1,720,988 research outputs found
Gas dynamic and acoustic interactions with supersonic free shear layers
No full textExperiments were conducted in a two-dimensional wind tunnel to study dynamics of large-scale structures in a planar compressible mixing layer. High-speed cinematic (300,000 frames/sec) Mie-scattering and shadowgraph techniques were employed to track the evolution of these structures in the mixing layer. A covariance algorithm was developed to obtain quantitative information about the size, shape, orientation and convection speed of these structures in the mixing layer from the digitally processed temporally resolved cinematographic images. The results for the pressure matched flows, show large ellipsoidal eddies in the mixing layer oriented at angles ranging from 25\sp\circ-50\sp\circ, which tilt away from and then towards the streamwise direction as they convect downstream. For the pressure mismatched cases, the eddies showed fluctuations in their angular orientation with time which scaled with the eddy passage frequency. The amplitude of these angular fluctuations was found to be a strong function of the non-dimensional streamwise pressure gradient parameter across the compression/expansion waves. The convection speed of the large-scale structures in pressure matched flows was found to be higher than that predicted by the isentropic relations, whereas for the pressure mismatched cases the convection speeds were found to be lower than the isentropic relation.Finally, the planar mixing layer formed by the Mach 1.67 stream bounded on one side, was passively excited using feedback acoustic waves. This was achieved without the aid of the helical and flapping instability modes present in other unbounded free jet flows. The present mixing augmentation was achieved by the formation of large vortical structures, which resulted in nearly a increase in shear layer thickness. This excitation technique involves judicious placement of acoustically reflective surfaces near (but not within) the shear layer. At least two acoustically reflective surfaces, one on the downstream side and the other acting as the lower wall are necessary for the excitation to occur. It was also confirmed that the excitation was caused by an upstream traveling wave initiated at the downstream surface. A plausible model was developed to explain the aero-acoustic excitation mechanism.Made available in DSpace on 2011-05-07T13:33:27Z (GMT). No. of bitstreams: 2
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Previous issue date: 1995Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:56:15Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:26:17-05:00
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Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Unstructured grid simulations of shocks and detonations in two-phase flows
No full textA series of numerical simulations have been made of compressible two-phase flows containing inert particles. An existing conservative, monotonic compressible flow solver with an unstructured, adaptive mesh was used as the basis for code development. This flow solver incorporated the Finite Element Method-Flux Corrected Transport scheme, which has shown excellent predictive capability of various compressible flows which include both strong and weak shocks. Two separate two-phase flow techniques have been added to this solver to allow simulations of both Eulerian-Eulerian treatment and Eulerian-Lagrangian treatment. Both methods were used to study one-dimensional shock wave attenuation in two-phase flow containing gas and particles. The results show good agreement with experiment for both methods. The Lagrangian particle method tracked groups of particles as parcels and implemented a parcel adaptation method to ensure adequate parcel distribution throughout the adaptive mesh. This resulted in very large savings in CPU and memory requirements as compared to a conventional (non-parcel-adaptive) Lagrangian technique. The Eulerian particle method was found to produce cleaner solutions and required about half the memory and CPU time as the Lagrangian particle method.In the second thrust of this study, an inexpensive two-step induction parameter model was added to the Eulerian-Eulerian two-phase code to model the combustion of the gas phase. This combustion model was used to study shock initiated detonations of a hypothetical H\sb{2}:O\sb{2} mixture in a shock tube both with and without inert glass particles. The single-phase studies were used to document the detonation pattern resulting from the hypothetical combustion model and to provide grid resolution studies. In the two-phase detonations, it was found that decreasing the particle diameter while holding the particle mass loading constant, results in higher drag, work, and heat loadings that the particles place on the gas. Below a certain diameter, the detonations were seen to fail as indicated by a monotonic growth in the length of the induction zone.Made available in DSpace on 2011-05-07T12:13:09Z (GMT). No. of bitstreams: 2
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Previous issue date: 1995Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:38:15Z
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Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Very large eddy simulations of spatially evolving supersonic turbulent shear layers
No full textThe objective of this research was to further the understanding of the fundamental physical mechanisms which control turbulence and entrainment at different levels of compressibility for supersonic shear flows. Very Large Eddy Simulations (VLES) were performed based on the two-dimensional unsteady Euler equations without any empirical coefficients for turbulence modeling. These equations were used to study the spatially evolving mixing characteristics of unforced, planar, confined shear layers formed by two parallel streams of supersonic air that come into contact after passing over a splitter plate. The computations were performed using the conservative Finite Element Method-Flux Corrected Transport (FEM-FCT) scheme with unstructured adaptive grids. In general, it was found that the highest simulation fidelity was obtained by using a full turbulence spectra consistent with measured turbulent kinetic energy levels and an incompressible wavelength distribution. This model is the most physically consistent and yields the best comparison with the downstream experimental data. The computational investigation noted modifications of organized coherent structures as well asymmetric entrainment, i.e. the high speed fluid is entrained and convoluted to a greater extent than that of the lower speed stream, and their subsequent importance in mixing. The round vortex shape of incompressible circular eddies was modified to