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  • 1.
    Altimira, Mireia
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Teaching Research Methodologies2016In: INTED2016 Proceedings, IATED , 2016Conference paper (Refereed)
    Abstract [en]

    This paper presents the methodology employed in the Research Methodology course, part of the Master Program in Engineering Mechanics of the Royal Institute of Technology (KTH). As a higher education institution, KTH aims at excellence in both generation and dissemination of knowledge. Even though these two activities are traditionally seen as independent –or even mutually exclusive-, there is a positive correlation between performance in research andin pedagogical activities, reinforced by the fact that inquiry-based or researchbased learning activities enhance deep learning among the students. The course Research Methodology in Engineering Mechanics poses a unique opportunity to engage the students to research in different areas through diversity-oriented learning activities.The course’s main learning outcome is that the students become acquainted with the most common concepts and research methodologies used in the fields of Fundamental Mechanics, Solid Mechanics, Fluid Mechanics, Acoustics and Biomechanics. After the completion of the course, the student should also be able to identify and analyze the methodologies in a given published work.The course consists of 9 lectures and a group project, with an estimation of the total dedication time of 80h (3hp). One lecture is focused on research ethics;while in the other 8 researchers from KTH present their areas of expertise, introducing the most relevant methodologies applied. The attendance is about40 students.In the last two years, and based on the course assessment survey, the structure of the course has been continuously shifting from traditional lectures to cover a broader range of teaching activities. In this way, different learning styles are covered and the learning outcomes can be achieved by as many students as possible. In this regard, traditional lectures are combined with problem-based or case-based lectures, and role-play. Additionally, in the content of the lectures we also try to keep a balance between experimental and numerical research methods of the different disciplines, in such a way that the students get a holistic view of the research in that particular field.This project involves reading a research journal article in the subject of engineering mechanics and presenting, in written and oral form, a critical analysis of the methodologies employed. In the beginning of their project, the students give an oral presentation of their article to another group. At the end of this activity, each group is asked to mention a positive aspect of the presentation they just heard and something that they believe should be improved. As an additional task, each group is asked to peer-review another group’s report. To do so, the students are given a document with some guidelines and evaluation criteria. Special instructions are given to make sure positive feedback is also included in the review. With this, the students get to read the work of others, learn to apply quality criteria and give feedback, and self-reflect on their own work after the review process.

  • 2.
    Håkansson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fall, Andreas
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Yu, Sun
    DESY, Hamburg Germany.
    Krywka, Christina
    Institute of experimental and applied physics. Kiel Germany.
    Roth, Stephan
    DESY, Hamburg Germany.
    Santoro, Gonzalo
    DESY, Hamburg Germany.
    Kvick, Mathias
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Innventia AB, Stockholm Sweden.
    Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments2014In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 4018-Article in journal (Refereed)
    Abstract [en]

    Cellulose nanofibrils can be obtained from trees and have considerable potential as a building block for biobased materials. In order to achieve good properties of these materials, the nanostructure must be controlled. Here we present a process combining hydrodynamic alignment with a dispersion-gel transition that produces homogeneous and smooth filaments from a low-concentration dispersion of cellulose nanofibrils in water. The preferential fibril orientation along the filament direction can be controlled by the process parameters. The specific ultimate strength is considerably higher than previously reported filaments made of cellulose nanofibrils. The strength is even in line with the strongest cellulose pulp fibres extracted from wood with the same degree of fibril alignment. Successful nanoscale alignment before gelation demands a proper separation of the timescales involved. Somewhat surprisingly, the device must not be too small if this is to be achieved.

  • 3.
    Håkansson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Kvick, Mathias
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Measurement of width and streakiness of particle streaks in turbulent flowsArticle in journal (Other academic)
  • 4.
    Håkansson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fall, Andreas B.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Continuous assembly of aligned nanofibrils into a micro filamentManuscript (preprint) (Other academic)
  • 5.
    Håkansson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Alignment of cellulose nanofibrils in a flow focusing device: mea-surements and calculations of flow and orientationManuscript (preprint) (Other academic)
  • 6.
    Håkansson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wågberg, Lars
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Orientation of nano-fibrillated cellulose in accelerated flowManuscript (preprint) (Other academic)
  • 7.
    Håkansson, Karl M. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Kvick, Mathias
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Measurement of width and intensity of particle streaks in turbulent flows2013In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 6, p. 1555-Article in journal (Refereed)
    Abstract [en]

    Fibre streaks are observed in experiments with fibre suspensions in a turbulent half-channel flow. The preferential concentration methods, most commonly used to quantify preferential particle concentration, are in one dimension found to be concentration dependent. Two different new streak quantification methods are evaluated, one based on Voronoi analysis and the other based on artificial particles with an assigned fixed width. The width of the particle streaks and a measure of the intensity of the streaks, i.e. streakiness, are sought. Both methods are based on the auto-correlation of a signal, generated by summing images in the direction of the streaks. Common for both methods is a severe concentration dependency, verified in experiments keeping the flow conditions constant while the (very dilute) concentration of fibres is altered. The fixed width method is shown to be the most suitable method, being more robust and less computationally expensive. By assuming the concentration dependence to be related to random noise, an expression is derived, which is shown to make the streak width and the streakiness independent of the concentration even at as low concentrations as 0.05 particles per pixel column in an image. The streakiness is obtained by applying an artificial particle width equal to 20 % of the streak width. This artificial particle width is in this study found to be large enough to smoothen the correlation without altering the streakiness nor the streak width. It is concluded that in order to make quantitative comparisons between different experiments or simulations, the evaluation has to be performed with care and be very well documented.

