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Effect of surface tension on global modes of confined wake flows
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.ORCID iD: 0000-0003-3737-0091
2011 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 23, no 1, 014108- p.Article in journal (Refereed) Published
Abstract [en]

Many wake flows are susceptible to self-sustained oscillations, such as the well-known von Karman vortex street behind a cylinder that makes a rope beat against a flagpole at a distinct frequency on a windy day. One appropriate method to study these global instabilities numerically is to look at the growth rates of the linear temporal global modes. If all growth rates for all modes are negative for a certain flow field then a self-sustained oscillation should not occur. On the other hand, if one growth rate for one mode is slightly positive, the oscillation will approximately obtain the frequency and shape of this global mode. In our study, we first introduce surface tension between two fluids to the wake-flow problem. Then we investigate its effects on the global linear instability of a spatially developing wake with two co-flowing immiscible fluids. The inlet profile consists of two uniform layers, which makes the problem easily parametrizable. The fluids are assumed to have the same density and viscosity, with the result that the interface position becomes dynamically important solely through the action of surface tension. Two wakes with different parameter values and surface tension are studied in detail. The results show that surface tension has a strong influence on the oscillation frequency, growth rate, and shape of the global mode(s). Finally, we make an attempt to confirm and explain the surface-tension effect based on a local stability analysis of the same flow field in the streamwise position of maximum reverse flow.

Place, publisher, year, edition, pages
2011. Vol. 23, no 1, 014108- p.
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-31601DOI: 10.1063/1.3540686ISI: 000287424200043Scopus ID: 2-s2.0-79551558463OAI: oai:DiVA.org:kth-31601DiVA: diva2:406316
Funder
Swedish Research Council
Note
QC 20110325Available from: 2011-03-25 Created: 2011-03-21 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Numerical stability studies of one-phase and immiscible two-phase jets and wakes
Open this publication in new window or tab >>Numerical stability studies of one-phase and immiscible two-phase jets and wakes
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The initial linear instability growth of two-dimensional plane wakes and jetsis investigated, by temporal two-dimensional global modes, and local spatialstability analysis. Comparisons are also made to experiments, direct numericalsimulations, and methods designed for weakly-non-parallel flows. The studiesproceed through three different flow setups with increasing complexity.The first flow analysed is a convectively unstable liquid sheet surroundedby a stagnant or co-flowing gas. The experimentally measured growth rates arefound to be in excellent agreement with spatial stability calculations, if the airboundary layer is taken into account, and not otherwise. The stabilizing effectof moderate air co-flow is quantified in the numerical study, and the governingparameters found to be the speed difference between water and air, and theshear from air at the water surface (inversely proportional to the air boundarylayer thickness).The second flow case is a one-phase confined wake, i.e. a wake in a channel.The effect of confinement (wall distance) on the global stability of wakes isanalysed by linear global modes, and compared to the results from DNS andweakly-non-parallel theory. At Re = 100, confinement is globally stabilizing,mostly due to a faster development towards a parabolic profile for confinedflows. The stabilizing effect of confinement almost disappears at Re ≈ 400.However, when the structural sensitivity of the wakes is analysed by an adjointbasedapproach, fundamental differences are seen in the global wavemakers ofconfined and unconfined wakes at Re ≈ 400.The third and most complex flow case is immiscible two-fluid wakes andjets. A parallel multi-domain spectral code is developed, where the kinematicand dynamic conditions on the interface are imposed as coupling conditions. Itis shown that intermediate values of surface tension can destabilize stable wakesand jets. In addition, surface tension has a considerable influence on the globaloscillation frequency and spatial shape of the global mode for unstable wakes.The character of the mode is gradually changed from a wake instability to aglobal shear layer instability. Both symmetric and antisymmetric modes areencountered for both wakes and jets, depending on the strength of the surfacetension (value of the Weber number) and the flow case.iii

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. ix, 93 p.
Series
Trita-MEK, ISSN 0348-467X ; 11:07
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-34149 (URN)978-91-7501-051-9 (ISBN)
Public defence
2011-06-13, F3, Lindstedsvägen 206, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish e‐Science Research Center
Note
QC 20110530Available from: 2011-05-30 Created: 2011-05-26 Last updated: 2012-05-24Bibliographically approved

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Söderberg, L. Daniel

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