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Kern, S., Blanco, D. C., Cavalieri, A. V., Negi, P., Hanifi, A. & Henningson, D. S. (2024). Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels. Journal of Fluid Mechanics, 986, Article ID A3.
Open this publication in new window or tab >>Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels
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2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 986, article id A3Article in journal (Refereed) Published
Abstract [en]

Thin airfoil dynamic stall at moderate Reynolds numbers is typically linked to the sudden bursting of a small laminar separation bubble close to the leading edge. Given the strong sensitivity of laminar separation bubbles to external disturbances, the onset of dynamic stall on a NACA0009 airfoil section subject to different levels of low-amplitude free stream disturbances is investigated using direct numerical simulations. The flow is practically indistinguishable from clean inflow simulations in the literature for turbulence intensities at the leading edge of Tu = 0.02 %. At slightly higher turbulence intensities of Tu = 0.05 %, the bursting process is found to be considerably less smooth and strong coherent vortex shedding from the laminar separation bubble is observed prior to the formation of the dynamic stall vortex (DSV). This phenomenon is considered in more detail by analysing its appearance in an ensemble of simulations comprising statistically independent realisations of the flow, thus proving its statistical relevance. In order to extract the transient dynamics of the vortex shedding, the classical proper orthogonal decomposition method is generalised to include time in the energy measure and applied to the time-resolved simulation data of incipient dynamic stall. Using this technique, the dominant transient spatiotemporally correlated features are distilled and the wave train of the vortex shedding prior to the emergence of the main DSV is reconstructed from the flow data exhibiting dynamics of large-scale coherent growth and decay within the turbulent boundary layer.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
boundary layer separation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-346305 (URN)10.1017/jfm.2024.314 (DOI)001209573200001 ()2-s2.0-85192671697 (Scopus ID)
Note

QC 20240513

Available from: 2024-05-13 Created: 2024-05-13 Last updated: 2024-05-24Bibliographically approved
Kern, J. S., Lupi, V. & Henningson, D. S. (2024). Floquet stability analysis of pulsatile flow in toroidal pipes. Physical Review Fluids, 9(4), Article ID 043906.
Open this publication in new window or tab >>Floquet stability analysis of pulsatile flow in toroidal pipes
2024 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 9, no 4, article id 043906Article in journal (Refereed) Published
Abstract [en]

The linear temporal stability of the fully developed pulsatile flow in a torus with high curvature is investigated using Floquet theory. The baseflow is computed via a Newton-Raphson iteration in frequency space to obtain basic states at supercritical Reynolds numbers in the steady case for two curvatures, δ=0.1 and 0.3, exhibiting structurally different linear instabilities for the steady flow. The addition of a pulsatile component is found to be overall stabilizing over a wide range of pulsation amplitudes, in particular for high values of curvature. The pulsatile flows are found to be at most transiently stable with large intracyclic growth rate variations even at small pulsation amplitudes. While these growth rates are likely insufficient to trigger an abrupt transition at the parameters in this work, the trends indicate that this is indeed likely for higher pulsation amplitudes, similar to pulsatile flow in straight pipes. At the edge of the considered parameter range, subharmonic eigenvalue orbits in the local spectrum of the time-periodic operator, recently found in pulsating channel flow, have been confirmed also for pulsatile flow in toroidal pipes, underlining the generality of this phenomenon.

Place, publisher, year, edition, pages
American Physical Society (APS), 2024
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-345693 (URN)10.1103/PhysRevFluids.9.043906 (DOI)2-s2.0-85190067564 (Scopus ID)
Note

QC 20240418

Available from: 2024-04-18 Created: 2024-04-18 Last updated: 2024-04-18Bibliographically approved
Blanco, D. C. P., Hanifi, A., Henningson, D. S. & Cavalieri, A. V. G. (2024). Linear and nonlinear receptivity mechanisms in boundary layers subject to free-stream turbulence. Journal of Fluid Mechanics, 979, Article ID A31.
Open this publication in new window or tab >>Linear and nonlinear receptivity mechanisms in boundary layers subject to free-stream turbulence
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 979, article id A31Article in journal (Refereed) Published
Abstract [en]