  • 8.
    Håkansson, Karl M. O.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl-Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Nanofibril Alignment in Flow Focusing: Measurements and Calculations2016In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 120, no 27, p. 6674-6686Article in journal (Refereed)
    Abstract [en]

    Alignment of anisotropic supermolecular building blocks is crucial to control the properties of many novel materials. In this study, the alignment process of cellulose nanofibrils (CNFs) in a flow-focusing channel has been investigated using small-angle X-ray scattering (SAXS) and modeled using the Smoluchowski equation, which requires a known flow field as input. This flow field was investigated experimentally using microparticle-tracking velocimetry and by numerically applying the two-fluid level set method. A semidilute dispersion of CNFs was modeled as a continuous phase, with a higher viscosity as compared to that of water. Furthermore, implementation of the Smoluchowski equation also needed the rotational Brownian diffusion coefficient, which was experimentally determined in a shear viscosity measurement. The order of the nanofibrils was found to increase during extension in the flow-focusing channel, after which rotational diffusion acted on the orientation distribution, driving the orientation of the fibrils toward isotropy. The main features of the alignment and dealignment processes were well predicted by the numerical model, but the model overpredicted the alignment at higher rates of extension. The apparent rotational diffusion coefficient was seen to increase steeply as the degree of alignment increased. Thus, the combination of SAXS measurements and modeling provides the necessary framework for quantified studies of hydrodynamic alignment, followed by relaxation toward isotropy.

  • 9. Jadoon, A.
    et al.
    Prahl, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Revstedt, J.
    Dynamic interaction of fixed dual spheres for several configurations and inflow conditions2010In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 29, no 1, p. 43-52Article in journal (Refereed)
    Abstract [en]

    The changes in force characteristics as well as the shedding patterns for various dual sphere configurations are studied. The Reynolds numbers considered are 300, 600 and two different inflow conditions are used: steady and pulsating. The sphere formations are defined by the separation distance D-0 between the spheres and the angle between the line connecting the centres of the spheres and the main flow direction, gamma. The position of one of the spheres is varied in the range 0 degrees-90 degrees using a 15 degrees increment. Two separation distances are studied; 1.5D and 3D. The method used for the simulations is the Volume of Solid (VOS) approach, a method based on Volume of Fluid (VOF). A major conclusion from this work is that the sphere interaction alters the wake dynamics by obstructing the vortex shedding (generating a steady wake or a wake with lower Strouhal number) and by changing the direction of the lift force so that it in most cases is directed in the plane containing the sphere centres. The results also show that changing the inflow condition gives the same relative change in drag and lift as for a single sphere. The drag is substantially reduced by placing the sphere downstream in a tandem arrangement and slightly increased in a side-by-side arrangement. However, the effect is decreased by increasing separation distance and increasing Reynolds number.

  • 10.
    Kekesi, Timea
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Drop deformation and breakup in flows with shear2016In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 140, p. 319-329Article in journal (Refereed)
    Abstract [en]

    A Volume of Fluid (VOF) method is applied to study the deformation and breakup of a single liquid drop in shear flows superimposed on uniform flow. The effect of shearing on the breakup mechanism is investigated as a function of the shear rate. Sequential images are compared for the parameter range studied; density ratios of liquid to gas of 20, 40, and 80, viscosity ratios in the range 0.5-50, Reynolds numbers between 20, a constant Weber number of 20, and the non-dimensional shear rate of the flow G = 0-2.1875. It is found that while shear breakup remains similar for all values of shear rate considered, other breakup modes observed for uniform flows are remarkably modified with increasing shear rate. The time required for breakup is significantly decreased in strong shear flows. A simple model predicting the breakup time as a function of the shear rate and the breakup time observed in uniform flows is suggested.

  • 11. Krochak, P.
    et al.
    Fasci, Giuseppe Carmine
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Norman, B.
    Prahl-Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Bridging chemical dosage, mixing quality, and variability in paper sheets2015In: TAPPI Journal, ISSN 0734-1415, Vol. 14, no 5, p. 311-320Article in journal (Refereed)
    Abstract [en]

    In the first part of this work, a series of paper production trials were performed on a forming experimental (FEX) pilot machine to investigate the distribution of additives in the final product. In these trials, a blue color was dosed into the stock before the headbox instead of a retention aid. Fine paper sheets were produced using twin-wire forming. Visual inspection of the sheets revealed surprisingly high levels of variability of the blue color. In the second part, the effect of different dosage nozzle configurations on downstream mixing quality of a single-component, polyacrylamide retention aid was studied using two-phase computational fluid dynamics. A non-Newtonian model for this phase was implemented using rheological parameters obtained through a combination of numerical and experimental analysis. Dosage was made into a turbulent pipe flow under typical industrial approach flow conditions. The effect of the number of dosage points, impingement angle, dosage location, and dosage speed on mixing uniformity was investigated qualitatively and quantitatively. Results from these studies indicate the existence of optimal dosage configurations and point toward strong coupling between chemical addition strategy, mixing quality, and chemical variability in final products. Application: Mills can gain valuable information, including dosage nozzle configuration and dosage conditions, for optimizing mixing of retention aids in the approach flow during paper production.

  • 12. Krochak, P.
    et al.
    Fasci, Giuseppe Carmini
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Norman, B.
    Prahl-Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Bridging chemical dosage, mixing quality and variability in paper sheets2013In: Pap. Conf. Trade Show, PaperCon, 2013, p. 1057-1069Conference paper (Refereed)
  • 13.
    Kvick, Mathias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Håkansson, Karl
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fibre orientation and fibre streaks in turbulent wall bounded flowManuscript (preprint) (Other academic)
  • 14.
    Kvick, Mathias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Håkansson, Karl
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, Daniel
    Innventia.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fibre streaks in wall turbulent flow2010In: 7th Int. Conference on Multiphase Flow, Tampa, Florida, USA, may 30 - June 4, 2010, ICMF , 2010Conference paper (Refereed)
  • 15.
    Kvick, Mathias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    Effect of fibres on hydrodynami stability in a curved rotating channel2013In: ICMF2013, 2013, p. 674-Conference paper (Refereed)
  • 16.
    Kvick, Mathias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    Effect of fibrils on curvature- and rotation-induced hydrodynamic stability2013In: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 224, no 10, p. 2249-2261Article in journal (Refereed)
    Abstract [en]

