| 1. |
- Alfredsson, Henrik, et al.
(författare)
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The Diagnostic Plot—A Tutorial with a Ten Year Perspective
- 2021
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Ingår i: Progress in Turbulence IX. - Cham : Springer Nature. ; , s. 125-135
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Konferensbidrag (refereegranskat)abstract
- The diagnostic plot was introduced in 2010 (Eur. J. Mech. B/Fluids 29: 403–406) but was used already in 2008 during a large measurement campaign as a litmus test to determine if tripped zero-pressure gradient turbulent boundary layers fulfilled basic criteria of being canonical. It used the rms-level of streamwise velocity (urms ) in the outer part of the boundary layer, a region where urms can give clear indications if insufficient or too tough tripping has been used. In standard plots one needs both the friction velocity and measurement of the full velocity and turbulence profiles. By instead plotting urms/ U∞ as a function of U/ U∞, it was found that this gives rise to a well-defined distribution that could be used as a canonical measure. It was later discovered that it is possible to extend the description to the near wall region. It has also been extended to boundary layers over rough surfaces and with pressure gradients, and some further uses. This paper aims to be both a review of the development of the method during the last 10+ years and a tutorial for those who want to employ it in their research and maybe also find new uses of the methodology.
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| 2. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new formulation for the streamwise turbulence intensity distribution
- 2011
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Ingår i: 13th European Turbulence Conference (ETC13). - : Institute of Physics Publishing (IOPP). ; , s. 022002-
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Konferensbidrag (refereegranskat)abstract
- Numerical and experimental data from zero pressure-gradient turbulent boundary layers over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & Orlu [Eur. J. Fluid Mech. B/Fluids, 4 2, 403 (2010)]. In the diagnostic plot the local turbulence intensity is shown as a function of the local mean velocity normalized with a reference velocity scale. In the outer region of the boundary layer a universal linear decay of the turbulence intensity is observed independent of Reynolds number. The deviation from this linear region appears in the buffer region and seems to be universal when normalized with the friction velocity. Therefore, a new empirical fit for the streamwise velocity turbulence intensity distribution is proposed and the results are compared with up to date reliable high-Reynolds number experiments and extrapolated towards Reynolds numbers relevant to atmospherical boundary layers.
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| 3. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new formulation for the streamwise turbulence intensity distribution in wall-bounded turbulent flows
- 2012
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 36, s. 167-175
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of the streamwise velocity turbulence intensity has recently been discussed in several papers both from the viewpoint of new experimental results as well as attempts to model its behavior. In the present paper numerical and experimental data from zero pressure-gradient turbulent boundary layers, channel and pipe flows over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & ÖrlÌ [P.H. Alfredsson, R. ÖrlÌ, The diagnostic plot-a litmus test for wall bounded turbulence data, Eur. J. Mech. B Fluids 29 (2010) 403-406]. In the diagnostic plot the local turbulence intensity is plotted as function of the local mean velocity normalized with a reference velocity scale. Alfredsson et al. [P.H. Alfredsson, A. Segalini, R. ÖrlÌ, A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the outer peak, Phys. Fluids 23 (2011) 041702] observed that in the outer region of the boundary layer a universal linear decay of the turbulence intensity independent of the Reynolds number exists. This approach has been generalized for channel and pipe flows as well, and it has been found that the deviation from the previously established linear region appears at a given wall distance in viscous units (around 120) for all three canonical flows. Based on these results, new empirical fits for the streamwise velocity turbulence intensity distribution of each canonical flow are proposed. Coupled with a mean streamwise velocity profile description the model provides a composite profile for the streamwise variance profile that agrees nicely with existing numerical and experimental data. Extrapolation of the proposed scaling to high Reynolds numbers predicts the emergence of a second peak of the streamwise variance profile that at even higher Reynolds numbers overtakes the inner one.
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| 4. |
- Alfredsson, P. Henrik, et al.
(författare)
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A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the "outer" peak
- 2011
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 23:4, s. 041702-
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Tidskriftsartikel (refereegranskat)abstract
- One recent focus of experimental studies of turbulence in high Reynolds number wall-bounded flows has been the scaling of the root mean square of the fluctuating streamwise velocity, but progress has largely been impaired by spatial resolution effects of hot-wire sensors. For the near-wall peak, recent results seem to have clarified the controversy; however, one of the remaining issues in this respect is the emergence of a second (so-called outer) peak at high Reynolds numbers. The present letter introduces a new scaling of the local turbulence intensity profile, based on the diagnostic plot by Alfredsson and Orlu [Eur. J. Mech. B/Fluids 42, 403 (2010)], which predicts the location and amplitude of the "outer" peak and suggests its presence as a question of sufficiently large scale separation.
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| 5. |
- Alfredsson, P. Henrik, et al.
(författare)
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Turbulent boundary layers over flat plates and rotating disks-The legacy of von Karman : A Stockholm perspective
- 2013
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 40, s. 17-29
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Tidskriftsartikel (refereegranskat)abstract
- Many of the findings and ideas of von Karman are still of interest to the fluid dynamics community. For instance, his result that the mean velocity distribution in turbulent flows has a logarithmic behavior with respect to the distance from the centreline is still a cornerstone for everybody working in wall-bounded turbulence and was first presented to an international audience in Stockholm at the Third International Congress for Applied Mechanics in 1930. In this paper we discuss this result and also how the so-called von Karman constant can be determined in a new simple way. We also discuss the possibility of a second (outer) maximum of the streamwise velocity fluctuations, a result that was implicit in some of the assumptions proposed by von Karman.
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| 6. |
- Alfredsson, P. Henrik, et al.
