolves according to the specific properties, a proper coupling must be introduced for modeling the diffusivity. In recent years, a discrete kinetic scheme for multi-component flows has been proposed which was able to solve the Maxwell-Stefan system of equations for any number of species. However, reacting flows lead to additional challenges and have seldom been studied by lattice Boltzmann method. The aim of the present work is to implement this model in the lattice Boltzmann solver, extend it to take account multiple chemical reactions. The temperature is modeled through separate distribution function and the flow distribution function is assumed to be independent of temperature. The performance has been checked for a binary diffusion flow and a counter-current propane/air reacting flow. The obtained results show that this model able to deal with multispecies flows and solves the multi-component system of equations. [ABSTRACT FROM AUTHOR]

We consider the numerical solution of competitive exothermic and endothermic reactions in the presence of a chaotic advection flow. The resulting behaviour is characterized by a strong dependence on the competitive reaction history. The burnt temperature is not immediately connected to simple enthalpy calculations, so there is a subtlety in the interplay between the major parameters, notably the Damköhler number, the ratio of the heats of exothermic and endothermic reactions, as well as the ratio of their respective activation energies. This paper seeks to explore the way these parameters affect the steady states of these reaction fronts and their stability. [ABSTRACT FROM AUTHOR]

• A numerical model for simulation of sub-critical cryogenic jets is constructed. • The proposed model is able to solve multi-physics processes involving moving interface, phase transition and combustion. • Hybrid thermodynamic closure is proposed for the mixture of liquid and reactive species under the sub-critical condition. • A low dissipation Riemann solver and a homogenous reconstruction strategy are adopted. In order to simulate cryogenic H 2 − O 2 jets under subcritical condition, a numerical model is constructed to solve compressible reactive multi-component flows which involve complex multi-physics processes such as moving material interfaces, shock waves, phase transition and combustion. The liquid and reactive gaseous mixture are described by a homogeneous mixture model with diffusion transport for heat, momentum and species. A hybrid thermodynamic closure strategy is proposed to construct an equation of state (EOS) for the mixture. The phase transition process is modeled by a recent fast relaxation method which gradually reaches the thermo-chemical equilibrium without iterative process. A simplified transport model is also implemented to ensure the accurate behavior in the limit of pure fluids and maintain computational efficiency. Last, a 12-step chemistry model is included to account for hydrogen combustion. Then the developed numerical model is solved with the finite volume method where a low dissipation AUSM (advection upstream splitting method) Riemann solver is extended for multi-component flows. A homogeneous reconstruction strategy compatible with the homogeneous mixture model is adopted to prevent numerical oscillations across material interfaces. Having included these elements, the model is validated on a number of canonical configurations, first for multiphase flows, and second for reactive flows. These tests allow recovery of the expected behavior in both the multiphase and reactive limits, and the model capability is further demonstrated on a 2D burning cryogenic H 2 − O 2 jet, in a configuration reminiscent of rocket engine ignition. [ABSTRACT FROM AUTHOR]

The present article deals with the spatiotemporal reduction of a reacting, supersonic, turbulent jet. A flowfield dataset is first obtained from a LES simulation including chemical reactions. The spatial reduction is accomplished by performing successively a Fourier transform in the azimuthal direction and a proper orthogonal decomposition (POD), while the temporal reduction is obtained through a selection of Fourier modes in the leading temporal POD modes (chronos). A prior low-pass temporal filter has been used to eliminate a significant portion of high-frequency content, the resulting reduced-order model (ROM) focusing only on the low-frequency, large-scale structures of the jet. The leading axisymmetric ( m = 0 ) POD mode describes a very low-frequency pulsation of the shock cells. The second and third axisymmetric POD modes are paired and describe a wave packet amplified along the potential core and with a rapid decay downstream. The two leading helical ( m = 1 ) POD modes, which are complex modes, appear to be mixed and together describe convecting wave packets of a longer wavelength and a lower frequency. They can be associated to the instability of the shear layer downstream of the potential core. Strong global cross-correlations are observed between the mass fractions and the axial velocity and temperature fields, which lead to similar energy decay rates and POD modes when they are included in the POD. The temporal reduction of the leading sets of chronos via an energy-based selection of Fourier modes has been shown to complement efficiently the spatial reduction, providing a complete spatiotemporal ROM of the flowfield. [ABSTRACT FROM AUTHOR]