    Flow of a suspension of water and nano-fibrillated cellulose (NFC) in a curved and rotating channel is studied experimentally and theoretically. The aim is to investigate how NFC affects the stability of the flow. This flow is subject to a centrifugal instability creating counter-rotating vortices in the flow direction. These rolls can be both stabilised and destabilised by system rotation, depending on direction and velocity of the rotation. Flow visualisation images with pure water and an NFC/water suspension are categorised, and stability maps are constructed. A linear stability analysis is performed, and the effect of fibrils is taken into account assuming straight fibrils and constant orientation distributions, i.e., without time-dependent flow-orientation coupling. The results show that NFC has a less stabilising effect on the primary flow instability than indicated from the increase in viscosity measured by a rotary viscometer, but more than predicted from the linear stability analysis. Several unknown parameters (the most prominent being fibril aspect ratio and the interaction parameter in the rotary diffusion) appear in the analysis.

  • 17.
    Kvick, Mathias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of nano-fibrillated cellulose on curvature- and rotation-induced instabilities in channel flowManuscript (preprint) (Other academic)
  • 18.
    Kvick, Mathias
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability of the flow in a flow-focusing deviceManuscript (preprint) (Other academic)
  • 19.
    Kékesi, Timea
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Drop deformation and breakup2014In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 66, p. 1-10Article in journal (Refereed)
    Abstract [en]

    A Volume of Fluid (VOF) method is applied to investigate the deformation and breakup of an initially spherical drop in the bag- and shear breakup regimes, induced by steady disturbances. The onset of breakup is sought by studying steady-shape deformations while increasing the Weber number until breakup occurs. A parameter study is carried out applying different material properties and a wide range of drop Reynolds numbers in the steady wake regime. Density ratios of liquid to gas of 20, 40, and 80, viscosity ratios in the range 0.5-50, and Reynolds numbers between 20 and 200 are investigated for a constant Weber number of 20. The critical Weber number is found to be 12, in agreement with observations of earlier studies. For Weber number of 20 varying density, viscosity ratios and Reynolds numbers, interesting mixed breakup modes are discovered. Moreover, a new regime map including all modes observed is presented. A criterion for the transition between bag-and shear breakup is defined relating the competing inertial and shear forces appearing in the flow. Furthermore, results on breakup times and the time history of the drag coefficient are presented; the latter is concluded to be a potential parameter to indicate the occurrence of breakup. (C) 2014 Elsevier Ltd. All rights reserved.

  • 20.
    Kékesi, Timea
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Corrigendum to: "Drop deformation and breakup". Int. J. Multiphase Flow, 66, (2014) 1-102016In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533Article in journal (Refereed)
  • 21.
    Kékesi, Tímea
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
    Altimira, Mireia
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Amberg, Gustav
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Interaction between two deforming liquid drops in tandem and various off-axis arrangements subject to uniform flowManuscript (preprint) (Other academic)
    Abstract [en]

    A Volume of Fluid (VOF) method is applied to study the interaction between two liquid drops with the same initial diameter in uniform flow. Various arrangements of the drops are studied, based on two parameters, namely the initial separation distance and the angle between the line connecting the centres of the drops and the free-stream direction. Specifically, initial separation distances of l = 1.5 − 5D drop diamters, and angles between β = 0◦ − 90◦ are considered. Simulations for a Weber number of W e = 20, two Reynolds numbers Re = 20 and 50, and density and viscosity ratios in the range ρ∗ = 20 − 80 and μ∗ = 0.5 − 50 are performed. The movement of the secondary drop with respect to the primary drop, and the time required for the breakup of the secondary drop as compared to those observed for single drops are evaluated. It is found that the drops collide only in cases corresponding to the shortest initial displacements, while in others they deform and break up independently, similar or identical to single drops. The same behaviour is reflected in the time required for breakup. Cases where the drops behave independently show breakup times close to those observed for single drops.

  • 22.
    Lacagnina, Giovanni
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Institute of Sound and Vibration Research (ISVR), University of Southampton, United Kingdom.
    Szász, Robert-Zoltán
    Department of Energy Sciences, LTH Faculty of Engineering, Lund University.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Experimental study on the forcing design for an intermittent injection2018In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 59, no 8, article id 123Article in journal (Refereed)
    Abstract [en]

    In this paper we consider the effects of acceleration and deceleration on the forcing of an intermittent jet. This experimental study specifically focuses on the effect of the acceleration and deceleration on the mixing of an intermittent jet with the ambient fluid and on the growth of disturbances that may lead to turbulence. The influence of different injection strategies has been evaluated. The results show that the deceleration phase may be able to contribute significantly to enhance the mixing of the jet with the ambient fluid. This effect is manifested primarily around the tail of the jet, towards the end of injection. The acceleration phase on the other hand has mainly impact at the leading part of the jet, where the leading part of the jet forms a mushroom shaped structure with minor mixing effect.

  • 23.
    Mylavarapu, Goutham
    et al.
    Aerospace Engineering, University of Cincinnati.
    Mihaescu, Mihai
    Aerospace Engineering, University of Cincinnati.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Murugappan, Shanmugam
    Otolargyngology, Head and Neck Surgery, University of Cincinnati-Medical Center.
    Prahl Wittberg, Lisa
    Dept. Energy Sciences, Division of Fluid Mechanics, Lund University.
    Fuchs, Laszlo
    Dept. Energy Sciences, Division of Fluid Mechanics, Lund University.
    Papatziamos, Georgios
    Karolinska Hospital Solna.
    Importance of paranasal sinuses in computational modeling of nasal airflow2009In: 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, AIAA , 2009, p. 2009-0772-Conference paper (Refereed)
    Abstract [en]

    This paper investigates the importance of including paranasal sinuses in the computational modeling of the nasal airflow. Three dimensional models of human nasal airway with and without including paranasal sinuses were reconstructed from Computed Tomography (CT) axial images of a subject with healthy nasal airway. The reconstruction process was performed using MIMICS® software program. The airway volume was discretized using TGRID® mesh generator. Steady Reynolds-Averaged Navier-Stokes (RANS) simulations were carried in both inspiratory and expiratory phases of respiratory cycle at a peak flow rate of 15 L/min in FLUENT®. The results show that the left and right nasal resistances change with less than 11% when paranasal sinuses are included in the computational model of the nasal airway. The flow into the sinuses is characterized by very low velocities during both inspiration and expiration conditions. The velocity distributions in the main nasal passage show small change predominantly in regions closer to the paranasal sinuses when compared to the model where sinuses were not included.

