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Beneitez Galan, M., Duguet, Y., Schlatter, P. & Henningson, D. S. (2019). Edge tracking in spatially developing boundary layer flows. Journal of Fluid Mechanics, 881, 164-181
Open this publication in new window or tab >>Edge tracking in spatially developing boundary layer flows
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 881, p. 164-181Article in journal (Refereed) Published
Abstract [en]

Recent progress in understanding subcritical transition to turbulence is based on the concept of the edge, the manifold separating the basins of attraction of the laminar and the turbulent state. Originally developed in numerical studies of parallel shear flows with a linearly stable base flow, this concept is adapted here to the case of a spatially developing Blasius boundary layer. Longer time horizons fundamentally change the nature of the problem due to the loss of stability of the base flow due to Tollmien-Schlichting (TS) waves. We demonstrate, using a moving box technique, that efficient long-time tracking of edge trajectories is possible for the parameter range relevant to bypass transition, even if the asymptotic state itself remains out of reach. The flow along the edge trajectory features streak switching observed for the first time in the Blasius boundary layer. At long enough times, TS waves co-exist with the coherent structure characteristic of edge trajectories. In this situation we suggest a reinterpretation of the edge as a manifold dividing the state space between the two main types of boundary layer transition, i.e. bypass transition and classical transition.

Place, publisher, year, edition, pages
Cambridge University Press, 2019
Keywords
boundary layer stability, nonlinear dynamical systems, transition to turbulence, Aerodynamics, Boundary layer flow, Boundary layers, Dynamical systems, Parallel flow, Shear flow, Trajectories, Turbulence, Basins of attraction, Blasius boundary layer, Boundary layer stabilities, Boundary layer transitions, Classical transition, Subcritical transition, Tollmien-Schlichting waves, Atmospheric thermodynamics, boundary layer, fluid dynamics, fluid flow, nonlinearity
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263766 (URN)10.1017/jfm.2019.763 (DOI)2-s2.0-85074285559 (Scopus ID)
Note

QC 20191112

Available from: 2019-11-12 Created: 2019-11-12 Last updated: 2019-11-12Bibliographically approved
Negi, P., Hanifi, A. & Henningson, D. S. (2019). Global Stability of rigid-body-motion fluid-structure-interaction problems.
Open this publication in new window or tab >>Global Stability of rigid-body-motion fluid-structure-interaction problems
2019 (English)Report (Other academic)
Abstract [en]

A rigorous derivation and validation for linear fluid-structure-interaction (FSI) equations for a rigid-body-motion problem is performed in an Eulerian framework. We show that the “added-stiffness” terms arising in the formulation of Fanion et al. (2000) vanish at the FSI interface in a first-order approximation. Several numerical tests with rigid-body motion are performed to show the validity of the derived formulation by comparing the time evolution between the linear and non-linear equations when the base flow is perturbed by identical small-amplitude perturbations. In all cases both the growth rate and angular frequency of the instability matches within 0.1% accuracy. The derived formulation is used to investigate the phenomenon of symmetry breaking for a rotating cylinder with an attached splitter-plate. The results show that the onset of symmetry breaking can be explained by the existence of a zero-frequency linearly unstable mode of the coupled fluid-structure-interaction system. Finally, the structural sensitivity of the least stable eigenvalue is studied for an oscillating cylinder, which is found to change significantly when the fluid and structural frequencies are close to resonance.

Publisher
p. 38
Series
TRITA-SCI-RAP ; 2019:007
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262856 (URN)
Funder
Swedish National Infrastructure for Computing (SNIC)
Note

QC 20191025. QC 20191030

Available from: 2019-10-23 Created: 2019-10-23 Last updated: 2019-10-30Bibliographically approved
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
Brynjell-Rahkola, M., Hanifi, A. & Henningson, D. S. (2019). On the stability of a Blasius boundary layer subject to localised suction. Journal of Fluid Mechanics, 871, 717-741
Open this publication in new window or tab >>On the stability of a Blasius boundary layer subject to localised suction
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 871, p. 717-741Article in journal (Refereed) Published
Abstract [en]

In this study the origins of premature transition due to oversuction in boundary layers are studied. An infinite row of circular suction pipes that are mounted at right angles to a flat plate subject to a Blasius boundary layer is considered. The interaction between the flow originating from neighbouring holes is weak and for the parameters investigated, the pipe is always found to be unsteady regardless of the state of the flow in the boundary layer. A stability analysis reveals that the appearance of boundary layer transition can be associated with a linear instability in the form of two unstable eigenmodes inside the pipe that have weak tails, which extend into the boundary layer. Through an energy budget and a structural sensitivity analysis, the origin of this flow instability is traced to the structures developing inside the pipe near the pipe junction. Although the amplitudes of the modes in the boundary layer are orders of magnitude smaller than the corresponding amplitudes inside the pipe, a Koopman analysis of the data gathered from a nonlinear direct numerical simulation confirms that it is precisely these disturbances that are responsible for transition to turbulence in the boundary layer due to oversuction.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-264150 (URN)10.1017/jfm.2019.326 (DOI)000493076600011 ()2-s2.0-85066907512 (Scopus ID)
Note

