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Publications (10 of 13) Show all publications
Li, X., Schonecker, S., Li, W., Varga, L. K., Irving, D. L. & Vitos, L. (2018). Tensile and shear loading of four fcc high-entropy alloys: A first-principles study. Physical Review B, 97(9), Article ID 094102.
Open this publication in new window or tab >>Tensile and shear loading of four fcc high-entropy alloys: A first-principles study
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094102Article in journal (Refereed) Published
Abstract [en]

Ab initio density-functional calculations are used to investigate the response of four face-centered-cubic (fcc) high-entropy alloys (HEAs) to tensile and shear loading. The ideal tensile and shear strengths (ITS and ISS) of the HEAs are studied by employing first-principles alloy theory formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation. We benchmark the computational accuracy against literature data by studying the ITS under uniaxial [110] tensile loading and the ISS for the [11 (2) over tilde](111) shear deformation of pure fcc Ni and Al. For the HEAs, we uncover the alloying effect on the ITS and ISS. Under shear loading, relaxation reduces the ISS by similar to 50% for all considered HEAs. We demonstrate that the dimensionless tensile and shear strengths are significantly overestimated by adopting two widely used empirical models in comparison with our ab initio calculations. In addition, our predicted relationship between the dimensionless shear strength and shear instability are in line with the modified Frenkel model. Using the computed ISS, we derive the half-width of the dislocation core for the present HEAs. Employing the ratio of ITS to ISS, we discuss the intrinsic ductility of HEAs and compare it with a common empirical criterion. We observe a strong linear correlation between the shear instability and the ratio of ITS to ISS, whereas a weak positive correlation is found in the case of the empirical criterion.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350286 (URN)10.1103/PhysRevB.97.094102 (DOI)000426775200002 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-05-09Bibliographically approved
Huang, S., Huang, H., Li, W., Kim, D., Lu, S., Li, X., . . . Vitos, L. (2018). Twinning in metastable high-entropy alloys. Nature Communications, 9, Article ID 2381.
Open this publication in new window or tab >>Twinning in metastable high-entropy alloys
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2018 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 2381Article in journal (Refereed) Published
Abstract [en]

Twinning is a fundamental mechanism behind the simultaneous increase of strength and ductility in medium- and high-entropy alloys, but its operation is not yet well understood, which limits their exploitation. Since many high-entropy alloys showing outstanding mechanical properties are actually thermodynamically unstable at ambient and cryogenic conditions, the observed twinning challenges the existing phenomenological and theoretical plasticity models. Here, we adopt a transparent approach based on effective energy barriers in combination with first-principle calculations to shed light on the origin of twinning in high-entropy alloys. We demonstrate that twinning can be the primary deformation mode in metastable face-centered cubic alloys with a fraction that surpasses the previously established upper limit. The present advance in plasticity of metals opens opportunities for tailoring the mechanical response in engineering materials by optimizing metastable twinning in high-entropy alloys.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-358516 (URN)10.1038/s41467-018-04780-x (DOI)000435453200009 ()29915174 (PubMedID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research VINNOVA, 2014-03374The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Carl Tryggers foundation
Available from: 2018-09-03 Created: 2018-09-03 Last updated: 2018-09-03Bibliographically approved
Chen, W., Dolguntseva, I., Savin, A., Zhang, Y., Li, W., Svensson, O. & Leijon, M. (2017). Numerical modelling of a point-absorbing wave energy converter in irregular and extreme waves. Applied Ocean Research, 63, 90-105
Open this publication in new window or tab >>Numerical modelling of a point-absorbing wave energy converter in irregular and extreme waves
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2017 (English)In: Applied Ocean Research, ISSN 0141-1187, E-ISSN 1879-1549, Vol. 63, p. 90-105Article in journal (Refereed) Published
Abstract [en]

Based on the Navier-Stokes (RANS) equations, a three-dimensional (3-D) mathematical model for the hydrodynamics and structural dynamics of a floating point-absorbing wave energy converter (WEC) with a stroke control system in irregular and extreme waves is presented. The model is validated by a comparison of the numerical results with the wave tank experiment results of other researchers. The validated model is then utilized to examine the effect of wave height on structure displacements and connection rope tension. In the examined cases, the differences in WEC’s performance exhibited by an inviscid fluid and a viscous fluid can be neglected. Our results also reveal that the differences in behavior predicted by boundary element method (BEM) and the RANS-based method can be significant and vary considerably, depending on wave height.

