Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D17: Reacting Flows: Theory and Analysis |
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Chair: Peter Hamlington, University of Colorado, Boulder Room: D131 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D17.00001: Analysis of Turbulent Scales of Motion in Premixed Flames Using Structure Functions Peter Hamlington, Samuel Whitman, Colin Towery, Alexei Poludnenko Recently, multiscale turbulence-flame interactions in premixed reacting flows have been examined using both physical space and spectral approaches. However, there remains relatively little understanding of how turbulent scales of motion vary through the internal structure of the flame itself (i.e., through premixed flamelets). Such an analysis is made difficult by the inhomogeneity, small scale, and spatial locality of many premixed flames, particularly at high Damk\"{o}hler and low Karlovitz numbers. Conditional structure functions provide a possible solution to this analysis challenge, and in this talk we present results from the calculation of structure functions using data from highly-resolved direct numerical simulations (DNS) of turbulent premixed flames. The high resolution of the DNS allows structure functions to be calculated normally and tangentially to the local flame surface, revealing the specific effects of the flame on turbulent scales of motion near the scale of the local flame width. Moreover, the conditional nature of the analysis allows the effects of different flame regions (e.g., the preheat and reaction zones) on turbulence to be isolated. The implications of these results for the theory and modeling of turbulent flame physics are outlined. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D17.00002: Wavelet multi-resolution analysis of energy transfer in turbulent premixed flames Jeonglae Kim, Maxime Bassenne, Colin Towery, Alexei Poludnenko, Peter Hamlington, Matthias Ihme, Javier Urzay Direct numerical simulations of turbulent premixed flames are examined using wavelet multi-resolution analyses (WMRA) as a diagnostics tool to evaluate the spatially localized inter-scale energy transfer in reacting flows. In non-reacting homogeneous-isotropic turbulence, the net energy transfer occurs from large to small scales on average, thus following the classical Kolmogorov energy cascade. However, in turbulent flames, our prior work suggests that thermal expansion leads to a small-scale pressure-work contribution that transfers energy in an inverse cascade on average, which has important consequences for LES modeling of reacting flows. The current study employs WMRA to investigate, simultaneously in physical and spectral spaces, the characteristics of this combustion-induced backscatter effect. The WMRA diagnostics provide spatial statistics of the spectra, scale-conditioned intermittency of velocity and vorticity, along with energy-transfer fluxes conditioned on the local progress variable. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D17.00003: Three dimensional dynamic mode decomposition of premixed turbulent jet flames Temistocle Grenga, Jonathan MacArt, Michael Mueller Analysis of turbulent combustion DNS data largely focuses on statistical analyses. However, turbulent combustion is highly unsteady and dynamic. In this work, Dynamic Mode Decomposition (DMD) will be explored as a tool for dynamic analysis of turbulent combustion DNS data, specifically a series of low Mach number spatially-evolving turbulent planar premixed hydrogen/air jet flames. DMD decomposes data into coherent modes with corresponding growth rates and oscillatory frequencies. The method identifies structures unbiased by energy so is particularly well suited to exploring dynamic processes at scales smaller than the largest, energy-containing scales of the flow and that may not be co-located in space and time. The focus of this work will be on both the physical insights that can potentially be derived from DMD modes and the computational issues associated with applying DMD to large three-dimensional DNS datasets. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D17.00004: Lagrangian analysis of premixed turbulent combustion in hydrogen-air flames Ryan Darragh, Alexei Poludnenko, Peter Hamlington Lagrangian analysis has long been a tool used to analyze non-reacting turbulent flows, and has recently gained attention in the reacting flow and combustion communities. The approach itself allows one to separate local molecular effects, such as those due to reactions or diffusion, from turbulent advective effects along fluid pathlines, or trajectories. Accurate calculation of these trajectories can, however, be rather difficult due to the chaotic nature of turbulent flows and the added complexity of reactions. In order to determine resolution requirements and verify the numerical algorithm, extensive tests are described in this talk for prescribed steady, unsteady, and chaotic flows, as well as for direct numerical simulations (DNS) of non-reacting homogeneous isotropic turbulence. The Lagrangian analysis is then applied to DNS of premixed hydrogen-air flames at two different turbulence intensities for both single- and multi-step chemical mechanisms. Non-monotonic temperature and fuel-mass fraction evolutions are found to exist along trajectories passing through the flame brush. Such non-monotonicity is shown to be due to molecular diffusion resulting from large spatial gradients created by turbulent advection. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D17.00005: Lyapunov spectrum in turbulent combustion Malik Hassanaly, Venkat Raman Transient flame evolution is an important flow problem for many practical applications (for example high-altitude relight, ignition in internal combustion engines, unstart in scramjets). Current approaches to combustion modeling utilize assumptions that are valid mainly for statistically stationary processes. In order to understand the transient problem, a dynamic systems approach is followed here. The propagation of a flame in a turbulent channel flow is used as a canonical turbulent combustion system and is analyzed with the Lyapunov theory. In particular, the Lyapunov spectrum for this flow is computed using multiple coordinated simulations. For a range of flow conditions, dimensionality of the state-space is determined. It is shown that the internal structure of the flame plays a critical role in determining the response of the system to perturbations in the flow. