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
Simulations of turbulent boundary layers with suction and pressure gradients
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
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 [en]
boundary layers, near-wall turbulence, history effects, asymptotic suction boundary layers, large-eddy simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-185275ISBN: 978-91-7595-934-4 (print)OAI: oai:DiVA.org:kth-185275DiVA: diva2:919885
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
List of papers
1. Simulations of turbulent asymptotic suction boundary layers
Open this publication in new window or tab >>Simulations of turbulent asymptotic suction boundary layers
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
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-179851 (URN)10.1080/14685248.2015.1083574 (DOI)000366142800001 ()2-s2.0-85010976693 (Scopus ID)
Note

QC 20160107

Available from: 2016-01-03 Created: 2016-01-03 Last updated: 2017-05-22Bibliographically approved
2. On determining characteristic length scales in pressure-gradient turbulent boundary layers
Open this publication in new window or tab >>On determining characteristic length scales in pressure-gradient turbulent boundary layers
2016 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28Article in journal (Refereed) Published
Abstract [en]

In the present work we analyze three commonly used methods to determine the edge of pressure gradient turbulent boundary layers: two based on composite profiles, the one by Chauhan et al. (Fluid Dyn. Res. 41:021401, 2009) and the one by Nickels (J. Fluid Mech. 521:217–239, 2004), and the other onebased on the condition of vanishing mean velocity gradient. Additionally, a new method is introduced based on the diagnostic plot concept by Alfredsson et al. (Phys. Fluids 23:041702, 2011). The boundary layers developing over the suction and pressure sides of a NACA4412 wing section, extracted from a directnumerical simulation at chord Reynolds number Rec = 400, 000, is used as the test case, besides other numerical and experimental data from favorable, zero and adverse pressure-gradient flat-plate turbulent boundary layers. We find that all the methods produce robust results with mild or moderate pressure gradients, although the composite-profile techniques require data preparation, including initial estimations of fitting parameters and data truncation. Stronger pressure gradients (with a Rotta–Clauser pressure-gradient parameter β larger than around 7) lead to inconsistent results in all the techniques except the diagnosticplot. This method also has the advantage of providing an objective way of defining the point where the mean streamwise velocity is 99% of the edge velocity, and shows consistent results in a wide range of pressure gradient conditions, as well as flow histories. Collapse of intermittency factors obtained from a wide range of pressure-gradient and Re conditions on the wing further highlightsthe robustness of the diagnostic plot method to determine the boundary layert hickness (equivalent to δ99 ) and the edge velocity in pressure gradient turbulent boundary layers.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-185269 (URN)10.1063/1.4947532  (DOI)2-s2.0-84966546523 (Scopus ID)
Note

QC 20160526

Available from: 2016-04-15 Created: 2016-04-15 Last updated: 2017-11-30Bibliographically approved
3. Large-eddy simulations of adverse pressure gradient turbulent boundary layers
Open this publication in new window or tab >>Large-eddy simulations of adverse pressure gradient turbulent boundary layers
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Adverse pressure-gradient (APG) turbulent boundary layers (TBL) are studied by performing well-resolved large-eddy simulations. The pressure gradient is imposed by defining the free-stream velocity distribution with the description of a power law. Different inflow conditions, box sizes and upper boundary conditions are tested in order to determine the final set-up. The statistics ofturbulent boundary layers with three different power-law coefficients and thus magnitudes of adverse pressure gradients are then compared to zero pressure-gradient (ZPG) data. The effect of the APG on TBLs is manifested in the mean flow through a much more prominent wake region and in the Reynolds stresses through the existence of an outer peak. The pre-multiplied energy budgets shows the APG influence on the distribution of the turbulent kinetic energy transfer mechanism across the boundary layer.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-185271 (URN)
Note

QS 2016

Available from: 2016-04-15 Created: 2016-04-15 Last updated: 2016-04-18Bibliographically approved
4. History effects and near-equilibrium in adverse-pressure-gradient turbulent boundary layers
Open this publication in new window or tab >>History effects and near-equilibrium in adverse-pressure-gradient turbulent boundary layers
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This study deals with turbulent boundary layers under adverse-pressure gradients. Well-resolved large-eddy simulations (LES) were performed to assess the influence of the streamwise pressure development. The pressure gradient is imposed by prescribing the free-stream velocity in the free-stream above the layer. In order to fulfill the near-equilibrium conditions, the free-stream velocity has to follow a power-law distribution. The turbulence statistics pertaining tocases with a constant Clauser pressure-gradient parameter β were compared with cases with a non-constant pressure distribution at matched β and friction Reynolds number  Reτ. It was noticed that the non-constant cases appear toconverge slowly to a certain state of the boundary layer, which is uniquelycharacterised by β and Reτ . The investigations on the flat plate were extended to the flow around a wing section. Comparisons with the flat-plate cases revealed some interesting features: In turbulent boundary layers with strong pressure gradients in the development history the energy-carrying structures in the outerregion are strongly enhanced, which can be detected by the pronounced wake inthe mean velocity as well as the large second peak in the Reynolds stresses. This was also confirmed by one-dimensional energy spectra, where more energetic large structures were identified in the outer region for stronger pressure gradients overall. A scaling law suggested by Kitsios et al. (2015) was tested on a constant pressure gradient case. The mean velocity and Reynolds stress profiles were found to be dependent on the downstream development when they were scaled with the edge velocity and displacement thickness.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-185272 (URN)
Note

QS 2016

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

Open Access in DiVA

fulltext(4450 kB)234 downloads
File information
File name FULLTEXT01.pdfFile size 4450 kBChecksum SHA-512
f8c2996fb4cd9ceb4e4d6453296d415c75d1841b2feeb5b084d7036b846900d24573005c3baa3faaca7fee74da41428b2215a9f7250ba55d9c3bf20ff5843ecb
Type fulltextMimetype application/pdf

Authority records BETA

Bobke, Alexandra

Search in DiVA

By author/editor
Bobke, Alexandra
By organisation
Linné Flow Center, FLOWSeRC - Swedish e-Science Research CentreMechanics
Fluid Mechanics and Acoustics

Search outside of DiVA

GoogleGoogle Scholar
Total: 234 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 650 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