Shock wave structure in a lattice gas

The motion and structure of shock and expansion waves in a simple particle system, a lattice gas and cellular automaton, are determined in an exact computation. Shock wave solutions, also exact, of a continuum description, a model Boltzmann equation, are compared with the lattice results. The comparison demonstrates that, as proved by Caprino et al. [“A derivation of the Broadwell equation,” Commun. Math. Phys. 135, 443 (1991)] only when the lattice processes are stochastic is the model Boltzmann description accurate. In the strongest shock wave, the velocity distribution function is the bimodal function proposed by Mott-Smith

The Coherent Flame Model for Turbulent Chemical Reactions

A description of the turbulent diffusion flame is proposed in which the flame structure is composed of a distribution of laminar diffusion flame elements, whose thickness is small in comparison with the large eddies. These elements retain their identity during the flame development; they are strained in their own plane by the gas motion, a process that not only extends their surface area, but also establishes the rate at which a flame element consumes the reactants. Where this flame stretching process has produced a high flame surface density, the flame area per unit volume, adjacent flame elements may consume the intervening reactant, thereby annihilating both flame segments. This is the flame shortening mechanism which, in balance with the flame stretching process, establishes the local level of the flame density. The consumption rate of reactant is then given simply by the product of the local flame density and the reactang consumption rate per unit area of flame surface. The proposed description permits a rather complete separation of the turbulent flow structure, on one hand, and the flame structure, on the other, and in this manner permits the treatment of reactions with complex chemistry with a minimum of added labor. The structure of the strained laminar diffusion flame may be determined by analysis, numerical computation, and by experiment without significant change to the model

Supersonic Airfoils Simplified

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77574/1/AIAA-1520-593.pd

Triple flames and flame stabilization

It is now well established that when turbulent jet flames are lifted, combustion begins, i.e., the flame is stabilized, at an axial station where the fuel and air are partially premixed. One might expect, therefore, that the beginning of the combustion zone would be a triple flame. Such flames have been described; however, other experiments provide data that are difficult to reconcile with the presence of triple flames. In particular, laser images of CH and OH, marking combustion zones, do not exhibit shapes typical of triple flames, and, more significantly, the lifted flame appears to have a propagation speed that is an order of magnitude higher than the laminar flame speed. The speed of triple flames studied thus far exceeds the laminar value by a factor less than two. The objective of the present task is the resolution of the apparent conflict between the experiments and the triple flame characteristics, and the clarification of the mechanisms controlling flame stability. Being investigated are the resolution achieved in the experiments, the flow field in the neighborhood of the stabilization point, propagation speeds of triple flames, laboratory flame unsteadiness, and the importance of flame ignition limits in the calculation of triple flames that resemble lifted flames

Large‐scale structures and molecular mixing

Scalar mixing and chemical reactions in turbulent shear layers and jets are examined with emphasis on experimental results of high spatial and temporal resolution. Such measurements show that the notion of distinguishing fluids that are molecularly mixed from those that are simply stirred is valid and useful. Two models that seem especially suitable for implementing mixing analyses from this viewpoint are described and speculations on possible connections with the idea of chaotic advection offered

Chemical Reactions in Turbulent Mixing Flows

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

Chemical Reactions in Turbulent Mixing Flows

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

Local temperature measurements in supercritical counterflow in liquid helium II

In order to investigate the validity of appending the Gorter-Mellink friction term to the equations of motion of liquid helium, the temperature was measured along the axis of a channel carrying a supercritical heat current. A single thermomenter on a traversing assembly was used permitting local measurements both in the interior of the channel and in the jet formed in the free fluid. The temperature gradient in the interior of the channel is found to be in agreement with the Gorter-Mellink law up to the lambda point, but goes to zero within a channel radius, in the free jet. The Gorter-Mellink A (T ) was also measured up to the lambda point. A much stronger divergence is found as T λ is approached than was indicated by previous measurements

Irreversibility in a Reversible Lattice Gas

A simple lattice gas model, a microscopically reversible cellular automaton, is described and shown to exhibit thermodynamic irreversibility in processes similar to those in real gases. The model, which has no random elements, develops a long-lasting equilibrium state within a Poincaré cycle. This state is an attractor resulting from the nonlinear nature of the collective particle collisions and motions. The results illustrate how the Second Law of Thermodynamics applies to real systems governed by reversible microscopic dynamics