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Quaranta, H. U., Brynjell-Rahkola, M., Leweke, T. & Henningson, D. S. (2019). Local and global pairing instabilities of two interlaced helical vortices. Journal of Fluid Mechanics, 863, 927-955
Open this publication in new window or tab >>Local and global pairing instabilities of two interlaced helical vortices
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 863, p. 927-955Article in journal (Refereed) Published
Abstract [en]

We investigate theoretically and experimentally the stability of two interlaced helical vortices with respect to displacement perturbations having wavelengths that are large compared to the size of the vortex cores. First, existing theoretical results are recalled and applied to the present configuration. Various modes of unstable perturbations, involving different phase relationships between the two vortices, are identified and their growth rates are calculated. They lead to a local pairing of neighbouring helix loops, or to a global pairing with one helix expanding and the other one contracting. A relation is established between this instability and the three-dimensional pairing of arrays of straight parallel vortices, and a striking quantitative agreement concerning the growth rates and frequencies is found. This shows that the local pairing of vortices is the driving mechanism behind the instability of the helix system. Second, an experimental study designed to observe these instabilities in a real flow is presented. Two helical vortices are generated by a two-bladed rotor in a water channel and characterised through dye visualisations and particle image velocimetry measurements. Unstable displacement modes are triggered individually, either by varying the rotation frequency of the rotor, or by imposing a small rotor eccentricity. The observed unstable mode structure, and the corresponding growth rates obtained from advanced processing of visualisation sequences, are in good agreement with theoretical predictions. The nonlinear late stages of the instability are also documented experimentally. Whereas local pairing leads to strong deformations and subsequent breakup of the vortices, global pairing results in a leapfrogging phenomenon, which temporarily restores the initial double-helix geometry, in agreement with recent observations from numerical simulations.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
vortex flows, vortex instability, vortex interactions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-245126 (URN)10.1017/jfm.2018.904 (DOI)000458504100001 ()2-s2.0-85060944108 (Scopus ID)
Note

QC 20190315

Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-06-11Bibliographically approved
Morra, P., Semeraro, O., Henningson, D. S. & Cossu, C. (2019). On the relevance of Reynolds stresses in resolvent analyses of turbulent wall-bounded flows. Journal of Fluid Mechanics, 867, 969-984, Article ID PII S0022112019001964.
Open this publication in new window or tab >>On the relevance of Reynolds stresses in resolvent analyses of turbulent wall-bounded flows
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 867, p. 969-984, article id PII S0022112019001964Article in journal (Refereed) Published
Abstract [en]

The ability of linear stochastic response analysis to estimate coherent motions is investigated in turbulent channel flow at the friction Reynolds number Re-r = 1007. The analysis is performed for spatial scales characteristic of buffer-layer and large-scale motions by separating the contributions of different temporal frequencies. Good agreement between the measured spatio-temporal power spectral densities and those estimated by means of the resolvent is found when the effect of turbulent Reynolds stresses, modelled with an eddy-viscosity associate with the turbulent mean flow, is included in the resolvent operator. The agreement is further improved when the flat forcing power spectrum (white noise) is replaced with a power spectrum matching the measures. Such a good agreement is not observed when the eddy-viscosity terms are not included in the resolvent operator. In this case, the estimation based on the resolvent is unable to select the right peak frequency and wall-normal location of buffer-layer motions. Similar results are found when comparing truncated expansions of measured streamwise velocity power spectral densities based on a spectral proper orthogonal decomposition to those obtained with optimal resolvent modes.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
turbulent boundary layers
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-249766 (URN)10.1017/jfm.2019.196 (DOI)000463073000001 ()2-s2.0-85063881975 (Scopus ID)
Note

QC 20190429

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-04-29Bibliographically approved
Kleine, V., Kleusberg, E., Hanifi, A. & Henningson, D. S. (Eds.). (2019). Tip-vortex instabilities of two in-line wind turbines. Institute of Physics (IOP)
Open this publication in new window or tab >>Tip-vortex instabilities of two in-line wind turbines
2019 (English)Conference proceedings (editor) (Refereed)
Place, publisher, year, edition, pages
Institute of Physics (IOP), 2019
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251408 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved
Sasaki, K., Vinuesa, R., Cavalieri, A. V. G., Schlatter, P. & Henningson, D. S. (2019). Transfer functions for flow predictions in wall-bounded turbulence. Journal of Fluid Mechanics, 864, 708-745
Open this publication in new window or tab >>Transfer functions for flow predictions in wall-bounded turbulence
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2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 864, p. 708-745Article in journal (Refereed) Published
Abstract [en]

