Applied Mechanics and Materials Vols. 226-228

Abstract: In this paper, we derive the universal expression of the modal parameters for a damaged beam under arbitrary boundary conditions. The delta function is first employed to describe a notch damage in the beam and consequently to derive the governing equation for the damaged beam. Second, by virtue of the perturbation method, the eigenvalues and the corresponding mode shapes are obtained for the damaged beam. Finally, numerical examples are given for an elastically supported beam. It is believed that the proposed approach could provide the necessary theoretical background for damage identification in beam structures.

Abstract: In modal testing, the measured Frequency Response Functions (FRFs) are often inaccurate due to the adverse mechanical effects, such as mass-loading effects of transducers, shaker-structure interaction and the support effects. This paper deals with the elimination of the support effects from measured FRFs using dynamic substructure method. Both stiffness and damping are considered in the support effects. The validity of the method is proved in a simulated modal test. It is shown that with the given support conditions the affected FRFs can be corrected provided that some additional FRFs concerned with the support point are also measured. Finally, performance of this method is assessed with noisy measured data. Simulation shows that 1% white noise is acceptable in the proposed correction method.

Abstract: A numerical simulation method with gas-structure interaction to analyze 3D complex flow in rotating machinery was presented and the effects with different aerodynamic turbulence model for gas-structure interaction was also presented. The blades are an important component in rotating machinery. Gas flow is unsteady three-dimensional turbulence motion with transient and anisotropic. Then the gas flow and the vibration of rotating blades interfere with each other, resulting in a complex coupling effect. It affects the machine efficiency directly. For discussing the effects on flow field of the coupling field, the blade model was built. And flow around the blades was simulated by gas-structure interaction with three turbulence models respectively. The turbulence models were standard κ-ε, renormalization group κ-ε and Smagorinsky LES. A feasible method was provided for flow field analysis in rotating machinery.

Abstract: Oil whip, which is one of the typical types of steam turbine failure, may lead to large amplitude vibration of the rotor and even to catastrophic accident. In order to prevent and on-line eliminate the oil whip, researches on vibration of steam turbine caused by the oil whip and its characteristics have been elaborated from two respects of theory and experiment. The causes and the formation mechanism of the oil whip have been analyzed, and in certain conditions, a series of experiments for the oil whip on the rotor experiment rig have been made. Illustrating and summarizing the results of these experiments by change the rotor speed and the lubricant quantity. These researches on the oil whip could provide some theoretical bases for the practical work. Along with the expounded prevention measures of oil whip, some ideas on oil whip online elimination are put forward.

Abstract: It’s important to identify structural modal parameters, especially, accurate modal shapes in time domain for accurate damage identification and health monitoring of structures. The natural excitation vibration is used to identify the structural modal parameters. The power spectral density curve has obtained according to the measuring point vibration time domain curve, and then natural frequencies and vibration value are determined by the curve peak. The reference point is taken for the vibration unit values to normalize other modal shapes. The modal shapes are calculated using the phase measuring point relative to a reference point. After the finite element analysis and calculation, and the measured modal is compared with it. The results show that the dynamic performance of the bridge is reliable because the measurement method and the finite element method are basically consistent. The damping ratio of the bridge is larger dispersion but the average damping ratio is in line with the rules and scope of the cable-stayed bridge.

Abstract: The presented paper deals with the group delay in the digital filter induced instability of a two dimensional airfoil section active flutter suppression system. Firstly, the aeroelastic model of the airfoil with an ultrasonic motor actuated control surface is set up; secondly, both H_∞ and μ robust controllers are designed; and then, the group delay induced instability in wind tunnel test is presented; finally, through a combined theoretical and numerical study, the test phenomenon is well explained. Wind tunnel experiments and numerical simulations demonstrate that long enough group delay in digital filter can induce instability of flutter control system, the flutter under control will decrease first, and then become another flutter of lower frequency and moderated amplitude, and μ controller works better than H_∞ controller on the same condition.

Abstract: This paper puts forward pseudo-response spectrum method for calculating the dynamic response of structure caused by harmonic load on the floor, and proposes to use frequency ratio as a new modal truncation index. Combined with engineering practice, the analysis results of pseudo-response spectrum method were compared with the results of existing dynamic coefficient method and elastic time-history method. The contrast results show that dynamic coefficient method due to lacking of considering dynamic properties enough was not able to meet the real response and could not evaluate the comfort of the vibration. The results of pseudo-response spectrum method with suitable mode combination method have the same law of the maximum enveloping value as elastic time-history method. This proved that pseudo-response spectrum method is reasonable, safe and economical, which can provide the basis for engineering design.

Abstract: A two-phase wave equation of nearly saturated soils is proposed based on Biot's wave equations and Bardet's assumption regarding nearly saturated soils. Applying Helmholtz vector decomposition and the transmission reflection matrices method, the general solutions to the governing equations for the nearly saturated soils are obtained. Based on the general solutions and the fictitious pile methodology, the Fredholm integral equation for the second kind describing interaction between the pile and nearly saturated soils is established. Solution of the integral equation yields the dynamic response of a pile embedded in nearly saturated soils subjected to a top harmonic vertical loads. Results of this paper are compared with existing results, which shows that our solutions are in a good agreement with existing results. The numerical results of this study demonstrate that the saturation has a significant influence on the dynamic response of a pile in nearly saturated soils.

Abstract: In the present paper, the delayed feedback control is applied to suppress the amplitude of the vibration of a beam. The method of multiple scales is employed to obtain the analytical solutions when the primary resonance and 1:2 internal resonance occur simultaneously. The predictions from analytical solutions agree with the numerical simulations well. The analytical results show that the amplitude of the beam of the saturation control is much larger than its amplitude of the single-mode motion. The effects of the delayed feedback control on amplitude of the beam are investigated when the original system is in the saturation control. There is a tunable range of the delay could be used to suppress the amplitude of the beam for a fixed value of the gain. The amplitude of the beam can be suppressed from 0.20 to 0.10 when the gain and the delay are chosen appropriate values.

Abstract: A method which combines the estimation of machinery exciting force and mechano-acoustical transfer function is proposed in this paper to predict real-time radiated noise by ship structure. The measurement methods of mechano-acoustical and mechano-vibrational transfer functions which are used in radiated noise calculation are described. The feasibility and accuracy are verified by an underwater model experiment. Measurement and calculation results are consistent. The average error of the exciting force estimation is about 2dB, and the average error of radiated noise pressure calculation is about 3dB. The accuracy of the radiated noise calculation is directly affected by exciting forces estimation errors, which can be corrected by a correction factor to increase the accuracy of radiated noise calculation effectively.