Journal of Engineering and Thermal Sciences
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Phase-controlled vibrational laser percussion drilling
Get PDFIn this study, a phase-controlled vibration was applied to a laser percussion drilling process to improve the depth of penetration. To investigate the effects of phase-controlled vibration on the depth of penetration, a novel method that controls the phase offset between the accelerating motion and the emission of the laser beam was developed. The method is based on coaxial sensing of the working surface using a photodiode, coupled with microcontroller control of the drilling laser operation. Through real-time optical signal acquisition and analysis of laser machining processes, correlations between the accelerating motion and the emission of the laser beam were simultaneously obtained. All of the processing work was performed in air at standard atmospheric conditions, and gas assist was not used. This study showed that the application of phase-controlled vibration improved the depth of penetration in laser percussion machining and can contribute to the development of precision drilling in the industry
Experimental research on the dynamic characteristics of gas-hybrid bearing-flexible rotor system
Get PDFHigh-speed gas bearings for turbomachinery can dramatically improve the isentropic efficiency of turbine plants. Vibrations, especially low-frequency vibrations caused by the gas-film self-excited vibrations, significantly affect the safety and economy of the turbomachinery, which is dangerous at high rotational speeds. This experimental study focuses on the dynamic characteristics of a flexible rotor supported by gas hybrid bearing and driven by a radial turbine with high-pressure air. Cylindrical whirling, conical whirling, and first-order bending natural frequencies that change with bearing supply pressures were obtained by experimental modal analysis. Nonlinear mechanic behaviors, including gas film half-speed whirling and gas-film whip, in the experiments were identified by nonlinear measurements and analyses. The occurrence mechanism of gas-film whirling and whip is also discussed in this paper. Experiments for eliminating and restraining whirling and whip were conducted to investigate the gas bearing-rotor system stability effect of bearing supply pressure and elastic foundation. Some simple and effective measures were given to improve the stable rotational speed
Influence of seismic effect of bridge piles on a subway station
Get PDFAs separated platform subway station structures are close to bridge piles, their analysis under static and seismic loading is required. An artificial earthquake record, generated by using Kanai-Tajimi model and trigonometric series superposition method, is used to analyze the seismic performance of the subway station structure. Different construction sequences of bridge piles and subway stations are considered in the analysis of structural seismic performance. Irrespective of the type of analysis, most of the horizontal and the vertical stresses of the subway station with existing bridge piles show a decreasing trend as compared to those without bridge piles. The stresses associated with post-constructed bridge piles increase significantly as compared to those without piles, especially in areas of stress concentrations. A parametric analysis of the distance between the subway station and the bridge pile is also conducted. The percentage increase in horizontal and vertical stress of the subway station, with post-construction pile, gradually decreases with increasing distance. However, in case of existing piles the percentage increase in stress, as a function of distance, has an inflection point
Online milling tool condition monitoring with a single continuous hidden Markov models approach
Get PDFThe health status evolving from normal to broken condition of a milling tool is needed as an object of assessment in condition-based maintenance. This paper proposes continuous hidden Markov models (CHMM) to assess the status of the tool online based on the normal dataset in the same case. A wavelet-packet decomposition technology is used to feature extraction and the CHMM is trained by Baum-Welch algorithm. Finally, we compute the log-likelihood based on the trained CHMM for abnormal detection and health assessment in real time during the milling process. A case study on tool state estimation demonstrates the effectiveness and potential of this methodology
Vibration characteristics and environmental responses of different vehicle-track-ballast coupling systems in subway operation
Get PDFThe vibration characteristics of two ballast beds are analyzed in this study from five aspects including the amplitude-frequency characteristic curve of foundation reaction. This study also shows that the maximum ground Z vibration level caused by a normal monolithic ballast bed structure is 75 dB. The range of its vibration influence during daytime is approximately 30 m. The maximum ground Z vibration level caused by a rubber floating slab track structure is 52 dB, whereas that caused by a steel spring floating slab track structure is 57 dB. The maximum damping amount in horizontal speed of a rubber floating slab track structure is 74 %, whereas the reduction of vertical ground vibration speed and acceleration is 92 % and 93 %, respectively. The reduction in Z level is 37 %. The horizontal speed reduction in a steel spring floating slab track structure is 71 %, whereas the reduction of ground vertical vibration speed and acceleration is 83 % and 84 %, respectively. The reduction in Z level is 29 %
Aerodynamic simulation of wind turbine blade airfoil with different turbulence models
