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Direct numerical simulation of separated flow in a three-dimensional diffuser
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-9627-5903
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-7864-3071
2010 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 650, 307-318 p.Article in journal (Refereed) Published
Abstract [en]

A direct numerical simulation (DNS) of turbulent flow in a three-dimensional diffuser at Re = 10 000 (based on bulk velocity and inflow-duct height) was performed with a massively parallel high-order spectral element method running on up to 32 768 processors. Accurate inflow condition is ensured through unsteady trip forcing and a long development section. Mean flow results are in good agreement with experimental data by Cherry et al. (Intl J. Heat Fluid Flow, vol. 29, 2008, pp. 803-811), in particular the separated region starting from one corner and gradually spreading to the top expanding diffuser wall. It is found that the corner vortices induced by the secondary flow in the duct persist into the diffuser, where they give rise to a dominant low-speed streak, due to a similar mechanism as the 'lift-up effect' in transitional shear flows, thus governing the separation behaviour. Well-resolved simulations of complex turbulent flows are thus possible even at realistic Reynolds numbers, providing accurate and detailed information about the flow physics. The available Reynolds stress budgets provide valuable references for future development of turbulence models.

Place, publisher, year, edition, pages
2010. Vol. 650, 307-318 p.
Keyword [en]
Bulk velocity, Complex turbulent flows, Experimental data, Flow physics, Heat fluid flow, High-order, Inflow conditions, Mean flow, Reynolds stress, Separated flows, Separated region, Separation behaviour, Spectral element method
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-25925DOI: 10.1017/S0022112010000558ISI: 000278212500011Scopus ID: 2-s2.0-77952428627OAI: oai:DiVA.org:kth-25925DiVA: diva2:360863
Note
QC 20101105Available from: 2010-11-05 Created: 2010-11-05 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Spectral-element simulations of separated turbulent internal flows
Open this publication in new window or tab >>Spectral-element simulations of separated turbulent internal flows
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: US-AB, 2009. vii, 32 p.
Series
Trita-MEK, ISSN 0348-467X ; 2009:13
Keyword
spectral element method, direct numerical simulations (DNS), turbulence, dealiasing, three-dimensional separation, massively parallel simulations
Identifiers
urn:nbn:se:kth:diva-11643 (URN)978-91-7415-506-8 (ISBN)
Presentation
2009-12-15, E3, Osquars Backe 14, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20101105Available from: 2009-11-27 Created: 2009-11-27 Last updated: 2010-11-05Bibliographically approved
2. Spectral-element simulations of turbulent wall-bounded flows including transition and separation
Open this publication in new window or tab >>Spectral-element simulations of turbulent wall-bounded flows including transition and separation
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The spectral-element method (SEM) is used to study wall-bounded turbulent flowsin moderately complex geometries. The first part of the thesis is devoted to simulations of canonical flow cases, such as temporal K-type transitionand turbulent channel flow, to investigate general resolution requirements and computational efficiency of the numerical code nek5000. Large-eddy simulation (LES) is further performed of a plane asymmetric diffuser flow with an opening angle of 8.5 degrees, featuring turbulent flow separation. Good agreement with numerical studies of Herbst (2007) is obtained, and it is concluded that the use of a high-order method is advantageous for flows featuring pressure-induced separation. Moreover, it is shown, both a priori on simpler model problems and a posteriori using the full Navier--Stokes equations, that the numerical instability associated with SEM at high Reynolds numbers is cured either by employing over-integration (dealiasing) or a filter-based stabilisation, thus rendering simulations of moderate to high Reynolds number flows possible.

The second part of the thesis is devoted to the first direct numerical simulation (DNS) of a truly three-dimensional, turbulent and separated diffuser flow at Re = 10 000 (based on bulk velocity and inflow-duct height), experimentally investigated by Cherry et al. (2008). The massively parallel capabilities of the spectral-element method are exploited by running the simulations on up to 32 768 processors. Very good agreement with experimental mean flow data is obtained and it is thus shown that well-resolved simulations of complex turbulent flows with high accuracy are possible at realistic Reynolds numberseven in complicated geometries. An explanation for the discovered asymmetry of the mean separated flow is provided and itis demonstrated that a large-scale quasi-periodic motion is present in the diffuser.

In addition, a new diagnostic measure, based on the maximum vorticity stretching component in every spatial point, is designed and tested in a number of turbulent and transitional flows. Finally, Koopman mode decomposition is performed of a minimal channel flow and compared to classical proper orthogonal decomposition (POD).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. ix, 81 p.
Series
Trita-MEK, ISSN 0348-467X ; 2011:15
Keyword
spectral-element method, direct numerical simulation (DNS), large-eddy simulation (LES), turbulence, transition, over-integration, three-dimensional separation, massively parallel simulations, proper orthogonal decomposition (POD), Koopman modes, vorticity stretching, coherence
National Category
Engineering and Technology
Research subject
SRA - E-Science (SeRC)
Identifiers
urn:nbn:se:kth:diva-50294 (URN)978-91-7501-178-3 (ISBN)
Public defence
2011-12-16, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note
QC 20111206Available from: 2011-12-06 Created: 2011-12-04 Last updated: 2012-05-24Bibliographically approved

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Schlatter, PhilippHenningson, Dan S.

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