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Wavepackets in turbulent flows around airfoils
Divisão de Engenharia Aeronáutica, Instituto Tecnológico de Aeronáutica, 12228-900, São José dos Campos, SP, Brazil.
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), Engineering Mechanics, Fluid Mechanics and Engineering Acoustics.
Divisão de Engenharia Aeronáutica, Instituto Tecnológico de Aeronáutica, 12228-900, São José dos Campos, SP, Brazil.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Superseded Departments (pre-2005), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0001-9627-5903
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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: urn:nbn:se:kth:diva-267020OAI: oai:DiVA.org:kth-267020DiVA, id: diva2:1390046
Note

QC 20200511

Available from: 2020-01-31 Created: 2020-01-31 Last updated: 2020-05-11Bibliographically approved
In thesis
1. Studies on adverse-pressure-gradient turbulent boundary layers on wings
Open this publication in new window or tab >>Studies on adverse-pressure-gradient turbulent boundary layers on wings
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:nbn:se:kth:diva-267022 (URN)978-91-7873-436-8 (ISBN)
Presentation
2020-02-28, Seminarierum Faxén (rum 5316), KTH Mekanik, Teknikringen 8, KTH, Stockholm, 10:15 (English)
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Note

QC 20200131

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

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Tanarro, AlvaroSchlatter, PhilippVinuesa, RicardoHanifi, ArdeshirHenningson, Dan S.

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