  • 24.
    Nygård, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Altimira, M.
    Semlitsch, Bernhard
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Analysis of vortical structures in intermittent jets2016In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, p. 3-10Conference paper (Refereed)
    Abstract [en]

    The manipulation of jets has since long been subject to research, due to the wide range of industrial applications in which they are used. A vast number of numerical and experimental studies concerning the physics of the breakup process of continuous jets have been published. Improvements in mixing and ambient gas entrainment have been reported experimentally when using intermittent injection, although the responsible mechanisms have not yet been completely revealed. This work presents a systematic analysis of the mechanisms of jet breakup and mixing with the surrounding fluid and its relation to vorticity generation and transport. Comparisons aremade between the redistribution of vorticity and the engulfment of ambient fluid into the core region for different injection strategies. © Springer International Publishing Switzerland 2016.

  • 25.
    Nygård, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Altimira, Mireia
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Disintegration Mechanisms of Intermittent Liquid Jets2016In: SAE International Journal of Fuels and Lubricants, ISSN 1946-3952, E-ISSN 1946-3960, Vol. 9, no 1, p. 91-99Article in journal (Refereed)
    Abstract [en]

    It has been observed that intermittent injection leads to improved spray characteristics in terms of mixing and gas entrainment. Although some experimental work has been carried out in the past, the disintegration mechanisms that govern the breakup of intermittent jets remain unknown. In this paper we have carried out a systematic numerical analysis of the breakup of pulsated jets under different injection conditions. More specifically, the duty cycle (share of active injection during one cycle) is varied, while the total cycle time is kept constant. The advection of the liquid phase is handled through the Volume of Fluid approach and, in order to provide an accurate, yet computationally acceptable, resolution of the turbulent structures, the implicit Large Eddy Simulation has been adopted. The results show that the primary disintegration results from a combination of stretching, collision and aerodynamic interaction effects. Moreover, there exists a strong coupling between stretching and collision as stretching makes the pulse thinner prior to the contact between pulses. In this work, the purpose is to study the collision contribution to breakup in terms of the near nozzle pulse disintegration rate. When approaching the low duty cycle limit, this effect is significant because of the lower liquid volume of the pulse. In contrast, for a high duty cycle, the stretching effect is limited and a wide tail region remains as an obstruction for following pulses. However, the integral momentum of the pulse is maintained to a larger degree that has an adverse effect on the outcome of the collision event.

  • 26.
    Nygård, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Altimira, Mireia
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Quantifying primary breakup in pulsating liquid2014In: / [ed] A. Eriksson, A. Kulachenko, M. Mihaescu and G. Tibert, 2014Conference paper (Refereed)
  • 27.
    Nygård, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes.
    Altimira, Mireia
    Lund University.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Interaction between liquid pulses during intermittent injection2014In: Proceedings of the 26th ILASS-Europe 2014, 2014Conference paper (Refereed)
  • 28. Prahl, L.
    et al.
    Hoelzer, A.
    Arlov, D.
    Revstedt, J.
    Sommerfeld, M.
    Fuchs, Laszlo
    On the interaction between two fixed spherical particles2007In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 33, no 7, p. 707-725Article in journal (Refereed)
    Abstract [en]

    The variation of the drag (CD) and lift coefficients (CL) of two fixed solid spherical particles placed at different positions relative each other is studied. Simulations are carried out for particle Reynolds numbers of 50, 100 and 200 and the particle position is defined by the angle between the line connecting the centers of the particles and the free-stream direction (a) and the separation distance (do) between the particles. The flow around the particles is simulated using two different methods; the Lattice Boltzmann Method (LBM), using two different computational codes, and a conventional finite difference approach, where the Volume of Solid Method (VOS) is used to represent the particles. Comparisons with available numerical and experimental data show that both methods can be used to accurately resolve the flow field around particles and calculate the forces the particles are subjected to. Independent of the Reynolds number, the largest change in drag, as compared to the single particle case, occurs for particles placed in tandem formation. Compared to a single particle, the drag reduction for the secondary particle in tandem arrangement is as high as 60%, 70% and 80% for Re = 50, 100 and 200, respectively. The development of the recirculation zone is found to have a significant influence on the drag force. Depending on the flow Situation in-between the particles for various particle arrangements, attraction and repulsion forces are detected due to low and high pressure regions, respectively. The results show that the inter-particle forces are not negligible even under very dilute conditions.

  • 29.
    Prahl, Lisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Jadoon, A.
    Revstedt, J.
    Interaction between two spheres placed in tandem arrangement in steady and pulsating flow2009In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 35, no 10, p. 963-969Article in journal (Refereed)
    Abstract [en]

    The interaction among two spheres in tandem formation are studied for a Reynolds number of 300 using both steady and pulsating inflow conditions. The purpose is to further investigate the force characteristics as well as the shedding patterns of the two spheres as the separation distance is changed from 1.5 to 12. sphere diameters. The method used for the simulations is the volume of solid (VOS) method, an approach based on the volume of fluid (VOF) method. Comparisons with other computational methods have shown VOS to accurately resolve the flow field around solid spheres. The results show that the separation distance plays a significant role in changing the flow patterns and shedding frequencies at moderate separation distances, whereas effect on drag is observed even at a separation distance of 12 diameters.

