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Studies on adverse-pressure-gradient turbulent boundary layers on wings
KTH, School of Engineering Sciences (SCI), Engineering Mechanics.
2020 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The present licentiate thesis addresses the use of well-resolved simulations to simulate turbulent boundary layers (TBL) subjected to adverse pressure gradients. Within the thesis a wide variety of analyses are performed, and a method to improve the performance of the simulations is presented. The first aim of the thesis is to assess the effect of adverse pressure gradients and flow history on the development and fundamental characteristics of turbulent boundary layers. With this in mind, well-resolved large-eddy simulations (LES) of the turbulent boundary layers over two wing sections are performed using the spectral-element-method (SEM) code Nek5000. In order to assess the effects of the adverse pressure gradient on turbulent boundary layers, turbulence statistics are computed and time series are collected from the simulations. The turbulence statistics show a significant effect of the adverse pressure gradient on the mean velocity profiles, turbulent fluctuations and turbulent kinetic energy budgets. In addition, the time series are used to compute the power-spectral densities of the turbulent boundary layers and to analyse the effect of the adverse pressure gradient on the turbulent scales across the boundary layer. After having compared both wings at moderate Reynolds number Rec=400,000, the next goal is to perform high-resolution simulations of wings at higher Reynolds numbers in order to study conditions closer to those in reality, and to evaluate the effect of adverse pressure gradient with increasing Reynolds numbers. To achieve this, better and more efficient computational methods are required. In this thesis, the performance of the adaptive mesh refinement method recently implemented in Nek5000 is assessed for the first time on wing simulations. The obtained results show a large potential of this new method (which includes the use of non-conformal meshes) with respect to the previous simulations carried out with conformal meshes. Lastly, we performed a modal decomposition of the TBLs developing around both wing sections. To this end, we consider spectral proper orthogonal decomposition (SPOD), which can be used to identify the most energetic structures of the turbulent boundary layer.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 38
Series
TRITA-SCI-FOU ; 02
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-267022ISBN: 978-91-7873-436-8 (print)OAI: oai:DiVA.org:kth-267022DiVA, id: diva2:1390048
Presentation
2020-02-28, Seminarierum Faxén (rum 5316), KTH Mekanik, Teknikringen 8, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20200131

Available from: 2020-01-31 Created: 2020-01-31 Last updated: 2020-02-03Bibliographically approved
List of papers
1. Effect of adverse pressure gradients on turbulent wing boundary layers
Open this publication in new window or tab >>Effect of adverse pressure gradients on turbulent wing boundary layers
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, no A8, p. 1-28Article in journal (Refereed) Published
Abstract [en]

    The characteristics of turbulent boundary layers (TBLs) subjected to adverse pressure gradients are analysed through well-resolved large-eddy simulations. The geometries under study are the NACA0012 and NACA4412 wing sections, at 0 and 5 degrees angle of attack, respectively, both of them at a Reynolds number based on inflow velocity and chord length Rec = 400,000. The turbulence statistics show that adverse pressure gradients (APGs) have a significant effect on the mean velocity, velocity fluctuations and turbulent kinetic energy budget, and this effect is more prominent on the outer region of the boundary layer. Furthermore, the effect of flow history is assessed by means of an integrated Clauser pressure-gradient parameter, β, through the study of cases with matching local values of β and the friction Reynolds number, Reτ, to isolate this effect. Our results show a noticeable effect of the flow history on the outer region, however the differences in the near-wall peak of the tangential velocity fluctuations appear to be mostly produced by the local APG magnitude. The one-dimensional power-spectral density shows energetic small scales in the outer region of APG TBLs, whereas these energetic scales do not appear in zero-pressure-gradient (ZPG) TBLs, suggesting that small scales near the wall are advected towards the outer layer by the APG. Moreover, the linear coherence spectra show that the spectral outer peak of high-Reynolds-number ZPG TBLs is highly correlated with the near-wall region , unlike APG TBLs which do not show such a correlation. This result, together with the different two-dimensional spectra of APG and high-Reynolds-number ZPG TBLs, suggests different energisation mechanisms due to APG and increase in Reynolds number. To the authors' knowledge, this is the first in-depth analysis of the TBL characteristics over wings, including detailed single-point statistics, spectra and coherence.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-267017 (URN)10.1017/jfm.2019.838 (DOI)000508121500008 ()
Note

QC 20200203

Available from: 2020-01-31 Created: 2020-01-31 Last updated: 2020-02-17Bibliographically approved
2. Enabling adaptive mesh refinement for spectral-element simulations of turbulence around wing sections
Open this publication in new window or tab >>Enabling adaptive mesh refinement for spectral-element simulations of turbulence around wing sections
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(English)In: Article in journal (Other academic) Submitted
Abstract [en]

