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Sökning: WAKA:kon > Högskolan i Borås > Luleå tekniska universitet

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1.
  • Gantasala, Sudhakar, et al. (författare)
  • Aeroelastic simulations of wind turbine using 13 DOF rigid beam model
  • 2016
  • Ingår i: Open archives of the 16th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery. - : Symposia on Rotating Machinery.
  • Konferensbidrag (refereegranskat)abstract
    • The vibration behavior of wind turbine substructures is mainly dominated by their first few vibration modes because wind turbines operate at low rotational speeds. In this study, 13 degrees of freedom (DOF) model of a wind turbine is derived considering fundamental vibration modes of the tower and blades which are modelled as rigid beams with torsional springs attached at their root. Linear equations of motion (EOM) governing the structural behavior of wind turbines are derived by assuming small amplitude vibrations. This model is used to study the coupling between the structural and aerodynamic behavior of NREL 5 MWmodel wind turbine. Aeroelastic natural frequencies of the current model are compared with the results obtained from the finite element model of this wind turbine. Quasi-steady aerodynamic loads are calculated considering wind velocity changes due to height and tower shadow effects. In this study, vibration responses are simulated at various wind velocities. The derived 13 DOF simplified model of the wind turbine enables to simulate the influence ofchange in parameters and operating conditions on vibration behavior with less computational effort. Besides that, the results of the simplified models can be interpreted with much ease.
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3.
  • Luneno, Jean-Claude (författare)
  • Coupled vibrations in horizontal and vertical rotor-bearing systems
  • 2011
  • Konferensbidrag (refereegranskat)abstract
    • For dynamical systems having several degrees of freedom, motion in one direction can induce motion in the other. This means that there is a certain coupling between these two motions. Coupling can in some cases be a source of instability that causes self-excited vibrations in rotating machinery. In classical modeling of rotor systems, couplings other than those that are the result of gyroscopic effect are normally not considered. This is due to thecomplexity of the reasons for coupling which mainly depends on machinery hardware, for example, the bearings’ design (type) and the asymmetry in machine components.Plain cylindrical hydrodynamic journal-bearings provide high damping to the rotorsystem, but they also cross couple the rotor translational motions. Cross coupling is the main source of oil induced instability; therefore, the rotor speed should not exceed the speed at which oil-induced instability occurs. The inherent nonlinearity of plain cylindrical hydrodynamic journal- bearings becomes strong for eccentricities greater than 60% of the bearing clearance, where most existing linear models are not able to accurately predict the rotor trajectory. Strong nonlinearities together with cross coupling are the source of complex dynamics in fluid-film journal bearings. The journal bearing impedance descriptions method, a method that is valid for all bearing aspect ratios and all eccentricities, was used to evaluate linear analysis of the rotor steady-state imbalance response. The results show that linear bearing models derived from the nonlinear impedance descriptions of the Moes-cavitated (π-film) finite-length bearing can predict the steady-state imbalance response of a rigid symmetric rotor that is supported by two identical journal-bearings at high eccentricities. This is, however, only the case when operating conditions are below the threshold speed of instability and when the system has period one solutions. The error increases in the vicinity of resonance speed. A combi-bearing is a fluid-film lubricated tilting-pad thrust and journal bearings combined together. Thrust bearing is used in vertical rotating machinery and shafts designed to transmit thrust. The total axial load is carried by the single thrust bearing. The analyzed combi-bearing is an existing machine component used in the hydropower unit Porjus U9 situated in northern Sweden. The linearized model shows that the combi-bearing couples the rotor’s lateral and angular motions. However, if the thrust bearing’s pads arrangement is not symmetrical or if all the pads are not angularly equidistant the rotor axial and angular motions are also coupled. This last case of coupling will also occur if the thrust bearing equivalent total stiffness is not evenly distributed over the thrust bearing. A defected pad or unequal hydrodynamic pressure distribution on the pads’ surfaces may be the cause. The Porjus U9’s simulation results show that the combi-bearing influences the dynamic behavior of the machine. The rotor motions’ coupling due to combi-bearing changes the system’s natural frequencies and vibration modes.