Direct Numerical Simulations (DNS)s of a low Karlovitz number turbulent premixed round jet flame are performed to determine the impact of the turbulence inflow conditions on the flame characteristics. This is accomplished by providing the first flame with a fully developed turbulent inflow via simulation of the upstream pipe and the coflow region, while the inflow boundary condition in the second flame is generated with a digital filter-based method without including the upstream region. The two databases are then thoroughly compared to reveal any differences resulting from the different inflow boundary conditions. The turbulence kinetic energy is observed to drop shortly downstream of the synthetic inflow, which results in a higher jet momentum and a longer flame compared to the flame with the fully developed turbulent inflow. The distributions of the flame front orientation in the near field are observed to differ between the two flames but differences decrease quickly downstream as distributions tend towards isotropy. The flame structure is largely affected by tangential straining close to the inflow while further downstream, the flame structure is similar to that in an unstrained laminar flame. While the difference between the inflow boundaries had a significant effect on global parameters such as flame length, the influence of the inflow conditions on local statistics related to turbulence-flame interaction appear to be limited to the near inflow region. [ABSTRACT FROM AUTHOR]

A Lattice-Boltzmann model for low-Mach reactive flows is presented, built upon our recently published model (Comb & Flame, 196, 2018). The approach is hybrid and couples a Lattice-Boltzmann solver for the resolution of mass and momentum conservation and a finite difference solver for the energy and species conservation. Having lifted the constant thermodynamic and transport properties assumptions, the model presented now fully accounts for the classical reactive flow thermodynamic closure: each component is assigned NASA coefficients for calculating its thermodynamic properties. A temperature-dependent viscosity is considered, from which are deduced thermo-diffusive properties via specification of Prandtl and component-specific Schmidt numbers. Another major improvement from our previous contribution is the derivation of an advanced collision kernel compatible of multi-component reactive flows stable in high shear flows. Validation is carried out first on premixed configurations, through simulation of the planar freely propagating flame, the growth of the associated Darrieus-Landau instability and three regimes of flame-vortex interaction. A double shear layer test case including a flow-stabilized diffusion flame is then presented and results are compared with DNS simulations, showing excellent agreement. [ABSTRACT FROM AUTHOR]

Given the complex geometries usually found in practical applications, the Lattice Boltzmann (LB) method is becoming increasingly attractive for flow simulations. In addition to the simple treatment of intricate geometrical configurations, LB solvers can be implemented on very large parallel clusters with excellent scalability. However, reacting flows lead to additional challenges and have seldom been studied by LB methods. In this study, an in-house low Mach number Lattice Boltzmann solver, ALBORZ, has been extended to take into account multiple chemical components and reactions. For this purpose, the temperature and species of each mass fraction field are modeled through separate distribution functions. The flow distribution function is assumed to be independent of temperature and species mass fractions, which is valid in the limit of weak density variations. In order to compute reaction terms as well as variable thermodynamic and transport properties, the LB code has been coupled to another library of our group, REGATH. In this manner, LB simulations with detailed chemical kinetics and thermo-chemical models become possible. Since the code is currently limited to weak density variation, its performance has been checked for a laminar premixed as well as non-premixed counter-flow Ozone/Air reacting flow, describing kinetics with 4 species and 18 elementary reactions. Comparisons of the obtained reacting flow structures with results from classical finite-difference simulations show excellent agreement. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a model-free algorithm, robust mode analysis (RMA), to extract primary constituents in a fluid or reacting flow directly from high-frequency, high-resolution experimental data. It is expected to be particularly useful in studying strongly driven flows, where nonlinearities can induce chaotic and irregular dynamics. The lack of precise governing equations and the absence of symmetries or other simplifying constraints in realistic configurations preclude the derivation of analytical solutions for these systems; the presence of flow structures over a wide range of scales handicaps finding their numerical solutions. Thus, the need for direct analysis of experimental data is reinforced. RMA is predicated on the assumption that primary flow constituents are common in multiple, nominally identical realizations of an experiment. Their search relies on the identification of common dynamic modes in the experiments, the commonality established via proximity of the eigenvalues and eigenfunctions. Robust flow constituents are then constructed by combining common dynamic modes that flow at the same rate. We illustrate RMA using reacting flows behind a symmetric bluff body. Two robust constituents, whose signatures resemble symmetric and von Karman vortex shedding, are identified. It is shown how RMA can be implemented via extended dynamic mode decomposition in flow configurations interrogated with a small number of time-series. This approach may prove useful in analyzing changes in flow patterns in engines and propulsion systems equipped with sturdy arrays of pressure transducers or thermocouples. Finally, an analysis of high Reynolds number jet flows suggests that tests of statistical characterizations in turbulent flows may best be done using non-robust components of the flow.