(författare)
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Wind farms in complex terrains : an introduction
- 2017
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Ingår i: Philosophical Transactions. Series A. - : ROYAL SOC. - 1364-503X .- 1471-2962. ; 375:2091
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Tidskriftsartikel (refereegranskat)abstract
- 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'.
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| 7. |
- Alveroglu, B., et al.
(författare)
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An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces
- 2017
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 61, s. 310-315
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Tidskriftsartikel (refereegranskat)abstract
- ⋯ 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.
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| 8. |
- Alveroglu, B., et al.
(författare)
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An energy analysis of convective instabilities of the Bödewadt and Ekman boundary layers over rough surfaces
- 2019
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Ingår i: 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.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 9. |
- Alveroglu, B., et al.
(författare)
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The effect of surface roughness on the convective instability of the BEK family of boundary-layer flows
- 2016
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 56, s. 178-187
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Tidskriftsartikel (refereegranskat)abstract
- A Chebyshev polynomial discretisation method is used to investigate the effect of both anisotropic (radially and azimuthally) and isotropic surface roughnesses on the convective instability of the BEK family of rotating boundary-layer flows. The mean-flow profiles for the velocity components are obtained by modelling surface roughness with a partial-slip approach. A linear stability analysis is then performed to investigate the effect of roughness on the convective instability characteristics of the inviscid Type I (cross-flow) instability and the viscous Type II instability. It is revealed that all roughness types lead to a stabilisation of the Type I mode in all flows within the BEK family, with the exception of azimuthally-anisotropic roughness (radial grooves) within the Bödewadt layer which causes a mildly destabilising effect. In the case of the Type II mode, the results reveal the destabilising effect of radially-anisotropic roughness (concentric grooves) on all the boundary layers, whereas both azimuthally-anisotropic and isotropic roughnesses have a stabilising effect on the mode for Ekman and von Kármán layers. Complementary results are also presented by considering the effects of roughness on the growth rates of each instability mode within the Ekman layer.
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| 10. |
- Arnqvist, Johan, et al.
(författare)
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Wind Statistics from a Forested Landscape
- 2015
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 156:1, s. 53-71
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Tidskriftsartikel (refereegranskat)abstract
- An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40 wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.
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| 11. |
- Bailey, S. C. C., et al.
(författare)
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Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 715, s. 642-670
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Tidskriftsartikel (refereegranskat)abstract
- This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from Reθ = 11 100 to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here.
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| 12. |
- Bergström, Hans, 1952-, et al.
(författare)
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Wind power in forests : wind and effects on loads
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Within the project V-312, Wind power in forests, researchers and a PhD student at Uppsala University, WeatherTech Scandinavia, the Royal Institute of Technology (KTH), DTU Wind Energy in Denmark and Teknikgruppen have been cooperating. Within the project atmospheric turbulence measurements with high vertical resolution have been done, also down between the trees, to make it possible to give better theoretical descriptions of the observed properties. Several mesoscale models have also been used to model the above forest winds. The atmospheric measurements have been complemented by wind tunnel measurements using a wind tunnel floor designed with small cylindrical wooden sticks that should simulate the effect of the trees generating a known momentum sink able to affect the flow. The combined new knowledge about the forest boundary layer wind and turbulence properties have been used as input to a dynamical wind turbine computer model, used to simulate the turbine load response to the turbulent wind field.
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| 13. |
- Bourgoin, M., et al.
(författare)
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Investigation of the small-scale statistics of turbulence in the Modane S1MA wind tunnel
- 2018
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Ingår i: CEAS Aeronautical Journal. - : Springer. - 1869-5582 .- 1869-5590. ; 9:2, s. 269-281
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Braunbehrens, Robert, et al.
(författare)
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A statistical model for wake meandering behind wind turbines
- 2019
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Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : ELSEVIER. - 0167-6105 .- 1872-8197. ; 193
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 15. |
- Cai, Tengfei, et al.
(författare)
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Local topography-induced pressure gradient effects on the wake and power output of a model wind turbine
- 2021
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Ingår i: Theoretical and Applied Mechanics Letters. - : Elsevier. - 2095-0349. ; 11:5
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Tidskriftsartikel (refereegranskat)abstract
- Wind-tunnel experiments were performed to study the effect of favorable and adverse constant pressure gradients (PG) from local changes in the topography right downwind of a model wind turbine. Particle image velocimetry was used to characterize the near and intermediate wake regions. We explored five scenarios, two favorable, two adverse PG, and a case with negligible PG. Results show that the PGs induce a wake deflection and modulate the wake. They imposed a relatively small impact on the turbulence kinetic energy and kinematic shear stress but a comparatively dominant effect on the bulk flow on the flow recovery. Based on this, a simple formulation is used to describe the impact of PG on the wake. We modeled the base flow through a linearized perturbation method; the wake is obtained by solving a simplified, integrated streamwise momentum equation. This approach reasonably estimated the flow profile and PG-induced power output variations. (C) 2021 The Authors. Published by Elsevier Ltd on behalf of The Chinese Society of Theoretical and Applied Mechanics.
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| 16. |
- Campagne, Antoine, et al.
(författare)
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First report of the MILESTONE experiment: strongly stratified turbulence and mixing efficiency in the Coriolis platform
- 2016
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Ingår i: <em>VIIIth International Symposium on Stratified Flows (ISSF)</em>, 2016.
