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Segalini, A. & Dahlberg, J.-Å. (2020). Blockage effects in wind farms. Wind Energy, 23(2), 120-128
Open this publication in new window or tab >>Blockage effects in wind farms
2020 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 23, no 2, p. 120-128Article in journal (Refereed) Published
Abstract [en]

An experimental study of wind farm blockage has been performed to quantify the velocity decrease that the first row of a wind farm experiences due to the presence of the other turbines downstream. The general perception has been that turbines downstream of the first row are only influenced by the wakes from upstream turbines without any upstream effect. In the present study, an attempt is made to demonstrate the existence of a two-way coupling between individual turbines and turbines in the wind farm. Several staggered layouts were tested in the wind tunnel experiments by changing the spacing between rows, spacing between turbines in the rows, and the amount of wind turbines involved. The experiments focused on turbines located in the center of the first row as well as the two turbines located in the row edges, usually believed to experience a speedup. The present results show that no speedup is present and that all the turbines in the first row are subjected to a reduced wind speed. This phenomenon has been considered to be due to “global blockage.” An empirical correlation formula between spacing, number of rows, and velocity decrease is proposed to quantify such effect for the center turbine as well as for the turbines at the edges.

Place, publisher, year, edition, pages
John Wiley and Sons Ltd, 2020
Keywords
wind farm blockage, Wind, Wind power, Wind tunnels, Blockage effects, Empirical correlations, Two-way coupling, Wind farm, Wind speed, Wind tunnel experiment, Electric utilities, experimental study, perception, wind tunnel, wind turbine, wind velocity
National Category
Energy Systems
Identifiers
urn:nbn:se:kth:diva-267965 (URN)10.1002/we.2413 (DOI)000500087300001 ()2-s2.0-85076119060 (Scopus ID)
Note

QC 20200330

Available from: 2020-03-30 Created: 2020-03-30 Last updated: 2020-03-30Bibliographically approved
Matsubara, M., Alfredsson, P. H. & Segalini, A. (2020). Linear modes in a planar turbulent jet. Journal of Fluid Mechanics, 888, Article ID A26.
Open this publication in new window or tab >>Linear modes in a planar turbulent jet
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 888, article id A26Article in journal (Refereed) Published
Abstract [en]

A planar jet issuing from a fully developed two-dimensional turbulent channel flow is studied, with a focus on the transverse flapping of the jet core. The streamwise and transverse velocities were measured with hot-wire anemometry using an X-type probe. The mean velocity field and the velocity covariances were first characterised to assess the undisturbed flow field. Periodic excitations were introduced from a slot mounted at the channel exit and the coherent fluctuating part of the signal was obtained by using a phase-locked averaging technique, where the periodic initial forcing was used as trigger. This enabled the eduction of the coherent structure associated with the introduced perturbation. Its amplitude was found to be directly proportional to the intensity of the initial forcing and, within a certain range of the initial forcing amplitude, the growth curves were identical as well as the spatial distribution of the extracted fluctuations. Parallel and non-parallel linear stability theory captures qualitatively and quantitatively the features of the educed coherent structure. The existence of the linear mode in the turbulent jet implies that the large-scale perturbations observed in natural (unforced) jets can be regarded as an incoherent set of linear modes.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
jets, absolute, convective instability, shear layer turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-269021 (URN)10.1017/jfm.2020.25 (DOI)000512393600001 ()2-s2.0-85079328310 (Scopus ID)
Note

QC 20200316

Available from: 2020-03-16 Created: 2020-03-16 Last updated: 2020-03-16Bibliographically approved
Braunbehrens, R. & Segalini, A. (2019). A statistical model for wake meandering behind wind turbines. Journal of Wind Engineering and Industrial Aerodynamics, 193, Article ID UNSP 103954.
Open this publication in new window or tab >>A statistical model for wake meandering behind wind turbines
2019 (English)In: Journal of Wind Engineering and Industrial Aerodynamics, ISSN 0167-6105, E-ISSN 1872-8197, Vol. 193, article id UNSP 103954Article in journal (Refereed) Published
Abstract [en]

A new wake model is proposed to account for wake meandering in simulations of wind-turbine wakes performed on steady solvers, through a wake-meandering description based on the dispersion theory of Taylor (1921, P. Lond. Math Soc., vol. 20, pp. 196-211). Single-turbine simulations were performed by means of the linearised solver ORFEUS. By analysing the steady wake behind a turbine, a set of parameters describing the wake was first obtained and synthesised into a look-up table. The proposed meandering model extended the simulation results by superimposing the lateral and vertical meandering motions to the steady wake. As a result, the time-averaged velocity distribution of the wake was increased in width and reduced in intensity. Through this combination, the model provides rationale for the wake-deficit decrease and for the power underestimation effects of several wake models. The new wake model is validated against the Lillgrund and Horns Rev data sets.

