Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
The planar X-junction flow: stability analysis and control
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
Department of Engineering, University of Cambridge, Cambridge, UK.
DIIN, University of Salerno, Fisciano, Italy.
Department of Engineering, Univerisyt of Cambridge, Cambridge, UK.
Show others and affiliations
2014 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 753, 1-28 p.Article in journal (Refereed) Published
Abstract [en]

The bifurcations and control of the flow in a planar X-junction are studied via linear stability analysis and direct numerical simulations. This study reveals the instability mechanisms in a symmetric channel junction and shows how these can be stabilized or destabilized by boundary modification. We observe two bifurcations as the Reynolds number increases. They both scale with the inlet speed of the two side channels and are almost independent of the inlet speed of the main channel. Equivalently, both bifurcations appear when the recirculation zones reach a critical length. A two-dimensional stationary global mode becomes unstable first, changing the flow from a steady symmetric state to a steady asymmetric state via a pitchfork bifurcation. The core of this instability, whether defined by the structural sensitivity or by the disturbance energy production, is at the edges of the recirculation bubbles, which are located symmetrically along the walls of the downstream channel. The energy analysis shows that the first bifurcation is due to a lift-up mechanism. We develop an adjustable control strategy for the first bifurcation with distributed suction or blowing at the walls. The linearly optimal wall-normal velocity distribution is computed through a sensitivity analysis and is shown to delay the first bifurcation from Re = 82.5 to Re = 150. This stabilizing effect arises because blowing at the walls weakens the wall-normal gradient of the streamwise velocity around the recirculation zone and hinders the lift-up. At the second bifurcation, a three-dimensional stationary global mode with a spanwise wavenumber of order unity becomes unstable around the asymmetric steady state. Nonlinear three-dimensional simulations at the second bifurcation display transition to a nonlinear cycle involving growth of a three-dimensional steady structure, time-periodic secondary instability and nonlinear breakdown restoring a two-dimensional flow. Finally, we show that the sensitivity to wall suction at the second bifurcation is as large as it is at the first bifurcation, providing a possible mechanism for destabilization.

Place, publisher, year, edition, pages
2014. Vol. 753, 1-28 p.
Keyword [en]
flow control, instability, wakes/jets
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-139407DOI: 10.1017/jfm.2014.364ISI: 000341118600002Scopus ID: 2-s2.0-84904425092OAI: oai:DiVA.org:kth-139407DiVA: diva2:686951
Funder
EU, European Research Council, ALORS 2590620
Note

QC 20141007. Updated from submitted to published. Correction in: Journal of Fluid Mechanics, vol. 753, page: 560, WoS: 000341118600023

Available from: 2014-01-13 Created: 2014-01-13 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Global stability analysis of complex fluids
Open this publication in new window or tab >>Global stability analysis of complex fluids
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The main focus of this work is on the non-Newtonian effects on the inertial instabilities in shear flows. Both inelastic (Carreau) and elastic models (Oldroyd-B and FENE-P) have been employed to examine the main features of the non-Newtonian fluids; shear-thinning, shear-thickening and elasticity. Several classical configurations have been considered; flow past a circular cylinder, in a lid-driven cavity and in a channel. We have used a wide range of tools for linear stability analysis, modal, non-modal, energy and sensitivity analysis, to determine the instability mechanisms of the non-Newtonian flows and compare them with those of the Newtonian flows. Direct numerical simulations have been also used to prove the results obtained by the linear stability analysis.

