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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
Keyword
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
Segalini, A. (2017). Linearized simulation of flow over wind farms and complex terrains. Philosophical Transactions. Series A: Mathematical, physical, and engineering science, 375(2091), Article ID 20160099.
Open this publication in new window or tab >>Linearized simulation of flow over wind farms and complex terrains
2017 (English)In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, no 2091, article id 20160099Article in journal (Refereed) Published
Abstract [en]

The flow over complex terrains and wind farms is estimated here by numerically solving the linearized Navier-Stokes equations. The equations are linearized around the unperturbed incoming wind profile, here assumed logarithmic. The Boussinesq approximation is used to model the Reynolds stress with a prescribed turbulent eddy viscosity profile. Without requiring the boundary-layer approximation, two new linear equations are obtained for the vertical velocity and the wall-normal vorticity, with a reduction in the computational cost by a factor of 8 when compared with a primitive-variables formulation. The presence of terrain elevation is introduced as a vertical coordinate shift, while forestry or wind turbines are included as body forces, without any assumption about the wake structure for the turbines. The model is first validated against some available experiments and simulations, and then a simulation of a wind farm over a Gaussian hill is performed. The speed-up effect of the hill is clearly beneficial in terms of the available momentum upstream of the crest, while downstream of it the opposite can be said as the turbines face a decreased wind speed. Also, the presence of the hill introduces an additional spanwise velocity component that may also affect the turbines' operations. The linear superposition of the flow over the hill and the flow over the farm alone provided a first estimation of the wind speed along the farm, with discrepancies of the same order of magnitude for the spanwise velocity. Finally, the possibility of using a parabolic set of equations to obtain the turbulent kinetic energy after the linearized model is investigated with promising results. This article is part of the themed issue 'Wind energy in complex terrains'.

Place, publisher, year, edition, pages
ROYAL SOC, 2017
Keyword
wind farms, linearized methods, complex terrains
National Category
Energy Systems
Identifiers
urn:nbn:se:kth:diva-206250 (URN)10.1098/rsta.2016.0099 (DOI)000397879700004 ()2-s2.0-85015884941 (Scopus ID)
Note

QC 20170512

Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-06-30Bibliographically approved
Segalini, A. & Garrett, S. J. (2017). On the non-parallel instability of the rotating-sphere boundary layer. Journal of Fluid Mechanics, 818, 288-318.
Open this publication in new window or tab >>On the non-parallel instability of the rotating-sphere boundary layer
2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 818, p. 288-318Article in journal (Refereed) Published
Abstract [en]

We present a new solution for the steady boundary-layer flow over the rotating sphere that also accounts for the eruption of the boundary layer at the equator and other higher-order viscous effects. Non-parallel corrections to the local type I and type II convective instability modes of this flow are also computed as a function of spin rate. Our instability results are associated with the previously observed spiral vortices and remarkable agreement between our predictions of the number of vortices and experimental observations is found. Vortices travelling at 70 %-80 % of the local surface speed are found to be the most amplified for sufficient spin rates, also consistent with prior experimental observations.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2017
Keyword
absolute/convective instability, boundary layer stability
National Category
Fusion, Plasma and Space Physics Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-206240 (URN)10.1017/jfm.2017.131 (DOI)000398508300017 ()2-s2.0-85016389202 (Scopus ID)
Note

QC 20170517

Available from: 2017-05-17 Created: 2017-05-17 Last updated: 2017-05-19Bibliographically approved
Alfredsson, B. & Segalini, A. (2017). Wind farms in complex terrains: an introduction. Philosophical Transactions. Series A: Mathematical, physical, and engineering science, 375(2091).
Open this publication in new window or tab >>Wind farms in complex terrains: an introduction
2017 (English)In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, no 2091Article in journal (Refereed) Published
Abstract [en]

Wind energy is one of the fastest growing sources of sustainable energy production. As more wind turbines are coming into operation, the best locations are already becoming occupied by turbines, and wind-farm developers have to look for new and still available areas-locations that may not be ideal such as complex terrain landscapes. In these locations, turbulence and wind shear are higher, and in general wind conditions are harder to predict. Also, the modelling of the wakes behind the turbines is more complicated, which makes energy-yield estimates more uncertain than under ideal conditions. This theme issue includes 10 research papers devoted to various fluid-mechanics aspects of using wind energy in complex terrains and illustrates recent progress and future developments in this important field. This article is part of the themed issue 'Wind energy in complex terrains'.