Place, publisher, year, edition, pages
ELSEVIER, 2019
Keywords
Numerical wake model, Linearised RANS, Wake meandering, Atmospheric dispersion
National Category
Mechanical Engineering
Research subject
Machine Design
Identifiers
urn:nbn:se:kth:diva-262777 (URN)10.1016/j.jweia.2019.103954 (DOI)000488140800014 ()2-s2.0-85071502422 (Scopus ID)
Note

QC 20191022

Available from: 2019-10-22 Created: 2019-10-22 Last updated: 2019-11-26Bibliographically approved
Alveroglu, B., Segalini, A. & Garrett, S. J. (2019). An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces. In: Open Archives of the 16th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016: . Paper presented at 16th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016, Honolulu, United States, 10-15 April 2016. International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016
Open this publication in new window or tab >>An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces
2019 (English)In: Open Archives of the 16th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016, International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016 , 2019Conference paper, Published paper (Refereed)
Abstract [en]

An energy balance equation for the three-dimensional Bödewadt and Ekman layers of the so called “BEK family" of rotating boundary-layer flows is derived. A Chebyshev discretisation method is used to solve the equations and investigate the effect of surface roughness on the physical mechanisms of transition. All roughness types lead to a stabilization of the Type I (cross-flow) instability mode for both flows, with the exception of azimuthally-anisotropic roughness (radial grooves) within the Bödewadt layer which is destabilising. In the case of the viscous Type II instability mode, the results predict a destabilisation effect of radially-anisotropic roughness (concentric grooves) on both flows, whereas both azimuthally-anisotropic roughness and isotropic roughness have a stabilisation effect. The results presented here confirm the results of our prior linear stability analyses.

Place, publisher, year, edition, pages
International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016, 2019
Keywords
BEK family, Energy, Roughness, Anisotropy, Boundary layer flow, Boundary layers, Rotating machinery, Stabilization, Transport properties, Convective instabilities, Discretisation method, Ekman boundary layers, Energy balance equations, Physical mechanism, Radially anisotropic, Surface roughness
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-262390 (URN)2-s2.0-85069863343 (Scopus ID)
Conference
16th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC 2016, Honolulu, United States, 10-15 April 2016
Note

QC 20191028

Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-10-28Bibliographically approved
Segalini, A. & Camarri, S. (2019). Flow induced by a rotating cone: Base flow and convective stability analysis. Physical Review Fluids, 4(8), Article ID 084801.
Open this publication in new window or tab >>Flow induced by a rotating cone: Base flow and convective stability analysis
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 8, article id 084801Article in journal (Refereed) Published
Abstract [en]

The steady flow over a rotating cone is investigated theoretically and numerically in order to improve the traditional von Karman solution by proposing a self-similar correction which is an explicit function of the cone angle. The effect of the correction on the linear stability analysis of the rotating-cone flow is successively investigated through a weakly divergent approach. Both the base flow correction and the results of the stability analysis are validated against dedicated numerical simulations. As for the base flow, the comparison shows a clear improvement obtained by using the proposed correction in comparison with the classical von Karman solution. As for the stability properties of the flow, the comparison against the reference simulations shows a good agreement among all the approaches for large azimuthal wave numbers, but a better performance is obtained with the weakly divergent approach for lower wave numbers. The latter approach provides a lower critical Reynolds number than that predicted by parallel theory and, most importantly, changes the interplay between modes I and II with respect to what predicted by the parallel stability calculations. Finally, it is observed that the proposed correction of base flow has a slight effect on the stability analysis of the considered cases, but it may have important effects for low cone angles. Thus, while the classical Karman solution is appropriate for large cone angles, the proposed correction is recommended for future stability analyses of slender cones.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-256251 (URN)10.1103/PhysRevFluids.4.084801 (DOI)000479008200003 ()2-s2.0-85072024382 (Scopus ID)
Note