Get PDFThe different turbulence models have significant impacts on the aerodynamic performance of wind turbine blade airfoil. A kind of wind turbine blade airfoil was applied as the research object, in order to analyze the impacts of three different turbulence models which are S-A, k-εRNG, k-ωSST on the aerodynamic performance of wind turbine airfoil under different attack angles. By comparing the aerodynamic simulation results with the theoretical values of the lift coefficients, drag coefficients and the ratio of lift coefficient to drag coefficient for the forecast of best angle of attack, the effects of these three turbulence models on the blade airfoil aerodynamic performance were estimated in detail. The simulation of lift coefficient of wind turbine blade airfoil was verified with the flow field simulation of blade airfoil. A combined turbulence model, using different turbulence model for different angle of attack, was put forward. The simulation results demonstrate that, for the selected blade airfoil, using S-A turbulence model before the best attack angle and k-εRNG turbulence model after the best attack angle respectively, can make the simulation of blade airfoil aerodynamic performance much more accurate than the aerodynamic performance simulation using one single turbulence model, with the acceptable iterative time and the acceptable ratio of lift coefficient to drag coefficient. Therefore, the combined turbulence model can overcome the shortcomings when using only a traditional single turbulence model to simulate the aerodynamic performance of wind turbine blade airfoil, which will have a development and application value in the future
Development of vibrating bracelet for the actuation of the blood circulation at capillaries
Get PDFAccording to Katsuzo Nishi proposed capillarity exercise based on limbs vibrations the equipment to compensate obstruction of blood circulation is proposed. This equipment is composed from the vibrating bracelet for disable people and simplify exercise execution for healthy people as well. A need to create small size and low voltage device was identified. Previous studies have showed that it is recommended to avoid high frequencies because of possible harm for human. According to this requirements two high RPM, small size and low voltage motors were chosen. The rotors unbalances are working in beating regime, which increases the excentrical forces actuating human limbs. After processing gathered data proper frequencies for vibrating bracelet were chosen
Identification of second-order kernels in aerodynamics
Get PDFVolterra series is one of the powerful system identification methods for representing the nonlinear dynamic system behavior. The methods of step response and impulse response are commonly applied to a discrete aerodynamic Computational Fluid Dynamic (CFD) to identify the first- and second-order Volterra kernels. A critical problem, however, is the difficulty of identifying the second-order Volterra kernels correctly in CFD-based method. In this paper the second-order Volterra kernel function is expanded in terms of Chebyshev functions to reduce the size of the problem and the accuracy of the identification is also improved based on a third-order reduced model of Volterra series
Fuzzy semi-active control for seismic response reduction with magnetorheological dampers
Get PDFBased on the dynamic characteristics obtained with a three-dimension finite element method, a reduced mechanical model and parameters for the ship lift can be modified and obtained. Simulated analysis shows that the reduced mechanical model can adequately represent the dynamic characteristics and earthquake responses. The reduced mechanical model can be used to control the seismic whiplash effect of the top workshop of the ship lift with intelligent control methods. A modified Bingham model was proposed to simulate the machinery property of a magnetorheological smart damper. A roof intelligent isolation system was also proposed. Seismic simulation analysis of the ship lift was conducted using a fuzzy control strategy. Simulation analysis results show that the fuzzy semi-active control with a magnetorheological smart damper is beneficial in suppressing the seismic whiplash effect on the top workshop and confirm that the fuzzy semi-active control strategy is valid in this scenario
Seismic demand for eccentric wall structures subjected to velocity pulse-like ground motions
Get PDFThe elastic and inelastic seismic demand of shear wall structures, with stiffness, strength and combined-stiffness-and-strength eccentricity, subjected to velocity pulse-like ground motions are investigated. Based on the axial load-bending moment interaction model and eight pulse-like ground motions, nonlinear dynamic time history analyses are conducted to single-story RC eccentric wall structures. The seismic demand is discussed in terms of the displacement, floor rotation and ductility, and the influence mechanism of different eccentricity types is revealed. The results show that the eccentric systems for pulse-like cases experience much higher elastic and inelastic seismic demand comparing to those for non-pulse-like cases. The axial compression ratio has certain effect on the inelastic seismic demand. The stiffness eccentricity is the key factor to the elastic seismic demand, while the strength eccentricity influences the inelastic seismic demand most. It is suggested that the strength eccentricity be added as a parameter in the inelastic analysis of eccentric structures, and the influence of axial load as well as velocity pulse-like effect of ground motions also be accounted in