Place, publisher, year, edition, pages
ROYAL SOC, 2017
Keyword
atmospheric boundary layer, wind-turbine aerodynamics, wind-farm modelling
National Category
Energy Systems
Identifiers
urn:nbn:se:kth:diva-206249 (URN)10.1098/rsta.2016.0096 (DOI)000397879700001 ()2-s2.0-85015929530 (Scopus ID)
Note

QC 20170512

Available from: 2017-05-12 Created: 2017-05-12 Last updated: 2017-06-30Bibliographically approved
Segalini, A., Nakamura, T. & Fukagata, K. (2016). A Linearized k-ϵ Model of Forest Canopies and Clearings. Boundary-layer Meteorology, 161(3), 439-460.
Open this publication in new window or tab >>A Linearized k-ϵ Model of Forest Canopies and Clearings
2016 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 161, no 3, p. 439-460Article in journal (Refereed) Published
Abstract [en]

A linearized analysis of the Reynolds-averaged Navier–Stokes (RANS) equations is proposed where the (Formula presented.) turbulence model is used. The flow near the forest is obtained as the superposition of the undisturbed incoming boundary layer plus a velocity perturbation due to the forest presence, similar to the approach proposed by Belcher et al. (J Fluid Mech 488:369–398, 2003). The linearized model has been compared against several non-linear RANS simulations with many leaf-area index values and large-eddy simulations using two different values of leaf-area index. All the simulations have been performed for a homogeneous forest and for four different clearing configurations. Despite the model approximations, the mean velocity and the Reynolds stress (Formula presented.) have been reasonably reproduced by the first-order model, providing insight about how the clearing perturbs the boundary layer over forested areas. However, significant departures from the linear predictions are observed in the turbulent kinetic energy and velocity variances. A second-order correction, which partly accounts for some non-linearities, is therefore proposed to improve the estimate of the turbulent kinetic energy and velocity variances. The results suggest that only a region close to the canopy top is significantly affected by the forest drag and dominated by the non-linearities, while above three canopy heights from the ground only small effects are visible and both the linearized model and the simulations have the same trends there.

Place, publisher, year, edition, pages
Springer Netherlands, 2016
Keyword
Canopy flows, Forest clearings, Linearized Reynolds-averaged Navier–Stokes equations, Atmospheric thermodynamics, Boundary layers, Computational fluid dynamics, Drag, Forestry, Kinetic energy, Kinetics, Large eddy simulation, Linearization, Reynolds equation, Reynolds number, Turbulence models, Turbulent flow, Velocity, Linearized analysis, Model approximations, Second-order correction, Stokes equations, Turbulent kinetic energy, Velocity perturbation, Navier Stokes equations
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-197136 (URN)10.1007/s10546-016-0190-5 (DOI)000388586100003 ()2-s2.0-84978657959 (Scopus ID)
Funder
Swedish Energy AgencyStandUp for Wind
Note

QC 20161213

Available from: 2016-12-13 Created: 2016-11-30 Last updated: 2017-11-29Bibliographically approved
Alveroglu, B., Segalini, A. & Garrett, S. J. (2016). An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces. European journal of mechanics. B, Fluids.
Open this publication in new window or tab >>An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces
2016 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390Article in journal (Refereed) Published
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 discretization 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 destabilizing. In the case of the viscous Type II instability mode, the results predict a destabilization effect of radially-anisotropic roughness (concentric grooves) on both flows, whereas both azimuthally-anisotropic roughness and isotropic roughness have a stabilization effect. The results presented here confirm the results of our prior linear stability analyses. © 2016 Elsevier Masson SAS.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
BEK family, Energy, Roughness, Anisotropy, Boundary layer flow, Discrete event simulation, Stabilization, Surface roughness, Convective instabilities, Destabilization effects, Discretization method, Ekman boundary layers, Energy balance equations, Stabilization effects, Boundary layers
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-194587 (URN)10.1016/j.euromechflu.2016.09.006 (DOI)2-s2.0-84991273900 (Scopus ID)
Note

