| 1. |
- Amini, Kasra, et al.
(author)
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Scaling laws for near-wall flows of thixo-elasto-viscoplastic fluids in a millifluidic channel
- 2024
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In: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 36:2
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Journal article (peer-reviewed)abstract
- Thixo-elasto-viscoplastic (TEVP) fluids are very complex fluids. In addition to elasticity and viscoplasticity, they exhibit thixotropy, i.e., time-dependent rheology due to breakdown and recovery of internal structures at different length- and timescales. General and consistent methods for a priori flow prediction of TEVP fluids based on rheological characteristics are yet to be developed. We report a combined study of the rheology and flow of 18 samples of different TEVP fluids (three yogurts and three concentrations of Laponite and Carbopol, respectively, in water in both the unstirred and a stirred state). The rheology is determined both with standard protocols and with an ex situ protocol aiming at reproducing the shear history of the fluid in the flow. Micrometer resolution flow measurements in a millimeter scale rectangular duct are performed with Doppler Optical Coherence Tomography (D-OCT). As expected, the results show the existence of a plug flow region for samples with sufficiently high yield stress. At low flow rates, the plug extends almost all the way to the wall and the extent of the plug decreases not only with increased flow rate but also with increased thixotropy. The ex situ rheology protocol enables estimation of the shear rate and shear stress close to the wall, making it possible to identify two scaling laws that relates four different non-dimensional groups quantifying the key properties wall-shear stress and slip velocity. The scaling laws are suggested as an ansatz for a priori prediction of the near-wall flow of TEVP fluids based on shear flow-curves obtained with a rheometer.
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| 2. |
- Leskovec, Martin, et al.
(author)
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Pipe flow with large particles and their impact on the transition to turbulence
- 2020
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In: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 5:11
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Journal article (peer-reviewed)abstract
- The classical transition from laminar to turbulent flow is affected if solid particles are added. The transition behavior is a function of particle size d and solid volume fraction phi and the flow undergoes a smooth transition, as opposed to intermittent, if phi exceeds a certain threshold. In this work we show that, for particle-laden pipe flows with large particle-to-pipe diameter ratios d/D, the phi threshold for altering the transition is much lower than previously reported for smaller particles. Magnetic resonance velocimetry reveals that particles introduce turbulent-like fluid velocity fluctuations in laminar flow. Factors that might control the limits between "classical" and "smooth" transition in the state space spanned by d/D and phi are discussed based on scaling analyses.
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| 3. |
- Shemesh, Noam, et al.
(author)
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Conventions and nomenclature for double diffusion encoding NMR and MRI.
- 2016
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In: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 75:1, s. 82-87
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Research review (peer-reviewed)abstract
- Stejskal and Tanner's ingenious pulsed field gradient design from 1965 has made diffusion NMR and MRI the mainstay of most studies seeking to resolve microstructural information in porous systems in general and biological systems in particular. Methods extending beyond Stejskal and Tanner's design, such as double diffusion encoding (DDE) NMR and MRI, may provide novel quantifiable metrics that are less easily inferred from conventional diffusion acquisitions. Despite the growing interest on the topic, the terminology for the pulse sequences, their parameters, and the metrics that can be derived from them remains inconsistent and disparate among groups active in DDE. Here, we present a consensus of those groups on terminology for DDE sequences and associated concepts. Furthermore, the regimes in which DDE metrics appear to provide microstructural information that cannot be achieved using more conventional counterparts (in a model-free fashion) are elucidated. We highlight in particular DDE's potential for determining microscopic diffusion anisotropy and microscopic fractional anisotropy, which offer metrics of microscopic features independent of orientation dispersion and thus provide information complementary to the standard, macroscopic, fractional anisotropy conventionally obtained by diffusion MR. Finally, we discuss future vistas and perspectives for DDE. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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| 4. |
- Abbasi Hoseini, A., et al.