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Konferensbidrag (refereegranskat)abstract
- Strongly stratified turbulence is a possible interpretation of oceanic and atmospheric mea-surements. However, this regime has never been produced in a laboratory experiment be-cause of the two conditions of very small horizontal Froude number Fh and large buoyancy Reynolds number R which require a verily large experimental facility. We present a new attempt to study strongly stratified turbulence experimentally in the Coriolis platform.The flow is forced by a slow periodic movement of an array of six vertical cylinders of 25 cm diameter with a mesh of 75 cm. Five cameras are used for 3D-2C scanned horizontalparticles image velocimetry (PIV) and stereo 2D vertical PIV. Five density-temperatureprobes are used to measure vertical and horizontal profiles and signals at fixed positions.The first preliminary results indicate that we manage to produce strongly stratified tur-bulence at very small Fh and large R in a laboratory experiment.
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| 17. |
- Castellani, Francesco, et al.
(författare)
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Aerodynamic Analysis of a Wind-Turbine Rotor Affected by Pitch Unbalance
- 2021
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Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 14:3
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Tidskriftsartikel (refereegranskat)abstract
- The aerodynamics of a rotor with pitch imbalance has been investigated experimentally and numerically in the present work. The comparison of mean velocity and turbulence intensity in the balanced and unbalanced cases indicated that a pitch imbalance modifies both the mean velocity and the turbulent activity; the latter is weakly increased by the imbalance. Spectral analysis indicated that the dynamics of the wake is also affected by the pitch imbalance since the tip vortices loose strength and disorganise more quickly than in the balanced case. The pitch imbalance has, however, a detrimental effect on the power coefficient and it affects the thrust coefficient as well. Only the blade affected by the imbalance shows significant modifications of the applied load, while the other blades operate with the same loading conditions.
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| 18. |
- Castellani, F., et al.
(författare)
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Wake losses from averaged and time-resolved power measurements at full scale wind turbines
- 2017
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 854:1
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Tidskriftsartikel (refereegranskat)abstract
- This work deals with the experimental analysis of wake losses fluctuations at full-scale wind turbines. The test case is a wind farm sited on a moderately complex terrain: 4 turbines are installed, having 2 MW of rated power each. The sources of information are the time-resolved data, as collected from the OPC server, and the 10-minutes averaged SCADA data. The objective is to compare the statistical distributions of wake losses for far and middle wakes, as can be observed through the "fast" lens of time-resolved data, for certain selected test-case time series, and through the "slow" lens of SCADA data, on a much longer time basis that allow to set the standards of the mean wake losses along the wind farm. Further, time-resolved data are used for an insight into the spectral properties of wake fluctuations, highlighting the role of the wind turbine as low-pass filter. Summarizing, the wind rose, the layout of the site and the structure of the data sets at disposal allow to study middle and far wake behavior, with a "slow" and "fast" perspective.
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| 19. |
- Castro, Ian P., et al.
(författare)
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Outer-layer turbulence intensities in smooth- and rough-wall boundary layers
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 727, s. 119-131
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Tidskriftsartikel (refereegranskat)abstract
- Clear differences in turbulence intensity profiles in smooth, transitional and fully rough zero-pressure-gradient boundary layers are demonstrated, using the diagnostic plot introduced by Alfredsson, Segalini & Orlu (Phys. Fluids, vol. 23, 2011, p. 041702) u'/U versus U/U-e, where u' and U are the local (root mean square) fluctuating and mean velocities and U-e is the free stream velocity. A wide range of published data are considered and all zero-pressure-gradient boundary layers yield outer flow u'/U values that are roughly linearly related to U/U-e, just as for smooth walls, but with a significantly higher slope which is completely independent of the roughness morphology. The difference in slope is due largely to the influence of the roughness parameter (Delta U+ in the usual notation) and all the data can be fitted empirically by using a modified form of the scaling, dependent only on Delta U/U-e. The turbulence intensity, at a location in the outer layer where U/U-e is fixed, rises monotonically with increasing Delta U/U-e which, however, remains of O(1) for all possible zero-pressure-gradient rough-wall boundary layers even at the highest Reynolds numbers. A measurement of intensity at a point in the outer region of the boundary layer can provide an indication of whether the surface is aerodynamically fully rough, without having to determine the surface stress or effective roughness height. Discussion of the implication for smooth/rough flow universality of differences in outer-layer mean velocity wake strength is included.
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| 20. |
- Chicchiero, Claudio, et al.
(författare)
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Triple-deck analysis of the steady flow over a rotating disk with surface roughness
- 2021
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Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- The effect of surface roughness on the steady laminar flow induced by a rotating disk submerged by fluid otherwise at rest is investigated here theoretically and numerically. A theory is proposed where a triple-deck analysis is applied leading to a fast evaluation of the steady-flow modification due to the rough surface. The theory assumes that the roughness is much smaller than the boundary-layer height and is characterized by a significantly longer length scale (slender roughness). Only the leading-order correction is developed here, corresponding to a velocity-field correction that is linear with the roughness height. The proposed theory neglects some curvature terms (here partially accounted by means of a stretching of the radial coordinate and of a scaling of the dependent variables). Numerical simulations performed with different roughness geometries (axisymmetric roughness, radial grooves, and localized bumps) have been used to validate the theory. Results indicate that the proposed theory leads to a good quantification of the flow modifications due to surface roughness at a very low computational cost. A demonstration of the capabilities of the theory is finally proposed where the statistical effects on the flow due to a random (but statistically known) roughness distributed on the surface of a rotating disk are characterized.
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| 21. |
- Chougule, A., et al.
