1,721,085 research outputs found
Novel lubrication system to improve the excessive wear in wind turbine yaw and pitch gears
Full text linkTesi per compendi de publicacions. La consulta íntegra de la tesi, inclosos els articles no comunicats públicament per drets d'autor, es pot realitzar prèvia petició a l'Arxiu de la UPCManufacturers of wind turbines have observed a new phenomenon that appears in high power wind turbines: excessive wear in the teeth located at 0º in the pitch bearing.
In order to design more efficient wind turbines, manufacturers are increasing the rotor diameter to capture more kinetic energy from the wind to generate more energy, therefore the stress in the joints/unions is increased, which leads to the elastic deformation of the system. These stresses and deformations increase in all parts of the wind turbine, the foundations and the tower, yaw, nacelle, drive train and blade unions. All these cases are mainly static unions, except the drive train that transmits torque, the yaw system that turns the nacelle and the pitch system that turns each blade around its axis.
The weight of the blades under movement, always working in the most efficient position, causes micro-movements allowed by the elastic deformations and the backlash of the gear transmission that induce an excessive wear at the zero degree position. The same phenomenon is observed in the yaw system, though to a lesser extent. Despite the manufacturer's efforts, a solution that could be implemented in the near future or easily retrofitted in the wind turbines does not yet exist.
The aim of this PhD project is to improve the lubrication of the pitch and yaw gear systems of wind turbines through the use of a novel lubrication system based on an array of micro-fabricated channels fitted at the gears' root (dedendum). A micro-nozzle to continuously inject fresh grease in between the teeth in contact has been designed, manufactured and installed in a test bench of a 2 MW wind turbine pitch system. The test bench has been used to characterize the fatigue behavior of the gear surface using conventional wind turbine greases under real cyclic loads, showing a delay of 2x10^4 cycles in the appearance of wear.
The proposed micro-nozzle is expected to be compatible with and easily implemented into both newly designed and in-market models. This novel lubrication system will inject fresh lubricant to the gear contact area even when the wind turbine is generating electricity.Postprint (published version
Modelling of Lubricating Grease FlowUsing Computational Fluid Dynamics [Elektronisk resurs]
No full textIn this paper, numerical simulations of lubricating grease flow usingComputational Fluid Dynamics is presented. The grease is treated as asingle-phase Herschel-Bulkley fluid, where three different rheologicalproperties, corresponding to NLGI grade 00, 1 and 2, respectively, havebeen considered in two different configurations comprised by arestricted straight channel, and a double restriction seal with andwithout ring. The numerical code and rheology model have beenvalidated with analytical solutions and flow measurements using microparticleimage velocimetry. The grease velocity distribution in thegeometries are obtained and the motion of contaminant particlesinserted into the flow is investigated. The latter is of special interest inseal geometries which intend to protect the moving mechanicalcomponents from contamination.</p
Solar wind interaction with the terrestrial magnetopause
No full textThe solar wind interaction with the terrestrial magnetosphere is a source for many spectacular phenomena on or close the Earth's surface. A key question during the last fifty years have been how the solar wind plasma can enter the terrestrial magnetic shield represented by the magnetosphere and its outermost layer called the magnetopause. This have been the seed for many controversies among researchers throughout the years. Today we know that there are several possibilities for the solar wind to break through the magnetic boundary of the Earth. The main plasma transport mechanism at the magnetopause is called magnetic reconnection, where the magnetic energy stored in the solar wind is converted to kinetic energy through a localized break-down of the ideal frozen-in condition of the magnetic field within the plasma. Since its introduction to the space-physical community in the late 1950's, reconnection research have had its primary focus on understanding the onset mechanisms inside the diffusion region where the solar wind magnetic field is reconnected with the magnetospheric magnetic field. In this thesis work we put the context well out of the diffusion region and focuses on the implications of magnetic reconnection onto the surrounding solar wind plasma, rather than on the main mechanisms which initiates the process. We present solutions for the structure of the plasma flow through the magnetopause surface during conditions of ongoing reconnection. This is done through viscous-resistive reconnection models together with models where finite gyro-radius effects are considered. In order to validate the viscous-resistive model we also couple the analytical solutions with \textit{in situ} measurements made by the Cluster spacecraft fleet. This results in an entirely new way of determining the magnetopause transition layer thickness and the location of the reconnection site from spacecraft data.Godkänd; 2007; 20070904 (pafi