(author)
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Finite-length effects on dynamical behavior of rod-like particles in wall-bounded turbulent flow
- 2015
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In: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 76, s. 13-21
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Journal article (peer-reviewed)abstract
- Combined Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) measurements have been performed in dilute suspensions of rod-like particles in wall turbulence. PIV results for the turbulence field in the water table flow apparatus compared favorably with data from Direct Numerical Simulations (DNS) of channel flow turbulence and the universality of near-wall turbulence justified comparisons with DNS of fiber-laden channel flow. In order to examine any shape effects on the dynamical behavior of elongated particles in wall-bounded turbulent flow, fibers with three different lengths but the same diameter were used. In the logarithmic part of the wall-layer, the translational fiber velocity was practically unaffected by the fiber length l. In the buffer layer, however, the fiber dynamics turned out to be severely constrained by the distance z to the wall. The short fibers accumulated preferentially in low-speed areas and adhered to the local fluid speed. The longer fibers (l/z > 1) exhibited a bi-modal probability distribution for the fiber velocity, which reflected an almost equal likelihood for a long fiber to reside in an ejection or in a sweep. It was also observed that in the buffer region, high-speed long fibers were almost randomly oriented whereas for all size cases the slowly moving fibers preferentially oriented in the streamwise direction. These phenomena have not been observed in DNS studies of fiber suspension flows and suggested l/z to be an essential parameter in a new generation of wall-collision models to be used in numerical studies.
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| 5. |
- Ahlberg, Charlotte, 1974-
(author)
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An experimental study of fiber suspensions between counter-rotating discs
- 2009
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Licentiate thesis (other academic/artistic)abstract
- The behavior of fibers suspended in a flow between two counter-rotating discs has been studied experimentally. This is inspired by the refining process in the papermaking process where cellulose fibers are ground between discs in order to change performance in the papermaking process and/or qualities of the final paper product. To study the fiber behavior in a counter-rotating flow, an experimental set-up with two glass discs was built. A CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the discs. Experiments were performed for gaps of 0.1-0.9 fiber lengths, and for equal absolute values of the angular velocities for the upper and lower disc. The aspect ratios of the fibers were 7, 14 and 28. Depending on the angular velocity of the discs and the gap between them, the fibers were found to organize themselves in fiber trains. A fiber train is a set of fibers positioned one after another in the tangential direction with a close to constant fiber-to-fiber distance. In the fiber trains, each individual fiber is aligned in the radial direction (i.e. normal to the main direction of the train). The experiments show that the number of fibers in a train increases as the gap between the discs decreases. Also, the distance between the fibers in a train decreases as the length of the train increases, and the results for short trains are in accordance with previous numerical results in two dimensions.Furthermore, the results of different aspect ratios imply that there are three-dimensional fiber end-effects that are important for the forming of fiber trains.
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| 6. |
- Ahlberg, Charlotte, et al.
(author)
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Self-organization of fibers in a suspension between two counter-rotating discs
- 2009
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In: Proceedings of the ASME Fluids Engineering Division Summer Conference 2009. - NEW YORK : AMER SOC MECHANICAL ENGINEERS. - 9780791843727 ; , s. 585-592
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Conference paper (peer-reviewed)abstract
- The behavior of fibers suspended in a flow between two flat counter-rotating discs has been studied experimentally. Captured images of the fibers in the flow were analyzed by steerable filters, to extract positions and orientations of the fibers. Experiments were performed for gaps between the discs of less than one fiber length, and for equal absolute values of the angular velocities for the discs. The length-to-diameter ratio of the fibers was approximately 14. During certain conditions, the fibers organized themselves in a distinct manner, which we will denote as fiber trains, in which three or more fibers are aligned next to each other, at the same radial position, with a short fiber-to-fiber distance. The direction of the individual fibers is radial and the direction of the whole train is tangential. Trains containing more than 60 fibers have been observed and are quite impressing.
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| 7. |
- Ananthaseshan, S., et al.
