1,721,024 research outputs found
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
Experimental Investigation of Highly Irregular Roughness Effects in Wall Turbulence
No full textFinally, the spatial structure of flow over the RF1 surface is contrasted with that over the smooth wall, both within and outside the roughness sublayer, to further assess the possibility of structural modifications due to roughness. Hairpin vortex packets, commonly observed in smooth-wall turbulence, are observed often in the outer layer of the rough-wall flow. As in smooth-wall flow, these large-scale features are also found to contribute heavily to the Reynolds shear stress. However, while qualitative consistency is observed between the structure of smooth- and rough-wall flow, some quantitative differences are observed. For example, a slight reduction in the streamwise extent of two-point correlations of streamwise velocity, rhouu, is noted both within the roughness sublayer and the log layer of the flow. This reduction could be indicative of roughness-induced modifications of outer-layer vortex organization. This reduction in streamwise extent is also noted in spatial correlations of swirling strength. However, spatial correlations of the other velocity components, which are more representative of the smaller-scale features of outer-layer organization, show little sensitivity to roughness as they collapse well with smooth-wall results. Proper orthogonal decomposition (POD) is also employed to aid in studying the large- and small-scale features of these flows and this analysis reveals that the larger scales structures of the flow are most sensitive to roughness. Despite this sensitivity, the contributions of larger- and smaller-scales to the turbulent stresses are quite similar in the smooth- and rough-wall cases. Finally, conditional averaging reveals the importance of hairpin-like vortices and their organization into larger-scale packets to the production of Reynolds shear stress in both flows, particularly through the generation of intense ejection and sweep events.Made available in DSpace on 2015-09-28T16:23:48Z (GMT). No. of bitstreams: 2
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Previous issue date: 2008Embargo set by: Seth Robbins for item 89030
Lift date: Forever
Reason: Restricted to the U of I community idenfinitely during batch ingest of legacy ETDsRestricted to the U of I community idenfinitely during batch ingest of legacy ETDsU of I Only322 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008
Spatially and temporally resolved measurements of turbulent Rayleigh-Bénard convection by Lagrangian particle tracking of long-lived helium-filled soap bubbles
Get PDFRayleigh-Bénard convection (RBC), where a fluid is heated from below and cooled from above, is a prevalent model system to study the fundamentals of thermal convection. Typical for the turbulent RBC system is the occurrence of a large-scale circulation (LSC), which develops by self-organization of thermal plumes, erupted from the thermal boundary layers. In cylindrical samples of aspect ratios close to unity with a high degree of symmetry, the LSC reveals complex short- and long-term dynamics, which has been studied extensively in the past. Direct volumetric measurements of the LSC, however, allowing for a direct insight into the underlying turbulent processes are still rare. To bridge this gap, we performed Lagrangian Particle Tracking (LPT) by using a multi-camera setup, long-lasting, helium-filled soap bubbles and high-power LED arrays. With the "Shake-The-Box" Lagrangian particle tracking algorithm, we were able to instantaneously track up to 560,000 particles in the complete sample volume (~ 1 m³), corresponding to mean inter-particle distances down to 6-8 Kolmogorov lengths. We used the data assimilation scheme ‘FlowFit’, which involves continuity and Navier-Stokesconstraints, to map the scattered velocity and acceleration data on cubic grids, herewith recovering the smallest flow scales. Lagrangian and Eulerian visualizations reveal the dynamics of the large-scale circulation and its interplay with small scale structures, such as thermal plumes and turbulent background fluctuations. As a result, the complex time-dependent behavior of the LSC comprising azimuthal rotations, torsional oscillation and sloshing can be extracted from the data. Further, we found more seldom dynamic events, such as spontaneous reorientations of the LSC in the data from long-term measurements
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
Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB
Get PDFBioaerosol transport, dispersion and (local) accumulation processes in closed rooms have been identified as the main reasons for indirect or airborne SARS-CoV-2 transmission from person to person (Morawska and Cao 2020). Understanding short- and long-term airborne transport mechanisms of viruses via small bioaerosol particles (< 5 µm) inside populated rooms is an important key factor for optimizing various mitigations strategies (Morawska et al. 2020). Therefore, a large-scale 3D Lagrangian Particle Tracking (LPT) experiment enabling the instantaneous tracking of up to ~3 million submillimeter helium-filled-soap-bubbles (HFSBs) as passive tracers in a 12 m³ generic test room has been performed at DLR Göttingen, which allows to fully resolve the flow field in the complete room around a heated cyclically breathing thermal manikin with and without mouth-nose-masks or shields applied (see Figure 1). Six high-resolution CMOS streaming cameras (50 and 25 MP), an array of powerful pulsed LEDs and the Shake-The-Box (Schanz et al. 2016) LPT algorithm have been combined
Structure of turbulent channel flow perturbed by cylindrical roughness elements
Get PDFThe 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
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University of Illinois Theses & Dissertations (ID: 1)