Large-eddy simulations of a flat-plate boundary layer, without a leading edge, subject to multiple levels of incoming free-stream turbulence are considered in the present work. Within an input-output model, where nonlinear terms of the incompressible Navier-Stokes equations are treated as an external forcing, we manage to separate inputs related to perturbations coming through the intake of the numerical domain, whose evolution represents a linear mechanism, and the volumetric nonlinear forcing due to triadic interactions. With these, we perform the full reconstruction of the statistics of the flow, as measured in the simulations, to quantify pairs of wavenumbers and frequencies more affected by either linear or nonlinear receptivity mechanisms. Inside the boundary layer, different wavenumbers at near-zero frequency reveal streaky structures. Those that are amplified predominantly via linear interactions with the incoming vorticity occur upstream and display transient growth, while those generated by the nonlinear forcing are the most energetic and appear in more downstream positions. The latter feature vortices growing proportionally to the laminar boundary layer thickness, along with a velocity profile that agrees with the optimal amplification obtained by linear transient growth theory. The numerical approach presented is general and could potentially be extended to any simulation for which receptivity to incoming perturbations needs to be assessed.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
boundary layer receptivity, low-dimensional models
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-343253 (URN)10.1017/jfm.2023.1035 (DOI)001143358600001 ()2-s2.0-85183918235 (Scopus ID)
Note

QC 20240209

Available from: 2024-02-09 Created: 2024-02-09 Last updated: 2024-02-15Bibliographically approved
Kern, S., Negi, P., Hanifi, A. & Henningson, D. S. (2024). Onset of absolute instability on a pitching aerofoil. Journal of Fluid Mechanics, 988, Article ID A8.
Open this publication in new window or tab >>Onset of absolute instability on a pitching aerofoil
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 988, article id A8Article in journal (Refereed) Published
Abstract [en]

A global transient linear stability analysis of the three-dimensional time-dependent flow around an aerofoil undergoing small-amplitude pitching motion is performed using the optimally time-dependent (OTD) framework. The most salient linear instabilities associated with the instantaneous basic state are computed and tracked over time. The resulting OTD modes reflect the variations in the basic state and can be used as predictors of its spatial and temporal evolution, including the formation of a laminar separation bubble and its gradual spanwise modulation via primary global instability, leading to secondary instability and finally rapid breakdown to turbulence. The study confirms and expands upon earlier stability analyses of the same case based on the local properties of spanwise averaged velocity profiles in the bubble that predicted the onset of absolute instability soon followed by rapid breakdown of the separation bubble. The three-dimensional structure of the most unstable OTD mode is extracted, which compares well with both the locally absolutely unstable mode and the evolution of the basic state itself.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2024
Keywords
absolute/convective instability, boundary layer stability
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-347625 (URN)10.1017/jfm.2024.407 (DOI)001233796800001 ()2-s2.0-85195049078 (Scopus ID)
Note

QC 20240613

Available from: 2024-06-12 Created: 2024-06-12 Last updated: 2024-06-18Bibliographically approved
Alarcón, J. F., Cavalieri, A. V. .., Hanifi, A. & Henningson, D. S. (2024). Role of streak secondary instabilities on free-stream turbulence-induced transition. Journal of Fluid Mechanics, 988, Article ID A6.
Open this publication in new window or tab >>Role of streak secondary instabilities on free-stream turbulence-induced transition
2024 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 988, article id A6Article in journal (Refereed) Published
Abstract [en]

We study the stability of a zero-pressure gradient boundary layer subjected to free-stream disturbances by means of local stability analysis. The dataset under study corresponds to a direct numerical simulation (DNS) of a flat plate with a sharp leading edge in realistic wind tunnel conditions, with a turbulence level of 3.45 % at the leading edge. We present a method to track the convective evolution of the secondary instabilities of streaks by performing sequential stability calculations following the wave packet, connecting successive unstable eigenfunctions. A scattered nature, in time and space, of secondary instabilities is seen in the stability calculations. These instabilities can be detected before they reach finite amplitude in the DNS, preceding the nucleation of turbulent spots, and whose appearance is well correlated to the transition onset. This represents further evidence regarding the relevance of secondary instabilities of streaks in the bypass transition in realistic flow conditions. Consistent with the spatio-temporal nature of this problem, our approach allows us to integrate directly the local growth rates to obtain the spatial amplification ratio of the individual instabilities, where it is shown that instabilities reaching an -factor in the range [2.5,4] can be directly correlated to more than 65 % of the nucleation events. Interestingly, it is found that high amplification is not only attained by modes with high growth rates, but also by instabilities with sustained low growth rates for a long time.

Place, publisher, year, edition, pages
Cambridge University Press, 2024
Keywords
absolute/convective instability, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-347686 (URN)10.1017/jfm.2024.433 (DOI)001232840700001 ()2-s2.0-85195056445 (Scopus ID)
Note

QC 20240613

Available from: 2024-06-13 Created: 2024-06-13 Last updated: 2024-06-13Bibliographically approved
De Vincentiis, L., Durovic, K., Lengani, D., Simoni, D., Pralits, J., Henningson, D. S. & Hanifi, A. (2023). Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade. Journal of turbomachinery, 145(5), Article ID 051011.
Open this publication in new window or tab >>Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade
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2023 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 145, no 5, article id 051011Article in journal (Refereed) Published
Abstract [en]

In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the low- and high-speed regions associated with the wakes.