Three methods are evaluated to estimate the streamwise velocity fluctuations of a zero-pressure-gradient turbulent boundary layer of momentum-thickness-based Reynolds number up to using as input velocity fluctuations at different wall-normal positions. A system identification approach is considered where large-eddy simulation data are used to build single and multiple-input linear and nonlinear transfer functions. Such transfer functions are then treated as convolution kernels and may be used as models for the prediction of the fluctuations. Good agreement between predicted and reference data is observed when the streamwise velocity in the near-wall region is estimated from fluctuations in the outer region. Both the unsteady behaviour of the fluctuations and the spectral content of the data are properly predicted. It is shown that approximately 45 % of the energy in the near-wall peak is linearly correlated with the outer-layer structures, for the reference case. These identified transfer functions allow insight into the causality between the different wall-normal locations in a turbulent boundary layer along with an estimation of the tilting angle of the large-scale structures. Differences in accuracy of the methods (single- and multiple-input linear and nonlinear) are assessed by evaluating the coherence of the structures between wall-normally separated positions. It is shown that the large-scale fluctuations are coherent between the outer and inner layers, by means of an interactions which strengthens with increasing Reynolds number, whereas the finer-scale fluctuations are only coherent within the near-wall region. This enables the possibility of considering the wall-shear stress as an input measurement, which would more easily allow the implementation of these methods in experimental applications. A parametric study was also performed by evaluating the effect of the Reynolds number, wall-normal positions and input quantities considered in the model. Since the methods vary in terms of their complexity for implementation, computational expense and accuracy, the technique of choice will depend on the application under consideration. We also assessed the possibility of designing and testing the models at different Reynolds numbers, where it is shown that the prediction of the near-wall peak from wall-shear-stress measurements is practically unaffected even for a one order of magnitude change in the corresponding Reynolds number of the design and test, indicating that the interaction between the near-wall peak fluctuations and the wall is approximately Reynolds-number independent. Furthermore, given the performance of such methods in the prediction of flow features in turbulent boundary layers, they have a good potential for implementation in experiments and realistic flow control applications, where the prediction of the near-wall peak led to correlations above 0.80 when wall-shear stress was used in a multiple-input or nonlinear scheme. Errors of the order of 20 % were also observed in the determination of the near-wall spectral peak, depending on the employed method.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
turbulence modelling, turbulent boundary layers
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-245119 (URN)10.1017/jfm.2019.27 (DOI)000458488900001 ()2-s2.0-85061456245 (Scopus ID)
Note

QC 20190315

Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-03-15Bibliographically approved
Muhle, F., Schottler, J., Bartl, J., Futrzynski, R., Evans, S., Bernini, L., . . . Saetran, L. (2018). Blind test comparison on the wake behind a yawed wind turbine. Wind Energy Science, 3(2), 883-903
Open this publication in new window or tab >>Blind test comparison on the wake behind a yawed wind turbine
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2018 (English)In: Wind Energy Science, ISSN 2213-3968, E-ISSN 2366-7443, Vol. 3, no 2, p. 883-903Article in journal (Refereed) Published
Abstract [en]

This article summarizes the results of the "Blind test 5" workshop, which was held in Visby, Sweden, in May 2017. This study compares the numerical predictions of the wake flow behind a model wind turbine operated in yaw to experimental wind tunnel results. Prior to the workshop, research groups were invited to predict the turbine performance and wake flow properties using computational fluid dynamics (CFD) methods. For this purpose, the power, thrust, and yaw moments for a 30 degrees yawed model turbine, as well as the wake's mean and turbulent streamwise and vertical flow components, were measured in the wind tunnel at the Norwegian University of Science and Technology (NTNU). In order to increase the complexity, a non-yawed downstream turbine was added in a second test case, while a third test case challenged the modelers with a new rotor and turbine geometry. Four participants submitted predictions using different flow solvers, three of which were based on large eddy simulations (LES) while another one used an improved delayed detached eddy simulation (IDDES) model. The performance of a single yawed turbine was fairly well predicted by all simulations, both in the first and third test cases. The scatter in the downstream turbine performance predictions in the second test case, however, was found to be significantly larger. The complex asymmetric shape of the mean streamwise and vertical velocities was generally well predicted by all the simulations for all test cases. The largest improvement with respect to previous blind tests is the good prediction of the levels of TKE in the wake, even for the complex case of yaw misalignment. These very promising results confirm the mature development stage of LES/DES simulations for wind turbine wake modeling, while competitive advantages might be obtained by faster computational methods.