• The evolution of gas-dynamic flow affects the ignition process in the shock tube. • The non-steady flow dynamics determines the origins of ignition kernels. • The reaction wave is forming at the background of non-uniform flow field. • The detonation arises as a result of transient behavior of reaction wave. The paper analyses in details and describes the process of ignition kernel formation and subsequent detonation onset behind the shock wave propagating in the shock tube. To get the overall pattern of the process a series of one- and two-dimensional calculations are carried out with the use of a dissipation-free numerical technique. It is shown that one of the leading roles in the process of ignition kernel formation belongs to the non-steady flow dynamics establishing behind the shock wave. The development of the boundary layer determines both the temperature re-distribution in the near-wall region and the conditions for gas-dynamic acceleration of the flow. With an account of thermal runaway, the most intensively heated region corresponds to the area between the inner margin of the boundary layer and the contact surface separating driver gas and the test mixture. After localized ignition takes place the forming reaction wave propagates out from the ignition epicenter. Reaction wave propagates behind the outrunning compression wave through the already reacting mixture. Shock-induced compression of the test mixture provides conditions for the self-sustained acceleration of the combustion wave, and finally, the detonation onset takes place. [ABSTRACT FROM AUTHOR]

We investigate the behaviour of a reactive plume in the two limiting cases of slow and instantaneous chemical reactions. New laboratory measurements show that, whereas the slow reaction between the source and entrained chemical species takes place within the whole volume of each eddy in the plume, the fast reaction develops preferentially at the periphery of the eddies. We develop a new model that quantifies the mixing of the reactive buoyant fluids at the Batchelor scale and thereby the progress of the fast reaction. We present a series of new experimental results that suggest that a critical distance from the source, $z_{crit}$ , exists at which the volume of fluid that is entrained from the ambient is equal to that which is mixed within the plume at the Batchelor scale. For $z>z_{crit}$ , only a fraction of the entrained fluid is rapidly mixed and reacts with the plume fluid. The results of the new experiments enable us to quantify the distance from the source at which an instantaneous reaction reaches completion, and show that it can be significantly larger than the distance $L_{s}$ at which the stoichiometric dilution of the plume fluid is achieved. In the limit of an instantaneous reaction, the longitudinal profiles of source chemical concentration in the plume depend on $(z_{crit}/L_{s})^{5/6}$. The predictions of the model are validated against the experimental results, and the profiles of source chemical concentration in the plume for slow and fast reactions are compared. [ABSTRACT FROM AUTHOR]

A linear numerical study is conducted to quantify the effect of swirl on the response behaviour of premixed lean flames to general harmonic excitation in the inlet, upstream of combustion. This study considers axisymmetric M-flames and is based on the linearised compressible Navier-Stokes equations augmented by a simple one-step irreversible chemical reaction. Optimal frequency response gains for both axisymmetric and non-axisymmetric perturbations are computed via a direct-adjoint methodology and singular value decompositions. The high-dimensional parameter space, containing perturbation and base-flow parameters, is explored by taking advantage of generic sensitivity information gained from the adjoint solutions. This information is then tailored to specific parametric sensitivities by first-order perturbation expansions of the singular triplets about the respective parameters. Valuable flow information, at a negligible computational cost, is gained by simple weighted scalar products between direct and adjoint solutions. We find that for non-swirling flows, a mode with azimuthal wavenumber mD2 is the most efficiently driven structure. The structural mechanism underlying the optimal gains is shown to be the Orr mechanism for m=0 and a blend of Orr and other mechanisms, such as lift-up, for other azimuthal wavenumbers. Further to this, velocity and pressure perturbations are shown to make up the optimal input and output showing that the thermoacoustic mechanism is crucial in large energy amplifications. For m=0 these velocity perturbations are mainly longitudinal, but for higher wavenumbers azimuthal velocity fluctuations become prominent, especially in the non-swirling case. Sensitivity analyses are carried out with respect to the Mach number, Reynolds number and swirl number, and the accuracy of parametric gradients of the frequency response curve is assessed. The sensitivity analysis reveals that increases in Reynolds and Mach numbers yield higher gains, through a decrease in temperature diffusion. A rise in mean-flow swirl is shown to diminish the gain, with increased damping for higher azimuthal wavenumbers. This leads to a reordering of the most effectively amplified mode, with the axisymmetric (m=0) mode becoming the dominant structure at moderate swirl numbers. [ABSTRACT FROM AUTHOR]

