Journal of Vibroengineering
Not a member yet
3189 research outputs found
Sort by
Improving the rotordynamic stability of short labyrinth seals using positive preswirl
Get PDFIntroducing a negative preswirl at the upstream of annular gas seals has been considered as an effective way to improve the system stability. This paper demonstrates a stability enhancement approach for a short labyrinth seal using positive preswirls. The static and dynamic characteristics of the labyrinth seal with various blade numbers (5, 10, 15), inlet preswirl ratios (–0.3, –0.15, 0, 0.15, 0.3) were studied. Results show that the inlet preswirl ratio has a dramatic effect on the circumferential location of the high-pressure spot for each seal cavity, particularly for the first cavity. The inlet preswirl ratio has opposite effects on the system stability due to the difference of high-pressure spot locations between the first cavity and the others. An increasing positive inlet preswirl could improve the system stability for the labyrinth seal with fewer blades (e.g. 5 blades). Its characteristics is mainly dominated by the first seal cavity. For the labyrinth seal with 10 blades, the system characteristics shows slight dependency on the inlet preswirl ratio. For the labyrinth seal with more blades (e.g. 15 blades), the negative inlet preswirl still increases the system stability, which agrees with the conventional conclusion. The paper provides a deeper understanding on the stability improvement of the labyrinth seal
Design of a self-tunable, variable-length pendulum for harvesting energy from rotational motion
Get PDFIn this paper, a self-tunable energy harvester based on pendulum oscillations with a mechanical motion rectifier (MMR) system, which can convert vibration into electrical energy, is proposed. The harvester is composed of a pendulum excited by a slider-crank mechanism. The pendulum system is designed to automatically adjust its own natural frequency to match that of the imposed base excitation. Frequency adjustment in a proposed pendulum-type energy harvester is achieved by varying the length of the pendulum rod through changing the position of pendulum mass which mounted at its tip. The pendulum mass is driven by a ball screw through a stepper motor which controls the length of the pendulum automatically in accordance with the frequency of the external motion. The base motion frequency is detected by an infrared sensor. An ultrasonic distance sensor is used to detect the length of the pendulum rod and feeds this information to a microcontroller to obtain the corresponding natural frequency from a lookup table. The microcontroller calculates the frequency difference between natural frequency and excitation frequency and converts this value into a length difference through another lookup table. The microcontroller then gives instructions to drive a stepper motor through a sequence of steps to achieve the target length and keeps the device in resonance state to harvest maximum power during operation. Different time detection intervals were studied to investigate their effect on the tuning process. This study showed that the longer time intervals increase the detection accuracy for the calculation of low excitation frequency. The amount of energy consumed during the tuning process to adjust the pendulum length is presented. In this context, the consumed energy is only needed until the resonance of the device matches the excitation frequency. The harvester system was studied numerically and experimentally. Based on the findings of this work, the natural frequency of the harvester is successfully tuned below 0.7 Hz, when the length of pendulum rod is changed from 550 mm to 900 mm, generating power from 1.78 W to 4.1 W at an optimal load resistance value of 10 Ω and 3 Ω respectively at maximum excitation amplitude of 120 mm. Therefore, the proposed pendulum system can be used as an efficient harvester for producing power in low-frequency applications (< 1 Hz)
Effects of fatigue on biomechanics of forehand smash in badminton
Get PDFSports fatigue will cause deformation of actions. In this study, badminton was analyzed, and the influence which was brought by fatigue was studied from the perspective of biomechanics. The forehand smash of professional badminton athletes under normal and fatigue states was tested. The biomechanical indexes of athletes were obtained by infrared remote shooting test system and plantar pressure test system for comparison. The results showed that the forehand smash effect of athletes was significantly worse under fatigue state, the maximum gravity center height decreased to 1.22 ± 0.14 m, and the maximum gravity center speed decreased to 2.33 ± 0.12 m/s, which showed significant decreases compared to the normal state (P< 0.05). Moreover the maximum velocity of the right upper arm, right forearm, elbow joint, wrist joint and knee joint decreased significantly (P< 0.5), the maximum angle of the right hip joint, knee joint and left ankle joint also decreased significantly. The pressure of the inner side of the forefoot and the pressure in the middle of the forefoot increased (P< 0.05). The biomechanical analysis of forehand smash under fatigue condition reveals the relationship between fatigue and movement, which provides some scientific bases for the reasonable control of sports load
