Change search
Link to record
Permanent link

Direct link
BETA
Alternative names
Publications (10 of 48) Show all publications
Nygård, A., Altimira, M., Semlitsch, B., Wittberg, L. P. & Fuchs, L. (2016). Analysis of vortical structures in intermittent jets. In: Springer Proceedings in Physics: . Paper presented at 5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015 (pp. 3-10). Springer Science+Business Media B.V..
Open this publication in new window or tab >>Analysis of vortical structures in intermittent jets
Show others...
2016 (English)In: Springer Proceedings in Physics, Springer Science+Business Media B.V., 2016, p. 3-10Conference paper, Published 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.

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2016
Keyword
Industrial research, Mixing, Transport properties, Vorticity, Wakes, Ambient fluids, Ambient gas, Continuous jets, Intermittent injection, Numerical and experimental study, Systematic analysis, Vortical structures, Vorticity generation, Fighter aircraft
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-195137 (URN)10.1007/978-3-319-30602-5_1 (DOI)000387431400001 ()2-s2.0-84978998403 (Scopus ID)9783319306001 (ISBN)
Conference
5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015
Note

Correspondence Address: Nygård, A.; Department of Mechanics, KTHSweden; email: alexander@mech.kth.se. QC 20161116

Available from: 2016-11-16 Created: 2016-11-02 Last updated: 2017-01-10Bibliographically approved
Kékesi, T., Amberg, G. & Wittberg, L. P. (2016). Corrigendum to: "Drop deformation and breakup". Int. J. Multiphase Flow, 66, (2014) 1-10. International Journal of Multiphase Flow.
Open this publication in new window or tab >>Corrigendum to: "Drop deformation and breakup". Int. J. Multiphase Flow, 66, (2014) 1-10
2016 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-194588 (URN)10.1016/j.ijmultiphaseflow.2016.02.002 (DOI)2-s2.0-84964816414 (Scopus ID)
Note

Correspondence Address: Kékesi, T.email: timea@mech.kth.se. QC 20161102

Available from: 2016-11-02 Created: 2016-10-31 Last updated: 2017-11-29Bibliographically approved
Kekesi, T., Amberg, G. & Wittberg, L. P. (2016). Drop deformation and breakup in flows with shear. Chemical Engineering Science, 140, 319-329.
Open this publication in new window or tab >>Drop deformation and breakup in flows with shear
2016 (English)In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 140, p. 319-329Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Drop deformation, Drop breakup, Shear flow, Volume of Fluid (VOF)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-180581 (URN)10.1016/j.ces.2015.10.019 (DOI)000367117300028 ()2-s2.0-84946594865 (Scopus ID)
Note

QC 20160121

Available from: 2016-01-21 Created: 2016-01-19 Last updated: 2017-11-30Bibliographically approved
Prahl Wittberg, L., van Wyk, S., Fuchs, L., Gutmark, E., Backeljauw, P. & Gutmark-Little, I. (2016). Effects of aortic irregularities on blood flow. Biomechanics and Modeling in Mechanobiology, 15(2).
Open this publication in new window or tab >>Effects of aortic irregularities on blood flow
Show others...
2016 (English)In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 15, no 2Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Springer, 2016
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-174626 (URN)10.1007/s10237-015-0692-y (DOI)000372165100006 ()2-s2.0-84961128676 (Scopus ID)
Note

QC 20160411

Available from: 2015-12-03 Created: 2015-10-07 Last updated: 2017-12-01Bibliographically approved
Krochak, P., Fasci, G. C., Norman, B. & Prahl-Wittberg, L. (2015). Bridging chemical dosage, mixing quality, and variability in paper sheets. TAPPI Journal, 14(5), 311-320.
Open this publication in new window or tab >>Bridging chemical dosage, mixing quality, and variability in paper sheets
2015 (English)In: TAPPI Journal, ISSN 0734-1415, Vol. 14, no 5, p. 311-320Article in journal (Refereed) Published
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.

