The Experts below are selected from a list of 306 Experts worldwide ranked by ideXlab platform
Qingjie Cao - One of the best experts on this subject based on the ideXlab platform.
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A three-degree-of-Freedom Model for vortex-induced vibrations of turbine blades
Meccanica, 2016Co-Authors: Dan Wang, Yushu Chen, Marian Wiercigroch, Qingjie CaoAbstract:A new three-degree-of-Freedom Model simulating vortex-induced vibrations (VIVs) of turbine blades is proposed. Equations of motions include the coupling for bending and torsion of a blade as well as the fluid-blade interactions, which is described by a van der Pol oscillator. The 1:1 internal resonance analysis is carried out with the multiple scale method, and modulation equations are derived. Bifurcation curves for responses with respect to the detuning parameter and dimensionless freestream velocity are obtained. Effects of the system parameters including the lift and moment coefficients, structural damping, cubic nonlinearity and coupling parameters on the responses are investigated. It is found that dynamic behaviours such as the saddle-node and Hopf bifurcations can occur for certain values of the system parameters. The approximate solutions obtained by the multiple scale method are validated by a direct numerical simulation. The results indicate that the proposed three-degree-of-Freedom Model can be useful to explain the dynamic response characteristics of blades and to optimize the blade design.
Imran Akhtar - One of the best experts on this subject based on the ideXlab platform.
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Single-degree-of-Freedom Model of displacement in vortex-induced vibrations
Nonlinear Dynamics, 2021Co-Authors: Muhammad R. Hajj, Arshad Mehmood, Imran AkhtarAbstract:In contrast to the approach of coupling a nonlinear oscillator that represents the lift force with the cylinder’s equation of motion to predict the amplitude of vortex-induced vibrations, we propose and show that the displacement can be directly predicted by a nonlinear oscillator without a need for a force Model. The advantages of the latter approach include reducing the number of equations and, subsequently, the number of coefficients to be identified to predict displacements associated with vortex-induced vibrations. The implemented single-equation Model is based on phenomenological representation of different components of the transverse force as required to initiate the vibrations and to limit their amplitude. Three different representations for specific flow and cylinder parameters yielding synchronization for Reynolds numbers between 104 and 114 are considered. The method of multiple scales is combined with data from direct numerical simulations to identify the parameters of the proposed Models. The variations in these parameters with the Reynolds number, reduced velocity or force coefficient over the synchronization regime are determined. The predicted steady-state amplitudes are validated against those obtained from high-fidelity numerical simulations. The capability of the proposed Models in assessing the performance of linear feedback control strategy in reducing the vibrations amplitude is validated with performance as determined from numerical simulations.
Dan Wang - One of the best experts on this subject based on the ideXlab platform.
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A three-degree-of-Freedom Model for vortex-induced vibrations of turbine blades
Meccanica, 2016Co-Authors: Dan Wang, Yushu Chen, Marian Wiercigroch, Qingjie CaoAbstract:A new three-degree-of-Freedom Model simulating vortex-induced vibrations (VIVs) of turbine blades is proposed. Equations of motions include the coupling for bending and torsion of a blade as well as the fluid-blade interactions, which is described by a van der Pol oscillator. The 1:1 internal resonance analysis is carried out with the multiple scale method, and modulation equations are derived. Bifurcation curves for responses with respect to the detuning parameter and dimensionless freestream velocity are obtained. Effects of the system parameters including the lift and moment coefficients, structural damping, cubic nonlinearity and coupling parameters on the responses are investigated. It is found that dynamic behaviours such as the saddle-node and Hopf bifurcations can occur for certain values of the system parameters. The approximate solutions obtained by the multiple scale method are validated by a direct numerical simulation. The results indicate that the proposed three-degree-of-Freedom Model can be useful to explain the dynamic response characteristics of blades and to optimize the blade design.
Walter Herzog - One of the best experts on this subject based on the ideXlab platform.
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Velocity-dependent cost function for the prediction of force sharing among synergistic muscles in a one degree of Freedom Model.
