Abstract: Significant progress was made in the third year of an interdisciplinary experimental, numerical and theoretical program to extend the state of knowledge and understanding of the effects of chemical reactions in hypervelocity flows. The program addressed the key problems in aerothermochemistry that arise from.the interaction between the three strongly nonlinear effects: Compressibility; vorticity; and chemistry. Important new results included: • New data on transition in hypervelocity carbon dioxide flows • New method of free-piston shock tunnel operation for lower enthalpy • Accurate new method for computation of self-similar flows • New experimental data on flap-induced separation at high enthalpy • Insight into mechanisms active in reacting shear layers from comparison of experiment and computation • Extensive new data from Rayleigh scattering diagnostics of supersonic shear layer • Comparison of new experiments and computation of hypervelocity double-wedge flow yielded important differences • Further first-principles computations of electron collision cross-sections of CO, N_2 and NO • Good agreement between EFMO computation and experiment of flow over a cone at high incidence • Extension of LITA diagnostics to high temperature.

ID: CaltechAUTHORS:20141111-111211793

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Abstract: The present work is concerned with the extension of the theory of characteristics to conservation laws with source terms in one space dimension, such as the Euler equations for reacting flows. New spacetime curves are introduced on which the equations decouple to the characteristic set of O.D.E's for the corresponding homogeneous laws, thus allowing the introduction of functions analogous to the Riemann Invariants. The geometry of these curves depends on the spatial gradients for the solution. This particular decomposition can be used in the design of efficient unsplit algorithms for the numerical integration of the equations. As a first step, these ideas are implemented for the case of a scalar conservation law with a nonlinear source term. The resulting algorithm belongs to the class of MUSCL-type, shock-capturing schemes. Its accuracy and robustness are checked through a series of tests. The aspect of the stiffness of the source term is also studied. Then, the algorithm is generalized for a system of hyperbolic equations, namely the Euler equations for reacting flows. An extensive numerical study of unstable detonations is performed.

ID: CaltechAUTHORS:20141111-103448502

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Abstract: This is the final report of our program on "Chemical Reactions in Turbulent Mixing Flows," supported under the AFOSR Grant No. F49620-92-J-0290, which was granted a no-cost extension to permit the completion of the Supersonic Shear Layer Facility upgrade that extended the operating envelope to higher Mach-number flows. As part of this upgrade, a variety of new diagnostic and safety features were also implemented in this unique facility. The purpose of this program has been to conduct fundamental investigations of turbulent mixing, chemical reaction and combustion processes in turbulent, subsonic and supersonic flows. Scientific progress in these areas was documented in our most recent Annual Report (Dimotakis & Leonard 1994).

ID: CaltechAUTHORS:20141111-095158441

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Abstract: A new approach for studying wave propagation phenomena in an inviscid gas is presented. This approach can be viewed as the extension of the method of characteristics to the general case of unsteady multidimensional flow. The general case of the unsteady compressible Euler equations in several space dimensions is examined. A family of spacetime manifolds is found on which an equivalent one-dimensional problem holds. Their geometry depends on the spatial gradients of the flow, and they provide, locally, a convenient system of coordinate surfaces for spacetime. In the case of zero entropy gradients, functions analogous to the Riemann invariants of 1-D gas dynamics can be introduced. These generalized Riemann Invariants are constant on these manifolds and, thus, the manifolds are dubbed Riemann Invariant Manifolds (RIM). In this special case of zero entropy gradients, the equations of motion are integrable on these manifolds, and the problem of computing the solution becomes that of determining the manifold geometry in spacetime. This situation is completely to the traditional method of characteristics in one-dimensional flow. Explicit espressions for the local differential geometry of these manifolds can be found directly from the equations of motion. The local direction and speed of propagation of the waves that these manifolds represent, can be found as a function of the local spatial gradients of the flow. Their geometry is examined, and in particular, their relation to the characteristic surfaces. It turns out that they can be space-like or time-like, depending on the flow gradients. Wave propagation can be viewed as a superposition of these Riemann Invariant waves, whenever appropriate conditions of smoothness are met. This provides a means for decomposing the equations into a set of convective scalar fields in a way which is different and potentially more useful than the characteristic decomposition. The two decompositions become identical in the special case of one-dimellsional flow. This different approach can be used for computational purposes by discretizing the equivalent set of scalar equations. Such a computational application of this theory leads to the possibility of determining the solution at points in spacetime using information that propagates faster than the local characteristic speed, i.e., using information outside the domain of dependence. This possibility and its relation to the uniqueness theorems is discussed.

ID: CaltechAUTHORS:20141110-161414265

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Abstract: The purpose of this research is to conduct fundamental investigations of turbulent mixing, chemical reaction and combustion processes in turbulent, subsonic and supersonic flows. The program during this reporting period was comprised of several parts: a. an experimental effort, b. a numerical simulation effort, and c. an effort to develop instrumentation and diagnostics; flow and combustion facilities; and data-acquisition systems. The latter as dictated by the specific needs of the experimental part of the program. Our approach in this research has been to carry out a series of detailed theoretical and experimental studies of turbulent mixing in primarily in two, well-defined, fundamentally important flow fields: free-shear layers and axisymmetric jets. To elucidate molecular transport effects, experiments and theory concern themselves with both reacting and non-reacting flows of liquids and gases, in fully-developed turbulent flows, i.e., in moderate to high Reynolds number flows. A criterion for fully-developed turbulence was recently developed and will be presented below. The computational studies are, at present, focused at fundamental formulation and implementation issues pertaining to the computational simulation of both compressible and incompressible flows characterized by strong fronts, such as shock waves and flames. Our diagnostic development efforts have recently been focused on improving the signal-to-noise ratio of flow images, in both gas- and liquid-phase flows, as well as the continuing development of data-acquisition electronics to meet very high-speed, high-volume data requirements; the acquisition of single, or pairs, of two-dimensional images in rapid succession; and the acquisition of data from arrays of supersonic flow sensors.