Correspondence Address: Alveroglu, B.email: ba123@le.ac.uk. QC 20161102

Available from: 2016-11-02 Created: 2016-10-31 Last updated: 2017-11-29Bibliographically approved
Sarmast, S., Segalini, A., Mikkelsen, R. F. & Ivanell, S. (2016). Comparison of the near-wake between actuator-line simulations and a simplified vortex model of a horizontal-axis wind turbine. Wind Energy, 19(3), 471-481.
Open this publication in new window or tab >>Comparison of the near-wake between actuator-line simulations and a simplified vortex model of a horizontal-axis wind turbine
2016 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 19, no 3, p. 471-481Article in journal (Refereed) Published
Abstract [en]

The flow around an isolated horizontal-axis wind turbine is estimated by means of a new vortex code based on the Biot-Savart law with constant circulation along the blades. The results have been compared with numerical simulations where the wind turbine blades are replaced with actuator lines. Two different wind turbines have been simulated: one with constant circulation along the blades, to replicate the vortex method approximations, and the other with a realistic circulation distribution, to compare the outcomes of the vortex model with real operative wind-turbine conditions (Tjaereborg wind turbine). The vortex model matched the numerical simulation of the turbine with constant blade circulation in terms of the near-wake structure and local forces along the blade. The results from the Tjaereborg turbine case showed some discrepancies between the two approaches, but overall, the agreement is qualitatively good, validating the analytical method for more general conditions. The present results show that a simple vortex code is able to provide an estimation of the flow around the wind turbine similar to the actuator-line approach but with a negligible computational effort.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2016
Keyword
wind turbine wake, actuator-line model, vortex model, CFD, large eddy simulation, near-wake
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-182765 (URN)10.1002/we.1845 (DOI)000368991200006 ()2-s2.0-84955752555 (Scopus ID)
Funder
StandUp for WindSwedish e‐Science Research Center
Note

QC 20160223

Available from: 2016-02-23 Created: 2016-02-23 Last updated: 2017-11-30Bibliographically approved
Segalini, A. (2016). Experimental and analytical investigation of forest clearings. In: Springer Proceedings in Physics: . Paper presented at 5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015 (pp. 339-346). Springer, 185.
Open this publication in new window or tab >>Experimental and analytical investigation of forest clearings
2016 (English)In: Springer Proceedings in Physics, Springer, 2016, Vol. 185, p. 339-346Conference paper, Published paper (Refereed)
Abstract [en]

A numerical model of forest clearings is proposed based on the linearised Euler equations. The forest is replaced by distributed body forces that decrease the flow momentum entrained inside the canopy. The model is compared with available wind-tunnel measurements of clearings areas. It is observed that the model deviates from themeasurements over the forest surface, but the performance improve over the clearing surface, probably associated to an incorrect evaluation of the forest intensity. The proposed model suggests that the clearing is associated to an internal boundary layer with intensity function of the clearing length.

Place, publisher, year, edition, pages
Springer, 2016
Series
Springer Proceedings in Physics, ISSN 0930-8989 ; 185
Keyword
Boundary layers, Fighter aircraft, Wakes, Wind tunnels, Analytical investigations, Body forces, Flow momentums, Intensity functions, Internal Boundary layer, Wind tunnel measurements, Forestry
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-194583 (URN)10.1007/978-3-319-30602-5_43 (DOI)000387431400043 ()2-s2.0-84979030800 (Scopus ID)9783319306001 (ISBN)
Conference
5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015
Funder
StandUp for Wind
Note