Modelling of extended de-weight fuzzy control for an upper-limb exoskeleton
Get PDFPerforming heavy physical tasks, overhead work and long working hours are some examples of activities that can lead to musculoskeletal problems in humans. To overcome this issue, automated robots such as the upper-limb exoskeleton is used to assist humans while performing tasks. However, several concerns in developing the exoskeleton have been raised such as the control strategies used. In this study, a control strategy known as the extended de-weight fuzz was proposed to ensure that the exoskeleton could be maneuvered to the desired position with the least number of errors and minimum torque requirement. The extended de-weight fuzzy is a combination of the fuzzy-based PD and fuzzy-based de-weight controller systems. The extended de-weight fuzzy was then compared with the fuzzy-based PD and PID controllers, and the performances of these controllers were compared in terms of their deviations and required torques to perform tasks. The findings show that the proposed control strategy performs better than the fuzzy-based PD and PID controller systems
The influence of the controlling delay time on two-degree-of-freedom system with a high-static-low-dynamic-stiffness isolator
Get PDFIn order to research the influence of the control delay time on the vibration isolation system, the dynamic characteristic analysis of a two-degree-of-freedom vibration system, based on high-static-low-dynamic-stiffness vibration isolator (HSLDS-VI) with time-delayed feedback control is presented. The two-degree-of-freedom nonlinear vibration isolation dynamical system model comprising the delay time parameter is also derived. Then, the effects of feedback gain coefficient and delay parameter on the dynamic characteristics are analyzed with multiple scales method. The results demonstrate that the amplitude can be suppressed by the time-delay variation for a fixed feedback gain. The system can exhibit periodic motion or chaotic vibration when the feedback gain coefficient and time-delay parameter are adjusted, which is beneficial to the application of high-static-low-dynamic-stiffness vibration isolator
Effects of blunt trailing-edge optimization on aerodynamic characteristics of NREL phase VI wind turbine blade under rime ice conditions
Get PDFTo reduce the adverse effects of the ice on aerodynamic characteristics, a new NREL Phase VI wind turbine blade which is suitable to rime ice environments is developed through the blunt trailing-edge optimization. The parametric control equations of blunt trailing-edge airfoil are established by adopting the airfoil profile integration theory and B-spline curve, and the curve fitting of the airfoil’s rime ice from LEWICE software is carried out using the linear interpolation algorithm with equidistant and equiangular step lengths. The S809 airfoil under rime ice conditions is optimized to maximize the lift coefficient by the particle swarm optimization (PSO) coupled with GAMBIT and FLUENT, and a NREL Phase VI blade is formed with the optimized airfoil S809-BT (with BT the blunt trailing-edge). The blade’s rime ice is obtained through using the polynomial fitting to deal with projection point coordinates of airfoils’ ice shapes in lagging and flapping surfaces, and the pressure coefficient, flow characteristics, torque and output power of icy sharp and blunt trailing-edge blades are investigated. The results indicate that in rime ice conditions, compared with those of sharp trailing-edge blade, the pressure difference and vortex size of blunt trailing-edge blade become larger, and the torque and output power increase by 4.36 %, 1.55 % and 2.88 % at v= 7 m/s, 15 m/s and 20 m/s, respectively. The research provides significant guidance for improving the aerodynamic performance of wind turbine blade considering the icing effects
Dynamic responses of the flexible beam in a three-axis centrifugal environment
