Alexei Y. Poludnenko

The interaction of high-speed turbulence with flames: Global properties and internal flame structure

We study the dynamics and properties of a turbulent flame, formed in the presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type turbulence in an unconfined system. Direct numerical simulations are performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive flow code. A simplified reaction-diffusion model represents a stoichiometric H2–air mixture. The system being modeled represents turbulent combustion with the Damkohler number Da=0.05 and with the turbulent velocity at the energy injection scale 30 times larger than the laminar flame speed. The simulations show that flame interaction with high-speed turbulence forms a steadily propagating turbulent flame with a flame brush width approximately twice the energy injection scale and a speed four times the laminar flame speed. A method for reconstructing the internal flame structure is described and used to show that the turbulent flame consists of tightly folded flamelets. The reaction zone structure of these is virtually identical to that of the planar laminar flame, while the preheat zone is broadened by approximately a factor of two. Consequently, the system evolution represents turbulent combustion in the thin reaction zone regime. The turbulent cascade fails to penetrate the internal flame structure, and thus the action of small-scale turbulence is suppressed throughout most of the flame. Finally, our results suggest that for stoichiometric H2–air mixtures, any substantial flame broadening by the action of turbulence cannot be expected in all subsonic regimes.

Hydrodynamic Interaction of Strong Shocks with Inhomogeneous Media. I. Adiabatic Case

Many astrophysical flows occur in inhomogeneous (clumpy) media. We present results of a numerical study of steady, planar shocks interacting with a system of embedded cylindrical clouds. Our study uses a two-dimensional geometry. Our numerical code uses an adaptive mesh refinement, allowing us to achieve sufficiently high resolution both at the largest and the smallest scales. We neglect any radiative losses, heat conduction, and gravitational forces. Detailed analysis of the simulations shows that interaction of embedded inhomogeneities with the shock/postshock wind depends primarily on the thickness of the cloud layer and arrangement of the clouds in the layer. The total cloud mass and the total number of individual clouds is not a significant factor. We define two classes of cloud distributions: thin and thick layers. We define the critical cloud separation along the direction of the flow and perpendicular to it, distinguishing between the interacting and noninteracting regimes of cloud evolution. Finally, we discuss mass loading and mixing in such systems.

Interactions between turbulence and flames in premixed reacting flows

The interactions between turbulence and flames in premixed reacting flows are studied for a broad range of turbulence intensities by analyzing scalar (reactant mass-fraction) gradient, vorticity, and strain rate fields. The analysis is based on fully compressible, three-dimensional numerical simulations of H2-air combustion in an unconfined domain. For low turbulence intensities, a flame reconstruction method based on the scalar gradient shows that the internal flame structure is similar to that of a laminar flame, while the magnitudes of the vorticity and strain rate are suppressed by heat release and there is substantial anisotropy in the orientation of intense vortical structures. As the turbulence intensity increases, the local flame orientation becomes increasingly isotropic, and the flame preheat zone is substantially broadened. There is, however, relatively little broadening of the reaction zone, even for high intensities. At high turbulence intensities, the vorticity and strain rate are only weakly affected by the flame, and their interactions with the scalar gradient are similar to those found in nonreacting turbulence. A decomposition of the total strain rate into components due to turbulence and the flame shows that vorticity suppression depends on the relative alignment between vorticity and the flame surface normal. This effect is used to explain the anisotropy of intense vortices at low intensities. The decomposition also reveals the separate effects of turbulent and dilatational straining on the flame width.

Spontaneous transition of turbulent flames to detonations in unconfined media.

A deflagration-to-detonation transition (DDT) can occur in environments ranging from experimental and industrial systems to astrophysical thermonuclear (type Ia) supernovae explosions. Substantial progress has been made in explaining the nature of DDT in confined systems with walls, internal obstacles, or preexisting shocks. It remains unclear, however, whether DDT can occur in unconfined media. Here we use direct numerical simulations (DNS) to show that for high enough turbulent intensities unconfined, subsonic, premixed, turbulent flames are inherently unstable to DDT. The associated mechanism, based on the nonsteady evolution of flames faster than the Chapman-Jouguet deflagrations, is qualitatively different from the traditionally suggested spontaneous reaction-wave model. Critical turbulent flame speeds, predicted by this mechanism for the onset of DDT, are in agreement with DNS results.

Intermittency in premixed turbulent reacting flows

Intermittency in premixed reacting flows is studied using numerical simulations of premixed flames at a range of turbulence intensities. The flames are modeled using a simplified reaction mechanism that represents a stoichiometric H2-air mixture. Intermittency is associated with high probabilities of large fluctuations in flow quantities, and these fluctuations can have substantial effects on the evolution and structure of premixed flames. Intermittency is characterized here using probability density functions (pdfs) and moments of the local enstrophy, pseudo-dissipation rate (strain rate magnitude), and scalar (reactant mass fraction) dissipation rate. Simulations of homogeneous isotropic turbulence with a nonreacting passive scalar are also carried out in order to provide a baseline for analyzing the reacting flow results. In the reacting flow simulations, conditional analyses based on local, instantaneous values of the scalar are used to study variations in the pdfs, moments, and intermittency through ...

Shock propagation in deuterium-tritium-saturated foam

Adaptive-mesh-refinement hydrodynamic simulations have been performed of cross sections of fibrous foams saturated with cryogenic deuterium and tritium (DT). Material tracking indicates that the fibers and DT mix rapidly behind the shock. In addition, fluctuation decay lengths are on the order of a micron even in the absence of radiative and thermal energy transport. Outside the mix region, the Rankine–Hugoniot equations are satisfied to the degree to which the turbulence and transverse motion decay, a few percent or less. Simulations also show that the shock-front perturbations decay rapidly after the shock leaves the foam and enters a layer of DT ice, suggesting that the foam microstructure will not contribute to feedthrough.

