| 15571. |
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| 15572. |
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| 15573. |
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| 15574. |
- Lokatt, Mikaela
(författare)
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On a Viscous-Inviscid Interaction Model for Aeronautical Applications
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis concerns the development of a viscous-inviscid interaction model for prediction of viscous flow properties in aeronautical applications. The inviscid model is based on an existing potential flow model and the thesis thus focuses on the development of a viscous model based on the three-dimensional integral boundary layer equations. The model is to be applicable to complex geometries with unstructured meshes and this requirement, in combination with the fairly complex character of the integral boundary layer equations, sets high standards for the discretization. In order to arrive at a stable and well-conditioned scheme a number of topics related to the formulation and discretization of the integral model are analyzed and discussed. These include the challenge of finding a discretization which ensures flow conservation on curved surfaces and provides a stable and well-conditioned discretization for mixed-hyperbolic systems of conservation laws. Different coupling strategies for the viscous and inviscid models are discussed and analyzed and so are the singularities in the integral boundary layer equations in separated flow regions. The predictions of the resulting viscous-inviscid coupling scheme are validated by comparison to experimental measurements as well as to predictions from other numerical models. The currently developed coupled model is found to provide reasonably accurate predictions of viscous flow properties in laminar as well as turbulent flow regions while being stable and convergent in separated flow regions.
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| 15575. |
- Lokatt, Mikaela
(författare)
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On Aerodynamic and Aeroelastic Modeling for Aircraft Design
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis was performed with the aim of developing improved prediction methods for aerodynamic and aeroelastic analysis to be used in aircraft design. The first part of the thesis concerns the development of a viscous-inviscid interaction model for steady aerodynamic predictions. Since an inviscid, potential flow, model already is available, the main focus is on the development of a viscous model consisting of a three-dimensional integral boundary layer model. The performance of the viscous-inviscid interaction model is evaluated and it is found that the accuracy of the predictions as well as the computational cost appear to be acceptable for the intended application. The presented work also includes an experimental study aimed at analyzing steady and unsteady aerodynamic characteristics of a laminar flow wing model. An enhanced understanding of these characteristics is presumed to be useful for the development of improved aerodynamic prediction models. A combination of nearly linear as well as clearly nonlinear aerodynamic variations are observed in the steady as well as in the unsteady experimental results and it is discussed how these may relate to boundary layer properties as well as to aeroelastic stability characteristics. Aeroelastic considerations are receiving additional attention in the thesis, as a method for prediction of how flutter characteristics are affected by modeling uncertainties is part of the presented material. The analysis method provides an efficient alternative for obtaining increased information about, as well as enhanced understanding of, aeroelastic stability characteristics.
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| 15576. |
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| 15577. |
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| 15578. |
- Long, Jennifer, et al.
(författare)
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A definition of visual ergonomics
- 2014
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Ingår i: Applied Ergonomics. - : Elsevier BV. - 0003-6870 .- 1872-9126. ; 45:4, s. 1263-1264
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 15579. |
- Long, Mengni, 1987-, et al.
(författare)
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On the Attachment of Dart Lightning Leaders to Wind Turbines
- 2024
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Ingår i: Electric power systems research. - : Elsevier. - 0378-7796 .- 1873-2046.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Wind turbines are prone to damages due to lightning strikes and the blades are one of the most vulnerable components. Even though the blade tip is usually protected in standard designs,lightning damages several meters away from it have also beenobserved in field studies. However, these damages inboard fromthe tip cannot be explained by the attachment of downwardstepped leaders or the initiation of upward lightning alone. In this paper, the attachment of dart leaders in an upward lightning flashis investigated as a mechanism of strikes to inboard sections of the blade and the nacelle. Dart leaders in an upward lightning flashuse the channel previously ionized by the preceding stroke or thecontinuous current. The analysis is performed with the self-consistent leader inception and propagation model SLIM. A commercial wind turbine with 45 m long blades and hub height of 80 m is analysed as a case study. The impact of the prospective return stroke peak current, the rotation angle of the blade and the wind on the location of lightning strikes on this mechanism is analysed. The probability of lightning attachment of dart leaders along the blade for the case study is also calculated. It is shown that the dart leader attachment is a mechanism that can explain lightning strikes to the nacelle and to the inboard region several meters away from the blade tip. However, this mechanism cannot explain the lightning strikes observed in the close vicinity of theblade tip (in the region between 1.5 and 6 m from it). The modelling study here also shows that for the turbine under consideration, nacelle receptors intercept most of dart leaders,around 73%, even if the initial continuous currents flow to the blade tip. Overall, it is estimated that around 80% of strikes inupward lightning flashes attach to the tip receptor of the blade.
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| 15580. |
- Long, Mengni, 1987-
(författare)
-
On the Attachment of Lightning Flashes to Wind Turbines
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis aims at investigating the attachment of lightning flashes to wind turbines. Modern wind turbines are highly exposed to lightning strikes, due to the increase of their height and the rotation of the blades. Upward lightning is the dominant mechanism of lightning strikes to them. Therefore, this study evaluates the initiation of the initial upward leader discharge and the process of lightning attachment of dart leaders taking place prior to the first return stroke in upward flashes.This work extends the self-consistent leader inception and propagation model (SLIM) to evaluate the lightning attachment of dart and dart-stepped leaders to grounded objects. SLIM was originally proposed to evaluate the lightning attachment of stepped leaders. Unlike the well-studied lightning attachment of stepped leaders, upward connecting leaders initiated in response to dart and dart-stepped leaders develop under a significantly faster change of the ambient electric field. Additionally, these connecting leaders could develop in warm air pre-conditioned by the previous strokes in the same flash. An analytical expression to evaluate the charge required to thermalize the connecting leader per unit length is also developed in the extended model. This model is validated through the analysis of three attachment events recorded in rocket-triggered lightning experiments. Good agreement between the predicted properties of the upward leaders and the measurements has been found. The model is utilized to evaluate the different conditions where connecting leaders can develop prior to the return strokes in upward lightning.The extended model of SLIM is also applied to study the interception of lightning dart leaders by upward connecting leaders initiated from wind turbines. The evaluation considers the influence of the return stroke peak current, the blade rotation and wind on the attachment of lightning dart leaders to wind turbines. The probability of lightning strikes to the receptors along the blade and on the nacelle is calculated for upward lightning flashes. It is shown that the lightning attachment of dart leaders is a mechanism that can explain the lightning damages to the inboard region of the blades (more than 10 meters from the tip) and the nacelle of wind turbines.Furthermore, the critical stabilization electric field required to initiate upward lightning from wind turbines is evaluated for both ‘self-initiated’ and ‘other-triggered’ upward flashes. The calculation shows that the stabilization electric field of an operating wind turbine periodically changes due to the rotation of its blades. The initiation of upward lightning is greatly facilitated by the electric field change produced by nearby lightning events. However, the rate of rise of the electric field only has a weak impact on the stabilization electric field. The evaluation of the stabilization electric field provides essential information needed for the estimation of the incidence of upward lightning to wind turbines.
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