ID: CaltechAUTHORS:20141021-161231721

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Abstract: The purpose of this research is to conduct fundamental investigations of turbulent mixing, chemical reaction and combustion processes in turbulent, subsonic and supersonic flows. Our program is comprised of several parts: a. an experimental effort, b. an analytical effort, c. a computational effort, d. a modeling effort, and e. a diagnostics development and data-acquisition effort, the latter as dictated by specific needs of the experimental part of the overall program. Our approach has been to carry out a series of detailed theoretical and experimental studies of turbulent mixing in primarily in two, well-defined, fundamentally important flow fields: free shear layers and axisymmetric jets. To elucidate molecular transport effects, experiments and theory concern themselves with both reacting and non-reacting flows of liquids and gases, in fully-developed turbulent flows, i.e., in moderate to high Reynolds number flows. The computational studies are, at present, focused at fundamental issues pertaining to the computational simulation of both compressible and incompressible flows. Modeling has been focused on both shear layers and turbulent jets, with an effort to include the physics of the molecular transport processes, as well as formulations of models that permit the full chemical kinetics of the combustion process to be incorporated. Our primary diagnostic development efforts are currently focused on data-acquisition electronics to meet very high-speed, high-volume data requirements, the acquisition of single, or a sequence, of two-dimensional images, and the acquisition of data from arrays of supersonic flow sensors. Progress has also been made in the development of a dual-beam laser interferometer/correlator to measure convection velocities of large scale structures in supersonic shear layers and in a new method to acquire velocity field data using pairs of scalar images closely spaced in time.

ID: CaltechAUTHORS:20141020-101605714

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Abstract: The contour dynamics method is extended to vortex rings with vorticity varying linearly from the symmetry axis. An elliptic core model is also developed to explain some of the basic physics. Passage and collisions of two identical rings are studied focusing on core deformation, sound generation and stirring of fluid elements. With respect to core deformation, not only the strain rate but how rapidly it varies is important and accounts for greater susceptibility to vortex tearing than in two dimensions. For slow strain, as a passage interaction is completed and the strain relaxes, the cores return to their original shape while permanent deformations remain for rapidly varying strain. For collisions, if the strain changes slowly the core shapes migrate through a known family of two-dimensional steady vortex pairs up to the limiting member of the family. Thereafter energy conservation does not allow the cores to maintain a constant shape. For rapidly varying strain, core deformation is severe and a head-tail structure in good agreement with experiments is formed. With respect to sound generation, good agreement with the measured acoustic signal for colliding rings is obtained and a feature previously thought to be due to viscous effects is shown to be an effect of inviscid core deformation alone. For passage interactions, a component of high frequency is present. Evidence for the importance of this noise source in jet noise spectra is provided. Finally, processes of fluid engulfment and rejection for an unsteady vortex ring are studied using the stable and unstable manifolds. The unstable manifold shows excellent agreement with flow visualization experiments for leapfrogging rings suggesting that it may be a good tool for numerical flow visualization in other time periodic flows.

ID: CaltechAUTHORS:SHAnasatm102257

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Abstract: The purpose of this research has been to conduct fundamental investigations of turbulent mixing, chemical reaction and combustion processes in turbulent, subsonic and supersonic flows. Progress in this effort thus far has uncovered important deficiencies in conventional modeling of these phenomena, and offered alternative suggestions and formulations to address some of these deficiencies. This program is comprised of an experimental effort, an analytical modeling effort, a computational effort, and a diagnostics development and data-acquisition effort, the latter as dictated by specific needs of our experiments. Our approach has been to carry out a series of detailed theoretical and experimental studies primarily in two, well-defined, fundamentally important flow fields: free shear layers and axisymmetric jets. To elucidate molecular transport effects, experiments and theory concern themselves with both liquids and gases. Modeling efforts have been focused on both shear layers and turbulent jets, with an effort to include the physics of the molecular transport processes, as well as formulations of models that permit the full chemical kinetics of the combustion process to be incorporated. The computational studies are, at present, focused at fundamental issues pertaining to the computational simulation of both compressible and incompressible flows. This report includes an outline discussion of work completed under the sponsorship of this Grant, with six papers, which have not previously been included in past reports, or transmitted in reprint form, appended.

ID: CaltechAUTHORS:20141020-095543611

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Abstract: Work is continuing primarily in gas phase turbulent mixing and chemical reactions. The liquid phase work to date is in its final stages of being analyzed and documented for dissemination in the form of archival publications. In the gas phase shear layer work, our investigations are concentrating on shear layer free stream density ratio effects, finite kinetic rate (Damköhler number) effects, and a design effort in support of the planned extension of the work to supersonic flows. In jet flows, progress has been made in the gas phase laser Rayleigh scattering techniques developed for conserved scalar measurements down to diffusion space and time scales. A new technique has been developed under joint support with the Gas Research Institute that permits the imaging of soot sheets in turbulent flames and is being used to describe the combustion flame sheets in methane flames. Theoretical work in progress is addressing the finite chemical rate problem as well as the diffusion-limited shear layer mixing problem. Advances in our data acquisition capabilities during the last year are permitting higher temporal resolution measurements to be taken with digital image arrays.

ID: CaltechAUTHORS:20141020-092935685

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