• Heterogeneous catalysis plays important roles in hypersonic aerothermodynamics. • Gas-interface-solid coupling with heterogeneous catalysis modeled by interface balance was established. • Chemically reacting boundary layer features material-catalysis/gaseous-reactions codominance. • Upstream inert catalycity enhances downstream catalytic heating through directional transport of atoms. • Thermal protection coating layout can manage heat distribution using catalytic exothermicity. Heterogeneous catalysis plays an important role in causing the aerodynamic heating surge in the hypersonic nonequilibrium flow/thermal protection material interactions for high-speed aircrafts. To model the flow/material interaction problem, a gas-interface-solid coupling method for heterogeneous catalysis was established by introducing interface balance relation into the coupling framework of nonequilibrium Navier-Stokes equations and thermal conduction equation. The influence of coating heterogeneous catalysis on heat transfer into the heat shield of a blunt body was then numerically investigated to reveal the catalysis-induced aerothermodynamics. The coupling simulation results reveal that the codominant role of material mediated catalysis and gaseous reactions on boundary layer and their induced near-wall evolutions are the main features of the chemically reacting boundary layer that differ from the conventional boundary layer theory. It was found that catalytically inert coatings can weaken the local aerodynamic heating due to the low activity of local catalysis and enhance the downstream catalytic heating through driving atomic species to the downstream region. Appropriate layouts of thermal protection coatings can make full use of the nature of catalytic exothermicity to effectively manage hypersonic flow and catalysis-induced heat-transfer characteristics for the lightweight and low-redundancy thermal protection designs of future spacecraft. [ABSTRACT FROM AUTHOR]

An adaptive projection-based reduced-order model (ROM) formulation is presented for model-order reduction of problems featuring chaotic and convection-dominant physics. An efficient method is formulated to adapt the basis at every time-step of the on-line execution to account for the unresolved dynamics. The adaptive ROM is formulated in a Least-Squares setting using a variable transformation to promote stability and robustness. An efficient strategy is developed to incorporate non-local information in the basis adaptation, significantly enhancing the predictive capabilities of the resulting ROMs. A detailed analysis of the computational complexity is presented, and validated. The adaptive ROM formulation is shown to require negligible offline training and naturally enables both future-state and parametric predictions. The formulation is evaluated on representative reacting flow benchmark problems, demonstrating that the ROMs are capable of providing efficient and accurate predictions including those involving significant changes in dynamics due to parametric variations, and transient phenomena. A key contribution of this work is the development and demonstration of a comprehensive ROM formulation that targets predictive capability in chaotic, multi-scale, and transport-dominated problems. • An adaptive reduced-order model formulation is derived to enable true predictions of chaotic and convection-dominant physics. • A compact method is formulated to update the basis to account for unresolved scales at every time step. • A full state estimation strategy is developed to incorporate non-local information in ROM adaptation. • The adaptive procedure significantly reduces the offline training cost, making it negligible compared to the online calculations. • Detailed evaluations show significant predictive accuracy in future state and parametric prediction in chaotic flows. [ABSTRACT FROM AUTHOR]