Keywords
Point-absorbing WEC, CFD, Irregular waves, Extreme waves, Connection rope tension, Survivability
National Category
Marine Engineering
Identifiers
urn:nbn:se:uu:diva-313250 (URN)10.1016/j.apor.2017.01.004 (DOI)000397367100008 ()
Funder
Swedish Energy AgencyStandUp
Available from: 2017-01-18 Created: 2017-01-18 Last updated: 2017-04-21Bibliographically approved
Wu, J., Yao, Y., Li, W., Zhou, L. & Göteman, M. (2017). Optimizing the Performance of Solo Duck Wave Energy Converter in Tide. Energies, 10(3), Article ID 289.
Open this publication in new window or tab >>Optimizing the Performance of Solo Duck Wave Energy Converter in Tide
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2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 3, article id 289Article in journal (Refereed) Published
Abstract [en]

The high efficiency performance of the Edinburgh Duck wave energy converter (WEC) in 2D regular wave tests makes it a promising wave energy conversion scheme. A solo Duck WEC will be able to apply the point absorber effect to further enhance its performance. Since released degree of freedom will decrease the efficiency, a Duck WEC with fixed pitching axis will be a better option. However, for fixed supported WECs, tide is a non-ignorable consideration. In this paper, a movable mass method is utilized in the whole tidal range to not only balance the Duck to appropriate beak angles, but also follow the variation of hydrodynamic coefficients to keep cancelling the reactance of the system impedance so that complex conjugate control can be realized to optimize the power capture performance of the Duck WEC in tide. Results show that the beak angle should be adjusted to as large a value as possible so that the response amplitude of the Duck at maximum relative capture width will be reasonable small, and the lowest weight of the movable mass is found when its designed position locates at the center of the Duck profile.

Place, publisher, year, edition, pages
MDPI AG, 2017
Keywords
solo Duck, hydrodynamic coefficients, complex conjugate control, maximum relative capture width, movable mass method
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-321162 (URN)10.3390/en10030289 (DOI)000398736700029 ()
Available from: 2017-05-02 Created: 2017-05-02 Last updated: 2017-11-29Bibliographically approved
Wu, J., Yao, Y., Zhou, L., Chen, N., Yu, H., Li, W. & Göteman, M. (2017). Performance analysis of solo Duck wave energy converter arrays under motion constraints. Energy, 139, 155-169
Open this publication in new window or tab >>Performance analysis of solo Duck wave energy converter arrays under motion constraints
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2017 (English)In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 139, p. 155-169Article in journal (Refereed) Published
Abstract [en]

This paper studies the power capture performance of solo Duck wave energy converter (WEC) arrays. The barrier function method combined with a quasi-Newton BFGS optimization algorithm is applied to find the maximum captured power of the array when the Ducks are under motion constraints. Based on this optimized maximum captured power, the effects of separation distance, wave period, incident wave direction and Duck width on the array performance are investigated. For the two Ducks array, results show that the alternative constructive and destructive interaction stripes in the contour plot of the q-factor variation with non-dimensional separation distance are resulted from the diffracted wave pattern from each Duck, and the hydrodynamic interaction strength is reduced when constraints affect the performance. For the three Ducks array, the middle Duck shows larger variability of captured power than the side Ducks due to experiencing double in phase diffracted wave from the side ones. The captured power of the solo Duck WEC array is sensitive to incident wave direction, and arrays with Ducks of smaller width are found to have better performance in power capture efficiency.