Solar wind interaction with the terrestrial magnetopause [Elektronisk resurs]
No full textMagnetic reconnection plays an important role in the transfer of mass, energy and momentum from the solar wind to the terrestrial magnetosphere. The earliest contributions to the theory of magnetic reconnection dates from the beginning of the 1930's. However, it took until the end of the 1950's when Sweet and Parker made their first reconnection model, for the concept to reach a somewhat solid ground. During the years since then magnetic reconnection has walked through the phase of reaching mythical proportions where some researchers believed in it, and some not, to the acceptance it has today where the main issue not is whether the process happens or not, but rather the main physical properties and the implications of it. During the last fifteen years much of the research due to the rapid increase in computer capacity, treats numerical simulations of magnetic reconnection. Theoretical analysis keeps though its position as a cornerstone for the understanding of the process. But also for the base of new implemented models. Much of the theoretical work accomplished to this day has its focus on magnetic reconnection itself; applications for different conditions, and the onset of the process - something which still is under much discussion among researchers. This work focuses on the implications of magnetic reconnection in combination with the outer magnetosheath flow. The analysis treats a two-dimensional and three-dimensional case. For the 3D case, the magnetosheath plasma flow is considered to be incompressible, while we for the 2D case also treat a compressible magnetosheath plasma. Magnetic reconnection is assumed to occur in a region stretching from the sub-solar point to the north, at an arbitrary point for the 2D case, and along a line parallel to the y-axis for the 3D case. The analysis is based on the MHD equations including dissipative effects such as viscosity and resistivity, where the equations are solved approximately by the use of an ordinary perturbation expansion for large Reynolds and Lundqvist numbers. The objective of the 2D study treating an incompressible plasma flow, is to get a description of the current transition layer in combination with the outer magnetosheath and boundary layer flow. The solutions are asymptotically matched with an existing model for the magnetosheath magnetic field. For the 2D compressible case and 3D analysis, the objective is to study the development of the magnetic field and total velocity during the transition from the magnetosheath to the magnetosphere.</p
Modelling of Lubricating Grease FlowUsing Computational Fluid Dynamics
No full textIn this paper, numerical simulations of lubricating grease flow usingComputational Fluid Dynamics is presented. The grease is treated as asingle-phase Herschel-Bulkley fluid, where three different rheologicalproperties, corresponding to NLGI grade 00, 1 and 2, respectively, havebeen considered in two different configurations comprised by arestricted straight channel, and a double restriction seal with andwithout ring. The numerical code and rheology model have beenvalidated with analytical solutions and flow measurements using microparticleimage velocimetry. The grease velocity distribution in thegeometries are obtained and the motion of contaminant particlesinserted into the flow is investigated. The latter is of special interest inseal geometries which intend to protect the moving mechanicalcomponents from contamination
Solar wind interaction with the terrestrial magnetopause [Elektronisk resurs]
No full textThe solar wind interaction with the terrestrial magnetosphere is a source for many spectacular phenomena on or close the Earth's surface. A key question during the last fifty years have been how the solar wind plasma can enter the terrestrial magnetic shield represented by the magnetosphere and its outermost layer called the magnetopause. This have been the seed for many controversies among researchers throughout the years. Today we know that there are several possibilities for the solar wind to break through the magnetic boundary of the Earth. The main plasma transport mechanism at the magnetopause is called magnetic reconnection, where the magnetic energy stored in the solar wind is converted to kinetic energy through a localized break-down of the ideal