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Structure of turbulent channel flow perturbed by cylindrical roughness elements
Full text linkThe current study investigates the structural modifications imposed in fully-developed turbulent channel flow by an isolated, wall-mounted circular cylinder. The cylinder height is chosen to specifically extend into the logarithmic layer of the flow in order to study its perturbation of the larger flow scales that embody a significant fraction of the turbulent kinetic energy. Hot-wire measurements were made in the wake of the wall-mounted circular cylinder at multiple wall-normal and streamwise positions. Mean streamwise statistics (mean velocity and Reynolds normal stress) and pre-multiplied spectra of perturbed and unperturbed flow were computed, and influence of the cylinder on these statistics were analyzed. The influence of such perturbations on the inner-outer interactions of the channel flow were also investigated.
Besides the mean velocity deficit in the wake of the cylinder, a new peak in streamwise Reynolds normal stress away from the wall was observed, coupled with the suppression of the near-wall peak native to the incoming unperturbed flow. Pre-multiplied spectra elaborated on these energy modifications, specifically the occurrence of an energy peak corresponding to a wavelength (λx) ~ 0.45 times the channel half-height (h), an attenuation of large-scale energy close to the wall, and a tertiary peak at two-third's the cylinder height corresponding to a length scale of λx ~ 10h. Further, amplitude modulation effects of the large-scale motions on small scales close to the wall, representative of inner-outer interactions, was found to be greatly enhanced in the near-wall region. All the perturbations were found to decay with streamwise distance downstream towards the unperturbed flow.
A clear persistence of the structures at the aforementioned tertiary peak, similar to the wavelengths of the very large scale motions (VLSMs) in canonical wall turbulence, tends to suggest an environment in turbulent flows preferring structures of such wavelengths. Possible mechanisms for the observed suppression of near-wall cycle and the enhanced inner-outer interactions are suggested. The influence of cylinder aspect ratio on the characteristics of perturbed flow are evaluated, and a distinction in wake structure is identified. The necessity of future studies to further understand these significant attributes of perturbation response and recovery of the turbulent wall bounded flows is highlighted.Item withdrawn by Mark Zulauf ([email protected]) on 2013-07-17T19:45:45Z
Item was in collections:
University of Illinois Theses & Dissertations (ID: 1)
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Inner-outer interactions in a rough-wall turbulent boundary layer
Full text linkThe primary goal of the present effort is to enhance the current understanding of inner-outer interactions in rough-wall turbulent boundary layers. These interactions were recently established over smooth-wall turbulent boundary layers as modulating interactions, where the outer layer large scales amplitude and frequency modulate the near-wall small scales. Given that the outer layer dynamics responsible for these modulations are identical in most high-Reynolds-number (Re) rough-wall flows as well, similar modulation interactions are are explored to identify the similarities and differences of these interactions and establish a spatio-temporal description of the same. This is particularly important given the engineering significance of the flows over rough walls.
This work was performed as two parts. In the first part, high temporal-resolution boundary layer hot-wire measurements were made in a wind tunnel, that fully resolved all dynamical scales temporally at fixed points in the flow. Flows over smooth and rough walls were investigated, with the latter being a complex topography indicative of a realistic roughness commonly encountered in engineering applications. Single- and two-probe measurements provided a dual perspective on the large scales, and enabled analysis of analytical techniques commonly employed. With these measurements, it was found that the nature of amplitude and frequency modulation occur even over this complex topography, and that the structure is very similar to that observed in smooth-wall flow. Further, the simultaneous two-probe measurements enabled the investigation of predictive models, which interestingly suggested a possibly stronger modulation in rough-wall flow compared to the smooth-wall case. A `quasi-steady, quasi-homogenous' theory previously developed for smooth-wall flow showed promising predictions of the calibrations constants even in rough-wall flow, lending additional support to the mechanisms speculating that the small scales respond in a quasi-steady manner, irrespective of the origin, to the large scales.
With these inner-outer interactions established, the second part of the current work aimed to develop a spatio-temporal description of the modulating mechanisms using high frame-rate particle-image velocimetry (PIV). The experiments, performed in a refractive-index-matched flow facility, enabled the measurements very close to the surface without being impeded by the near-wall reflections that are common in smooth- and rough-wall PIV experiments. Following a preliminary demonstration of the relevant physics observed via point measurements, a representative large-scale structure was defined using conditional averaging. The associated changes to the small-scale turbulence close to the wall indicated similar modulation interactions, and provided a spatial tool to investigate the same. Further, the large-scale -small-scale interaction structure lended support to the speculations made on the same using hot-wire measurements in the current work and in the recent literature for smooth- and rough-wall flow.
These experiments identify and emphasize the significance of inner-outer interactions over rough-wall flows, and the necessity to accurately model them to enhance the fidelity of any high Re simulations over rough walls.Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2017-09-29 without embargo termsThe student, Gokul Pathikonda, accepted the attached license on 2017-07-14 at 09:40.The student, Gokul Pathikonda, submitted this Dissertation for approval on 2017-07-14 at 09:51.This Dissertation was approved for publication on 2017-07-14 at 14:38.DSpace SAF Submission Ingestion Package generated from Vireo submission #11466 on 2017-09-29 at 11:30:30Made available in DSpace on 2017-09-29T17:56:54Z (GMT). No. of bitstreams: 8
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Previous issue date: 2017-07-1
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Coherent Organization of Passive Scalar from a Point-Source in a Turbulent Boundary Layer
Full text linkThe spatial organization of a passive scalar plume originating from a point
source in a boundary layer is studied to understand its meandering
characteristics. We focus shortly downstream of the isokinetic injection
(, being boundary layer thickness) where the
scalar concentration is highly intermittent, the plume rapidly meanders, and
breaks-up into concentrated scalar pockets due to the action of turbulent
structures. Two injection locations were considered: the center of
logarithmic-region and the wake-region of the boundary layer. Simultaneous
quantitative planar laser-induced fluorescence (PLIF) and particle-image
velocimetry (PIV) were performed in a wind-tunnel, using acetone vapors to
measure scalar mixture fraction and velocity fields. Single- and multi-point
statistics were compared to established works to validate the diagnostic
novelties. Additionally, the spatial characteristics of the plume intermittency
were quantified using `blob' size, shape, orientation and mean concentration.
It was observed that straining and break up were the primary plume-evolution
modes in this region, with little small-scale homogenization. Further, the
dominant role of coherent vortex motions in meandering and break-up of the
plume was evident. Their action is found to be the primary mechanism by which
the scalar injected within the log-region is transported away from the wall
(`large meander events'). Strong spatial correlation was observed in both
instantaneous and conditional fields between the high scalar concentration
regions and the individual vortex heads. This coherent transport was weaker for
wake-injection case, where the plume only interacts with outer vortex motions.
A coherent-structure based mechanism is suggested to explain these transport
mechanisms
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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