Place, publisher, year, edition, pages
ASME International, 2023
Keywords
boundary layer development, computationalfluiddynamics(CFD)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-326643 (URN)10.1115/1.4056108 (DOI)000964312500012 ()2-s2.0-85144290077 (Scopus ID)
Note

QC 20230509

Available from: 2023-05-09 Created: 2023-05-09 Last updated: 2023-05-09Bibliographically approved
Coelho Leite Fava, T., Lobo, B. A., Nogueira, P. A., Schaffarczyk, A. P., Breuer, M., Henningson, D. S. & Hanifi, A. (2023). Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake. Paper presented at 8th Wake Conference 2023, Visby, Sweden, 20 - 22 June 2023. Journal of Physics, Conference Series, 2505(1), 012002-012002
Open this publication in new window or tab >>Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake
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2023 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 2505, no 1, p. 012002-012002Article in journal (Refereed) Published
Abstract [en]

Free-stream turbulence (FST) alters the boundary layer of wind turbine blades, changing the hydrodynamic stability and near wake. Large-eddy simulations (LES) of a blade section with a laminar separation bubble for several turbulence intensities (TI) and a Reynolds number of 100,000 are performed. The effects of boundary-layer streaks generated by FST on Tollmien-Schlichting (TS) and Kelvin-Helmholtz (KH) instabilities are analyzed with a model based on the parabolized stability equations (PSE). Two competing effects on flow stability are identified. The spanwise-averaged mean-flow distortion stabilizes primary TS/KH modes for increasing TI. However, this contribution seems dominant only for TI ≥ 8.6%. For lower TI, the spanwise-oscillating distortion caused by streaks destabilizes the flow, and the growth rates of secondary modal instabilities increase with the streak amplitude. The destabilization occurs mainly at spanwise locations with negative streaks since the inflection point shifts away from the wall, enhancing inviscid instabilities. Inflection points in the spanwise direction formed by the streaks also contribute to the destabilization. The modal structures from PSE and LES agree. Finally, the trailing-edge near-wake coherent structures are more energetic for TI ≥ 8.6% due to the partial stabilization of modal instabilities, delaying the turbulent breakdown.

Place, publisher, year, edition, pages
IOP Publishing, 2023
National Category
Fluid Mechanics and Acoustics
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-337105 (URN)10.1088/1742-6596/2505/1/012002 (DOI)001004334300002 ()2-s2.0-85163429561 (Scopus ID)
Conference
8th Wake Conference 2023, Visby, Sweden, 20 - 22 June 2023
Funder
StandUpStandUp for Wind
Note

QC 20231030

Available from: 2023-09-25 Created: 2023-09-25 Last updated: 2023-10-30Bibliographically approved
Beneitez Galan, M., Duguet, Y., Schlatter, P. & Henningson, D. S. (2023). Instability of the optimal edge trajectory in the Blasius boundary layer. Journal of Fluid Mechanics, 971, Article ID A42.
Open this publication in new window or tab >>Instability of the optimal edge trajectory in the Blasius boundary layer
2023 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 971, article id A42Article in journal (Refereed) Published
Abstract [en]

In the context of linear stability analysis, considering unsteady base flows is notoriously difficult. A generalisation of modal linear stability analysis, allowing for arbitrarily unsteady base flows over a finite time, is therefore required. The recently developed optimally time-dependent (OTD) modes form a projection basis for the tangent space. They capture the leading amplification directions in state space under the constraint that they form an orthonormal basis at all times. The present numerical study illustrates the possibility to describe a complex flow case using the leading OTD modes. The flow under investigation is an unsteady case of the Blasius boundary layer, featuring streamwise streaks of finite length and relevant to bypass transition. It corresponds to the state space trajectory initiated by the minimal seed; such a trajectory is unsteady, free from any spatial symmetry and shadows the laminar-turbulent separatrix for a finite time only. The finite-time instability of this unsteady base flow is investigated using the 8 leading OTD modes. The analysis includes the computation of finite-time Lyapunov exponents as well as instantaneous eigenvalues, and of the associated flow structures. The reconstructed instantaneous eigenmodes are all of outer type. They map unambiguously the spatial regions of largest instantaneous growth. Other flow structures, previously reported as secondary, are identified with this method as relevant to streak switching and to streamwise vortical ejections. The dynamics inside the tangent space features both modal and non-modal amplification. Non-normality within the reduced tangent subspace, quantified by the instantaneous numerical abscissa, emerges only as the unsteadiness of the base flow is reduced.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2023
Keywords
boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-338062 (URN)10.1017/jfm.2023.545 (DOI)001119286200001 ()2-s2.0-85172862327 (Scopus ID)
Note