Place, publisher, year, edition, pages
COPERNICUS GESELLSCHAFT MBH, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-239766 (URN)10.5194/wes-3-883-2018 (DOI)000450295200001 ()
Note

QC 20190109

Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-05-14Bibliographically approved
Dadfar, R., Hanifi, A. & Henningson, D. S. (2018). Control of instabilities in an unswept wing boundary layer. AIAA Journal, 56(5), 1750-1759
Open this publication in new window or tab >>Control of instabilities in an unswept wing boundary layer
2018 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 56, no 5, p. 1750-1759Article in journal (Refereed) Published
Abstract [en]

Linear control theory is used to construct an output feedback controller to attenuate the amplitude of the Tollmien–Schlichting waves inside the boundary layer developing over an unswept wing. The analysis is based on direct numerical simulations. The studied scenario includes the impulse response of the system to a generic disturbance in the freestream, which triggers a Tollmien–Schlichting wave packet inside the boundary layer. The performance of a linear quadratic Gaussian controller is analyzed to suppress the amplitude of the Tollmien–Schlichting wave packet using a row of sensors and plasma actuators localized at the wall. The target of the controller is chosen as a subset of proper orthogonal decomposition modes describing the dynamics of the unstable disturbances. The plasma actuators are implemented as volume forcing. To account for the limitations of the plasma actuators concerning a unidirectional forcing, several strategies are implemented in the linear quadratic Gaussian framework. Their performances are compared with that for classical linear quadratic Gaussian controller. These controllers successfully reduced the amplitude of the wave packet.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics Inc., 2018
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-228955 (URN)10.2514/1.J056415 (DOI)000432661400005 ()2-s2.0-85046622897 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20180530

Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-06-25Bibliographically approved
Morra, P., Sasaki, K., Cavalieri, A., Hanifi, A. & Henningson, D. S. (2018). Control of streaky disturbances in the boundary layer over a flat plate. In: 31st Congress of the International Council of the Aeronautical Sciences, ICAS 2018: . Paper presented at 31st Congress of the International Council of the Aeronautical Sciences, ICAS 2018, 9 September 2018 through 14 September 2018. International Council of the Aeronautical Sciences
Open this publication in new window or tab >>Control of streaky disturbances in the boundary layer over a flat plate
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2018 (English)In: 31st Congress of the International Council of the Aeronautical Sciences, ICAS 2018, International Council of the Aeronautical Sciences , 2018Conference paper, Published paper (Refereed)
Abstract [en]

The present work considers control of perturbations in the boundary layer over a flat plate by means of adaptive methods. In particular, we focus our attention on a control law based on a multi-input-multi-output (MIMO) filtered-x least-mean-square (fxLMS) adaptive algorithm. The studies are performed through direct numerical simulations. The perturbation field studied here mimics those generated by freestream turbulence with different amplitude and scales. Plasma actuators and shear-stress sensors are considered to mimic a real case scenario.

Place, publisher, year, edition, pages
International Council of the Aeronautical Sciences, 2018
Keywords
Adaptive control, Feedforward control, Flow control, Streaks, Adaptive algorithms, Boundary layers, MIMO systems, Shear stress, Wave plasma interactions, Adaptive methods, Filtered x least mean squares, Freestream turbulence, Multi input multi output, Real case scenarios, Shear-stress sensors
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-247422 (URN)2-s2.0-85060468653 (Scopus ID)9783932182884 (ISBN)
Conference
31st Congress of the International Council of the Aeronautical Sciences, ICAS 2018, 9 September 2018 through 14 September 2018
Note