Significant modifications/alterations in the instability of the different flows have been observed under the action of the non-Newtonian effects. In general, shear-thinning/shear-thickening effects destabilize/stabilize the flow around the cylinder and in a lid driven cavity. Viscoelastic effects both stabilize and destabilize the channel flow depending on the ratio between the viscoelastic and flow time scales. The instability mechanism is just slightly modified in the cylinder flow whereas new instability mechanisms arise in the lid-driven cavity flow. We observe that the non-Newtonian effect can alter the inertial flow at both baseflow and perturbation level (e.g. Carreau fluid past a cylinder or in a lid driven cavity) or it may just affect the perturbations (e.g. Oldroyd-B fluid in channel). In all the flow cases studied, the modifications in the instability dynamics are shown to be strongly connected to the contribution of the different terms in the perturbation kinetic energy budget.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. viii, 26 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2013:20
Keyword
non-Newtonian flow, Carreau model, Oldroyd-B model, FENE-P model, modal analysis, nonmodal analysis, sensitivity analysis
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-139405 (URN)978-91-7501-958-1 (ISBN)
Presentation
2013-12-13, Q2, Osquldasväg 10, Stockholm, 10:15
Opponent
Supervisors
Note

QC 20140113

Available from: 2014-01-13 Created: 2014-01-13 Last updated: 2014-01-13Bibliographically approved
2. Stability analysis and inertial regimes in complex  flows
Open this publication in new window or tab >>Stability analysis and inertial regimes in complex  flows
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this work we rst study the non-Newtonian effects on the inertial instabilities in shear flows and second the inertial suspensions of finite size rigid particles by means of numerical simulations.

In the first part, both inelastic (Carreau) and elastic models (Oldroyd-B and FENE-P) have been employed to examine the main features of the non-Newtonian fluids in several congurations; flow past a circular cylinder, in a lid-driven cavity and in a channel. In the framework of the linear stability analysis, modal, non-modal, energy and sensitivity analysis are used to determine the instability mechanisms of the non-Newtonian flows. Signicant modifications/alterations in the instability of the different flows have been observed under the action of the non-Newtonian effects. In general, shear-thinning/shear-thickening effects destabilize/stabilize the flow around the cylinder and in a lid driven cavity. Viscoelastic effects both stabilize and destabilize the channel flow depending on the ratio between the viscoelastic and flow time scales. The instability mechanism is just slightly modied in the cylinder flow whereas new instability mechanisms arise in the lid-driven cavity flow.

In the second part, we employ Direct Numerical Simulation together with an Immersed Boundary Method to simulate the inertial suspensions of rigid spherical neutrally buoyant particles in a channel. A wide range of the bulk Reynolds numbers, 500<Re<5000, and particle volume fractions, 0<\Phi<3, is studied while fixing the ratio between the channel height to particle diameter, 2h/d = 10. Three different inertial regimes are identied by studying the stress budget of two-phase flow. These regimes are laminar, turbulent and inertial shear-thickening where the contribution of the viscous, Reynolds and particle stress to transfer the momentum across the channel is the strongest respectively. In the inertial shear-thickening regime we observe a signicant enhancement in the wall shear stress attributed to an increment in particle stress and not the Reynolds stress. Examining the particle dynamics, particle distribution, dispersion, relative velocities and collision kernel, confirms the existence of the three regimes. We further study the transition and turbulence in the dilute regime of finite size particulate channel flow. We show that the turbulence can sustain in the domain at Reynolds numbers lower than the one of the unladen flow due to the disturbances induced by particles.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. x, 60 p.
Series
TRITA-MEK, ISSN 0348-467X
Keyword
non-Newtonian flow, global stability analysis, inertial suspensions, particle dynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-177850 (URN)978-91-7595-782-1 (ISBN)
Public defence
2015-12-18, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20151127

Available from: 2015-11-27 Created: 2015-11-27 Last updated: 2015-11-27Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full textScopus

Authority records BETA

Brandt, Luca

Search in DiVA

By author/editor
Lashgari, ImanBrandt, Luca
By organisation
MechanicsLinné Flow Center, FLOWSeRC - Swedish e-Science Research Centre
In the same journal
Journal of Fluid Mechanics
Mechanical Engineering

Search outside of DiVA

GoogleGoogle Scholar

doi
urn-nbn

Altmetric score

doi
urn-nbn
Total: 76 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf