2,112 research outputs found
Vortex organization in a turbulent boundary layer overlying sparse roughness elements
No full textVortex organization in the outer layer of a turbulent boundary layer overlying sparse, hemispherical roughness elements is explored with two-component particle-image velocimetry (PIV) in multiple streamwise-wall-normal measurement planes downstream and between elements. The presence of sparse roughness elements causes a shortening of the streamwise length scale in the near-wall region. These measurements confirm that vortex packets exist in the outer layer of flow over rough walls, but that their organization is altered, and this is interpreted as the underlying cause of the length-scale reduction. In particular, the elements shed vortices which appear to align in the near-wall region, but are distinct from the packets. Further, it is observed that ejection events triggered in the element wakes are more intense compared to the ejection events in smooth wall. We speculate that this may initiate a self-sustaining mechanism leading to the formation of hairpin packets as a much more effective instability compared to those typical of smooth-wall turbulence.This is an Author's Accepted Manuscript of an article published as Michele Guala , Christopher D. Tomkins , Kenneth T. Christensen , Ronald J. Adrian. (2012) Vortex organization in a turbulent boundary layer overlying sparse roughness elements, Journal of Hydraulic Research, 50(5), 465-481 http://dx.doi.org/10.1080/00221686.2012.729229
Copyright Taylor & Francis, available online at: http://www.tandfonline.com/doi/abs/10.1080/00221686.2012.72922
Reynolds-stress enhancement associated with a short fetch of roughness in wall turbulence
No full textParticle-image velocimetry experiments are performed to study the response of smooth-wall turbulent channel flow to a short fetch of roughness (ten outer length scales long). The roughness studied herein is replicated from a surface scan of a damaged turbine blade and contains both large- and small-scale surface defects attributable to pitting, deposition and spallation. Quadrant analysis is used to investigate the characteristics of Reynolds-stress-producing events within the internal layer that develops over the roughness. The total mean Reynolds stress is dramatically increased in the presence of the roughness as compared to the smooth-wall baseline owing to an increased number of extremely intense ejections and sweeps. In contrast, inward and outward interactions, as well as relatively weak ejection and sweep events, are found to be insensitive to the surface conditions. While the stress and space fractions for all Reynolds-stress-producing events are found to be insensitive to the surface topology, the most intense ejection and sweep events yield stress and space fractions that vary significantly with the local surface topology.is peer reviewedSubmitted by Sarah Shreeves ([email protected]) on 2007-01-18T22:37:05Z
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Previous issue date: 2006-01published or submitted for publicatio
Liftings for noncomplete probability spaces
No full textThe current state of knowledge concerning liftings for noncomplete probability spaces is discussed. This is a somewhat expanded version of the author's talk given at the 1991 Summer Conference on General Topology and Applications in Honor of Mary Ellen Rudin and Her Work.PT: S; CR: BURKE MR, IN PRESS P AM MATH S BURKE MR, 1991, ISRAEL J MATH, V73, P33 BURKE MR, 1992, ISRAEL J MATH, V79, P289 CARLSON T, THEOREM LIFTING CHRISTENSEN JPR, 1974, TOPOLOGY BOREL STRUC FREMLIN DH, 1989, HDB BOOLEAN ALGEBRAS, P877 INOESCUTULCEA A, 1966, 5TH P BERK S MATH ST, V2 IONESCUTULCEA A, 1967, CONTRIBUTIONS PROB 1, P63 IONESCUTULCEA A, 1969, TOPICS THEORY LIFTIN JECH TJ, 1978, SET THEORY JOHNSON RA, 1980, P AM MATH SOC, V80, P234 JUST W, IN PRESS T AM MATH S KUPKA J, 1983, INDIANA U MATH J, V32, P717 LOSERT V, 1983, LNM, V1080, P95 MAHARAM D, 1958, P AM MATH SOC, V9, P987 SHELAH S, 1983, ISRAEL J MATH, V45, P90 TALAGRAND M, 1982, P AM MATH SOC, V84, P379 VONNEUMANN J, 1931, CRELLES J MATH, V165, P109; NR: 18; TC: 0; J9: ANN N Y ACAD SCI; PG: 4; GA: BZ86BSource type: Electronic(1
Measurement of instantaneous Eulerian acceleration fields by particle-image accelerometry: Methods and accuracy
No full textAcceleration is a fundamental quantity in fluid mechanics because it reflects the sum of all forces (pressure and viscous) present within the flow. However, measurements of acceleration have been difficult to achieve relative to the ease with which fluid velocity can be measured. A particle-image accelerometer (PIA) has been developed to measure Eulerian acceleration fields by time-differencing successive measurements of the Eulerian velocity field as measured by particle-image velocimetry (PIV). The measurements can also be made in uniformly translating frames. With current video camera technology, it is often not possible to measure the two velocity fields with a time separation sufficiently small enough to permit accurate finite difference approximation of the time derivative. A two-CCD-camera system has been developed to alleviate this limitation. Polarization filtering is utilized to separate the particle images viewed by each camera. The polarization filtering is achieved using cross-polarized light-sheets and a polarization filter just upstream of the imaging optics of the cameras. In this manner, PIV measurements can be achieved easily at time delays several orders of magnitude smaller than the shutter-time of the CCD cameras. The accuracy of the acceleration measurements is determined by numerical finite differencing errors and random noise and bias errors associated with the measurement of velocity. These errors, and methods of compensating for them, are studied.is peer reviewedMade available in DSpace on 2007-03-08T21:29:06Z (GMT). No. of bitstreams: 1
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Previous issue date: 2001-12published or submitted for publicatio
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
No full textRayleigh-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
Large-scale 3D flow investigations around a cyclically breathing thermal manikin in a 12 m³ room using HFSB and STB
No full textBioaerosol 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
No full textThe 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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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
No full textA 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
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