Coherent Structures in Turbulent Shear Layers and Relation to Mixing Reaction and Combustion


Book Description

A number of numerical analyses served as experiments which led to a broad range of ideas about the dynamics of mixing layer. One theme which emerged early was the adversary role of vorticity and of deformation - two quanities which attempt to subjugate each other and whose competition is the subject matter of the development and growth to the shear flow. Another strand of the work follows a number of instabilities (two and three dimensional) which lead to recognizable non-linear motions. A third focus of the work relates vorticity and the mixing of scalars. The work started out to attempt to explain striking experimental results and ended by making a number of predictions which have not yet been experimentally established. A significant fraction, though not all of the typical motions which are dynamically plausible in a shear layer have been examined and given simple analytic representation. Finally the basis for the effective control of the shear layer has been laid.




Turbulent Mixing in Nonreactive and Reactive Flows


Book Description

Turbulence, mixing and the mutual interaction of turbulence and chemistry continue to remain perplexing and impregnable in the fron tiers of fluid mechanics. The past ten years have brought enormous advances in computers and computational techniques on the one hand and in measurements and data processing on the other. The impact of such capabilities has led to a revolution both in the understanding of the structure of turbulence as well as in the predictive methods for application in technology. The early ideas on turbulence being an array of complicated phenomena and having some form of reasonably strong coherent struc ture have become well substantiated in recent experimental work. We are still at the very beginning of understanding all of the aspects of such coherence and of the possibilities of incorporating such structure into the analytical models for even those cases where the thin shear layer approximation may be valid. Nevertheless a distinguished body of "eddy chasers" has come into existence. The structure of mixing layers which has been studied for some years in terms of correlations and spectral analysis is also getting better understood. Both probability concepts such as intermittency and conditional sampling as well as the concept of large scale structure and the associated strain seem to indicate possibilities of distinguishing and synthesizing 'engulfment' and molecular mixing.




Simulation of Turbulent Flows with and without Combustion with Emphasis on the Impact of Coherent Structures on the Turbulent Mixing


Book Description

The analysis of turbulent mixing in complex turbulent flows is a challenging task. The effective mixing of entrained fluids to a molecular level is a vital part of the dynamics of turbulent flows, especially when combustion is involved. The work has shown the limitations of the steady-state simulations and acknowledged the need of applying high-fidelity unsteady methods for the calculation of flows with pronounced unsteadiness promoted by large-scale coherent structures or other sources.







An Experimental Investigation of Structure, Mixing and Combustion in Compressible Turbulent Shear Layers [microform]


Book Description

Two-dimensional, compressible, turbulent shear layers are studied in a new wind tunnel facility. Both reacting and non-reacting flows are investigated, with one free stream velocity supersonic and the other subsonic. The combustion experiments are based on use of low concentrations of hydrogen, nitric oxide and fluorine gases. Side-view Schlieren photographs of these reacting and non-reacting flows appear devoid of the 2-D, large scale structures seen in incompressible flow. Comparison with all-subsonic flows produced in the same facility suggests that this lack of two-dimensional structure is due to the presence of the supersonic high-speed free stream velocity. Travelling shock and expansion waves are observed in the high compressibility flows, evidently created by turbulent structures convecting at supersonic velocities. Such waves are seen only in the low-speed fluid, with apparent convection velocities much higher than those predicted on the basis of isentropic pressure-matching arguments. The measured shear layer growth rates agree with previous results by other experiments, except for a few cases at low compressibility and low density ratio. The fast chemistry regime is attained in some of the high compressibility flows tested. 'Flip' experiments conducted in this regime indicated that the volume fraction of mixed fluid in the layer is substantially reduced as compared to previous incompressible results. These same flip experiments also reveal that compressibility significantly alters the entrainment ratio.




An Experimental Investigation of Structure, Mixing and Combustion in Compressible Turbulent Shear Layers


Book Description

Two-dimensional, compressible, turbulent shear layers are studied in a new wind tunnel facility. Both reacting and non-reacting flows are investigated, with one free stream velocity supersonic and the other subsonic. The combustion experiments are based on use of low concentrations of hydrogen, nitric oxide and fluorine gases. Side-view Schlieren photographs of these reacting and non-reacting flows appear devoid of the 2-D, large scale structures seen in incompressible flow. Comparison with all-subsonic flows produced in the same facility suggests that this lack of two-dimensional structure is due to the presence of the supersonic high-speed free stream velocity. Travelling shock and expansion waves are observed in the high compressibility flows, evidently created by turbulent structures convecting at supersonic velocities. Such waves are seen only in the low-speed fluid, with apparent convection velocities much higher than those predicted on the basis of isentropic pressure-matching arguments. The measured shear layer growth rates agree with previous results by other experiments, except for a few cases at low compressibility and low density ratio. The fast chemistry regime is attained in some of the high compressibility flows tested. 'Flip' experiments conducted in this regime indicated that the volume fraction of mixed fluid in the layer is substantially reduced as compared to previous incompressible results. These same flip experiments also reveal that compressibility significantly alters the entrainment ratio.




IUTAM Symposium on Turbulent Mixing and Combustion


Book Description

The goals of the Symposium were to draw together researchers in turbulence and combustion so as to highlight advances and challenge the boundaries to our understanding of turbulent mixing and combus tion from both experimental and simulation perspectives; to facilitate cross-fertilization between leaders in these two fields. These goals were noted to be important given that turbulence itself is viewed as the last great problem in classical physics and the addition of chemical reaction amplifies the difficulties enormously. The papers that have been included here reflect the richness of our subject. Turbulence is rich and complex in its own right. And, its inner structure, hidden in the morass of scales, large and small, can dominate transport. Earlier IUTAM Symposia have considered this field, Eddy Structure Identification in Free Turbulent Flows, Bonnet and Glauser (eds) 1992 and Simulation and Identification of Organized Structures in Flows, Sorensen, Hopfinger and Aubry (eds) 1997. The combustion community is well served by its specialized events, most notable is the bi annual International Combustion Symposium, held under the auspices of the Combustion Institute. Mixing is often considered somewhere in between these two. This broad landscape was addressed in this Sym posium in a somewhat temporal linear fashion of increasing complexity. The lectures considered the many challenges posed by adding one ele ment to the base formed by others: turbulence and turbulent mixing in the absence of combustion through to turbulent mixing dominated by chemistry and combustion.




Eddy Structure Identification in Free Turbulent Shear Flows


Book Description

The existence and crucial role played by large-scale, organized motions in turbulent flows are now recognized by industrial, applied and fundamental researchers alike. It has become increasingly evident that coherent structures influence mixing, noise, vibration, heat transfer, drag, etc... The accelera tion of the development of both experimental and computational programs devoted to this topic has been evident at several recent international meet ings. One of the first questions which experimentalists or numerical analysts are faced with is: how can these structures be separated from the background turbulence? This is a nontrivial task because the coherent structures are gen erally embedded in a random field and the technique used to determine when and where certain structures are passing, or their averaged characteristics (in the more probable or dominant role sense) is directly related to the definition of the coherent structure. Several methods or approaches are available and the choice of a particular one is generally dependent on the desired informa tion. This choice depends not only on the definition of the structure, but also on the experimental and numerical capabilities available to the researcher.







Computational Mechanics


Book Description

Computational Mechanics is the proceedings of the International Symposium on Computational Mechanics, ISCM 2007. This conference is the first of a series created by a group of prominent scholars from the Mainland of China, Hong Kong, Taiwan, and overseas Chinese, who are very active in the field. The book includes 22 full papers of plenary and semi-plenary lectures and approximately 150 one-page summaries.