(författare)
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Spectral tensor parameters for wind turbine load modeling from forested and agricultural landscapes
- 2015
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 18:3, s. 469-481
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Tidskriftsartikel (refereegranskat)abstract
- A velocity spectral tensor model was evaluated from the single-point measurements of wind speed. The model contains three parameters representing the dissipation rate of specific turbulent kinetic energy, a turbulence length scale and the turbulence anisotropy. Sonic anemometer measurements taken over a forested and an agricultural landscape were used to calculate the model parameters for neutral, slightly stable and slightly unstable atmospheric conditions for a selected wind speed interval. The dissipation rate above the forest was nine times that at the agricultural site. No significant differences were observed in the turbulence length scales between the forested and agricultural areas. Only a small difference was observed in the turbulence anisotropy at the two sites, except near the surface, where the forest turbulence was more isotropic. The turbulence anisotropy remained more or less constant with height at the forest site, whereas the turbulence became more isotropic with height for the agricultural site. Using the three parameters as inputs, we quantified the performance of the model in coherence predictions for vertical separations. The model coherence of all the three velocity components was overestimated for the analyzed stability classes at both sites. As expected from the model approximations, the model performed better at both sites for neutral stability than slightly stable and unstable conditions. The model prediction of coherence of the along-wind and vertical components was better than that of the cross-wind component. No significant difference was found between the performance of the model at the forested and the agricultural areas.
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| 22. |
- Ebenhoch, Raphael, et al.
(författare)
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A linearized numerical model of wind-farm flows
- 2017
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 20:5, s. 859-875
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 23. |
- Fiorini, T., et al.
(författare)
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Turbulent pipe flow near-wall statistics
- 2017
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Ingår i: Progress in Turbulence VII. - Cham : Springer Science+Business Media B.V.. - 9783319579337 ; , s. 89-94
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Konferensbidrag (refereegranskat)abstract
- Results from the first experimental campaign in the Long Pipe facility of the CICLoPE laboratory are reported. Single hot-wire profile measurements are presented, taken from the wall up to one third of the pipe radius, with the friction Reynolds number Reτ ranging from 6.5 × 103 up to 3.8 × 104 . Measurements of the pressure drop along the pipe are presented together with an estimation of its uncertainty. Mean and variance of the streamwise velocity fluctuations are examined and compared with the findings from other facilities. The amplitude of the inner-scaled near-wall peak of the variance, after being corrected for spatial resolution effects, shows an increasing trend with Reynolds number, in accordance with low Reynolds number experiments and simulations.
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| 24. |
- Forsting, Alexander R. Meyer, et al.
(författare)
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On the accuracy of predicting wind-farm blockage
- 2023
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Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 214, s. 114-129
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Tidskriftsartikel (refereegranskat)abstract
- To assess the uncertainty in blockage quantification, this study proposes a comparison of farm blockage predictions from wind-tunnel experiments, Reynolds Averaged Navier-Stokes based simulations using multiple numerical setups, and analytical models. The influence of the numerical setup is demonstrated to be small if a consistent definition of blockage (able to sort out systematic errors) is used. The effect of domain confinement and turbulence intensity is investigated assessing their range of variability. Different analytical models performed similarly in comparison to the numerical data, demonstrating the best accuracy for realistic spacing between the turbines and supporting their use as reliable engineering tools.
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| 25. |
- Garrett, S. J., et al.
(författare)
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On the stability of von Kármán rotating-disk boundary layers with radial anisotropic surface roughness
- 2016
-
Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 28:1
-
Tidskriftsartikel (refereegranskat)abstract
- We summarise results of a theoretical study investigating the distinct convective instability properties of steady boundary-layer flow over rough rotating disks. A generic roughness pattern of concentric circles with sinusoidal surface undulations in the radial direction is considered. The goal is to compare predictions obtained by means of two alternative, and fundamentally different, modelling approaches for surface roughness for the first time. The motivating rationale is to identify commonalities and isolate results that might potentially represent artefacts associated with the particular methodologies underlying one of the two modelling approaches. The most significant result of practical relevance obtained is that both approaches predict overall stabilising effects on type I instability mode of rotating disk flow. This mode leads to transition of the rotating-disk boundary layer and, more generally, the transition of boundary-layers with a cross-flow profile. Stabilisation of the type 1 mode means that it may be possible to exploit surface roughness for laminar-flow control in boundary layers with a cross-flow component. However, we also find differences between the two sets of model predictions, some subtle and some substantial. These will represent criteria for establishing which of the two alternative approaches is more suitable to correctly describe experimental data when these become available.
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| 26. |
- Helvig, Sanne de Jong, et al.
(författare)
-
A comparison of lab-scale free rotating wind turbines and actuator disks
- 2021
-
Ingår i: Journal of Wind Engineering and Industrial Aerodynamics. - : Elsevier BV. - 0167-6105 .- 1872-8197. ; 209
-
Tidskriftsartikel (refereegranskat)abstract
- Planar particle image velocimetry was conducted upstream and in the near-wake of a lab-scale free-rotating wind turbine model and compared to several actuator disks with the same dimensions. The Reynolds number of the incoming flow is order 10(4). Actuator disks with different designs and solidities were tested, and the process of actuator disk selection is explicitly shown. The drag, mean velocity and mean vorticity in the wake of the disks were compared to that of the rotating model. For the disk that was the best match, the Reynolds stresses and swirling strength are also presented. The instantaneous swirling strength illustrated that despite similar mean fields, the instantaneous phenomena were significantly different. Distinct tip vortices were present in the wake of the rotating model but were absent from the wake of the actuator disk. Proper orthogonal decomposition was used to further investigate the underlying phenomena in the two flows, again demonstrating the importance of tip vortices when studying the rotating model and the lack of such distinct vortices when using the actuator disk. Hence, despite well-matched mean characteristics, the instantaneous structures in the two flows remain distinct.