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4.
  • Luneno, Jean-Claude, et al. (författare)
  • Effects of shaft flexibility and gyroscopic coupling on instability threshold speeds of rotor-bearing systems
  • 2010
  • Ingår i: 13th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery 2010 (ISROMAC-13). - Red Hook, NY : Curran Associates, Inc.. - 9781617388484 ; , s. 537-543
  • Konferensbidrag (refereegranskat)abstract
    • The driving speeds at which self-excited motions occur in rotor-bearing systems are commonly referred to as "instability threshold". These speeds and the magnitude of rotor (journal) trajectories are two important variables characterising the limits and states of a rotating machinery. The hydrodynamic lubrication in journal-bearing provides damping and reduces friction on rotor systems; therefore the journal amplitude should not exceed the bearing radial clearance. Linear bearing models are not able to accurately predict the journal trajectories for rotor-bearing system operating in conditions where the system does not have period one solutions, or when the journal motion is larger than 20-30% of the bearing radial clearance. Therefore the nonlinear bearing impedance descriptions method was used to model the hydrodynamic reaction forces. Two cases were analysed: 1) a rigid non-symmetric rotor and 2) a flexible non-symmetric rotor. The two models consist of a rotor supported by two identical finite-length hydrodynamic journal bearings of length to diameter ratio L/D=1, with same lubricant properties. The flexible non-symmetric rotor was modelled by the finite element method (FEM). Simulation results show that the instability threshold of the rigid non-symmetric rotor-bearing system (case1) depends on the low stability characteristics of the less loaded bearing. But when the shaft flexibility and the gyroscopic coupling effect are taken into account; the instability threshold increases for the flexible non-symmetric rotor-bearing system (case2). The gyroscopic coupling effect does not only increase the instability threshold, but the journal trajectories magnitude has also significantly increased. This is normally not a preferable condition since high vibrations will induce heat and stress in babbited bearing.
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5.
  • Luneno, Jean-Claude, et al. (författare)
  • Oil induced instability : analytic study and experimental verification on flexible rotor supported by a journal-bearing at one end
  • 2010
  • Ingår i: Proceedings of the 8th IFToMM International Conference on Rotordynamics.
  • Konferensbidrag (refereegranskat)abstract
    • Oil induced instability, is a frequently encountered phenomenon causing system instability for rotors supported by hydrodynamic journal-bearings. In this paper a flexible rotor, simply supported at one end and with oil lubricated journal-bearing at the other, is analytically modelled. The rotor system is modelled in two ways namely as a discrete system by finite element method (FEM) with nonlinear journal-bearing and as a lumped inertia system with linear journal-bearing. The analysed rotor-bearing system is a Bently Nevada Rotor Kit Model RK4 with Oil whirl/whip option. Results obtained from the simulation of the discrete rotor model with a nonlinear journal-bearing indicate at which rotational speed the oil induced instability (oil whirl) will occur. Campbell diagrams are shown for the lumped inertia rotor model with linear journal-bearing and the critical speeds are predicted. From the results the accuracy of the analytical speed-dependent bearing coefficients are evaluated. These coefficients were derived from the nonlinear bearing impedance descriptions by D. Childs. The bearing impedance descriptions method is a method valid for all L/D (length to diameter) ratios, and all journal eccentricities. The simulation time is significantly reduced by using a lumped inertia rotor model with linear journal-bearing. Critical speed obtained from Campbell diagram predicts a threshold speed of instability which is about 0.35% higher than that predicted by the discrete rotor model with a nonlinear journal-bearing. Compared with results collected from experiment, the simulation results predict a threshold speed of instability which is about 5.69% higher (linear analysis), or 5.36% higher (nonlinear analysis).
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  • Resultat 1-5 av 5
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Luneno, Jean-Claude (4)
Aidanpää, Jan-Olov (3)
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Cho, S. W. (1)
Mathew, Aji P. (1)
Skrifvars, M. (1)
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Oksman, Kristiina (1)
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