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Stability and sensitivity of a cross-flow-dominated Falkner-Skan-Cooke boundary layer with discrete surface roughness
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.ORCID-id: 0000-0001-9446-7477
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre. KTH, Skolan för teknikvetenskap (SCI), Mekanik.ORCID-id: 0000-0001-9627-5903
KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Centra, SeRC - Swedish e-Science Research Centre.ORCID-id: 0000-0002-5913-5431
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2017 (Engelska)Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 826, s. 830-850Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

With the motivation of determining the critical roughness size, a global stability and sensitivity analysis of a three-dimensional Falkner-Skan-Cooke (FSC) boundary layer with a cylindrical surface roughness is performed. The roughness size is chosen such that breakdown to turbulence is initiated by a global version of traditional secondary instabilities of the cross-flow (CF) vortices instead of an immediate flow tripping at the roughness. The resulting global eigenvalue spectra of the systems are found to be very sensitive to numerical parameters and domain size. This sensitivity to numerical parameters is quantified using the epsilon-pseudospectrum, and the dependency on the domain is analysed through an impulse response, structural sensitivity analysis and an energy budget. It is shown that while the frequencies remain relatively unchanged, the growth rates increase with domain size, which originates from the inclusion of stronger CF vortices in the baseflow. This is reflected in a change in the rate of advective energy transport by the baseflow. It is concluded that the onset of global instability in a FSC boundary layer as the roughness height is increased does not correspond to an immediate flow tripping behind the roughness, but occurs for lower roughness heights if sufficiently long domains are considered. However, the great sensitivity results in an inability to accurately pinpoint the exact parameter values for the bifurcation, and the large spatial growth of the disturbances in the long domains eventually becomes larger than can be resolved using finite-precision arithmetic.

Ort, förlag, år, upplaga, sidor
Cambridge University Press, 2017. Vol. 826, s. 830-850
Nyckelord [en]
absolute/convective instability, boundary layer stability, transition to turbulence
Nationell ämneskategori
Maskinteknik
Identifikatorer
URN: urn:nbn:se:kth:diva-214322DOI: 10.1017/jfm.2017.466ISI: 000407571200038Scopus ID: 2-s2.0-85029412275OAI: oai:DiVA.org:kth-214322DiVA, id: diva2:1141238
Forskningsfinansiär
Swedish e‐Science Research Center
Anmärkning

QC 20170914

Tillgänglig från: 2017-09-14 Skapad: 2017-09-14 Senast uppdaterad: 2017-11-23Bibliografiskt granskad
Ingår i avhandling
1. Studies on instability and optimal forcing of incompressible flows
Öppna denna publikation i ny flik eller fönster >>Studies on instability and optimal forcing of incompressible flows
2017 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

This thesis considers the hydrodynamic instability and optimal forcing of a number of incompressible flow cases. In the first part, the instabilities of three problems that are of great interest in energy and aerospace applications are studied, namely a Blasius boundary layer subject to localized wall-suction, a Falkner–Skan–Cooke boundary layer with a localized surface roughness, and a pair of helical vortices. The two boundary layer flows are studied through spectral element simulations and eigenvalue computations, which enable their long-term behavior as well as the mechanisms causing transition to be determined. The emergence of transition in these cases is found to originate from a linear flow instability, but whereas the onset of this instability in the Blasius flow can be associated with a localized region in the vicinity of the suction orifice, the instability in the Falkner–Skan–Cooke flow involves the entire flow field. Due to this difference, the results of the eigenvalue analysis in the former case are found to be robust with respect to numerical parameters and domain size, whereas the results in the latter case exhibit an extreme sensitivity that prevents domain independent critical parameters from being determined. The instability of the two helices is primarily addressed through experiments and analytic theory. It is shown that the well known pairing instability of neighboring vortex filaments is responsible for transition, and careful measurements enable growth rates of the instabilities to be obtained that are in close agreement with theoretical predictions. Using the experimental baseflow data, a successful attempt is subsequently also made to reproduce this experiment numerically.

In the second part of the thesis, a novel method for computing the optimal forcing of a dynamical system is developed. The method is based on an application of the inverse power method preconditioned by the Laplace preconditioner to the direct and adjoint resolvent operators. The method is analyzed for the Ginzburg–Landau equation and afterwards the Navier–Stokes equations, where it is implemented in the spectral element method and validated on the two-dimensional lid-driven cavity flow and the flow around a cylinder.

Ort, förlag, år, upplaga, sidor
Stockholm, Sweden: KTH Royal Institute of Technology, 2017. s. 47
Serie
TRITA-MEK, ISSN 0348-467X ; 2017:19
Nyckelord
hydrodynamic stability, optimal forcing, resolvent operator, Laplace preconditioner, spectral element method, eigenvalue problems, inverse power method, direct numerical simulations, Falkner–Skan–Cooke boundary layer, localized roughness, crossflow vortices, Blasius boundary layer, localized suction, helical vortices, lid-driven cavity, cylinder flow
Nationell ämneskategori
Strömningsmekanik och akustik
Forskningsämne
Teknisk mekanik
Identifikatorer
urn:nbn:se:kth:diva-218172 (URN)978-91-7729-622-5 (ISBN)
Disputation
2017-12-14, D3, Lindstedtsvägen 5, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Anmärkning

QC 20171124

Tillgänglig från: 2017-11-24 Skapad: 2017-11-23 Senast uppdaterad: 2017-11-27Bibliografiskt granskad

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Brynjell-Rahkola, MattiasShahriari, NimaSchlatter, PhilippHanifi, ArdeshirHenningson, Dan S.

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Brynjell-Rahkola, MattiasShahriari, NimaSchlatter, PhilippHanifi, ArdeshirHenningson, Dan S.
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Linné Flow Center, FLOWSeRC - Swedish e-Science Research CentreMekanik
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