(author)
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Red blood cell distribution width is associated with increased interactions of blood cells with vascular wall
- 2022
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In: Scientific Reports. - : Springer Nature. - 2045-2322. ; 12:1
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Journal article (peer-reviewed)abstract
- The mechanism underlying the association between elevated red cell distribution width (RDW) and poor prognosis in variety of diseases is unknown although many researchers consider RDW a marker of inflammation. We hypothesized that RDW directly affects intravascular hemodynamics, interactions between circulating cells and vessel wall, inducing local changes predisposing to atherothrombosis. We applied different human and animal models to verify our hypothesis. Carotid plaques harvested from patients with high RDW had increased expression of genes and proteins associated with accelerated atherosclerosis as compared to subjects with low RDW. In microfluidic channels samples of blood from high RDW subjects showed flow pattern facilitating direct interaction with vessel wall. Flow pattern was also dependent on RDW value in mouse carotid arteries analyzed with Magnetic Resonance Imaging. In different mouse models of elevated RDW accelerated development of atherosclerotic lesions in aortas was observed. Therefore, comprehensive biological, fluid physics and optics studies showed that variation of red blood cells size measured by RDW results in increased interactions between vascular wall and circulating morphotic elements which contribute to vascular pathology.
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| 8. |
- Arnelo, Urban, et al.
(author)
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Single-operator pancreatoscopy is helpful in the evaluation of suspected intraductal papillary mucinous neoplasms (IPMN)
- 2014
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In: Pancreatology (Print). - : Elsevier. - 1424-3903 .- 1424-3911. ; 14:6, s. 510-514
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Journal article (peer-reviewed)abstract
- BACKGROUND AND OBJECTIVE: Even when advanced cross-sectional imaging modalities have been employed, endoscopic evaluation of intraductal papillary mucinous neoplasms (IPMN) is often required in order to assess the final character and extent of lesions. The current study addresses the use of SpyGlass single-operator peroral pancreatoscopy in suspected IPMN.DESIGN: A prospective, non-randomized exploratory cohort study.SETTING: Single-center.PATIENTS AND INTERVENTION: A prospective study-cohort of 44 consecutive patients in a single tertiary referral center who underwent ERCP and peroral pancreatoscopy, was prospectively collected between July 2007 and March 2013 because of a radiological signs of IPMN. These IPMN-findings were discovered incidentally in 44% of the cases.MAIN OUTCOME MEASUREMENTS: Diagnostic accuracy (specificity & sensitivity) and complications.RESULTS: The targeted region of the pancreatic duct was reached with the SpyGlass system in 41 patients (median age 65 years, 41% female). Three patients were excluded from analysis because of failed deep cannulation of the pancreatic duct. Brush cytology was taken in 88% and direct biopsies in 41%. IPMN with intermediate or high-grade dysplasia was the main final diagnosis (76%) in 22 patients who had surgery. Out of the 17 patients with a final diagnosis of MD-IPMN, 76% were correctly identified by pancreatoscopy. Of the 9 patients with a final diagnosis of BD-IPMN, the pancreatoscopy identified 78% of the cases correctly.The incidence of post-ERCP pancreatitis was 17%. Pancreatoscopy was found to have provided additional diagnostic information in the vast majority of the cases and to affect clinical decision-making in 76%.LIMITATIONS: Single-center study.CONCLUSIONS: Single-operator peroral pancreatoscopy contributed to the clinical evaluation of IPMN lesions and influenced decision-making concerning their clinical management. The problem of post-procedural pancreatitis needs further attention.
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| 9. |
- Bagge, Joar, 1991-, et al.
(author)
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Parabolic velocity profile causes shape-selective drift of inertial ellipsoids
- 2021
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In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 926
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Journal article (peer-reviewed)abstract
- Understanding particle drift in suspension flows is of the highest importance in numerous engineering applications where particles need to be separated and filtered out from the suspending fluid. Commonly known drift mechanisms such as the Magnus force, Saffman force and Segre-Silberberg effect all arise only due to inertia of the fluid, with similar effects on all non-spherical particle shapes. In this work, we present a new shape-selective lateral drift mechanism, arising from particle inertia rather than fluid inertia, for ellipsoidal particles in a parabolic velocity profile. We show that the new drift is caused by an intermittent tumbling rotational motion in the local shear flow together with translational inertia of the particle, while rotational inertia is negligible. We find that the drift is maximal when particle inertial forces are of approximately the same order of magnitude as viscous forces, and that both extremely light and extremely heavy particles have negligible drift. Furthermore, since tumbling motion is not a stable rotational state for inertial oblate spheroids (nor for spheres), this new drift only applies to prolate spheroids or tri-axial ellipsoids. Finally, the drift is compared with the effect of gravity acting in the directions parallel and normal to the flow. The new drift mechanism is stronger than gravitational effects as long as gravity is less than a critical value. The critical gravity is highest (i.e. the new drift mechanism dominates over gravitationally induced drift mechanisms) when gravity acts parallel to the flow and the particles are small.