Get PDFThe loads acting on rapid maneuver aircrafts are characterized by high accelerations, high jerks and multiple directions in the space. The flight simulation tests for aircraft loads are usually carried out on a three-axis centrifuge. Due to the difference between flight and centrifugal environments, the loads in the simulation tests are not completely consistent with those in the actual flight environment. To verify that the dynamic responses of aircrafts in the three-axis centrifuge can be used to predict the responses in the flight environment, a beam installed in the three-axis centrifuge is considered. The velocity and acceleration models of the beam are established by the motion synthesis method. The rigid-flexible coupling dynamic equations of the beam are derived using the Kane’s method. Under different flight accelerations, the dynamic responses of beam in the three-axis centrifugal environment are simulated, which agree well with the responses in the flight environment. Besides, the influence of accelerations and jerks on the responses is analyzed. The results of this paper demonstrate that the present dynamic model can be used effectively to predict the experimental results in flight environments
Ride comfort performance of hydro pneumatic isolation for soil compactors cab in low frequency region
Get PDFThe hydro pneumatic isolation (HPI) of the cab combined by the high static stiffness and nonlinear viscous damping of the pneumatic isolation; and nonlinear adjustable damping of the hydraulic isolation are proposed. Based on the simulation and experimental studies, a nonlinear dynamic model of the soil compactor interacting with the deformable terrains is built to analyze the low frequency ride comfort of the HPI. The HPI’s performance for improving the ride comfort and health of the driver is evaluated via both the power-spectral-density and root-mean-square of acceleration responses of the driver’s seat heave, pitching and rolling cab angles. The research results show that the HPI’s characteristics with high static stiffness and nonlinear damping have an obvious impact on reducing low frequency vibration and controlling the cab shake of the vehicle in comparison with the traditional rubber mounts
Influence of vertical and horizontal whole-body vibration on selected muscles tension of employees age 50+ in relation to professional exposure to vibration (pilot study)
Get PDFThe pilot study on changes of selected muscles tension caused by vertical and horizontal whole-body vibration focused on employees belonging to two professional (occupational) groups. One of these groups consisted of men aged 50 years professionally exposed to vibration and the second were men not exposed to vibration at the workplace. Tests have been conducted in two series on special designed laboratory test bench for simulation of exposure to whole-body vibration. During the I series of tests, vertical vibration had acted on subjects and during II series horizontal lateral vibration had acted. The EMG signals were registered from muscles of the shoulder girdle and lower back, both sites of the body. Muscles tension values obtained during I and II series indicates that there are no statistical significant differences between reaction of subjects on vertical and horizontal vibration. However slightly lower EMG RMS values during II series were noticed. Exposure to whole-body vibration may cause changes in muscle tension both in employees (aged 50 years and older) occupationally exposed to vibration and not exposed to vibration. The observed changes vary in both groups of subjects. In occupationally exposed to vibration subjects the impact of exposure to vibration on muscle tension is less than in reference group
A new lateral load pattern for pushover analysis of asymmetric-plan structures
Get PDFPushover analysis has become one of the most commonly used nonlinear static procedures (NSP) for the seismic estimation of structures in engineering practice. In this paper, a new lateral load pattern is presented to enhance the accuracy of conventional pushover analysis (CPA) procedures for evaluating seismic behaviors of asymmetric-plan multistory buildings, which has considered the effect of torsion and higher modes. This spatially distributed lateral load pattern is proposed eventually by introducing and calculating the coefficients of adjustment and distribution respectively. The performance and accuracy of the proposed spatial pushover analysis (SPA) procedure is verified against two distinct multistory buildings with irregular plans subjected to five medium-to-strong ground motions. Furthermore, the peak responses in terms of base shear, roof displacements, interstory drifts, plastic hinge rotations and pushover capacity curves obtained from the SPA method are compared with those from nonlinear time history analyses (NTHA), and CPA procedures. The comparative results indicate that new lateral load pattern agrees very well with the NTHA procedure. The proposed SPA procedure shows its efficiency and overcomes the limitations of current extended pushover methods to assess the seismic responses of asymmetric-plan structures. It is strongly suggested that the new load pattern as an applicable method for pushover analysis of asymmetric-plan structures