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On the onset of transition in 90°-bend pipe flow
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 Engineering Sciences (SCI), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.ORCID iD: 0000-0001-9627-5903
2019 (English)In: 11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019, International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2019Conference paper, Published paper (Refereed)
Abstract [en]

The present work deals with the global stability analysis of the flow in a 90◦-bend pipe with curvature δ = R/Rc = 0.3, being R the radius of the cross-section of the pipe and Rc the radius of curvature at the pipe centreline. Direct numerical simulations (DNS) for values of the bulk Reynolds number Reb = U-------D/v between 2000 and 3000 are performed. The bulk Reynolds number is based on the bulk velocity Ub, the pipe diameter D, and the kinematic viscosity ν. It is found that the flow is steady for Reb ≤ 2500, and two pairs of symmetric, counter-rotating vortices are observed in the section of the pipe downstream of the bend. Moreover, two recirculation regions are present inside the bend, one on the outer wall and the other on the inner one. For Reb ≥ 2550, the flow becomes periodic, oscillating with a fundamental non-dimensional frequency St = fD/Ub = 0.23. A global stability analysis reveals a pair of complex conjugate eigenvalues with positive real part. The velocity components of the unstable direct and adjoint eigenmodes are investigated, and it is observed that a large spatial separation occurs because of the non-normality of the linearised Navier–Stokes operator. Thus, an analysis of the structural sensitivity of the unstable eigenmode to spatially localised feedbacks is performed, in order to identify the core of the instability, the so-called wavemaker. It is found that the region located 15° downstream of the bend inlet, on the outer wall, is where the instability originates. Since flow separation is observed in this region, it is concluded that the instability is linked with the strong shear by the backflow phenomena.

Place, publisher, year, edition, pages
International Symposium on Turbulence and Shear Flow Phenomena, TSFP , 2019.
Keywords [en]
Eigenvalues and eigenfunctions, Flow separation, Oscillating flow, Reynolds number, Stability, Turbulence, Complex conjugate eigenvalues, Counter-rotating vortices, Global stability analysis, Non-dimensional frequency, Radius of curvature, Recirculation regions, Structural sensitivity, Velocity components, Shear flow
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-268502Scopus ID: 2-s2.0-85073623679OAI: oai:DiVA.org:kth-268502DiVA, id: diva2:1421373
Conference
11th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2019, 30 July 2019 through 2 August 2019
Note

QC 20200402

Available from: 2020-04-02 Created: 2020-04-02 Last updated: 2020-04-02Bibliographically approved

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Lupi, ValerioSchlatter, Philipp

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