  • 30.
    Prahl Wittberg, Lisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Björkman, M.
    Khokhar, Gohar
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mohlin, U. -B
    Dahlkild, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Flow conditions in the grooves of a Low-Consistency refiner2012In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 2, p. 173-183Article in journal (Refereed)
    Abstract [en]

    The flow pattern in the grooves plays a major role for the homogeneity of refining as well as for the transfer and loading of fiber flocs in refining position on the bar edges. However, it is an area where very little information is available. In the present study, flow conditions in the grooves in a Low-Consistency (LC) - disc refiner were studied both experimentally and numerically. The experimental study involved high-speed imaging through a 3 cm peephole into a commercial refiner. The Computational Fluid Dynamics (CFD) simulation focused on the flow condition in a radial groove, considering both Newtonian and non-Newtonian flows. Flow conditions for stator and rotor grooves were modeled along the groove at different angular speeds and pressure differences over the refiner. Both the experimental and the modeling results show a dual flow pattern in the grooves; a rotational/spiral movement at the top of the groove and a flow in the direction of the groove at the bottom, which to the authors knowledge has not been reported in literature. The strong vortical motion at the top of the grooves observed both for the rotor and the stator are believed to be important for placing the fibers onto the bar edges and to induce shear forces in such a way that the fibers get treated. Moreover, a large sensitivity to suspension properties in terms of the development of flow pattern was detected.

  • 31.
    Prahl Wittberg, Lisa
    et al.
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Revstedt, Johan
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Fuchs, Laszlo
    Lunds Universitet, LTH, Div. Fluid mechanics.
    A study of the dynamics of dual-particles settling close to a vertical wall2005In: 4th interntional Conference on Turbulence and Shear Flow Phenomena, Williamsbrug, Viriginia, USA, June 27-29, 2005, 2005Conference paper (Refereed)
    Abstract [en]

    Simulations of single and multiple spherical particles settling under gravity in the presence of a vertical wall are performed. In this study, the Reynolds number based on a characteristic velocity set to unity and the diameter of the sphere is varied in the range from 1 to 1000 and the density ratio in the range from 1.5 to 8. Interaction between particles as well as with the wall is investigated. The particles are modeled by using the Volume of Solid (VOS) approach, a method based on the Volume of Fluid (VOF) approach.The simulations showed that the motion of both single and dual spheres falling side by side is affected by the wall. Also, a dominant frequency for the oscillatory movements perpendicular to the wall was detected.

  • 32.
    Prahl Wittberg, Lisa
    et al.
    Lunds Universitet, LTH, Avd. för Strömningsmekanik.
    Revstedt, Johan
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Fuchs, Laszlo
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Interaction among droplets in a uniform flow at intermediate Reynolds numbers2006In: 44th AIAA Aerospace and Science Meeting, Reno, Nevada, USA, Jan 9-12, 2006, American Institute of Aeronautics and Astronautics , 2006Conference paper (Refereed)
    Abstract [en]

    Simulations of single and dual droplets in a uniform flow for Reynolds numbers 100and Weber numbers of 0.1 and 1.0 are performed on a Cartesian grid using the Volume ofFluid method. The simulations are carried out to provide a detailed study of the interactionbetween droplets. Thus, the main focus is to investigate the forces a droplet is subjectedto as its position is changed in relation to a reference droplet. The results are comparedto simulations for solid particles using the Volume of Solid, a method based on the Volumeof Fluid approach. The results show the importance of accounting to the full interactionamong the droplets. Such interaction has to be included even for rather diluted two-phasesystems. The large number of calculations results in a data-base that can be used as alook-up table for accounting for the inter-droplet interaction (i.e. effects on lift- and dragcoefficients)in the frame-work of Lagrangian particle tracking approach. Additionally, wedo take into account also droplet deformation, which has significant effect for droplet inmany engineering applications.

  • 33.
    Prahl Wittberg, Lisa
    et al.
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Revstedt, Johan
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Fuchs, Laszlo
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Sphere wake dynamics2008In: Int. Conf. on Jets, Wakes and Separated Flows, Berlin, Germany, Sept 16-19, 2008, Technical University of Berlin , 2008Conference paper (Refereed)
    Abstract [en]

    Understanding the flow behavior around spheres is of great importance when it comes to improving the efficiency of any industrial application involving multiphase flow systems. The purpose with this study is to continue the work and further investigate the wake dynamics behind interacting spheres under steady and periodic free-stream conditions. Both single and dual sphere configurations are considered, focusing on Reynolds number at values around which, flow transitions have been found to occur.

    The results shows that "lock on" to the free-stream flow frequency is more likely to occur for free-stream frequencies greater than the frequency associated with the natural vortex shedding. The appearance of a low frequeny mode can often be related to a swirling motion of the wake. Also, the vortex shedding for tandem config-urations is delayed at short inter-particle distances and the vorticle strucctures in the flow field of two spheres in tandem are different as compared to those observed for a single sphere

  • 34.
    Prahl Wittberg, Lisa
    et al.
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Revstedt, Johan
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Fuchs, Laszlo
    Lunds Universitet, LTH, Div. Fluid mechanics.
    The interaction among two fixed spherical particles in an oscillatory flow2007In: 5th Conference on Bluff Body Wakes and Vortex-Induced Vibrations, Bahia, Brazil, Dec 12 -15, 2007, 2007Conference paper (Refereed)
  • 35.
    Prahl Wittberg, Lisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Revstedt, Johan
    Lunds Universitet, LTH, Div. Fluid mechanics.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Hydrodynamic interaction among multiple spherical particles2010In: 7 Int. Conference on Multiphase Flow, Tampa, Florida, USA, May 30 - June 4, 2010, 2010Conference paper (Refereed)
    Abstract [en]

    Multiple sphere formations are studied for Reynolds numbers of 100 − 300 in order to better understand the hydrodyanmicalinteraction among spheres. Spheres placed in tandem, diagonal as well as cluster formations including3–5 spheres are investigated using a conventional finite difference method where the Volume of Solid (VOS) methodis used to represent the spherical particles. The results show that, independent of sphere formation, an overall trend isthat the front spheres are the least affected by the inclusion of additional spheres in the formation. Although spherecharacteristics depending on position within a formation is found, the behavior of the trailing spheres changes withformation type, number of spheres, separation distances and Reynolds numbers.