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| 27. |
- Hyvärinen, Ann, et al.
(författare)
-
A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
- 2018
-
Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 21:8, s. 605-617
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 28. |
- Hyvärinen, Ann, et al.
(författare)
-
Effects From Complex Terrain on Wind-Turbine Performance
- 2017
-
Ingår i: Journal of energy resources technology. - : ASME Press. - 0195-0738 .- 1528-8994. ; 139:5
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, experimental measurements are made to study wind turbines over complex terrains and in presence of the atmospheric boundary layer. Thrust and power coefficients for single and multiple turbines are measured when introducing sinusoidal hills and spires inducing an artificial atmospheric boundary layer. Additionally, wake interaction effects are studied, and inflow velocity profiles are characterized using hot-wire anemometry. The results indicate that the introduced hills have a positive impact on the wind-turbine performance and that wake-interaction effects are significantly reduced during turbulent inflow conditions.
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| 29. |
- Hyvärinen, Ann, et al.
(författare)
-
Qualitative analysis of wind-turbine wakes over hilly terrain
- 2017
-
Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 854:1
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, wind-turbine wakes are studied over flat and hilly terrains. Measurements made by using stereoscopic PIV are compared to data obtained from numerical simulations using RANS equations and an actuator-disc method. The numerical and experimental data show similar qualitative trends, indicating that the wind-turbine wake is perturbed by the presence of the hills. Additionally, a faster flow recovery at hub height is seen with the hilly terrain, indicating that the hills presence is beneficial for downstream turbines exposed to wake-interaction effects. The Jensen wake model is implemented over the hilly terrain and it is shown that this model cannot accurately capture the wake modulations induced by the hills. However, by superimposing a wind-turbine wake simulated over flat terrain on the hilly-terrain flow field, it is illustrated that the commonly-used wake-superposition technique can yield reasonable results if the used wake model has sufficient accuracy.
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| 30. |
- Hyvärinen, Ann
(författare)
-
Wind turbines over a hilly terrain: performance and wake evolution
- 2018
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this licentiate thesis is to investigate wind-turbines placed in a complex-terrain environment. This is done by studying the flow around small-scale wind-turbine models placed over a landscape model with hills, and by comparing the results with corresponding data obtained over a flat terrain model. The studied flow features include the wind-turbine wake development and the turbine performance under different conditions, the effects from wake interactions, the influence of the ambient turbulence levels and the influence from a complex topography. Wind-tunnel measurements have been performed using particle image velocimetry and hot-wire anemometry to measure the velocity field. Additionally, numerical simulations, based on RANS modelling and actuator-disc techniques, have been made to support the experimental data and to gain further knowledge about the investigated flow cases.The results reveal that the hills promote a downward wake deflection behind the turbines and enhance the wind-turbine wake diffusion. As a consequence of this, and with the flow acceleration introduced by the hills, an improved power performance is seen for turbines exposed to wake-interference effects. A correlation is observed between the turbulence levels present in the flow, and the magnitude to which the hill-induced flow gradients influence the wake: Stronger wake deflections due to the hills are seen when the wind-turbine wake is more diffused. This is for instance the case when the wake of two tandem turbines is studied, or when higher ambient turbulence levels are present in the wind tunnel.A good qualitative agreement is seen when comparing the experimental and numerical results. The simulation results further indicate that the hills give rise to modulations of the wind-turbine wake. It is shown that these modulations can be reasonably captured by means of wake-superposition techniques, given that a wake model with sufficient accuracy is chosen.
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| 31. |
- Ito, T., et al.
(författare)
-
On Similarity of Turbulence Statistics of a Turbulent Planar Jet Taking the Static Pressure into Account
- 2021
-
Ingår i: Progress in Turbulence IX. - Cham : Springer Science and Business Media Deutschland GmbH. ; , s. 43-49
-
Konferensbidrag (refereegranskat)abstract
- The pressure gradient in a jet is usually regarded as negligibly small when deriving the streamwise velocity profile from the momentum equations. In addition one assumes that the bulk streamwise momentum is conserved in the streamwise direction. On the other hand, it is known that the pressure distribution in the jet is well balanced with the square of the lateral velocity fluctuation, indicating that pressure is not negligible in the lateral momentum equation. The purpose of this study is to determine the importance of the pressure in the jet by evaluating balances in the streamwise and lateral momentum equations from experimental data measured by a static pressure tube and an X-probe. The turbulence fluctuations and the static pressure indicate similarities in their lateral distributions and are well balanced in the streamwise and lateral momentum equations. Although the contribution of the static pressure to the streamwise momentum is small, it is of the same order as that of the turbulent statistics in the lateral momentum equation.
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| 32. |
- Kato, Kentaro, et al.
(författare)
-
Instabilities and Transition on a Rotating Cone-Old Problems and New Challenges
- 2022
-
Ingår i: Laminar-Turbulent Transition. - Cham : Springer Nature. ; , s. 203-213
-
Konferensbidrag (refereegranskat)abstract
- An experimental investigation of instabilities and transition in the boundary layer on a rotating broad (120 degrees apex angle) cone through hot-wire measurements combined with local linear stability analysis (LLSA) has been undertaken. The rotating-cone flow is susceptible to both cross-flow and centrifugal instabilities. For broad cones, the cross-flow instability dominates over the centrifugal instability, and vice versa for slender cones. Although stationary vortical disturbances from the cross-flow instability are dominant on the broad cone (in this case 24-26 vortices develop), we have identified an initially slowly growing nonstationary mode with a much smaller wavenumber, which close to transition increases its growth rate dramatically. We report on a detailed process to identify the wavenumber of the measured nonstationary disturbance, as well as quantitative comparisons between experimental results and LLSA.