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D17.00006: Optimal stretching of fluid for enhancing reaction growth Thomas Nevins, Douglas Kelley When a biological or chemical scalar grows in flowing fluid, the resulting reacted region is dependent on both the details of the flow, and the reaction kinetics. We simultaneously film reaction state and flow in a laboratory model of reactive mixing in order to examine reactive mixing in physical, time-dependent flows. Using the excitable Belousov-Zhabotinsky (BZ) reaction, we find an optimal stretching range in which the flow enhances reaction, but larger stretching causes reaction blowout. We observe the transition from flow mostly helping to mostly blowout is not associated with the transition to turbulence, and that stretching fields (closely related to finite-time Lyapunov exponents) inside the optimal range appear to have a large effect on reaction growth rate locally. We also present estimates of the optimal stretching for BZ, and hypothesize that it is a feature exclusive to excitable reactions. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D17.00007: Dissipation Element Analysis of Reacting- and Non-Reacting Flows. Dominik Denker, Jonas Boschung, Fabian Hennig, Heinz Pitsch Dissipation element analysis is a tried and tested method for analyzing scalar field in turbulent flows. Dissipation elements are defined as an ensemble of grid point whose gradient trajectories reach the same extremal points. Therefore, the scalar field can be compartmentalized in monotonous space filling regions. Dissipation elements can be described by two parameters, namely the Euclidean distance between their extremal points and their scalar difference in these points. The joint probability density function of these two parameters is expected to suffice for a statistical reconstruction of the scalar field. In addition, normalized dissipation element statistics show a remarkable invariance towards changes in Reynolds numbers. Dissipation element statistics of the passive scalar and the turbulent kinetic energy are compared for different flow configurations including reacting and non-reacting turbulent flows. Furthermore, the Reynolds number scaling of the dissipation element parameters is investigated. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D17.00008: Reaction front barriers in time aperiodic fluid flows. Rory Locke, Kevin Mitchell Many chemical and biological systems can be characterized by the propagation of a front that separates different phases or species. One approach to formalizing a general theory is to apply frameworks developed in nonlinear dynamics. It has been shown that invariant manifolds form barriers to passive transport in time-dependent or time-periodic fluid flows. More recently, analogous manifolds termed burning- invariant-manifolds (BIMs), have been shown to form one-sided barriers to reaction fronts in advection-reaction-diffusion (ARD) systems. To model more realistic time-aperiodic systems, recent theoretical work has suggested that similar one-sided barriers, termed burning Lagrangian coherent structures (bLCSs), exist for fluid velocity data prescribed over a finite time interval. In this presentation, we use a stochastic ”wind” to generate time dependence in a double-vortex channel flow and demonstrate the (locally) most attracting or repelling curves are the bLCSs. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D17.00009: Attached and lifted diffusion flames in a mixing layer Moshe Matalon, Zhanbin Lu Many practical combustion devices are concerned with the stabilization of diffusion flames that are formed by injecting gaseous fuels into a co-flowing stream containing an oxidizer. A primary concern of these configurations is the attachment and lift-off characteristics of the diffusion flame relative to the rim of the injector. In such circumstances, the edge of the flame, which possesses a distinct structure that combines characteristics of both premixed an diffusion flames, is found to play a crucial role in determining the stabilization of the diffusion flame. In this study, we examine the effect of streams of unequal flow rates on the structural and dynamical properties of the edge flame. We show that, depending on the stoichiometric conditions and the diffusive properties of the fuel and oxidizer, the diffusion flame may either be attached to the rim of the injector, lifted and stabilized at a downstream equilibrium position, or blown off by the flow. Under certain conditions the diffusion flame may undergo spontaneous oscillations, whereby the edge of the flame exhibits a back and forth motion along a direction that coincides with the diffusion flame surface. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D17.00010: Experimental Vortex Identification and Characterization in Reacting Jets in Crossflow Vedanth Nair, Ben Emerson, Timothy Lieuwen Reacting jets in crossflow (JICF) is an important canonical flow field in combustion problems where there is strong coupling between heat release and the evolution of vortical structures. We use vortex identification studies to experimentally characterize the spatial evolution of vortex dynamics in a reacting JICF. A vortex identification algorithm was designed to operate on particle image velocimetry (PIV) data and its raw Mie scattering images. The algorithm uses the velocity fields to obtain comparisons between the strain rate and the rotation rate. Additionally, the algorithm uses the raw Mie scattering data to identify regions where the high acceleration at vortex cores has centrifuged seeding particles out of the vortex cores. Together, these methods are used to estimate the vortex location and circulation. Analysis was done on 10 kHz PIV data from a reacting JICF experiment, and the resulting vortex trajectory, and growth rate statistics are presented. Results are compared between non-reacting JICF and reacting studies performed with different jet density ratios and different levels of acoustic forcing. We observed how the density ratio, the frequency and amplitude of the acoustic forcing affected the vortex characteristics and growth rate. [Preview Abstract] |
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