In this study, a new set of direct numerical simulations is generated and used to examine the influence of mixture composition heterogeneities on the propagation of a premixed iso-octane/air spherical turbulent flame, with a representative chemical description. The dynamic effects of both turbulence and combustion heterogeneities are considered, and their competition is assessed. The results of the turbulent homogeneous case are compared with those of heterogeneous cases which are characterized by multiple stratification length scales and segregation rates in the regime of a wrinkled flame. The comparison reveals that stratification does not alter turbulent flame behaviors such as the preferential alignment of the convex flame front with the direction of the compression. However, we find that the overall flame front propagation is slower in the presence of heterogeneities because of the differential on speed propagation. Furthermore, analysis of different displacement speed components is performed by taking multi-species formalism into account. This analysis shows that the global flame propagation front slows down due to the heterogeneities caused by the reaction mechanism and the differential diffusion accompanied by flame surface density variations. Quantification of the effects of each of these mechanisms shows that their intensity increases with the increase in stratification’s length scale and segregation rate.

Abstract A new Lattice-Boltzmann model for low-Mach reactive flows is presented. Based on standard lattices, the model is easy to implement, and is the first, to the authors’ knowledge, to pass the classical freely propagating flame test case as well as the counterflow diffusion flame, with strains up to extinction. For this presentation, simplified transport properties are considered, each species being assigned a separate Lewis number. In addition, the gas mixture is assumed to be calorically perfect. Comparisons with reference solutions show excellent agreement for mass fraction profiles, flame speed in premixed mixtures, as well as maximum temperature dependence with strain rate in counterflow diffusion flames. [ABSTRACT FROM AUTHOR]

In this paper we study the essential role of Darrieus–Landau (DL), hydrodynamic, cellular flame-front instability in the propagation of expanding turbulent flames. First, we analyse and compare the characteristic time scales of flame wrinkling under the simultaneous actions of DL instability and turbulent eddies, based on which three turbulent flame propagation regimes are identified, namely, instability dominated, instability–turbulence interaction and turbulence dominated regimes. We then perform experiments over an extensive range of conditions, including high pressures, to promote and manipulate the DL instability. The results clearly demonstrate the increase in the acceleration exponent of the turbulent flame propagation as these three regimes are traversed from the weakest to the strongest, which are respectively similar to those of the laminar cellularly unstable flame and the turbulent flame without flame-front instability, and thus validating the scaling analysis. Finally, based on the scaling analysis and the experimental results, we propose a modification of the conventional turbulent flame regime diagram to account for the effects of DL instability. [ABSTRACT FROM AUTHOR]

The present work focuses on the flame-wall interaction (FWI) based on direct numerical simulations (DNS) of a head-on premixed flame quenching configuration at the statistically stationary state. The effects of FWI on the turbulent flame temperature, wall heat flux, flame dynamics and flow structures were investigated. In turbulent head-on quenching, particularly for high turbulence intensity, the distorted flames generally consist of the head-on flame part and the entrained flame part. The flame properties are jointly influenced by turbulence, heat generation from chemical reactions and heat loss to the cold wall boundary. For the present FWI configuration, as the wall is approached, the 'influence zone' can be identified as the region within which the flame temperature, scalar gradient and flame dilatation start to decrease, whereas the wall heat flux tends to increase. As the distance to the wall drops below the flame-quenching distance, approximately where the wall heat flux reaches its maximum value, chemical reactions become negligibly weak inside the 'quenching zone'. A simplified counter-flow model is also proposed. With the reasonably proposed relation between the flame speed and the flame temperature, the model solutions match well with the DNS results, both qualitatively and quantitatively. Moreover, near-wall statistics of some important flame properties, including the flame dilatation, reaction progress variable gradient, tangential strain rate and curvature were analysed in detail under different wall boundary conditions. [ABSTRACT FROM AUTHOR]