Keyword
Chemical bonds, Computational fluid dynamics, Mixing, Nozzles, Paper, Paper products, Papermaking, Turbulent flow, Two phase flow, Chemical variability, Mixing uniformities, Non-Newtonian models, Nozzle configuration, Numerical and experimental analysis, Rheological parameter, Single components, Turbulent pipe flow
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-170290 (URN)000355323600004 ()2-s2.0-84930366957 (Scopus ID)
Note

QC 20150630

Available from: 2015-06-30 Created: 2015-06-29 Last updated: 2017-12-04Bibliographically approved
van Wyk, S., Prahl Wittberg, L., Bulusu, K. V., Fuchs, L. & Plesniak, M. W. (2015). Non-Newtonian perspectives on pulsatile blood-analog flows in a 180 degrees curved artery model. Physics of fluids, 27(7), Article ID 071901.
Open this publication in new window or tab >>Non-Newtonian perspectives on pulsatile blood-analog flows in a 180 degrees curved artery model
Show others...
2015 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, no 7, article id 071901Article in journal (Refereed) Published
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.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-172725 (URN)10.1063/1.4923311 (DOI)000358872200003 ()
Funder
Swedish Research Council
Note

QC 20150828

Available from: 2015-08-28 Created: 2015-08-27 Last updated: 2017-12-04Bibliographically approved
Van Wyk, S., Prahl Wittberg, L. & Fuchs, L. (2014). Atherosclerotic indicators for blood-like fluids in 90-degree arterial-like bifurcations. Computers in Biology and Medicine, 50, 56-69.
Open this publication in new window or tab >>Atherosclerotic indicators for blood-like fluids in 90-degree arterial-like bifurcations
2014 (English)In: Computers in Biology and Medicine, ISSN 0010-4825, E-ISSN 1879-0534, Vol. 50, p. 56-69Article in journal (Refereed) Published
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.

Keyword
Bifurcation, Atherosclerosis, Wall shear stress, Wall shear stress gradients, CFD, Blood, Atherogenic indicator, OSI, Residence time
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-136675 (URN)10.1016/j.compbiomed.2014.03.006 (DOI)000338606900008 ()2-s2.0-84900803305 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140811. Updated from manuscript to article in journal.

Available from: 2013-12-06 Created: 2013-12-06 Last updated: 2017-12-06Bibliographically approved
Kékesi, T., Amberg, G. & Prahl Wittberg, L. (2014). Drop deformation and breakup. International Journal of Multiphase Flow, 66, 1-10.
Open this publication in new window or tab >>Drop deformation and breakup
2014 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 66, p. 1-10Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Droplet, Deformation, Breakup, Regime map, Breakup time, Volume of Fluid (VOF)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-154737 (URN)10.1016/j.ijmultiphaseflow.2014.06.006 (DOI)000342548300001 ()2-s2.0-84904904082 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20141119. QC 20160113

Available from: 2014-11-19 Created: 2014-10-27 Last updated: 2017-12-05Bibliographically approved
Söder, M., Prahl Wittberg, L., Lindgren, B. & Fuchs, L. (2014). Effect of Swirl/Tumble (Tilt) Angle on Flow Homogeneity, Turbulence and Mixing Properties. Paper presented at SAE 2014 International Powertrain, Fuels & Lubricants Meeting, 20 October 2014 - 22 October 2014. SAE technical paper series, 2014-October.
Open this publication in new window or tab >>Effect of Swirl/Tumble (Tilt) Angle on Flow Homogeneity, Turbulence and Mixing Properties
2014 (English)In: SAE technical paper series, ISSN 0148-7191, Vol. 2014-OctoberArticle in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
SAE International, 2014
Keyword
Computational fluid dynamics, Diesel engines, Engine cylinders, Engines, Kinetic energy, Kinetics, Large eddy simulation, Mixing, Powertrains, Solid lubricants, Turbomachinery, Vorticity, Effect of swirls, Engine development, Heavy-duty diesel engine, Large scale structures, Non-uniformities, Theoretical aspects, Turbulence level, Turbulent kinetic energy, Turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-176217 (URN)10.4271/2014-01-2579 (DOI)2-s2.0-84938545225 (Scopus ID)
Conference
SAE 2014 International Powertrain, Fuels & Lubricants Meeting, 20 October 2014 - 22 October 2014
Note

QC 20151123

Available from: 2015-11-23 Created: 2015-11-02 Last updated: 2017-12-01Bibliographically approved
Håkansson, K., Fall, A., Lundell, F., Yu, S., Krywka, C., Roth, S., . . . Söderberg, D. (2014). Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments. Nature Communications, 5, 4018.
Open this publication in new window or tab >>Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments
Show others...
2014 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 4018-Article in journal (Refereed) Published
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.

Keyword
Current International Research, Wood Cell-Walls, Rotational Diffusion, Microfibril Angle, Fibers, Flow, Nanopaper, Nanocomposites, Birefringence, Microchannels
National Category
Chemical Sciences Fluid Mechanics and Acoustics Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-133940 (URN)10.1038/ncomms5018 (DOI)000338836700002 ()2-s2.0-84901950560 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20140812. Updated from manuscript to article in journal.

Available from: 2013-11-13 Created: 2013-11-13 Last updated: 2017-12-06Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9976-8316

Search in DiVA

Show all publications