frozen-in condition of the magnetic field within the plasma. Since its introduction to the space-physical community in the late 1950's, reconnection research have had its primary focus on understanding the onset mechanisms inside the diffusion region where the solar wind magnetic field is reconnected with the magnetospheric magnetic field. In this thesis work we put the context well out of the diffusion region and focuses on the implications of magnetic reconnection onto the surrounding solar wind plasma, rather than on the main mechanisms which initiates the process. We present solutions for the structure of the plasma flow through the magnetopause surface during conditions of ongoing reconnection. This is done through viscous-resistive reconnection models together with models where finite gyro-radius effects are considered. In order to validate the viscous-resistive model we also couple the analytical solutions with \textit{in situ} measurements made by the Cluster spacecraft fleet. This results in an entirely new way of determining the magnetopause transition layer thickness and the location of the reconnection site from spacecraft data.</p
MR-fluid Technology for Reduced Wear in Windturbine Bearings
No full textMagnetorheology (MR) is a technology to enable active control of the viscosity of a material - typically afluid, but also semi-solid materials such as lubricating greases. Magnetic particles of nano to micron scale aremixed with the fluid and when subjected to a magnetic field the particles are aligned with the field, inducing ashear resistance - i.e. the effect of an increased viscosity - if applied in appropriate direction. This technology isfor example used in active dampers in sports cars, where electromagnets activated by a sensor systemcontrols the viscosity of the damper oil and the pressure in the dampers. This paper proposes a method ofusing MR technology to reduce wear in wind turbine bearings by enhancing the lubricant film in the maximumHertzian contact. Electromagnets are used to control the sequence of demagnetization and magnetization ofthe lubricant. Here the wear particles existing in the lubricant acts as guiding particles.</p
Solar wind interaction with the terrestrial magnetopause
No full textMagnetic reconnection plays an important role in the transfer of mass, energy and momentum from the solar wind to the terrestrial magnetosphere. The earliest contributions to the theory of magnetic reconnection dates from the beginning of the 1930's. However, it took until the end of the 1950's when Sweet and Parker made their first reconnection model, for the concept to reach a somewhat solid ground. During the years since then magnetic reconnection has walked through the phase of reaching mythical proportions where some researchers believed in it, and some not, to the acceptance it has today where the main issue not is whether the process happens or not, but rather the main physical properties and the implications of it. During the last fifteen years much of the research due to the rapid increase in computer capacity, treats numerical simulations of magnetic reconnection. Theoretical analysis keeps though its position as a cornerstone for the understanding of the process. But also for the base of new implemented models. Much of the theoretical work accomplished to this day has its focus on magnetic reconnection itself; applications for different conditions, and the onset of the process - something which still is under much discussion among researchers. This work focuses on the implications of magnetic reconnection in combination with the outer magnetosheath flow. The analysis treats a two-dimensional and three-dimensional case. For the 3D case, the magnetosheath plasma flow is considered to be incompressible, while we for the 2D case also treat a compressible magnetosheath plasma. Magnetic reconnection is assumed to occur in a region stretching from the sub-solar point to the north, at an arbitrary point for the 2D case, and along a line parallel to the y-axis for the 3D case. The analysis is based on the MHD equations including dissipative effects such as viscosity and resistivity, where the equations are solved approximately by the use of an ordinary perturbation expansion for large Reynolds and Lundqvist numbers. The objective of the 2D study treating an incompressible plasma flow, is to get a description of the current transition layer in combination with the outer magnetosheath and boundary layer flow. The solutions are asymptotically matched with an existing model for the magnetosheath magnetic field. For the 2D compressible case and 3D analysis, the objective is to study the development of the magnetic field and total velocity during the transition from the magnetosheath to the magnetosphere.Godkänd; 2005; 20070116 (ysko
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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