an oblique flattened shape with decreased transverse height as convective Mach number (M\sb{\rm c}) increases. The high speed side convolutions of organized lumps of fluid are also decreased as M\sb{\rm c} increases. However, the overall vortex size are only slightly modified, whereas the angular orientation is significantly modified. The merging process is very different as a function of M\sb{\rm c}: the rotational vortex pairing process at low M\sb{\rm c} is modified to a slapping process at high M\sb{\rm c}. This slapping process, which results in eddy flattening and oblique angles at higher M\sb{\rm c}, reduces the degree of coherency. Turbulence statistics of velocity and mixture fraction also investigated for three different M\sb{\rm c}'s. The converged peak values of V\sbsp{\rm rms}{\prime}, f\sbsp{\rm rms}{\prime} and Reynolds stress are reduced, while U\sbsp{\rm rms}{\prime} peak values remain constant as M\sb{\rm c} increases (coherency decreases). Those observations are consistent with experimental results and observations of coherency effects.Made available in DSpace on 2011-05-07T13:14:58Z (GMT). No. of bitstreams: 2
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Previous issue date: 1994Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:52:16Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:24:03-05:00
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Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Direct numerical simulation of large bubbles in a free shear layer
No full textThe full Navier-Stokes equations were employed with a single-fluid model and a front tracking scheme to study large cylindrical, spherical, and ellipsoidal bubbles in a free shear layer. This approach allows direct simulation of the multiphase flow by wholly incorporating the bubble flow field in conjunction with the large scale vortical structures of the liquid. The role of large bubbles in modifying finite Reynolds number shear flow structures was investigated, specifically for bubbles whose diameter approaches the scale of the largest liquid eddies. Both two- and three-dimensional results indicate that duration of eddy crossing is the main mechanism for flow modulation, which is typically characterized by decreased vortex coherency and size, modified fluctuation statistics, and significant variations in pairing/merging phenomena. The comparison of fluctuating statistics and flow field visualization also allowed qualitative discrimination between the modulation of the non-linear eddy dynamics and fluctuations due simply to the random bubble induced perturbations. Increasing bubble deformation only has a minor effect on liquid flow modulation.In addition to the flow modulation studies, a general formulation based on Auton et al. (1988) of the hydrodynamic forces on a finite Reynolds number large bubble in an unsteady, non-uniform, and rotational flow was developed. This formulation is used to investigate the effects of non-linear spatial and temporal gradients on dispersion of large cylindrical, spherical, and ellipsoidal bubbles in a free shear layer. Both two- and three-dimensional results indicate that the bubble dispersion in the full Navier-Stokes solution was significantly different than that given by a conventional bubble dynamic equation based on linear spatial gradients and quasi-steady flow. This is due to the forces not accounted for by such a formulation, which are related to regions of high non-uniformity and unsteadiness. Comparison of the dispersion of highly deformed bubbles with negligibly deformed ones indicates significant differences. The adjunct forces in the drag/lift direction on both two- and three-dimensional bubbles (low or high deformation) were found to be correlated with rapid variations of relative bubble velocity and acceleration/high gradients of the liquid velocity.Made available in DSpace on 2011-05-07T12:18:21Z (GMT). No. of bitstreams: 2
license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5)
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Previous issue date: 1995Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T14:39:24Z
Item is restricted indefinitely.Restriction data tranferred 2014-07-01T11:16:34-05:00
Original Data
Group with Access UIUC Users [automated]
Release Date: none
Reason: ETDs are only available to UIUC Users without author permissionETDs are only available to UIUC Users without author permissionU of I Onl
Unstructured grid simulations of shocks and detonations in two-phase flows
No full textA series of numerical simulations have been made of compressible two-phase flows containing inert particles. An existing conservative, monotonic compressible flow solver with an unstructured, adaptive mesh was used as the basis for code development. This flow solver incorporated the Finite Element Method-Flux Corrected Transport scheme, which has shown excellent predictive capability of various compressible flows which include both strong and weak shocks. Two separate two-phase flow techniques have been added to this solver to allow simulations of both Eulerian-Eulerian treatment and Eulerian-Lagrangian treatment. Both methods were used to study one-dimensional shock wave attenuation in two-phase flow containing gas and particles. The results show good agreement with experiment for both methods. The Lagrangian particle method tracked groups of particles as parcels and implemented a parcel adaptation method to ensure adequate parcel distribution throughout the adaptive mesh. This resulted in very large savings in CPU and memory requirements as compared to a conventional (non-parcel-adaptive) Lagrangian technique. The Eulerian particle method was found to produce cleaner solutions and required about half the memory and CPU time as the Lagrangian particle method.In the second thrust of this study, an inexpensive two-step induction parameter model was added to the Eulerian-Eulerian two-phase code to model the combustion of the gas phase. This combustion model was used to study shock initiated detonations of a hypothetical H\sb{2}:O\sb{2} mixture in a shock tube both with and without inert glass particles. The single-phase studies were used to document the detonation pattern resulting from the hypothetical combustion model and to provide grid resolution studies. In the two-phase detonations, it was found that decreasing the particle diameter while holding the particle mass loading constant, results in higher drag, work, and heat loadings that the particles place on the gas. Below a certain diameter, the detonations were seen to fail as indicated by a monotonic growth in the length of the induction zone.U of I OnlyETDs are only available to UIUC Users without author permissio
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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