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| 33. |
- Kato, Kentaro, Dr, 1987-, et al.
(författare)
-
Instability and transition in the boundary layer driven by a rotating slender cone
- 2021
-
Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 915
-
Tidskriftsartikel (refereegranskat)abstract
- Instability and transition in the boundary layer on a slender cone (600 apex angle) rotating in still fluid are investigated using hot-wire anemometry as well as through linear stability analysis. In contrast to broad cones (including the disk), where a cross-flow instability dominates the transition and different studies report similar transition Reynolds numbers, the reported transition Reynolds numbers on slender cones are scattered. The present experiments provide quantitative experimental datasets and the stability and transition are evaluated based on both the Reynolds number and a Girder number. The results consistently show that the instability development depends on the Gortler number rather than the Reynolds number and that transition starts at a well-defined Gortler number, whereas the transition Reynolds number depends on the rotational rate. The measured disturbance that first grows in the laminar region has a frequency approximately the same as or twice the rotational rate of the cone, which according to the stability analysis corresponds to the critical frequency of a slightly inclined vortex structure with respect to the cone axis or an axisymmetric vortex structure. These structures are similar to those observed in the flow visualisations of Kobayashi & Izumi (J. Fluid Mech., vol. 127, 1983, pp. 353-364) and considered as being due to a centrifugal instability.
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| 34. |
- Laurantzon, Fredrik, 1979-, et al.
(författare)
-
Experimental analysis of turbocharger interaction with a pulsatile flow through time-resolved flow measurements upstream and downstream of the turbine
- 2012
-
Ingår i: Institution of Mechanical Engineers - 10th International Conference on Turbochargers and Turbocharging. - : Elsevier. - 9780857092090 ; , s. 405-415
-
Konferensbidrag (refereegranskat)abstract
- The inflow to and outflow from turbochargers are highly complex and, in particular, pulsating. Nevertheless, most studies of turbocharger performance are conducted under steady conditions. Hence, there is a great interest in determining and understanding turbocharger performance maps under pulsatile conditions. The highly complex flow field constitutes a challenge for time-resolved flow measurements by means of conventional measurement techniques. In a recent paper by Laurantzon et al [Meas. Sci. Technol. 20 123001 (2010)], time-resolved bulk flow measurements under pulsatile conditions have been obtained via wavelet analysis of the signal from a vortex flow meter. Here, this method has been used in order to obtain time-resolved performance maps based on the mass flow both upstream and down-stream of the turbine. The results show that the turbine has a large damping effect on the mass flow pulsations, but that the pulse shape is to a high degree preserved while passing through the turbine, and that the time-dependent filling and emptying of the turbine case make the quasi-steady assumption invalid, if the whole turbine stage is considered.
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| 35. |
- Laurantzon, Fredrik, 1979-, et al.
(författare)
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Time-resolved measurements with a vortex flowmeter in a pulsating turbulent flow using wavelet analysis
- 2010
-
Ingår i: Measurement science and technology. - : IOP Publishing. - 0957-0233 .- 1361-6501. ; 21:12, s. 123001-
-
Tidskriftsartikel (refereegranskat)abstract
- Vortex flowmeters are commonly employed in technical applications and are obtainable in a variety of commercially available types. However their robustness and accuracy can easily be impaired by environmental conditions, such as inflow disturbances and/or pulsating conditions. Various post-processing techniques of the vortex signal have been used, but all of these methods are so far targeted on obtaining an improved estimate of the time-averaged bulk velocity. Here, on the other hand, we propose, based on wavelet analysis, a straightforward way to utilize the signal from a vortex shedder to extract the time-resolved and thereby the phase-averaged velocity under pulsatile flow conditions. The method was verified with hot-wire and laser Doppler velocimetry measurements.
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| 36. |
- Matsubara, Masaharu, et al.
(författare)
-
Linear modes in a planar turbulent jet
- 2020
-
Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 888
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 37. |
- Mazzeo, Francesco, et al.
(författare)
-
An Experimental Study on a Wind Turbine Rotor Affected by Pitch Imbalance
- 2022
-
Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 15:22
-
Tidskriftsartikel (refereegranskat)abstract
- An experimental and numerical investigation about the pitch imbalance effect on a wind turbine model is performed. The characterization of the power losses and loads generated on a small-scale model and the validation of an analytical framework for the performance of unbalanced rotors are proposed. Starting from the optimal collective pitch assessment (performed to identify the condition with the maximum power coefficient), the pitch of just one blade was systematically changed: it is seen that the presence of a pitch misalignment is associated with a degradation of the turbine performance, visible both from experiments and from Blade Element Momentum (BEM) calculations (modified to account for the load asymmetry). Up to 30% power losses and a 15% thrust increase are achievable when an imbalanced rotor operates at tip speed ratios around five, clearly highlighting the importance of avoiding this phenomenon when dealing with industrial applications. The numerical model predicts this result within 5% accuracy. Additional numerical simulations showed that, away from the optimal collective pitch, the blade imbalance can provide a power increase or a power decrease with respect to the balanced case, suggesting how an operator can maximise the production of an unbalanced rotor. An analysis of the axial and lateral forces showed a sensitivity of the loads' standard deviation when imbalance is present. An increase of the lateral loads was observed in all unbalanced cases.
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| 38. |
- Medici, D., et al.
(författare)
-
On the determination of stability conditions over forested areas from velocity measurements
- 2014
-
Ingår i: European Wind Energy Association Conference and Exhibition 2014. - : European Wind Energy Association.