The implementation of adaptive mesh refinement (AMR) in the spectral-element method code Nek5000 is used for the first time on the well-resolved large-eddy  simulation (LES) of the turbulent flow over wings. In particular, the flow over a NACA4412 profile with a 5° angle of attack at chord-based Reynolds number Rec=200,000 is analysed in the present work. The mesh, starting from a coarse resolution, is progressively refined by means of AMR, which allows for high resolution near the wall and wake whereas significantly larger elements are used in the far-field. The resulting mesh is of higher resolution than those in previous conformal cases, and it allows for the use of larger computational domains, avoiding the use of precursor RANS simulations to determine the boundary conditions. All of this with, approximately, 3 times lower total number of grid points if the same spanwise length is used. Turbulence statistics obtained in the AMR simulation show good agreement with the ones obtained with the conformal mesh. Finally, using AMR on wings will enable simulations at Rec beyond 1 million, thus allowing the study of pressure-gradient effects at high Reynolds numbers relevant for practical applications.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-267019 (URN)
Note

QC 20200204

Available from: 2020-01-31 Created: 2020-01-31 Last updated: 2020-02-04Bibliographically approved
3. Wavepackets in turbulent flows around airfoils
Open this publication in new window or tab >>Wavepackets in turbulent flows around airfoils
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Motivated by the recent analysis by Sano et al. 2019, Phys. Rev. Fluids, vol. 4, p. 094602, of spanwise-coherent structures in the turbulent flow around airfoils and their connection to trailing-edge noise, we carry out a thorough characterisation of such structures in three simulation databases. We analyse two different numerical simulations of incompressible flow in turbulent regime, both at chord Reynolds number of 400,000: a large-eddy simulation for a NACA 0012 profile at zero angle of attack, and a direct numerical simulation for a NACA 4412 airfoil with an angle of attack of 5 degrees. Snapshots of the flow field were analysed using Spectral Proper Orthogonal Decomposition (SPOD), in order to extract the dominant coherent structures of the flow. Focus is given to  the aforementioned spanwise-coherent fluctuations, which two-dimensional disturbances in the computational domain due to the use of periodic boundary conditions. The leading SPOD modes show that the most energetic coherent structures are wavepackets, extending over the whole turbulent boundary layers around the airfoils with significant amplitudes near the trailing-edge. Higher amplitudes are observed in the region of  stronger adverse pressure gradient at the suction side of the NACA 4412 airfoil. To understand how such structures in the turbulent field can be modelled, the linear response of the flow using the singular value decomposition of the linearised resolvent operator was performed, using the mean field as a base flow and considering a locally parallel approximation. Such analysis shows that the leading SPOD modes can be associated to optimal, linearised flow responses, particularly for stations far from the trailing edge; the latter introduces a discontinuity in boundary conditions, and the locally parallel approximation becomes questionable. We then focus on evaluating the dependence of such wavepackets on the domain size, to ensure that these structures are not an artifact of the use of periodic boundary conditions in small computational boxes. To do so, we performed an incompressible LES of a zero-pressure gradient turbulent boundary layer (ZPGTBL), for three different spanwise sizes: Lz=32 δ*, Lz=64 δ* and Lz=128 δ*, where δ* is a reference displacement thickness in a region of developed turbulent flow, with Reynolds number matching the values in the airfoil simulations. The signature of such wavepackets is seen in non-premultiplied spanwise wavenumber spectra, which reaches, for the three domain sizes, a plateau for spanwise wavelengths going to infinity (or wavenumbers going to zero); this plateau is representative of the spanwise-coherent structures seen in the airfoil simulations. Similar SPOD and resolvent analyses were carried out for the zero spanwise wavenumber of the ZPGTBL, and the same coherent wavepackets were observed for the three domains, with very similar amplitudes. Such wavepackets were also accurately modelled using the optimal resolvent response. These results confirm that the spanwise-elongated structures are not domain-size dependent for the studied simulations, and are thus a feature of turbulent boundary layers.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-267020 (URN)
Available from: 2020-01-31 Created: 2020-01-31 Last updated: 2020-01-31
4. Turbulence statistics in a spectral-element code: a toolbox for high-fidelity simulations
Open this publication in new window or tab >>Turbulence statistics in a spectral-element code: a toolbox for high-fidelity simulations
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-267021 (URN)
Note

QC 20200203

Available from: 2020-01-31 Created: 2020-01-31 Last updated: 2020-02-03Bibliographically approved

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