QC 20191022

Available from: 2019-10-22 Created: 2019-10-22 Last updated: 2019-10-22Bibliographically approved
Segalini, A. & Dahlberg, J.-A. -. (2019). Global Blockage Effects in Wind Farms. In: Journal of Physics: Conference Series. 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 012021.
Open this publication in new window or tab >>Global Blockage Effects in Wind Farms
2019 (English)In: Journal of Physics: Conference Series, Institute of Physics Publishing (IOPP), 2019, Vol. 1256, no 1, article id 012021Conference paper, Published paper (Refereed)
Abstract [en]

An experimental and numerical study of wind-farm blockage has been performed to quantify the velocity reduction that the first row of a wind farm experiences due to other turbines downstream. In the present study, an attempt is made to demonstrate the existence of a two-way coupling between individual turbines and wind farm. Several staggered layouts were tested in the wind-tunnel experiments by changing the spacing between rows, spacing between the turbines in the same row and the amount of wind turbines involved. Three turbines located in the first row were monitored to assess their sensitivity to the turbines downstream. One of the experiments was replicated by means of numerical simulations performed in ORFEUS, a linearised code developed at KTH, in order to complement the experimental results. Simulations were performed at the same thrust coefficient and with a more distant ceiling to assess the eventual interference of the ceiling on the experimental results. Additionally, simulations performed at different thrust coefficients were done to assess its effect on the blockage phenomenon.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Series
Journal of Physics: Conference Series, ISSN 1742-6588 ; 1256
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-262575 (URN)10.1088/1742-6596/1256/1/012021 (DOI)2-s2.0-85069993609 (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
Hyvärinen, A., Lacagnina, G. & Segalini, A. (2018). A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills. Wind Energy, 21(8), 605-617
Open this publication in new window or tab >>A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
2018 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 21, no 8, p. 605-617Article in journal (Refereed) Published
Abstract [en]

In the present work, the wake development behind small-scale wind turbines is studied when introducing local topography variations consisting of a series of sinusoidal hills. Additionally, wind-tunnel tests with homogeneous and sheared turbulent inflows were performed to understand how shear and ambient turbulence influence the results. The scale of the wind-turbine models was about 1000times smaller than full-size turbines, suggesting that the present results should only be qualitatively extrapolated to real-field scenarios. Wind-tunnel measurements were made by means of stereoscopic particle image velocimetry to characterize the flow velocity in planes perpendicular to the flow direction. Over flat terrain, the wind-turbine wake was seen to slowly approach the ground while it propagated downstream. When introducing hilly terrain, the downward wake deflection was enhanced in response to flow variations induced by the hills, and the turbulent kinetic energy content in the wake increased because of the speed-up seen over the hills. The combined wake observed behind 2 streamwise aligned turbines was more diffused and when introducing hills, it was more prone to deflect towards the ground compared to the wake behind an isolated turbine. Since wake interactions are common at sites with multiple turbines, this suggested that it is important to consider the local hill-induced velocity variations when onshore wind farms are analysed. Differences in the flow fields were seen when introducing either homogeneous or sheared turbulent inflow conditions, emphasizing the importance of accounting for the prevailing turbulence conditions at a given wind-farm site to accurately capture the downstream wake development.

Place, publisher, year, edition, pages
John Wiley & Sons, 2018
Keywords
Wind turbines, Complex terrain, Wind-tunnel measurements
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-221704 (URN)10.1002/we.2181 (DOI)000438224900001 ()2-s2.0-85044577908 (Scopus ID)
Funder
Swedish Research CouncilStandUp for Wind
Note

QCR 20180122

Available from: 2018-01-22 Created: 2018-01-22 Last updated: 2019-09-06Bibliographically approved
Rajput, M. S., Burman, M., Segalini, A. & Hallström, S. (2018). Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites. Polymer testing, 68, 446-455
Open this publication in new window or tab >>Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites
2018 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 68, p. 446-455Article in journal (Refereed) Published
Abstract [en]