  • 36.
    Prahl Wittberg, Lisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    van Wyk, Stevin
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gutmark, E.
    Backeljauw, P.
    Gutmark-Little, I.
    Effects of aortic irregularities on blood flow2016In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 15, no 2Article in journal (Refereed)
    Abstract [en]

    Anatomic aortic anomalies are seen in many medical conditions and are known to cause disturbances in blood flow. Turner syndrome (TS) is a genetic disorder occurring only in females where cardiovascular anomalies, particularly of the aorta, are frequently encountered. In this study, numerical simulations are applied to investigate the flow characteristics in four TS patient- related aortic arches (a normal geometry, dilatation, coarctation and elongation of the transverse aorta). The Quemada viscosity model was applied to account for the non-Newtonian behavior of blood. The blood is treated as a mixture consisting of water and red blood cells (RBC) where the RBCs are modeled as a convected scalar. The results show clear geometry effects where the flow structures and RBC distribution are significantly different between the aortas. Transitional flow is observed as a jet is formed due to a constriction in the descending aorta for the coarctation case. RBC dilution is found to vary between the aortas, influencing the WSS. Moreover, the local variations in RBC volume fraction may induce large viscosity variations, stressing the importance of accounting for the non-Newtonian effects.

  • 37.
    Prahl-Wittberg, Lisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    van Wyk, Stevin
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Backeljauw, Philippe
    Cincinnati Children's Hospital.
    Gutmark-Little, Iris
    Cincinnati Children's Hospital.
    The Impact of Aortic Arch Geometry on Flow Characteristics2013In: / [ed] AIAA, AIAA, 2013Conference paper (Refereed)
    Abstract [en]

    Cardiovascular defects characterized by geometrical anomalies of the aorta and its eecton the blood ow is the focus of this study. Not only are the local ow characteristicsgeometry dependent, but they are also directly connected to the rheological properties ofblood. Flow characteristics such as wall shear stress are often postulated to play a centralrole in the development of vascular disease.In this study, blood is considered to be a non-Newtonian uid and modeled via theQuemada model, an empirical model that is valid for dierent red blood cell loading.Three patient-specic geometries of the aortic arch are investigated numerically. Thethree geometries investigated in this study all display malformations that are prevalent inpatients having the genetic disorder Turner syndrome. The results show a highly complexow with regions of secondary ow that are enhanced in two of the three aortas. Moreover,blood ow is clearly diverted due to the malformations, moving to a larger extent throughthe branches of the arch instead of through the descending aorta. The geometry havingan elongated transverse aorta is found to be subjected to larger areas of highly oscillatorylow wall shear stress.

  • 38. Söder, M.
    et al.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Lindgren, B.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effect of Swirl/Tumble (Tilt) Angle on Flow Homogeneity, Turbulence and Mixing Properties2014In: SAE technical paper series, ISSN 0148-7191, Vol. 2014-OctoberArticle in journal (Refereed)
    Abstract [en]

    In this work, the effect of swirl to tumble ratio on homogeneity, turbulence and mixing in a generic heavy duty Diesel engine during compression, is investigated using Large-Eddy Simulations. The main conclusion is that the relative importance of dilatation (relative volume change) increases whereas the effect of tumble breakdown decreases with the swirl to tumble ratio. In detail, we show that an increase in tumble raises the peak turbulence level and shifts the peak to earlier crank angles, which in turn leads to higher dissipation. Moreover, maximum turbulence level at top dead center is obtained for a combination of swirl and tumble rather than for pure tumble. Furthermore, it is observed that the peak turbulent kinetic energy displays levels three times greater than the initial kinetic energy of the tumble motion. Thus, energy is added to the flow (turbulence) by the piston through generation of vorticity by vorticity-dilatation interaction. Also, the intermediate swirl/tumble ratios are found to introduce large non-uniformity in the flow field, leading to a non-solid body like rotation. Swirl/tumble (tilt) angles larger than 19°are necessary for complete mixing of the gas within the engine cylinder. Taken together, the combined effect of a combination of swirl and tumble turbulence during compression is investigated. This knowledge is important both for engine development as well as more theoretical aspects regarding the breakdown of large scale structures in an engine.

  • 39.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden.
    Lindgren, Björn
    Scania CV, Sweden.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Laszlo, Fuchs
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Effect of swirl/tumble (Tilt) angle on flow homogeneity, turbulence and mixing propertiesManuscript (preprint) (Other academic)
    Abstract [en]

    In this work, the effect of swirl to tumble ratio on homogeneity, turbulence and mixing in a generic heavy duty Diesel engine during compression, is investigated using Large-Eddy Simulations. The main conclusion is that the relative importance of dilatation (relative volume change) increases whereas the effect of tumble breakdown decreases with the swirl to tumble ratio.In detail, we show that an increase in tumble raises the peak turbulence level and shifts the peak to earlier crank angles, which in turn leads to higher dissipation. Moreover, maximum turbulence level at top dead center is obtained for a combination of swirl and tumble rather than for pure tumble. Furthermore, it is observed that the peak turbulent kinetic energy displays levels three times greater than the initial kinetic energy of the tumble motion. Thus, energy is added to the flow (turbulence) by the piston through generation of vorticity by vorticity-dilatation interaction. Also, the intermediate swirl/tumble ratios are found to introduce large non-uniformity in the flow field, leading to a non-solid body like rotation. Swirl/tumble (tilt) angles larger than 19 deg are necessary for complete mixing of the gas within the engine cylinder. Taken together, the combined effect of a combination of swirl and tumble turbulence during compression is investigated. This knowledge is important both for engine development as well as more theoretical aspects regarding the breakdown of large scale structures in an engine.