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. The experiments were conducted in 2C2H2 + 5O2 + 21Ar and C3H8 + 5O2. Numerical simulations were also performed for weakly unstable cellular detonations to test the validity of the exponential horn geometry. The experiments and simulations demonstrated that such quasi-steady state detonations can be realized, hence permitting us 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 flow divergence obtained with the real thermo-chemical data of the mixtures. For the 2C2H2 + 5O2 + 21Ar system, reasonable agreement was found between the experiments and steady wave prediction, particularly for the critical divergence leading to failure. Observations of the reaction zone structure in these detonations indicated that all the gas reacted very close to the front, as the transverse waves were reactive. The experiments obtained in the much more unstable detonations in C3H8 + 5O2 showed significant differences between the experimentally derived D(K) curve and the prediction of steady wave propagation. The latter was found to significantly under-predict the detonability of cellular detonations. The transverse waves in this mixture were found to be non-reactive, hence permitting the shedding of non-reacted pockets, which burn via turbulent flames on their surface. It is believed that the large differences between experiment and the inviscid model in this class of cellular structures is due to the importance of diffusive processes in the burn-out of the non-reacted pockets. The empirical tuning of a global one-step chemical model to describe the macro-scale kinetics in cellular detonations revealed that the effective activation energy was lower by 14% in 2C2H2 + 5O2 + 21Ar and 54% in the more unstable C3H8 + 5O2 system. This confirms previous observations that diffusive processes in highly unstable detonations are responsible for reducing the thermal ignition character of the gases processed by the detonation front. [ABSTRACT FROM AUTHOR]

This paper presents 5 kHz stereo particle image velocimetry and OH planar laser induced fluorescence measurements of transversely forced swirl flames. The presence of transverse forcing on this naturally unstable flow both influences the natural instabilities, as well as amplifies disturbances that may not necessarily manifest themselves during natural oscillations. By manipulating the structure of the acoustic forcing field, both axisymmetric and helical modes are preferentially excited away from the frequency of natural instability. The paper presents a method for spatially interpolating the phase locked r-z and r-θ planar velocity and flame position data, extracting the full three-dimensional structure of the helical disturbances. These helical disturbances are also decomposed into symmetric and anti-symmetric disturbances about the jet core, showing the subsequent axial evolution (in magnitude and phase) of each of these underlying disturbances. It is shown that out-of-phase acoustic forcing excites m = ±1 modes, but the flow field preferentially amplifies the counter-winding, co-rotating helical disturbance over the co-winding, counter-rotating helical disturbance. This causes the flow and flame to transition from a transverse flapping near the jet exit to a precessing motion further downstream. In contrast, in-phase forcing promotes axisymmetric m = 0 disturbances which dominate the flow field over the entire axial domain. In both cases, the amplitudes of the anti-symmetric disturbances about the jet core grow with downstream distance before saturating and decaying, while the symmetric disturbances appear nearly negligible. It is suggested that this saturation and decay is due to linear effects (e.g. a negative spatial growth rate), rather than nonlinear interactions. [ABSTRACT FROM AUTHOR]

Given the complex geometries usually found in practical applications, the Lattice Boltzmann (LB) method is becoming increasingly attractive. In addition to the simple treatment of intricate geometrical configurations, LB solvers can be implemented on very large parallel clusters with excellent scalability. However, reacting flows and especially combustion lead to additional challenges and have seldom been studied by LB methods. Indeed, overall mass conservation is a pressing issue in modeling multi-component flows. The classical advection–diffusion LB model recovers the species transport equations with the generalized Fick approximation under the assumption of an incompressible flow. However, for flows involving multiple species with different diffusion coefficients and density fluctuations – as is the case with weakly compressible solvers like Lattice Boltzmann –, this approximation is known not to conserve overall mass. In classical CFD, as the Fick approximation does not satisfy the overall mass conservation constraint a diffusion correction velocity is usually introduced. In the present work, a local expression is first derived for this correction velocity in a LB framework. In a second step, the error due to the incompressibility assumption is also accounted for through a modified equilibrium distribution function. Theoretical analyses and simulations show that the proposed scheme performs much better than the conventional advection–diffusion Lattice Boltzmann model in terms of overall mass conservation. [ABSTRACT FROM AUTHOR]

Nine decades of rocket engine and gas turbine development have shown that thermoacoustic oscillations are difficult to predict but can usually be eliminated with relatively small ad hoc design changes. These changes can, however, be ruinously expensive to devise. This review explains why linear and nonlinear thermoacoustic behavior is so sensitive to parameters such as operating point, fuel composition, and injector geometry. It shows how nonperiodic behavior arises in experiments and simulations and discusses how fluctuations in thermoacoustic systems with turbulent reacting flow, which are usually filtered or averaged out as noise, can reveal useful information. Finally, it proposes tools to exploit this sensitivity in the future: adjoint-based sensitivity analysis to optimize passive control designs and complex systems theory to warn of impending thermoacoustic oscillations and to identify the most sensitive elements of a thermoacoustic system. [ABSTRACT FROM AUTHOR]