QC20190502

Available from: 2019-05-02 Created: 2019-05-02 Last updated: 2019-05-02Bibliographically approved
Eriksson, O., Laure, E., Lindahl, E., Henningson, D. S. & Ynnerman, A. (2018). e-Science in Scandinavia. Informatik-Spektrum, 41(6), 398-404
Open this publication in new window or tab >>e-Science in Scandinavia
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2018 (English)In: Informatik-Spektrum, ISSN 0170-6012, E-ISSN 1432-122X, Vol. 41, no 6, p. 398-404Article in journal (Refereed) Published
National Category
Computer and Information Sciences
Identifiers
urn:nbn:se:kth:diva-260432 (URN)10.1007/s00287-018-01133-2 (DOI)2-s2.0-85058461975 (Scopus ID)
Note

QC 20191011

Available from: 2019-09-30 Created: 2019-09-30 Last updated: 2019-10-11Bibliographically approved
Negi, P. S., Hanifi, A. & Henningson, D. S. (2018). LES of the unsteady response of a natural laminar flow airfoil. In: 2018 Applied Aerodynamics Conference: . Paper presented at 36th AIAA Applied Aerodynamics Conference, 2018, [state] GA, United States, 25 June 2018 through 29 June 2018. American Institute of Aeronautics and Astronautics
Open this publication in new window or tab >>LES of the unsteady response of a natural laminar flow airfoil
2018 (English)In: 2018 Applied Aerodynamics Conference, American Institute of Aeronautics and Astronautics, 2018Conference paper, Published paper (Refereed)
Abstract [en]

Large-eddy simulations are performed to investigate the dynamic response of a natural laminar flow airfoil undergoing harmonic pitch oscillations at a chord based Reynolds number of Rec= 750, 000. Large changes in the transition location are observed throughout the pitch cycles which leads to a non-linear response of the aerodynamic force coefficients. Preliminary results show that the evolution of the boundary layer over the airfoil can be modeled by using a simple phase-lag concept which implies that the boundary-layer evolution is quasi-steady in nature. A simple empirical model is developed based on this quasi-steady, phase-lag assumption which fits very well with the measured experimental data.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-234512 (URN)10.2514/6.2018-3824 (DOI)2-s2.0-85051738799 (Scopus ID)9781624105593 (ISBN)
Conference
36th AIAA Applied Aerodynamics Conference, 2018, [state] GA, United States, 25 June 2018 through 29 June 2018
Funder
VINNOVAEU, European Research CouncilSwedish e‐Science Research Center
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2018-09-07Bibliographically approved
Sasaki, K., Morra, P., Fabbiane, N., Cavalieri, A. V. G., Hanifi, A. & Henningson, D. S. (2018). On the wave-cancelling nature of boundary layer flow control. Theoretical and Computational Fluid Dynamics, 32(5), 593-616
Open this publication in new window or tab >>On the wave-cancelling nature of boundary layer flow control
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2018 (English)In: Theoretical and Computational Fluid Dynamics, ISSN 0935-4964, E-ISSN 1432-2250, Vol. 32, no 5, p. 593-616Article in journal (Refereed) Published
Abstract [en]

This work deals with the feedforward active control of Tollmien-Schlichting instability waves over incompressible 2D and 3D boundary layers. Through an extensive numerical study, two strategies are evaluated; the optimal linear-quadratic-Gaussian (LQG) controller, designed using the Eigensystem realization algorithm, is compared to a wave-cancellation scheme, which is obtained using the direct inversion of frequency-domain transfer functions of the system. For the evaluated cases, it is shown that LQG leads to a similar control law and presents a comparable performance to the simpler, wave-cancellation scheme, indicating that the former acts via a destructive interference of the incoming wavepacket downstream of actuation. The results allow further insight into the physics behind flow control of convectively unstable flows permitting, for instance, the optimization of the transverse position for actuation. Using concepts of linear stability theory and the derived transfer function, a more efficient actuation for flow control is chosen, leading to similar attenuation of Tollmien-Schlichting waves with only about 10% of the actuation power in the baseline case.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Boundary layer control, Flow control, Instability control, LQG controllers, Inversion controllers
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-235102 (URN)10.1007/s00162-018-0469-x (DOI)000443412500003 ()2-s2.0-85049125322 (Scopus ID)
Note

QC 20180917

Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-17Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-7864-3071

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