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RISS 인기검색어
- 검색키워드
- 저자 : S.R.K. Nielsen (검색결과 4 건)
- 무료
- 기관 내 무료
- 유료
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On the nonlinear structural analysis of wind turbine blades using reduced degree-of-freedom models
K. Holm-Jørgensen,J.W. Stærdahl,S.R.K. Nielsen 국제구조공학회 2008 Structural Engineering and Mechanics, An Int'l Jou Vol.28 No.1
Wind turbine blades are increasing in magnitude without a proportional increase of stiffness for which reason geometrical and inertial nonlinearities become increasingly important. Often these effects are analysed using a nonlinear truncated expansion in undamped fixed base mode shapes of a blade, modelling geometrical and inertial nonlinear couplings in the fundamental flap and edge direction. The purpose of this article is to examine the applicability of such a reduced-degree-of-freedom model in predicting the nonlinear response and stability of a blade by comparison to a full model based on a nonlinear co-rotating FE formulation. By use of the reduced-degree-of-freedom model it is shown that under strong resonance excitation of the fundamental flap or edge modes, significant energy is transferred to higher modes due to parametric or nonlinear coupling terms, which influence the response and stability conditions. It is demonstrated that the response predicted by such models in some cases becomes instable or chaotic. However, as a consequence of the energy flow the stability is increased and the tendency of chaotic vibrations is reduced as the number of modes are increased. The FE model representing the case of infinitely many included modes, is shown to predict stable and ordered response for all considered parameters. Further, the analysis shows that the reduced-degree-of-freedom model of relatively low order overestimates the response near resonance peaks, which is a consequence of the small number of included modes. The qualitative erratic response and stability prediction of the reduced order models take place at frequencies slightly above normal operation. However, for normal operation of the wind turbine without resonance excitation 4 modes in the reduced-degree-of-freedom model perform acceptable.
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Characterising and reducing the blank response from mercury vapour sorbent tubes
Brown, R. C.,Braysher, E.,McGhee, E.,Goddard, S.,Ent, H.,Kim, K. H.,Nielsen, J. Royal Society of Chemistry 2017 Analytical methods Vol.9 No.18
<P>An investigation into the factors contributing to the blank response of sorbent tubes used for sampling and measuring mercury vapour is presented. These contributing factors are quantified and strategies to mitigate or remove their effects have been proposed - the most effective of which on a routine operational basis is the cleaning of the sorbent tubes in air to remove surface adsorbed mercury and any organic contaminants that can be oxidised. Contributions of up to 175 pg of mercury, or mercury equivalent mass, were identified and removed. The largest contributors were deeply absorbed mercury and hydrocarbons and other organic compounds oxidised and removed by heating in air. Decreasing the blank response resulted in an improvement in detection limit of a factor of two. This estimate was corroborated by a novel technique for assessing the detection limit of analytical methods employing multiple desorptions that relies on determining when the ratio of the third desorption response was equivalent to the first desorption response.</P>
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On the nonlinear structural analysis of wind turbine blades using reduced degree-of-freedom models
Holm-Jorgensen, K.,Staerdahl, J.W.,Nielsen, S.R.K. Techno-Press 2008 Structural Engineering and Mechanics, An Int'l Jou Vol.28 No.1
Wind turbine blades are increasing in magnitude without a proportional increase of stiffness for which reason geometrical and inertial nonlinearities become increasingly important. Often these effects are analysed using a nonlinear truncated expansion in undamped fixed base mode shapes of a blade, modelling geometrical and inertial nonlinear couplings in the fundamental flap and edge direction. The purpose of this article is to examine the applicability of such a reduced-degree-of-freedom model in predicting the nonlinear response and stability of a blade by comparison to a full model based on a nonlinear co-rotating FE formulation. By use of the reduced-degree-of-freedom model it is shown that under strong resonance excitation of the fundamental flap or edge modes, significant energy is transferred to higher modes due to parametric or nonlinear coupling terms, which influence the response and stability conditions. It is demonstrated that the response predicted by such models in some cases becomes instable or chaotic. However, as a consequence of the energy flow the stability is increased and the tendency of chaotic vibrations is reduced as the number of modes are increased. The FE model representing the case of infinitely many included modes, is shown to predict stable and ordered response for all considered parameters. Further, the analysis shows that the reduced-degree-of-freedom model of relatively low order overestimates the response near resonance peaks, which is a consequence of the small number of included modes. The qualitative erratic response and stability prediction of the reduced order models take place at frequencies slightly above normal operation. However, for normal operation of the wind turbine without resonance excitation 4 modes in the reduced-degree-of-freedom model perform acceptable.
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Beam Dynamics in a Long-pulse Linear Induction Accelerator
Carl Ekdahl,E. O. Abeyta,P. Aragon,R. Archuleta,G. Cook,D. Dalmas,K. Esquibel,R. Gallegos,R. Garnett,J. Harrison,J. Johnson,E. Jacquez,B. Trent McCuistian,N. Montoya,S. Nath,K. Nielsen,D. Oro,C. Rose 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.61
The second axis of the Dual Axis Radiography of Hydrodynamic Testing (DARHT) facility produces up to four radiographs within an interval of 1.6 microseconds. It accomplishes this by slicing four micro-pulses out of a long 1.8-kA, 16.5-MeV electron beam pulse and focusing them onto a bremsstrahlung converter target. The long beam pulse is created by a dispenser cathode diode and accelerated by the unique DARHT Axis-II linear induction accelerator (LIA). Beam motion in the accelerator would be a problem for radiography. High frequency motion, such as from beam breakup instability, would blur the individual spots. Low frequency motion, such as produced by pulsed power variation, would produce spot to spot differences. In this article, we describe these sources of beam motion, and the measures we have taken to minimize it.
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