  • 40.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Investigating the dynamic effects on flow structures generated during the intake stroke in heavy-duty diesel engines using Large Eddy SimulationsManuscript (preprint) (Other academic)
    Abstract [en]

    The aim of this work is to investigate whether a quasi-steady assumption is applicable for the flow entering a heavy-duty diesel engine. That is, what errors can be expected when a steady swirl test rig is used to characterize the flow structures entering the cylinder. The main conclusion is that the dynamic effects have an effect on the flow generated during intake.In detail, we show that the swirl coefficient is higher during fluid deceleration as compared to during fluid acceleration. The swirl coefficient was also found to be around 40~\% higher during valve closing as compared to similar lifts during valve opening. The flow of angular momentum into the cylinder is found to be delayed by the time it takes to empty the volume above the valves. A new ratio, port delay ratio (Rp), is proposed to account for this phenomena. Minimizing the port delay ratio during engine design is likely to lead to a stronger and more homogeneously distributed swirling motion.Taken together, the dynamic effects on swirl and mass flow expected during the intake have been investigated. This knowledge is important when measured mean swirl numbers are used as initial condition in sector model simulations of combustion. Sector model simulations are very common during the optimization process of diesel combustion and a small difference between real and simulated swirl numbers may have a significant effect on engine performance.

  • 41.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Towards understanding the effect of compression on swirl and tumble in the context of turbulence and mixingManuscript (preprint) (Other academic)
    Abstract [en]

    The rotational motions of different strength and tilt angles in a generic heavy duty Diesel engine during compression are investigated using Large Eddy Simulations. The main conclusion is that the relative importance of dilatation (relative volume change) decreases whereas the effect of tumble breakdown increases with the tumble number. For rotational motions with similar BDC tumble numbers the presence of swirl has a dampening effect on peak turbulence during compression but not on turbulence level at TDC.In detail, we show that peak turbulence levels are strongly affected by BDC tumble number, while a local maximum turbulence level at TDC was found for a tilted rotational motion with a tilt angle of 61 deg and BDC tumble number of 0.9. The effect of tilt and BDC total kinetic energy on TDC total kinetic energy and mixing is presented. It is observed that a small tilt angle is necessary in order to obtain a radially stratified mixture, e.g. stratified EGR.The combined effect of a rotational strength and tilt on turbulence and mixing during compression is investigated. This knowledge is important both for a better theoretical understanding of the compression process, and for engine development.

  • 42.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Effects of compression on coherent structures in an enclosureManuscript (preprint) (Other academic)
    Abstract [en]

    The effects of compression on turbulent swirling flows are studied using Large-Eddy Simulations (LES). In this study, the geometry investigated is a cylinder with swirling motions of dierent strengths with superimposed isotropic turbulence. During compression the evolution of turbulence and vorticity is investigated. During early compression, rapid diusion of turbulence is found. In the later part of the compression an increase of turbulence and vorticity is observed and linked to vorticity-dilatation interaction. It is shown that the swirling motion suppresses turbulence and turbulent anisotropy. The longitudinal integral length scale of the tangential fluctuations is found to be approximately twice the transverse length scale. The longitudinal length scale is largely unaffected by compression whereas an effect on the transverse length scale is observed.

  • 43.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Lindgren, Björn
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Compression of a swirling and tumbling flow2013In: ASME 2013 Internal Combustion Engine Division Fall Technical Conference, ICEF 2013: Fuels; Numerical Simulation; Engine Design, Lubrication, and Applications, 2013Conference paper (Refereed)
    Abstract [en]

    The effect of compression on a swirling/tumbling flow is studied using Large-Eddy Simulations (LES). In this study the geometry investigated is a cylinder with an artificially created swirling/tumbling motion. During compression the evolution of turbulence and vorticity are investigated. An increase of turbulence and vorticity is observed and linked to vorticity-dilatation interaction. It is shown that for swirling/tumbling flows turbulent kinetic energy available at Top Dead Center (TDC) is introduced by the piston through the vorticity-dilatation interaction and that turbulence increases independently of the presence of instability of the large scale flow structures.

  • 44.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden.
    Vernet, Julie
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Lindgren, Björn
    Scania CV, Sweden.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Study of flow generated by the port in a heavy-duty diesel engine at different valve lifts using PIVManuscript (preprint) (Other academic)
    Abstract [en]

    In-cylinder flow structures, also prior to ignition, have a large effect on combustion efficiency and emissions. Therefore, understanding the mechanism of formation and changes in such structures is of great importance in the work of reducing fuel consumption and emissions. Here, the flow entering the cylinder was studied using stereoscopic Particle Image Velocimetry (PIV). The measurements were carried out on a steady swirl test rig, commonly used to measure engine characteristics such as the swirl number. In this study, fluctuations in swirl coefficient were found to be greater than the mean swirl at low valve lifts, the flow was found to be Reynolds number independent and the turbulent fluctuations were observed to be axisymmetric.

  • 45.
    Söder, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Vernet, Julie
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Study of in-cylinder ow structures using PIV and LESManuscript (preprint) (Other academic)
    Abstract [en]

    In-cylinder ow structures have a large eect on combustion eciency and emissions. Thus, understanding these structures is of great importance in the work of reducing fuel consumption and emissions. In this paper, the in-cylinder flow is studied using stereoscopic Particle Image Velocimetry (PIV) measurements and Large Eddy Simulations (LES). The measurements and simulations have been carried out on a steady swirl test rig, commonly used to measureengine characteristics such as the swirl number. In this study, the cause of the fluctuation in swirl is explained, showing that the fluctuation can be of greater importance as compared to the mean value of the swirl number. Moreover, we show how the flow from the inlet ports are distributed.