Strings in the η Carinae Nebula: Hypersonic Radiative Cosmic Bullets

We present the results of a numerical study focusing on the propagation of a hypersonic bullet subject to radiative cooling. Our goal is to explore the feasibility of such a model for the formation of “strings” observed in the Carinae Homunculus nebula. Our simulations were performed in cylindrical symmetry with the adaptive mesh refinement code AstroBEAR. The radiative cooling of the system was followed using the cooling curve by Dalgarno & McCray (1972). In this letter we discuss the evolution and overall morphology of the system as well as key kinematic properties. We find that radiative bullets can produce structures with properties similar to those of the Carinae strings, i.e. high length-to-width ratios and Hubble-type flows in the form of a linear velocity increase from the base of the wake to the bullet head. These features, along with the appearance of periodic “ringlike” structures, may also make this model applicable to other astrophysical systems such as planetary nebulae, e.g. CRL 618 and NGC 6543, young stellar objects, etc. Subject headings: circumstellar matter — stars: individual ( Carinae) — stars: mass-loss — ISM: jets and outflows — planetary nebulae: individual (CRL 618)

Computation of fluid flows in non-inertial contracting, expanding, and rotating reference frames

We present the method for computation of fluid flows that are characterized by the large degree of expansion/contraction and in which the fluid velocity is dominated by the bulk component associated with the expansion/contraction and/or rotation of the flow. We consider the formulation of Euler equations of fluid dynamics in a homologously expanding/contracting and/or rotating reference frame. The frame motion is adjusted to minimize local fluid velocities. Such approach allows to accommodate very efficiently large degrees of change in the flow extent. Moreover, by excluding the contribution of the bulk flow to the total energy the method eliminates the high Mach number problem in the flows of interest. An important practical advantage of the method is that it can be easily implemented with virtually any Eulerian hydrodynamic scheme and adaptive mesh refinement (AMR) strategy. We also consider in detail equation invariance and existence of conservative formulation of equations for special classes of expanding/contracting reference frames. Special emphasis is placed on extensive numerical testing of the method for a variety of reference frame motions, which are representative of the realistic applications of the method. We study accuracy, conservativity, and convergence properties of the method both in problems which are not its optimal applications as well as in systems in which the use of this method is maximally beneficial. Such detailed investigation of the numerical solution behavior is used to define the requirements that need to be considered in devising problem-specific fluid motion feedback mechanisms.

Turbulence and Scalar Gradient Dynamics in Premixed Reacting Flows

The interaction between turbulence and premixed flames is examined for a range of turbulence intensities using results from direct numerical simulations of premixed stoichiometric hydrogen-air combustion. Particular focus is placed on the interaction of the turbulent vorticity and strain rate fields with the scalar gradient in the presence of very intense turbulence, where the scalar is the reactant mass fraction. Analysis of the scalargradient magnitude shows that, for all turbulence intensities, the flame is broadened in the preheat zone, but remains close to the laminar flame width in the reaction zone. In agreement with previous studies, broadening of the reaction zone is not observed for even the largest intensities. Consideration of the terms in the scalar gradient transport equation shows that the strain rate acts to reduce the scalar-gradient magnitude for small turbulence intensities, while it increases the magnitude for large intensities. The vorticity and strain rate magnitudes are suppressed in the reaction zone of the flame, but this suppression decreases in strength as the turbulence intensity increases. The interactions between the vorticity, strain rate, and scalar gradient are substantially influenced by heat release for small turbulence intensities. For intense turbulence, however, these interactions are similar to those observed in nonreacting turbulence. This paper uses data from a series of direct numerical simulations to examine the interactions of flames and turbulence in premixed, stoichiometric hydrogen and air combustion. 1 Particular focus is placed on the properties of the gradient of the reactant mass fraction, Y , where this scalar gradient reflects properties of the flame. The present simulations are extensions of those described in Ref. [1], and are carried out for a range of turbulence intensities, from IT ≡ Urms/SL =2 .45 to 30.6, where Urms is the rms turbulent velocity in the unburned mixture and SL is the laminar flame speed. Previous studies of the scalar gradient in premixed flames 2–6 have typically considered lower turbulence intensities; for example, Chakraborty and Swaminathan 3 examined premixed flames where IT =1 . 4a nd 7.6, and Kim and Pitsch 4 examined IT =1 3.8 and 19.5. For the range of turbulence intensities examined here, substantial differences in the interaction between the flame and turbulence are expected, resulting in changes to the properties of both the turbulence and the scalar gradient. These changes, and their connection to variations in the turbulence intensity, are the focus of the present study. In order to examine the interaction between the flames and turbulence, here we consider the coupled dynamics of the turbulent vorticity and strain rate with the dynamics of the scalar gradient. The vorticity, ω(x,t), is defined in terms of the velocity, ui(x,t), as

Evolution and Fragmentation of Wide-Angle Wind Driven Molecular Outflows

We present two dimensional cylindrically symmetric hydrodynamic simulations and synthetic emission maps of a stellar wind propagating into an infalling, rotating environment. The resulting outflow morphology, collimation and stability observed in these simulations have relevance to the study of young stellar objects, Herbig-Haro jets and molecular outflows. Our code follows hydrogen gas with molecular, atomic and ionic components tracking the associated time dependent molecular chemistry and ionization dynamics with radiative cooling appropriate for a dense molecular gas. We present tests of the code as well as new simulations which indicate the presence of instabilities in the wind-blown bubble’s swept-up shell.