National Category
Energy Systems Marine Engineering Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-327262 (URN)10.1016/j.energy.2017.07.152 (DOI)000414879500013 ()
Funder
Swedish Research Council, 2015-04657
Available from: 2017-08-07 Created: 2017-08-07 Last updated: 2018-02-22Bibliographically approved
Li, W., Isberg, J., Waters, R., Engström, J., Svensson, O. & Leijon, M. (2016). Statistical Analysis of Wave Climate Data Using Mixed Distributions and Extreme Wave Prediction. Energies, 9(6), Article ID 396.
Open this publication in new window or tab >>Statistical Analysis of Wave Climate Data Using Mixed Distributions and Extreme Wave Prediction
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2016 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 9, no 6, article id 396Article in journal (Refereed) Published
Abstract [en]

The investigation of various aspects of the wave climate at a wave energy test site is essential for the development of reliable and efficient wave energy conversion technology. This paper presents studies of the wave climate based on nine years of wave observations from the 2005-2013 period measured with a wave measurement buoy at the Lysekil wave energy test site located off the west coast of Sweden. A detailed analysis of the wave statistics is investigated to reveal the characteristics of the wave climate at this specific test site. The long-term extreme waves are estimated from applying the Peak over Threshold (POT) method on the measured wave data. The significant wave height and the maximum wave height at the test site for different return periods are also compared. In this study, a new approach using a mixed-distribution model is proposed to describe the long-term behavior of the significant wave height and it shows an impressive goodness of fit to wave data from the test site. The mixed-distribution model is also applied to measured wave data from four other sites and it provides an illustration of the general applicability of the proposed model. The methodologies used in this paper can be applied to general wave climate analysis of wave energy test sites to estimate extreme waves for the survivability assessment of wave energy converters and characterize the long wave climate to forecast the wave energy resource of the test sites and the energy production of the wave energy converters.

Keywords
wave climate, wave energy converter, ocean wave modelling, mixed-distribution model, extreme wave
National Category
Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-300069 (URN)10.3390/en9060396 (DOI)000378854400009 ()
Funder
StandUpSwedish Energy Agency
Available from: 2016-08-02 Created: 2016-08-02 Last updated: 2017-11-28Bibliographically approved
Li, W., Isberg, J., Engström, J., Waters, R. & Leijon, M. (2015). Optimization of the Power Absorption for a Linear Generator Wave Energy Converter. In: : . Paper presented at Proceedings of the Twenty-fifth (2015) International Ocean and Polar Engineering Conference, Kona, Big Island, Hawaii, USA, June 21-26, 2015.
Open this publication in new window or tab >>Optimization of the Power Absorption for a Linear Generator Wave Energy Converter
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2015 (English)Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents performance optimization studies on a linear generator wave energy converting system developed by Uppsala University of Sweden. The optimization is focused on the investigation of the power absorption with regard to the wave climate at the Lysekil test site for the developed wave energy converter. A frequency domain simplified numerical model is built based on the equation of motion of the system for simulating the behavior of the wave energy converter. The power take-off damping coefficient is chosen as the control parameter in this study. The result shows that the power take-off damping coefficient can be further optimized with regard to the characteristic of the wave climate at the test site for increasing the power absorption capacity of the linear generator wave energy converter. This study will provide significant performance optimization information to the development of the linear generator wave energy converting technology.

Keywords
Wave energy converter; Linear generator; Power absorption; Wave spectrum; Frequency domain; WAMIT
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-265484 (URN)978-1-880653-89-0 (ISBN)
Conference
Proceedings of the Twenty-fifth (2015) International Ocean and Polar Engineering Conference, Kona, Big Island, Hawaii, USA, June 21-26, 2015
Available from: 2015-10-29 Created: 2015-10-29 Last updated: 2016-02-23
Li, W., Isberg, J., Engström, J., Waters, R. & Leijon, M. (2015). Parametric Study of the Power Absorption for a Linear Generator Wave Energy Converter. Journal of Ocean and Wind Energy, 4
Open this publication in new window or tab >>Parametric Study of the Power Absorption for a Linear Generator Wave Energy Converter
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2015 (English)In: Journal of Ocean and Wind Energy, ISSN 0305-182X, E-ISSN 2245-408X, Vol. 4Article in journal, News item (Refereed) Published
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-268486 (URN)10.17736/jowe.2015.jcr30 (DOI)
Available from: 2015-12-06 Created: 2015-12-06 Last updated: 2017-12-01
Li, W., Isberg, J., Engström, J., Waters, R. & Leijon, M. (2015). Study of the foundation design for a linear generator wave energy converter using stochastic methods. Journal of Renewable and Sustainable Energy, 7(6), Article ID 063112.
Open this publication in new window or tab >>Study of the foundation design for a linear generator wave energy converter using stochastic methods
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2015 (English)In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012, E-ISSN 1941-7012, Vol. 7, no 6, article id 063112Article in journal, News item (Refereed) Published
Abstract [en]