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| 10. |
- Bellani, Gabriele, 1981-
(author)
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Experimental Studies of Complex Flows through Image-Based Techniques
- 2011
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Doctoral thesis (other academic/artistic)abstract
- This thesis deals with the development of experimental techniques for the study of complex flows inspired to a large extent by the papermaking process. In particular one part of this thesis is devoted to the development of laboratory experiments based on index-of-refraction matching and imaging techniques to study the behavior of dilute and concentrated suspension of elongated particles. Another part is aimed at exploring the potential of the synergy between experiments and numerical simulations to access quantities otherwise not-measurable in complex flows. Highspeedimaging experiments have been specifically designed for this purpose. The first of the Refractive IndexMatching (RIM) experiment was aimed at studying the flow generated during the filtration of a fiber suspension using Particle Image Velocimetry (PIV) and pressure drop measurements. The experiments were performed in a vertical laboratory filtration device. Index of refraction matching of fibers and fluids allowed measurements to be performed in the proximity and, to some extent, in the forming network during filtration. The area over which the forming network induces velocity gradients has been measured and have been found to be independent of the Reynolds number but dependent on the fiber length and the structure of the network. Analysis of the flow scales in the proximity of the network showed that the signature of the mesh used to filter the suspension is never completely suppressed as the network thickness increases. Also, pressure drop measurements over a static fiber network have been performed. A linear dependence of the pressure drop with the basis weight (mass of fibers in the network per unit area) and a non-dimensional filtration resistance independent of filtration velocity and network thickness (if network compressibility is accounted for) was found. These findings can help explain characteristics that are observed on paper sheets and help improvede watering efficiency. The second RIM experiment was aimed at measuring the interactions of Taylorscale elongated particles with turbulence. RIM particles with embedded tracers and Stereoscopic PIV were combined to simultaneously measure fluid phase and particle velocity. The novelty of this technique is that it allows to measure the three-dimensional angular velocity vector of arbitrarily shaped particles. This technique was applied to study the interaction of neutrally buoyant ellipsoidal particles with stationary homogeneous isotropic turbulence. The results were compared to the case of spherical particles. The main result is that both spherical and ellipsoidal particles provide enhancement of the small scales and reduction of the large scales at volume concentrations as low as 0.1%. However, the reduction of the large scales was much more evident for spherical particles. These results highlight the fact that particle elongation introduces different mechanisms of turbulent modulation as compared to the spherical particles. The first of the high-speed imaging experiments was to provide a database for test and validation of a CFD-based flow observer for complex flows. For this purpose time resolved measurements of a turbulent confined jet have been performed with high-speed PIV. The measurements have been used both as a feedback signal and as a reference for the evaluation of a CFD-based estimator for complex flows. Furthermore, based on the measurements Kalman filters have been designed and implemented in the observer. The experimental data have also been used to compare two modal decompositions, namely Proper Orthogonal Decomposition and Dynamical Modal Decomposition and evaluate their ability to describe the global behavior of complex flow. The second of the high-speed imaging experiment was applied to study spreading of a droplet on a solid surface. These experiments have been performed with extremely high time-resolution (140000 fps), over a range of parameters (in terms of droplet viscosity, equilibrium contact angle and droplet size) larger than any other experiment reported in the literature in a single work. By combining the experiments and direct numerical simulations a dissipative mechanisms arising from the contact line movement has been identified and the corresponding macroscopic coefficient has been measured.i
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