QC 20231013

Available from: 2023-10-13 Created: 2023-10-13 Last updated: 2024-01-16Bibliographically approved
Nobis, H., Schlatter, P., Wadbro, E., Berggren, M. & Henningson, D. S. (2023). Modal laminar-turbulent transition delay by means of topology optimization of superhydrophobic surfaces. Computer Methods in Applied Mechanics and Engineering, 403, Article ID 115721.
Open this publication in new window or tab >>Modal laminar-turbulent transition delay by means of topology optimization of superhydrophobic surfaces
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2023 (English)In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 403, article id 115721Article in journal (Refereed) Published
Abstract [en]

When submerged under a liquid, the microstructure of a SuperHydrophobic Surface (SHS) traps a lubricating layer of gas pockets, which has been seen to reduce the skin friction of the overlying liquid flow in both laminar and turbulent regimes. More recently, spatially homogeneous SHS have also been shown to delay laminar-turbulent transition in channel flows, where transition is triggered by modal mechanisms. In this study, we investigate, by means of topology optimization, whether a spatially inhomogeneous SHS can be designed to further delay transition in channel flows. Unsteady direct numerical simulations are conducted using the spectral element method in a 3D periodic wall-bounded channel. The effect of the SHS is modelled using a partial slip length on the walls, forming a 2D periodic optimization domain. Following a density-based approach, the optimization procedure uses the adjoint-variable method to compute gradients and a checkpointing strategy to reduce storage requirements. This methodology is adapted to optimizing over an ensemble of initial perturbations.This study presents the first application of topology optimization to laminar-turbulent transition. We show that this method can design surfaces that delay transition significantly compared to a homogeneous counterpart, by inhibiting the growth of secondary instability modes. By optimizing over an ensemble of streamwise phase-shifted perturbations, designs have been found with comparable mean transition time and lower variance.

Place, publisher, year, edition, pages
Elsevier BV, 2023
Keywords
Topology optimization, Spectral element method, Laminar-turbulent transition, Direct numerical simulations, Channel flow, Super-hydrophobic surfaces
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-323419 (URN)10.1016/j.cma.2022.115721 (DOI)000906896000009 ()2-s2.0-85141501653 (Scopus ID)
Note

QC 20230201

Available from: 2023-02-01 Created: 2023-02-01 Last updated: 2023-11-25Bibliographically approved
Kleine, V., Hanifi, A. & Henningson, D. S. (2023). Non-iterative vortex-based smearing correction for the actuator line method. Journal of Fluid Mechanics, 961, Article ID A29.
Open this publication in new window or tab >>Non-iterative vortex-based smearing correction for the actuator line method
2023 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 961, article id A29Article in journal (Refereed) Published
Abstract [en]

The actuator line method (ALM) is used extensively in wind turbine and rotor simulations. However, its original uncorrected formulation overestimates the forces near the tip of the blades and does not reproduce well forces on translating wings. The recently proposed vortex-based smearing correction for the ALM is a correction based on physical and mathematical properties of the simulation that allows for a more accurate and general ALM. So far, to correct the forces on the blades, the smearing correction depended on an iterative process at every time step, which is usually slower, less stable and less deterministic than direct methods. In this work, a non-iterative process is proposed and validated. First, we propose a formulation of the nonlinear lifting line that is equivalent to the ALM with smearing correction, showing that the results are practically identical for a translating wing. Then, by linearizing the lifting line method, the iterative process of the correction is substituted by the direct solution of a small linear system. No significant difference is observed in the results of the iterative and non-iterative corrections, in both wing and rotor simulations. Additional contributions of the present work include the use of a more accurate approximation for the velocity induced by a smeared vortex segment and the implementation of a free-vortex wake model to define the vortex sheet, which contribute to the accuracy and generality of the method. The results presented here may motivate the adoption of the ALM by other communities, for example, in fixed-wing applications.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2023
Keywords
computational methods, vortex shedding
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-328432 (URN)10.1017/jfm.2023.237 (DOI)000976753400001 ()2-s2.0-85158156996 (Scopus ID)
Note

QC 20230612

Available from: 2023-06-12 Created: 2023-06-12 Last updated: 2023-06-12Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7864-3071

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