-
Konferensbidrag (refereegranskat)abstract
- Two proxies able to determine the sign of the atmospheric stability in the absence of temperature measurements were investigated using data from four forested sites in Sweden. The results indicate that the simple proxy based on the time of the day when the measurement was taken was sufficient to identify approximately 75% of the stable occurrences. However, also during daytime, stable stratification was not infrequent. A criterion based on a combination of the local turbulence intensity and shear exponent was therefore also investigated, and the combination of the two criteria was able to estimate approximately 90% of the overall stable data. When selecting data with mean wind speeds over 5 m/s at 80 m height, the presented data sets include mostly data where temperature effects are small (near-neutral), followed by stable data. Only few occurrences of unstable conditions were anyway observed. The investigated data sets show large variation of shear and turbulence intensity with increasing stability, indicating the need for proxies that also can be used to characterize the degree of atmospheric stability.
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| 39. |
- Micheletto, Derek, et al.
(författare)
-
Experimental analysis of the wake behind a small wind-turbine model in yaw
- 2023
-
Ingår i: Proceedings 8th Wake Conference 2023. - : IOP Publishing.
-
Konferensbidrag (refereegranskat)abstract
- In this work we study the wake of a yawed wind-turbine model immersed in an atmospheric boundary layer (ABL). The ABL is replicated in the wind tunnel by means of a barrier-spires and distributed roughness configuration and is representative of a rural terrain. We quantify the properties of the wake in the horizontal plane at hub height and compare the predictions of available wake models to our data for different yaw angles. It is found that the model based on lifting-line theory performs best in predicting the velocity deficit without the need of tuning the parameters to the current setup. However, the wake deflection is slightly underestimated, most notably at the transition between near and far wake. Furthermore, a comparison with the turbine in a uniform incoming flow highlights the enhanced downward deflection of the wake which results from its interaction with the ABL.
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| 40. |
- Micheletto, Derek, et al.
(författare)
-
Experimental Study of the Transient Behavior of a Wind Turbine Wake Following Yaw Actuation
- 2023
-
Ingår i: Energies. - : MDPI AG. - 1996-1073. ; 16:13
-
Tidskriftsartikel (refereegranskat)abstract
- Wind tunnel experiments were performed to investigate the response of a wind turbine model immersed in a replicated atmospheric boundary layer to dynamic changes in the yaw angle. Both the flow field in the wake and the operating properties of the turbine, namely its thrust force, torque, and angular velocity, were monitored during repeated yaw maneuvers for a variety of yaw angles. It was observed that the characteristic time scale of the transient experienced by the turbine scalar quantities was one order of magnitude larger than that of the yaw actuation and depended primarily on the inertia of the rotor and the generator. Furthermore, a Morlet wavelet analysis of the thrust signal showed a strong peak at the rotation frequency of the turbine, with the transient emergence of high activity at a lower frequency during the yaw maneuver. The insights provided by the proper orthogonal decomposition analysis performed on the wake velocity data enabled the development of a simple reduced-order model for the transient in the flow field based on the stationary states before and after the yaw maneuver. This model was then further improved to require only the final state, extending its applicability to any arbitrary wind farm as a dynamical surrogate of the farm behavior.
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| 41. |
- Mohr, Matthias, et al.
(författare)
-
Wind power in forests II : Forest wind
- 2018
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Sweden has good conditions for wind power. However, most of Sweden (ca. 70%) is covered by forest. Forests decrease wind speeds and create turbulence, something which is not favourable for wind power. Several Swedish wind maps have shown that forests in Nordic countries can be well suited for wind power (e.g. Bergström and Söderberg 2011, Byrkjedal and Åkervik 2009).At the same time, there is uncertainty over wind conditions over forests at very high altitudes (ca. 150 m above ground). How good do wind resource assessment models agree with measurements? How much energy is a wind turbine in forest going to produce and which loads will a wind turbine in forest experience?This project has investigated all these issues. Work was concentrated in the following work packages:Wind resource at very high heightsTurbulence- and wind measurements at very high heights above forestAnalysis of turbulence data from forestsModel simulations with wind flow modelsModel simulations with very-high-resolution weather forecast modelsModel simulations with Large Eddy Simulation (LES) modelsImproved specification of so-called “synthetic turbulence” over forestAnalysis of airborne laser altimeter measurements over forestForest’s effects on wind turbine energy productionLoad simulations for wind turbines over forestWP1 studies how wind speed and direction varies with height over forest (up to ca 150 m above ground and higher up). Several profile relations are studied here. Frequency distributions of wind shear and veer are presented. WP2 describes turbulence and wind measurements that have been carried out within the project at Hornamossen. Moreover, the measurement campaign that was carried out in a line over the Hornamossen-hill within the New European Wind Atlas project is described. WP3 analyses turbulence data from Hornamossen together with turbulence data from Ryningsnäs. Of special interest is how turbulence intensity decreases with height as well as if the IEC-standard class A, B or C for wind turbines is complied with at different heights. WP4 describes the newly developed linearised wind flow model ORFEUS with a dedicated forest module. WP5 describes model simulations with WRF and the MIUU model, their sensitivity for surface roughness and turbulence parameterisations. Mean wind profiles from the models are compared to Hornamossen. WP6 describes LES simulations with Chalmers LES model and WRF-LES. LES-resultats depend to a large degree on how the turbulent vortices are initialised at the inflow boundaries of the LES model. Several different methods for that are described. WP7 describes a new turbulence model (the Segalini & Arnqvist model) that includes atmospheric stability. This is a further development of the IEC turbulence model (=Mann model) for neutral stability. Coherence of turbulent winds as well as phase profiles are other improvements of the IEC model. WP8 describes a new method to compute leaf/needle/plant area density from laser scans of the Swedish forest and how one estimates surface roughness and zero plane displacement from that. The new method is compared with two other methods. Results are also compared with official forest data (“skoglig grunddata”). The effect on the wind profile is also shown. WP9 describes the new methods for estimating AEP from the Power Curve Working Group and the IEC standard for Power Performance Testing. Effects on estimated AEP are shown. A new simple model for calculating turbulence effects on energy production is developed and compared with data from a wind farm. Within WP10 a new generic open-source wind turbine is developed and used for load simulations with aero-elastic simulations. Results show that the new coherence model for turbulence gives much smaller loads than the turbulence model of the IEC standard.For more information on the different parts of the project the reader is referred to the report’s introduction, the ”Summary and Conclusions” of each chapter as well as the overall summary (”Executive Summary”) at the end of the report.