  • 46.
    Van Wyk, Stevin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bulusu, Kartik V.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Plesniak, Michael W.
    Non-Newtonian perspectives of pulsatile blood-like flows in a 180 degree tube bendManuscript (preprint) (Other academic)
  • 47.
    van Wyk, Stevin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Bulusu, Kartik V.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Plesniak, Michael W.
    Non-Newtonian perspectives on pulsatile blood-analog flows in a 180 degrees curved artery model2015In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 071901Article in journal (Refereed)
    Abstract [en]

    Complex, unsteady fluid flow phenomena in the arteries arise due to the pulsations of the heart that intermittently pumps the blood to the extremities of the body. The many different flow waveform variations observed throughout the arterial network are a result of this process and a function of the vessel properties. Large scale secondary flow structures are generated throughout the aortic arch and larger branches of the arteries. An experimental 180. curved artery test section with physiological inflow conditions was used to validate the computational methods implemented in this study. Good agreement of the secondary flow structures is obtained between experimental and numerical studies of a Newtonian blood-analog fluid under steady-state and pulsatile, carotid artery flow rate waveforms. Multiple vortical structures, some of opposite rotational sense to Dean vortices, similar to Lyne-type vortices, were observed to form during the systolic portion of the pulse. Computational tools were used to assess the effect of blood-analog fluid rheology ( i.e., Newtonian versus non-Newtonian). It is demonstrated that non-Newtonian, blood-analog fluid rheology results in shear layer instabilities that alter the formation of vortical structures during the systolic deceleration and onwards during diastole. Additional vortices not observed in the Newtonian cases appear at the inside and outside of the bend at various times during the pulsation. The influence of blood-analog shear-thinning viscosity decreases mean pressure losses in contrast to the Newtonian blood analog fluid.

  • 48.
    Van Wyk, Stevin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Do-Quang, Minh
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Rheology of red blood cell flow in large geometries2013Conference paper (Refereed)
    Abstract [en]

    When studying disease development in arteries, it is important to understand the local variations in blood rheology. Blood flow in large arteries is often assumed to behave as a homogeneous fluid, an assumption that is not entirely correct. The local viscosity changes with the local concentration of Red Blood Cells (RBCs) and the rate of shear strongly influences the Wall Shear Stress (WSS) and its gradients, physiological parameters important in the study of atherosclerosis. Moreover, the flow behavior of RBCs is influenced by the geometric structure of the flow environment. In experiment, rheological properties across a tube cross-section are difficult to measure if non-invasive techniques are to be used. Therefore, rheometric devices are constructed of simple geometries to measure the bulk rheology. In this study, the Lattice Boltzmann Method is used to model the blood as a particle suspension of RBCs. The RBC Volume Fractions (VF) investigated corresponds to 1, 2 and 5%, and both a channel and a tube flow are considered. The results display large differences in RBC distributions and velocity profiles. Estimated from existing viscosity models, the viscosity distributions are found to display variations of up to 5% when comparing the two geometries. This is of importance since errors in quantifying the viscosity can lead to miscalculations of the physiological variables.

  • 49.
    Van Wyk, Stevin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Atherosclerotic indicators for blood-like fluids in 90-degree arterial-like bifurcations2014In: Computers in Biology and Medicine, ISSN 0010-4825, E-ISSN 1879-0534, Vol. 50, p. 56-69Article in journal (Refereed)
    Abstract [en]

    The identification of regions prone to atherogenesis in the arterial network is compounded by the complex, slow interaction of mechanical and biomechanical processes. In recent times simplifications to the analysis of the near wall hemodynamics have been sought-after to identify plaque prone regions. Mean parameters have been defined to analyze the common fluid mechanical hypotheses considering the role of wall shear stress (WSS) variations in the pathological changes to the endothelium. In this study well known WSS indicators are applied to varying flow conditions of blood-like fluids in a 90-degree arterial bifurcation. The conventional indicators identify two distinct, focal regions that correlate with a known plaque prone location near arterial bifurcations. The results however demonstrate that the interpretation of the indicators can be difficult under varying flow conditions unless complementary parameters are considered simultaneously. A new indicator is also suggested that extracts the peaks of the temporal WSS gradients (PTWSSGs) and is shown to co-incide well with plaque prone regions. The PTWSSG could be used as a complimentary atherogenic indicator in bifurcating arteries, thereby expanding cardiovascular disease studies to the consideration of alternative fluid mechanical hypotheses. The inclusion of a non-Newtonian model is important in predicting the WSS and temporal WSS gradient distributions near the bifurcation due to the separation bubble induced fluctuations in the shear. Atherogenic indicators could be misleading if non-Newtonian effects are excluded.

  • 50.
    van Wyk, Stevin
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Prahl Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Haemodynamics in a 3D 90-degree bifurcation2011In: Proceedings of the ECCOMAS Thematic International Conference on Simulation and Modeling of Biological Flows, Brussels, Belgium, September 21-23, 2011, 2011Conference paper (Refereed)
    Abstract [en]

    The transport behaviour of the haematocrit in the larger arteries is important in defining the variations in viscosityof blood. In this study, a finite volume method is used in order to simulate the blood flow and haematocrit transportthrough a large 3D human-like 90-degree bifurcation. The simulations are carried out to investigate the importance ofexplicitly modelling the non-Newtonian viscosity of blood regarding defining the flow. It is expected to be especiallyimportant in the regions surrounding a bifurcation. The main focus is to compare non-Newtonian to Newtonianbehaviour of the flow through important parameters such as pressure losses, mean viscosity variations and bulktransport properties of haematocrit. The study considers a broad range of physiological and pulsatile flow conditions,and displays the importance of modelling blood flow as a non-Newtonian fluid. The results have a relevant impactregarding the possible discrepencies in important physiological parameters such as wall shear stress (WSS), whencoupling the haematocrit field data back to the viscosity models.

12 1 - 50 of 52
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