QC 20161102

Available from: 2016-11-02 Created: 2016-10-31 Last updated: 2017-01-18Bibliographically approved
Örlü, R., Segalini, A., Klewicki, J. & Alfredsson, P. H. (2016). Generalization of the diagnostic plot to higher-order moments in turbulent boundary layers. In: Springer Proceedings in Physics: . Paper presented at 5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015 (pp. 333-338). Springer.
Open this publication in new window or tab >>Generalization of the diagnostic plot to higher-order moments in turbulent boundary layers
2016 (English)In: Springer Proceedings in Physics, Springer, 2016, p. 333-338Conference paper, Published paper (Refereed)
Abstract [en]

The present work extends the diagnostic plot concept for the streamwise turbulence intensity in wall-bounded turbulent flows [Alfredsson and Örlü, Eur. J. Mech. B/Fluids 42, 403 (2010)], and generalizes it for higher-order (even and odd) moments, thereby providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layer data up to a friction Reynolds number of 20000 are employed and demonstrate the feasibility of the diagnostic plot to scale data throughout the logarithmic and wake regions. © Springer International Publishing Switzerland 2016.

Place, publisher, year, edition, pages
Springer, 2016
Keyword
Boundary layer flow, Boundary layers, Probability density function, Probability distributions, Reynolds number, Turbulence, Turbulent flow, Wakes, General description, Higher order moments, Higher-order, Probability density distribution, Stream-wise velocities, Turbulence intensity, Turbulent boundary layers, Wall-bounded turbulent flows, Atmospheric thermodynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-194624 (URN)10.1007/978-3-319-30602-5_42 (DOI)000387431400042 ()2-s2.0-84979052108 (Scopus ID)9783319306001 (ISBN)
Conference
5th International Conference on Jets, Wakes and Separated Flows, ICJWSF2015, 15 June 2015 through 18 June 2015
Note

Correspondence Address: Örlü, R.; Linné FLOW Centre, KTH MechanicsSweden; email: ramis@mech.kth.se. QC 20161101

Available from: 2016-11-01 Created: 2016-10-31 Last updated: 2016-12-22Bibliographically approved
Örlü, R., Segalini, A., Klewicki, J. & Alfredsson, P. H. (2016). High-order generalisation of the diagnostic scaling for turbulent boundary layers. Journal of turbulence, 17(7), 664-677.
Open this publication in new window or tab >>High-order generalisation of the diagnostic scaling for turbulent boundary layers
2016 (English)In: Journal of turbulence, ISSN 1468-5248, E-ISSN 1468-5248, Vol. 17, no 7, p. 664-677Article in journal (Refereed) Published
Abstract [en]

The diagnostic scaling concept, introduced for the streamwise turbulence intensity in wall-bounded turbulent flows (Alfredsson, Segalini and Örlü, Phys. Fluids 2011;23:041702), is here extended and generalised not only for the higher even-order central statistical moments, but also for the odd moments and thereby the probability density distribution of the streamwise velocity fluctuations. Turbulent boundary layer data up to a friction Reynolds number of 60,000 are employed and demonstrate the feasibility of the diagnostic scaling for the data throughout the logarithmic and wake regions. A comparison with the generalised logarithmic law for even-order moments by Meneveau and Marusic (J. Fluid Mech. 2013;719:R1) based on the attached-eddy hypothesis, is reported. The diagnostic plot provides an apparent Reynolds-number-independent scaling of the data, and is exploited to reveal the functional dependencies of the constants needed in the attached-eddy-based model. In particular, the invariance of the lowest order diagnostic scaling poses an intriguing incompatibility with the asymptotic constancy of the Townsend–Perry constant.

Place, publisher, year, edition, pages
Taylor & Francis, 2016
Keyword
turbulent boundary layers, Turbulent flows
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:kth:diva-187183 (URN)10.1080/14685248.2016.1169282 (DOI)000380171000003 ()2-s2.0-84964329594 (Scopus ID)
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Note

QC 20160830

Available from: 2016-05-20 Created: 2016-05-18 Last updated: 2017-11-30Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-8667-0520

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