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| 42. |
- Nakamura, T., et al.
(författare)
-
Effects of a forest clearing : An experimental and numerical assessment
- 2013
-
Ingår i: International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2013. - : TSFP-8. - 9780000000002
-
Konferensbidrag (refereegranskat)abstract
- The flow above a forest model with and without the presence of a clearing is investigated experimentally in a wind tunnel facility by means of Particle Image Velocimetry. The results have been analyzed in terms of basic velocity statistics and vertical integral length scale and a comparison of the two geometries is presented. In order to improve the current flow understanding and to complement the experimental results, Large Eddy Simulations of the same flows under similar operating conditions are planned. The experimental results indicate that the clearing is associated to a velocity defect in the mean velocity profile associated with a significant vertical momentum flux. The turbulence is redistributed amongst the various velocity components so that the streamwise velocity variance is reduced while the vertical velocity variance is enhanced. From the clearing windward edge the turbulence mixing contributes to the decrease of the vertical correlation length that increases again after the clearing trailing edge.
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| 43. |
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| 44. |
- Odemark, Ylva, et al.
(författare)
-
The effects of a model forest canopy on the outputs of a wind turbine model
- 2014
-
Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 555:1
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of a rough surface boundary layer on the outputs of a wind turbine model were investigated experimentally in a wind tunnel. The very rough surface consisted of cylindrical pins, in order to model a forest canopy. The hub height of the turbine model was varied in order to see the effect of the presence of the model forest in the power and thrust coefficients. A small effect of the hub height was observed in the averaged power coefficient, where the turbine produced less for the lowest hub height. The difference was however reduced when scaling the power output with the available power in the wind instead of using the velocity at hub height. Consistent trends were present in the standard deviation of the thrust coefficient and the rotational speed, which both increased by decreasing the hub height. This underlines the fact that not only the rotor but also the tower and the bearings of a wind turbine must withstand to increased loads when operating close to a canopy.
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| 45. |
- Rajput, Moeen S., et al.
(författare)
-
Design and evaluation of a novel instrumented drop-weight rig for controlled impact testing of polymer composites
- 2018
-
Ingår i: Polymer testing. - : Elsevier. - 0142-9418 .- 1873-2348. ; 68, s. 446-455
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 46. |
- Riboldi, Jacopo, et al.
(författare)
-
Multiple Large-Scale Dynamical Pathways for Pan-Atlantic Compound Cold and Windy Extremes
- 2023
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 50:10
-
Tidskriftsartikel (refereegranskat)abstract
- Winter cold spells over North America have been correlated with European wind extremes, but the physical mechanisms behind such pan-Atlantic compound extremes have not been clarified yet. In this study, we propose that pan-Atlantic cold and windy extremes occur following two possible dynamical pathways. The first one involves the propagation of a Rossby wave train from the Pacific Ocean, associated with windstorms over north-western Europe in the 5-10 days after the cold spell peak. The second is associated with a high-latitude anticyclone over the North Atlantic and an equatorward-shifted jet, leading to windstorms over south-western Europe already in the days preceding the cold spell peak. European windstorms are thus systematically tied to North American cold spells according to the different flow configuration. The analysis underscores that apparently similar surface extremes might be driven by different processes, and conflating them together could lead to misleading conclusions.
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| 47. |
- Riboldi, Jacopo, et al.
(författare)
-
Multiple Large‐Scale Dynamical Pathways for Pan–Atlantic Compound Cold and Windy Extremes
- 2023
-
Ingår i: Geophysical Research Letters. - : John Wiley & Sons. - 0094-8276 .- 1944-8007. ; 50:10
-
Tidskriftsartikel (refereegranskat)abstract
- Winter cold spells over North America have been correlated with European wind extremes, but the physical mechanisms behind such “pan-Atlantic” compound extremes have not been clarified yet. In this study, we propose that pan–Atlantic cold and windy extremes occur following two possible dynamical pathways. The first one involves the propagation of a Rossby wave train from the Pacific Ocean, associated with windstorms over north-western Europe in the 5–10 days after the cold spell peak. The second is associated with a high-latitude anticyclone over the North Atlantic and an equatorward-shifted jet, leading to windstorms over south-western Europe already in the days preceding the cold spell peak. European windstorms are thus systematically tied to North American cold spells according to the different flow configuration. The analysis underscores that apparently similar surface extremes might be driven by different processes, and conflating them together could lead to misleading conclusions.
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| 48. |
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| 49. |
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| 50. |
- Sarmast, Sasan, et al.
(författare)
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Comparison of the near-wake between actuator-line simulations and a simplified vortex model of a horizontal-axis wind turbine
- 2016
-
Ingår i: Wind Energy. - : Wiley-Blackwell. - 1095-4244 .- 1099-1824. ; 19:3, s. 471-481
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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