This paper presents design studies of the gravity-based foundation for a linear generator wave energy converter. The wave energy converter is based on a direct driven generator mounted on the gravity-based foundation located at the seabed. The linear generator is connected to a point absorbing buoy on the sea surface via a connection rope. Such a device, developed at Uppsala University, has been in operation on the Swedish west coast since 2006. Study is focused on the analysis of the impact from undesirable motions of the gravity-based foundation, particularly the study of the tip and lifts phenomena with regard to the heave and surge forces. Long-term extreme significant wave heights are extrapolated from the statistical analysis of the measured wave climate data in the test site where the wave energy converter is deployed. The joint distribution of the significant wave height and the zero-crossing period from the measured wave climate is also analyzed to estimate the associated periods with respect to the long-term extreme significant wave height. The 25 years return extreme significant wave height 4.8m which is associated with its mean zero-crossing period 8.25 s from the joint distribution is chosen to determine the characteristics of the possible maximum wave for the Lysekil test site. The estimated maximum wave 9.2m is used to estimate the extreme values of the heave and surge forces on the wave energy converter and the gravity-based foundation. The results with respect to the foundation of a new generation wave energy converter about 35 tons with the presented methodology indicate that a heavier foundation which is about 70 tons needs to be designed in terms of considering the stability of the mooring foundation for long term real sea operation. The purpose of this paper is to propose a reliable approach to estimate the appropriate dimensions for gravity-based foundation of the linear generator wave energy converter and provides a theoretical reference to the construction of the gravity-based foundation.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-268484 (URN)10.1063/1.4936420 (DOI)000368036500020 ()
Funder
StandUp
Available from: 2015-12-06 Created: 2015-12-06 Last updated: 2017-12-01Bibliographically approved
Parwal, A., Remouit, F., Hong, Y., Francisco, F., Castelucci, V., Hai, L., . . . Leijon, M. (2015). Wave Energy Research at Uppsala University and The Lysekil Research Site, Sweden: A Status Update. In: : . Paper presented at Proceedings of the 11th European Wave and Tidal Energy Conference. Nantes, France, September 2015.
Open this publication in new window or tab >>Wave Energy Research at Uppsala University and The Lysekil Research Site, Sweden: A Status Update
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2015 (English)Conference paper, Published paper (Refereed)
Abstract [en]

This paper provides a summarized status update ofthe Lysekil wave power project. The Lysekil project is coordinatedby the Div. of Electricity, Uppsala University since 2002, with theobjective to develop full-scale wave power converters (WEC). Theconcept is based on a linear synchronous generator (anchored tothe seabed) driven by a heaving point absorber. This WEC has nogearbox or other mechanical or hydraulic conversion systems,resulting in a simpler and robust power plant. Since 2006, 12 suchWECs have been build and tested at the research site located atthe west coast of Sweden. The last update includes a new andextended project permit, deployment of a new marine substation,tests of several concepts of heaving buoys, grid connection,improved measuring station, improved modelling of wave powerfarms, implementation of remote operated vehicles forunderwater cable connection, and comprehensive environmentalmonitoring studies.

Keywords
Wave energy, point absorber, experiments, arrays, generators, ROVs
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Ocean and River Engineering
Identifiers
urn:nbn:se:uu:diva-265218 (URN)
Conference
Proceedings of the 11th European Wave and Tidal Energy Conference. Nantes, France, September 2015
Available from: 2015-10-26 Created: 2015-10-26 Last updated: 2019-04-05Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1832-5850

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