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Measurement of width and intensity of particle streaks in turbulent flows
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. (Mekanik, Söderberg D)
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. (Mekanik, Söderberg D)
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-9976-8316
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2013 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 54, no 6, 1555- p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2013. Vol. 54, no 6, 1555- p.
Keyword [en]
Concentration dependence, Concentration-dependent, Low concentrations, Particle concentrations, Particle streak, Preferential concentration, Quantification methods, Quantitative comparison
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-125758DOI: 10.1007/s00348-013-1555-xISI: 000321262300005ScopusID: 2-s2.0-84878704508OAI: diva2:640595
Knut and Alice Wallenberg FoundationSwedish Research Council

QC 20130814

Available from: 2013-08-14 Created: 2013-08-13 Last updated: 2014-10-03Bibliographically approved
In thesis
1. Transitional and turbulent fibre suspension flows
Open this publication in new window or tab >>Transitional and turbulent fibre suspension flows
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis the orientation of macro-sized fibres in turbulent flows is studied, as well as the effect of nano-sized fibrils on hydrodynamic stability. The focus lies on enabling processes for new materials where cellulose is the main constituent. When fibres (or any elongated particles) are added to a fluid, the complexity of the flow-problem increases. The fluid flow will influence the rotation of the fibres, and therefore also effect the overall fibre orientation. Exactly how the fibres rotate depends to a large extent on the mean velocity gradient in the flow.

In addition, when fibres are added to a suspending fluid, the total stress in the suspension will increase, resulting in an increased apparent viscosity. The increase in stress is related to the direction of deformation in relation to the orientation of the particle, i.e. whether the deformation happens along the long or short axis of the fibre. The increase in stress, which in most cases is not constant neither in time nor space, will in turn influence the flow.

This thesis starts off with the orientation and spatial distribution of fibres in the turbulent flow down an inclined plate. By varying fibre and flow parameters it is discovered that the main parameter controlling the orientation distribution is the aspect ratio of the fibres, with only minor influences from the other parameters. Moreover, the fibres are found to agglomerate into streamwise streaks. A new method to quantify this agglomeration is developed, taking care of the problems that arise due to the low concentration in the experiments. It is found that streakiness, i.e. the tendency to agglomerate in streaks, varies with Reynolds number.

Going from fibre orientation to flow dynamics of fibre suspensions, the influence of cellulose nanofibrils (CNF) on laminar/turbulent transition is investigated in three different setups, namely plane channel flow, curved-rotating channel flow, and the flow in a flow focusing device. This last flow case is selected since it is can be used for assembly of CNF based materials. In the plane channel flow, the addition of CNF delays the transition more than predicted from measured viscosities while in the curved-rotating channel the opposite effect is discovered. This is qualitatively confirmed by linear stability analyses. Moreover, a transient growth analysis in the plane channel reveals an increase in streamwise wavenumber with increasing concentration of CNF. In the flow focusing device, i.e. at the intersection of three inlets and one outlet, the transition is found to mainly depend on the Reynolds number of the side flow. Recirculation zones forming downstream of two sharp corners are hypothesised to be the cause of the transition. With that in mind, the two corners are given a larger radius in an attempt to stabilise the flow. However, if anything, the flow seems to become unstable at a smaller Reynolds number, indicating that the separation bubble is not the sole cause of the transition. The choice of fluid in the core flow is found to have no effect on the stability, neither when using fluids with different viscosities nor when a non-Newtonian CNF dispersion was used. Thus, Newtonian model fluids can be used when studying the flow dynamics in this type of device.

As a proof of concept, a flow focusing device is used to produce a continuous film from CNF. The fibrils are believed to be aligned due to the extensional flow created in the setup, resulting in a transparent film, with an estimated thickness of 1 um.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. x, 56 p.
TRITA-MEK, ISSN 0348-467X ; 2014.22
Fluid mechanics, fibre suspension, turbulence, laminar-turbulent transition, image analysis, hydrodynamic stability, cellulose nanofibrils.
National Category
Applied Mechanics
Research subject
Engineering Mechanics
urn:nbn:se:kth:diva-153018 (URN)978-91-7595-287-1 (ISBN)
Public defence
2014-10-24, L1, Drottning Kristinas Väg 30, KTH, Stockholm, 10:15 (English)
Knut and Alice Wallenberg Foundation

QC 20141003

Available from: 2014-10-03 Created: 2014-10-02 Last updated: 2015-03-06Bibliographically approved

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Håkansson, Karl M. O.Kvick, MathiasLundell, FredrikPrahl Wittberg, LisaSöderberg, L. Daniel
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