An efficient formulation of computational singular perturbation (CSP) is proposed for toy problems that consist of three variables. The essence of the new CSP formulation is to take advantage of short species timescales that can be analytically derived or efficiently evaluated through numerical computations. Complication induced by the presence of partial equilibrium reactions is discussed. It is shown that linear combinations of fast variables may result in slow modes. Special analytic treatments are proposed for the toy problems, and such treatments may be extended for general stiff chemistry in future study. This work demonstrates the possibility of efficient analytic or semianalytic approaches to enable on-the-fly chemical stiffness removal using CSP. In contrast, the previous CSP methods involve expensive eigen-computation and are typically expensive for large-scale combustion simulations. [ABSTRACT FROM AUTHOR]

A one-dimensional, unsteady nozzle flow is modelled to identify the sources of indirect noise in multicomponent gases. First, from non-equilibrium thermodynamics relations, it is shown that a compositional inhomogeneity advected in an accelerating flow is a source of sound induced by inhomogeneities in the mixture (i) chemical potentials and (ii) specific heat capacities. Second, it is shown that the acoustic, entropy and compositional linear perturbations evolve independently from each other and they become coupled through mean-flow gradients and/or at the boundaries. Third, the equations are cast in invariant formulation and a mathematical solution is found by asymptotic expansion of path-ordered integrals with an infinite radius of convergence. Finally, the transfer functions are calculated for a supersonic nozzle with finite spatial extent perturbed by a methane-air compositional inhomogeneity. The proposed framework will help identify and quantify the sources of sound in nozzles with relevance, for example, to aeronautical gas turbines. [ABSTRACT FROM AUTHOR]

Unstructured meshes allow easily representing complex geometries and to refine in regions of interest without adding control volumes in unnecessary regions. However, numerical schemes used on unstructured grids have to be properly defined in order to minimise numerical errors. An assessment of a low Mach algorithm for laminar and turbulent flows on unstructured meshes using collocated and staggered formulations is presented. For staggered formulations using cell-centred velocity reconstructions, the standard first-order method is shown to be inaccurate in low Mach flows on unstructured grids. A recently proposed least squares procedure for incompressible flows is extended to the low Mach regime and shown to significantly improve the behaviour of the algorithm. Regarding collocated discretisations, the odd-even pressure decoupling is handled through a kinetic energy conserving flux interpolation scheme. This approach is shown to efficiently handle variable-density flows. Besides, different face interpolations schemes for unstructured meshes are analysed. A kinetic energy-preserving scheme is applied to the momentum equations, namely, the symmetry-preserving scheme. Furthermore, a new approach to define the far-neighbouring nodes of the quadratic upstream interpolation for convective kinematics scheme is presented and analysed. The method is suitable for both structured and unstructured grids, either uniform or not. The proposed algorithm and the spatial schemes are assessed against a function reconstruction, a differentially heated cavity and a turbulent self-igniting diffusion flame. It is shown that the proposed algorithm accurately represents unsteady variable-density flows. Furthermore, the quadratic upstream interpolation for convective kinematics scheme shows close to second-order behaviour on unstructured meshes, and the symmetry-preserving is reliably used in all computations. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This work presents the analysis of a transverse combustion instability in a reducedscale rocket engine. The study is conducted on a time-resolved database of three-dimensional fields obtained via large-eddy simulation. The physical mechanisms involved in the response of the coaxial hydrogen/oxygen flames are discussed through the analysis of the Rayleigh term in the disturbance-energy equation. The interaction between acoustics and vorticity, also explicit in the disturbance-energy balance, is shown to be the main damping mechanism for this instability. The relative contributions of Rayleigh and damping terms, depending on the position of the flame with respect to the acoustic field, are discussed. The results give new insight into the phenomenology of transverse combustion instabilities. Finally, the applicability of spectral analysis on the nonlinear Rayleigh and dissipation terms is discussed. [ABSTRACT FROM AUTHOR]