Journal of biomechanics, 2009Co-Authors: Gudrun Schappacher-tilp, Paul Binding, Elena Braverman, Walter HerzogAbstract:Prediction of accurate and meaningful force sharing among synergistic muscles is a major problem in biomechanics research. Given a resultant joint moment, a unique set of muscle forces can be obtained from this mathematically redundant system using nonlinear optimization. The classical cost functions for optimization involve a normalization of the muscle forces to the absolute force capacity of the target muscles, usually by the cross-sectional area or the maximal isometric force. In a one degree of Freedom Model this leads to a functional relationship between moment arms and the predicted muscle forces, such that for constant moment arms, or constant ratios of moment arms, agonistic muscle forces increase or decrease in unison. Experimental studies have shown however that the relationship between muscle forces is highly task-dependent often causing forces to increase in one muscle while decreasing in a functional agonist, likely because of the contractile conditions and contractile properties of the involved muscles. We therefore, suggest a modified cost function that accounts for the instantaneous contraction velocity of the muscles and its effect on the instantaneous maximal force. With this novel objective function, a task-dependent prediction of muscle force distribution is obtained that allows, even in a one degree of Freedom system, the prediction of force sharing loops, and simultaneously increasing and decreasing forces for agonist pairs of muscles.
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Analytic analysis of the force sharing among synergistic muscles in one- and two-degree-of-Freedom Models.
Journal of biomechanics, 2000Co-Authors: Paul Binding, Azim Jinha, Walter HerzogAbstract:Abstract Mathematical optimization of specific cost functions has been used in theoretical Models to calculate individual muscle forces. Measurements of individual muscle forces and force sharing among individual muscles show an intensity-dependent, non-linear behavior. It has been demonstrated that the force sharing between the cat Gastrocnemius, Plantaris and Soleus shows distinct loops that change orientation systematically depending on the intensity of the movement. The purpose of this study was to prove whether or not static, non-linear optimization could inherently predict force sharing loops between agonistic muscles. Using joint moment data from a step cycle of cat locomotion, the forces in three cat ankle plantar flexors (Gastrocnemius, Plantaris and Soleus) were calculated using two popular optimization algorithms and two musculo-skeletal Models. The two musculo-skeletal Models included a one-degree-of-Freedom Model that considered the ankle joint exclusively and a two-degree-of-Freedom Model that included the ankle and the knee joint. The main conclusion of this study was that solutions of the one-degree-of-Freedom Model do not guarantee force-sharing loops, but the two-degree-of-Freedom Model predicts force-sharing loops independent of the specific values of the input parameters for the muscles and the musculo-skeletal geometry. The predicted force-sharing loops were found to be a direct result of the loops formed by the knee and ankle moments in a moment–moment graph.
Masahiro Wakatani - One of the best experts on this subject based on the ideXlab platform.
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Onset of intermittent thermal transport by ion-temperature-gradient-driven turbulence based on a low-degree-of-Freedom Model
Physics of Plasmas, 2004Co-Authors: K. Takeda, Sadruddin Benkadda, Satoshi Hamaguchi, Masahiro WakataniAbstract:Anomalous heat transport due to ion-temperature-gradient- (ITG)-driven turbulence is studied with the use of a low-degree-of-Freedom Model composed of 18 ordinary differential equations (ODEs). When the system is slightly above the stability threshold of ITG mode, the system is in the convection regime and convective heat transport of the system is time-independent or periodically oscillates. As the ion-temperature gradient is increased further, the system bifurcates to the turbulent regime. In the strongly turbulent regime, edge localized mode (ELM)-like intermittent bursts (so-called avalanches) are observed. This intermittency is caused by the competition of the following three factors: (1) generation of sheared flows and suppression of ITG turbulence, (2) gradual reduction of the sheared flows due to viscosity, and (3) rapid re-growth of ITG modes due to the reduction of the sheared flows. We found that the Nusselt number Nu scales with the ion pressure gradient Ki as Nu∝Ki3 in the presence of intermi...