QC 20191209

Available from: 2019-12-09 Created: 2019-12-09 Last updated: 2019-12-09Bibliographically approved
Kleusberg, E., Benard, S. & Henningson, D. S. (2019). Tip-vortex breakdown of wind turbines subject to shear. Wind Energy, 22(12), 1789-1799
Open this publication in new window or tab >>Tip-vortex breakdown of wind turbines subject to shear
2019 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 22, no 12, p. 1789-1799Article in journal (Refereed) Published
Abstract [en]

Sheared velocity profiles pervade all wind-turbine applications, thus making it important to understand their effect on the wake. In this study, a single wind turbine is modeled using the actuator-line method in the incompressible Navier–Stokes equations. The tip vortices are perturbed harmonically, and the growth rate of the response is evaluated under uniform inflow and a linear velocity profile. Whereas previous investigations of this kind were conducted in the rotating frame of reference, this study evaluates the excitation response in the fixed frame of reference, thus necessitating a frequency transformation. It is shown that increasing the shear decreases the spatial growth rate in the upper half of the wake while increasing it in the lower half. When scaled with the local tip vortex parameters, the growth rate along the entire azimuth collapses to a single value for the investigated wavenumbers. We conclude that even though the tip-vortex breakdown is asymmetric in sheared flow, the scaled growth rates follow the behavior of axisymmetric helical vortices. An excitation amplitude reduction by an order of magnitude extends the linear growth region of the wake by one radius for uniform inflow. In the sheared setup, the linear growth region is extended further in the top half than in the bottom half because of the progressive distortion of the helical tip vortices. An existing model to determine the stable wake length was shown to be in close agreement with the observed numerical results when adjusted for shear.

Place, publisher, year, edition, pages
John Wiley & Sons, 2019
Keywords
actuator-line method, CFD, Nek5000, shear, wake breakdown, wind turbine, Actuators, Aerodynamics, Computational fluid dynamics, Flow control, Navier Stokes equations, Shearing, Wakes, Wind tunnels, Wind turbines, Excitation amplitudes, Frequency transformations, Line methods, Numerical results, Stokes equations, Turbine applications, Velocity profiles, Vortex flow
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263254 (URN)10.1002/we.2403 (DOI)000496648000010 ()2-s2.0-85071751470 (Scopus ID)
Note

QC 20191105

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2019-12-13Bibliographically 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
Kleine, V., Kleusberg, E., Hanifi, A. & Henningson, D. S. (2019). Tip-vortex instabilities of two in-line wind turbines. In: Wake Conference 201922–24 May 2019, Visby, Sweden: . Paper presented at Wake Conference 2019; Uppsala University's Gotland Campus, Visby; Sweden; 22 May 2019 through 24 May 2019. Institute of Physics Publishing (IOPP), 1256(1), Article ID 012015.
Open this publication in new window or tab >>Tip-vortex instabilities of two in-line wind turbines
2019 (English)In: Wake Conference 201922–24 May 2019, Visby, Sweden, Institute of Physics Publishing (IOPP), 2019, Vol. 1256, no 1, article id 012015Conference paper, Published paper (Refereed)
Abstract [en]

The hydrodynamic stability of a vortex system behind two in-line wind turbines operating at low tip-speed ratios is investigated using the actuator-line method in conjunction with the spectral-element flow solver Nek5000. To this end, a simplified setup with two identical wind turbine geometries rotating at the same tip-speed ratio is simulated and compared with a single turbine wake. Using the rotating frame of reference, a steady solution is obtained, which serves as a base state to study the growth mechanisms of induced perturbations to the system. It is shown that, already in the steady state, the tip vortices of the two turbines interact with each other, exhibiting the so-called overtaking phenomenon. Hereby, the tip vortices of the upstream turbine overtake those of the downstream turbine repeatedly. By applying targeted harmonic excitations at the upstream turbine's blade tips a variety of modes are excited and grow with downstream distance. Dynamic mode decomposition of this perturbed flow field showed that the unstable out-of-phase mode is dominant, both with and without the presence of the second turbine. The perturbations of the upstream turbine's helical vortex system led to the destabilization of the tip vortices shed by the downstream turbine. Two distinct mechanisms were observed: for certain frequencies the downstream turbine's vortices oscillate in phase with the vortex system of the upstream turbine while for other frequencies a clear out-of-phase behaviour is observed. Further, short-wave instabilities were shown to grow in the numerical simulations, similar to existing experimental studies [1].

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Series
Journal of Physics: Conference Series, ISSN 1742-6588
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-262574 (URN)10.1088/1742-6596/1256/1/012015 (DOI)2-s2.0-85070017009 (Scopus ID)
Conference
Wake Conference 2019; Uppsala University's Gotland Campus, Visby; Sweden; 22 May 2019 through 24 May 2019
Note

QC 20191024

Available from: 2019-10-24 Created: 2019-10-24 Last updated: 2019-10-24Bibliographically 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
Show others...
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
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ORCID iD: ORCID iD iconorcid.org/0000-0001-7864-3071

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