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An Experimental Study of Momentum and Thermal Transport in Flow Through Smooth- and Rough-Wall Microchannels
No full textLocal Nusselt number (Nu) for smooth-wall laminar flow in the range 200 ≤ Re ≤ Recr agree well with macroscale predictions in both the thermally-developing and -developed regimes With increasing roughness, while an enhancement in local Nu is noted in the thermally-developing regime, no measurable influence is observed upon attainment of a thermally-developed state. These observations are supported by temperature profiles which suggest that the thermal boundary layer may be regenerated locally by roughness in the thermal entrance region of the flow. In the transitional regime, mean temperature profiles for the smooth- and rough-wall cases show deviation from fully-developed laminar behavior for Re > Recr. Finally, estimates of bulk Nu indicate enhancement in convective heat transfer over the smooth-wall case with increasing surface roughness in the laminar, transitional and turbulent regimes, though the smooth-wall data are in excellent agreement with macroscale predictions for laminar and turbulent flow. While a shift in the transitional pathway of the thermal transport behavior toward lower Re appears partially attributable for this enhancement with increasing roughness, it appears that turbulent convection also contributes significantly once transition is initiated.Made available in DSpace on 2015-09-28T16:23:50Z (GMT). No. of bitstreams: 2
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Previous issue date: 2009Embargo set by: Seth Robbins for item 89040
Lift date: Forever
Reason: Restricted to the U of I community idenfinitely during batch ingest of legacy ETDsRestricted to the U of I community idenfinitely during batch ingest of legacy ETDsU of I Only210 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009
Investigation of turbulent boundary layer flows with adverse pressure gradient by means of 3D Lagrangian particle tracking with Shake-The-Box
Get PDFA large-scale 3D Lagrangian particle tracking (LPT) investigation of a turbulent boundary layer (TBL) flow developing across different pressure gradient regions is presented in this study. Three high-speed multi-camera imaging systems, LED illumination and helium-filled soap bubbles (HFSB) tracers have been adopted to produce time-resolved sequences of particle images over a large volume encompassing approximately 3 m in the streamwise direction, 0:8 m in the spanwise direction and 0:25 m in the wall-normal direction. Individual tracers have been reconstructed and tracked within the imaged volume by means of the Shake-The-Box algorithm (STB, Schanz et al. (2016)); the FlowFit data assimilation algorithm (Gesemann et al. (2016)) has been used to evaluate the spatial velocity gradients and to interpolate the scattered LPT results onto a regular grid. Thanks to the large size of the investigated volume and to the time-resolved nature of the recorded images, the entire spatial extent of the large-scale coherent motions within the logarithmic region of the TBL (i.e. superstructures) could be captured and their dynamics investigated during their development over several boundary layer thickness in the streamwise direction, from the zero pressure gradient region (ZPG) to the adverse pressure gradient region (APG). Two free-stream velocities were investigated, namely 7 and 14m=s, corresponding to Ret ~ 3,000 and 5,000 respectively.
The results confirm the location and scale of the elongated high- and low-momentum structures in the logarithmic region, as well as their meandering in the spanwise direction. Two-point correlation statistics show that the width and spacing of the superstructures are not affected by the transition from the ZPG to the APG region. The analysis of the instantaneous flow realizations from both a Lagrangian and Eulerian perspective indicates the presence of significant fluid particle elements exchange across the interfaces of the large-scale structures
Main results of the first Lagrangian Particle Tracking Challenge
Get PDFThis work presents the main results of the first Lagrangian Particle Tracking challenge, conducted within the framework of the European Union’s Horizon 2020 project HOMER (Holistic Optical Metrology for Aero-Elastic Research), grant agreement number 769237. The challenge, jointly organised by the research groups of DLR, ONERA and TU Delft, considered a synthetic experiment reproducing the wall-bounded flow in the wake of a cylinder which was simulated by LES. The participants received the calibration images and sets of particle images acquired by four virtual cameras, and were asked to produce as output the particles positions, velocities and accelerations (when possible) at a specific time instant. Four different image acquisition strategies were addressed, namely two-pulse (TP), four-pulse (FP) and time-resolved (TR) acquisitions, each with varying tracer particle concentrations (or number of particles per pixel, ppp). The participants’ outputs were analysed in terms of percentages of correctly reconstructed particles, missed particles, ghost particles, correct tracks and wrong tracks, as well as in terms of position, velocity and acceleration errors, along with their distributions. The analysis of the results showed that the best-performing algorithms allow for a correct reconstruction of more than 99% of the tracer particles with positional errors below 0.1 pixels even at ppp values exceeding 0.15, whereas other algorithms are more prone to the presence of ghost particles already for ppp < 0.1. While the velocity errors remained contained within a small percentage of the bulk velocity, acceleration errors as large as 50% of the actual acceleration magnitude were retrieved
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