A drop-weight rig (DWR) intended to test the true impact response of laminated compositematerials is presented. The test setup is designed to prevent the transfer of unwanted mechanical noise, e.g. vibrations, into the load cell that is used to measure the load during the impact event. A novel catch mechanism preventing secondary impact is also implemented in the DWR design. A detailed evaluation is performed both in terms of the experimental modal and uncertainty analysis of the measured results from the DWR. The results demonstrate that the rig is capable of capturing the true impact response, providing highly resolved and noise-free force-time measurements where even subtle details of the impact event are visible. The rig also enables impact testing with good repeatability.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Low velocity impact, Vibration, Uncertainty analysis, Modal analysis, Free-fall
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-228484 (URN)10.1016/j.polymertesting.2018.04.022 (DOI)000437076000053 ()2-s2.0-85047266915 (Scopus ID)
Projects
DAMTISS
Funder
VINNOVA, 2013-01132
Note

QC 20180525

Available from: 2018-05-25 Created: 2018-05-25 Last updated: 2018-11-09Bibliographically approved
Bourgoin, M., Baudet, C., Kharche, S., Mordant, N., Vandenberghe, T., Sumbekova, S., . . . Peinke, J. (2018). Investigation of the small-scale statistics of turbulence in the Modane S1MA wind tunnel. CEAS Aeronautical Journal, 9(2), 269-281
Open this publication in new window or tab >>Investigation of the small-scale statistics of turbulence in the Modane S1MA wind tunnel
Show others...
2018 (English)In: CEAS Aeronautical Journal, ISSN 1869-5582, Vol. 9, no 2, p. 269-281Article in journal (Refereed) Published
Abstract [en]

This article describes the planning, set-up, turbulence characterization and analysis of measurements of a passive grid turbulence experiment that was carried out in the S1MA wind-tunnel from ONERA in Modane, in the context of the ESWIRP European project. This experiment aims at a detailed investigation of the statistical properties of turbulent flows at large Reynolds numbers. The primary goal is to take advantage of the unequaled large-scale dimensions of the ONERA S1MA wind-tunnel facility, to make available to the broad turbulence community high-quality experimental turbulence data with unprecendented resolution (both spatial and temporal) and accuracy (in terms of statistical convergence). With this goal, we designed the largest grid-generated turbulence experiment planned and performed to date. Grid turbulence is a canonical flow known to produce almost perfectly homogeneous and isotropic turbulence (HIT) which remains a unique framework to investigate fundamental physics of turbulent flows. Here, we present a brief description of the measurements, in particular those based on hot-wire diagnosis. By comparing results from classical hot-wires and from a nano-fabricated wire (developed at Princeton University), we show that our goal of resolving down to the smallest dissipative scales of the flow has been achieved. We also present the full characterization of the turbulence here, in terms of turbulent energy dissipation rate, injection and dissipation scales (both spatial and temporal) and Reynolds number.

Place, publisher, year, edition, pages
Springer, 2018
Keywords
Turbulence
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-238212 (URN)10.1007/s13272-017-0254-3 (DOI)2-s2.0-85047544305 (Scopus ID)
Note

QC 20181120

Available from: 2018-11-20 Created: 2018-11-20 Last updated: 2018-11-20Bibliographically approved
Ebenhoch, R., Muro, B., Dahlberg, J.-Å., Berkesten Hägglund, P. & Segalini, A. (2017). A linearized numerical model of wind-farm flows. Wind Energy, 20(5), 859-875
Open this publication in new window or tab >>A linearized numerical model of wind-farm flows
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2017 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 20, no 5, p. 859-875Article in journal (Refereed) Published
Abstract [en]

A fast and reasonably accurate numerical three-dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary-layer approximation of the Navier-Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator-disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind-tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows.

Place, publisher, year, edition, pages
John Wiley & Sons, 2017
Keywords
blockage effect, numerical wake model, linearized RANs, internal boundary layer, dispersive stresses
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-208244 (URN)10.1002/we.2067 (DOI)000398845300007 ()2-s2.0-85000909957 (Scopus ID)
Funder
StandUp for Wind
Note

QC 20170627

Available from: 2017-06-27 Created: 2017-06-27 Last updated: 2017-06-27Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-8667-0520

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