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Simulations of turbulent asymptotic suction boundary layers
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-9833-9560
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-1663-3553
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.ORCID iD: 0000-0001-9627-5903
2015 (English)In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 17, 157-180 p.Article in journal (Refereed) Published
Abstract [en]

A series of large-eddy simulations of a turbulent asymptotic suction boundary layer (TASBL) was performed in a periodic domain, on which uniform suction was applied over a flat plate. Three Reynolds numbers (defined as ratio of free-stream and suction velocity) of Re = 333, 400 and 500 and a variety of domain sizes were considered in temporal simulations in order to investigate the turbulence statistics, the importance of the computational domain size, the arising flow structures as well as temporal development length required to achieve the asymptotic state. The effect of these two important parameters was assessed in terms of their influence on integral quantities, mean velocity, Reynolds stresses, higher order statistics, amplitude modulation and spectral maps. While the near-wall region up to the buffer region appears to scale irrespective of Re and domain size, the parameters of the logarithmic law (i.e. von Kármán and additive coefficient) decrease with increasing Re, while the wake strength decreases with increasing spanwise domain size and vanishes entirely once the spanwise domain size exceeds approximately two boundary-layer thicknesses irrespective of Re. The wake strength also reduces with increasing simulation time. The asymptotic state of the TASBL is characterised by surprisingly large friction Reynolds numbers and inherits features of wall turbulence at numerically high Re. Compared to a turbulent boundary layer (TBL) or a channel flow without suction, the components of the Reynolds-stress tensor are overall reduced, but exhibit a logarithmic increase with decreasing suction rates, i.e. increasing Re. At the same time, the anisotropy is increased compared to canonical wall-bounded flows without suction. The reduced amplitudes in turbulence quantities are discussed in light of the amplitude modulation due to the weakened larger outer structures. The inner peak in the spectral maps is shifted to higher wavelength and the strength of the outer peak is much less than for TBLs. An additional spatial simulation was performed, in order to relate the simulation results to wind tunnel experiments, which – in accordance with the results from the temporal simulation – indicate that a truly TASBL is practically impossible to realise in a wind tunnel. Our unique data set agrees qualitatively with existing literature results for both numerical and experimental studies, and at the same time sheds light on the fact why the asymptotic state could not be established in a wind tunnel experiment, viz. because experimental studies resemble our simulation results from too small simulation boxes or insufficient development times.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2015. Vol. 17, 157-180 p.
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-179851DOI: 10.1080/14685248.2015.1083574ISI: 000366142800001OAI: oai:DiVA.org:kth-179851DiVA: diva2:890427
Note

QC 20160107

Available from: 2016-01-03 Created: 2016-01-03 Last updated: 2016-04-18Bibliographically approved
In thesis
1. Simulations of turbulent boundary layers with suction and pressure gradients
Open this publication in new window or tab >>Simulations of turbulent boundary layers with suction and pressure gradients
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The focus of the present licentiate thesis is on the effect of suction and pressure gradients on turbulent boundary-layer flows, which are investigated separately through performing numerical simulations.The first part aims at assessing history and development effects on adverse pressure-gradient (APG) turbulent boundary layers (TBL). A suitable set-up was developed to study near-equilibrium conditions for a boundary layer developingon a flat plate by setting the free-stream velocity at the top of the domain following a power law. The computational box size and the correct definition of the top-boundary condition were systematically tested. Well-resolved large-eddy simulations were performed to keep computational costs low. By varying the free-stream velocity distribution parameters, e.g. power-law exponent and virtual origin, pressure gradients of different strength and development were obtained. The magnitude of the pressure gradient is quantified in terms of the Clauser pressure-gradient parameter β. The effect of the APG is closely related to its streamwise development, hence, TBLs with non-constant and constant β were investigated. The effect was manifested in the mean flow through a much more pronounced wake region and in the Reynolds stresses through the existence of an outer peak. The terms of the turbulent kinetic energy budgets indicate the influence of the APG on the distribution of the transfer mechanism across the boundary layer. Stronger and more energetic structures were identified in boundary layers with relatively stronger pressure gradients in their development history. Due to the difficulty of determining the boundary-layer thickness in flows with strong pressure gradients or over a curvedsurface, a new method based on the diagnostic-plot concept was introduced to obtain a robust estimation of the edge of a turbulent boundary layer.

In the second part, large-eddy simulations were performed on temporally developing turbulent asymptotic suction boundary layers (TASBLs). Findings from previous studies about the effect of suction could be confirmed, e.g. the reduction of the fluctuation levels and Reynolds shear stresses. Furthermore, the importance of the size of the computational domain and the time development were investigated. Both parameters were found to have a large impact on the results even on low-order statistics. While the mean velocity profile collapses in the inner layer irrespective of box size and development time, a wake region occurs for too small box sizes or early development time and vanishes once sufficiently large domains and/or integration times are chosen. The asymptotic state is charactersized by surprisingly thick boundary layers even for moderateReynolds numbers Re (based on free-stream velocity and laminar displacement thickness); for instance, Re = 333 gives rise to a friction Reynolds number Reτ = 2000. Similarly, the flow gives rise to very large structures in the outer region. These findings have important ramifications for experiments, since very large facilities are required to reach the asymptotic state even for low Reynolds numbers.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. 37 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:07
Keyword
boundary layers, near-wall turbulence, history effects, asymptotic suction boundary layers, large-eddy simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-185275 (URN)978-91-7595-934-4 (ISBN)
Presentation
2016-05-12, D3, Lindstedtsvägen 5, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20160418

Available from: 2016-04-18 Created: 2016-04-15 Last updated: 2016-04-18Bibliographically approved

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Bobke, AlexandraÖrlü, RamisSchlatter, Philipp
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