Increase In Damping Coefficient Research Articles

Investigation of static and rotordynamic characteristics of the segmented annular seals based on the differential quadrature method

Theoretically, a solution model for the static and rotordynamic characteristics of a segmented annular seal based on the local differential quadrature (LDQ) method was established. The impacts of node density and the number of local support domain nodes on the accuracy of the algorithm were analyzed. A test rig for the leakage of segmented annular seals was designed and built. The leakage results measured on the test rig were compared with the theoretical analysis results to demonstrate the accuracy of the local differential quadrature method. The influence of structural and operating parameters on the seal static and rotordynamic characteristics was analyzed Based on the work carried out above. The research results indicate that when the number of local support domain nodes is equal to 3, the static and rotordynamic characteristics of the segmented annular seal can be obtained accurately based on the solution model. As the rotational speed increases, the segmented annular seal’s lift force, leakage, stiffness coefficient, and damping coefficient increase. As the pressure ratio increases, the lift force and leakage of the segmented annular seal significantly increase while the stiffness and damping coefficients decrease. As the depth and width of the pocket increase, the lift force, stiffness coefficient, and damping coefficient of the segmented annular seal first increase and then decrease. As the length of the pocket increases, the lift force, stiffness coefficient, and damping coefficient of the segmented annular seal increase. The influence of the pocket depth, width, and length on the leakage is minimal.

Performance of rough secant slider bearing lubricated with couple stress fluid in the presence of magnetic field

The effect of surface roughness, magnetic field, and couple stress on the performance of secant slider bearing is studied in this paper. The effectiveness of the bearing is assessed by using a couple stress liquid as the lubricant in the process. Because of microstructural impacts, couple stress fluids have distinctive rheological characteristics. These features have the potential to make a major impact on the bearing’s performance. The novel aspect of this study is that the roughness of the secant slider is taken into consideration. The modified Reynolds equation is derived using Christensen’s stochastic theory for roughness. Here, both transverse and longitudinal roughness patterns are considered. The analytical solutions are obtained for steady-state pressure, load-carrying capacity, film stiffness, and damping coefficient. The results are presented for various parameters through graphs and tables. The combined effect of the magnetic field and couple stress is significant on bearing characteristics. It is noticed that the bearing features increase with the longitudinal roughness parameter, and the reverse phenomenon is observed with the transverse roughness parameter. When a transverse roughness pattern is assumed on a secant slider bearing, characteristics like pressure, load, stiffness of the film, and damping coefficient increase significantly.

Identification of the process damping coefficient in dry and wet machining of steel

Flank face chamfers are an effective way to suppress vibrations and increase the productivity of milling processes. The underlying process damping mechanism is the so-called indentation effect. The effect describes the process damping as a result of an additional force due to the indentation of workpiece material under the flank face. In literature, this force is commonly modeled by the volume indented under the flank face and a process damping coefficient. To determine the process damping coefficient, various approaches with partly contradictory results exist. In this paper, a novel method to calculate the process damping coefficient based on process forces measurements in orthogonal cutting is applied for steel machining. The method considers ploughing effects of flank face chamfer and cutting edge rounding as well as plastic deformation effects. In the current investigation, the approach is applied to different cooling strategies, chamfer widths, and cutting speeds. The results show that the cutting speed has the most significant influence on the process damping coefficient. With increasing cutting speed, the process damping coefficient increases, which can be attributed to strain rate hardening effects.

Study of the Influence of the Backplate Position on EMAT Thickness-Measurement Signals.

Nondestructive testing (NDT) is an essential method for assessing structural integrity in the oil and gas industry. Electromagnetic acoustic transducers (EMATs) have been extensively used to detect the wall-thickness reduction of plate-like structures, because they do not require direct contact. The pulse intervals of echoes are used to calculate the remnant thickness of structures. If the width of a single pulse is too large, multiple pulses will be superimposed, making it more difficult to extract the pulse interval. Thus, the width of a single pulse affects the resolution of measurements. This paper investigates the impacts of the backplate position on the pulse width and amplitude of thickness-measurement signals, using EMATs. By means of impedance modeling and measurement, it can be shown that the output impedance of the receiving coil is strongly influenced by the coil-backplate gap. With the increment in the coil-backplate gap, the signal amplitude and damping coefficient increase, while the self-resonant frequency decreases. By means of signal measurements on the specimen, it is shown that the pulse width and the signal amplitude can be significantly influenced by the backplate position. By reducing the coil-backplate gap, the pulse width can be reduced by over 80%, and by increasing the gap, the signal amplitude can be increased by over 300%. These research results can be used to optimize EMAT design, thereby suppressing the superposition of pulse echoes.

Effect of aluminum foam on pressure oscillation of premixed methane-air deflagrations

The effect of aluminum foam on the pressure oscillation of premixed methane-air deflagrations was investigated using experiments performed in an empty duct and a duct with aluminum foam attached to the upper and lower walls. For a fair comparison, the passage areas of the two ducts are identical when considering a 10 mm thickness of the porous medium. The flame propagation, pressure oscillation, and interaction mechanism between the acoustic wave and flame front are discussed by comparing the flame structure and pressure waveform during premixed methane-air mixture deflagration. The results demonstrate that for the empty duct configuration, the flame propagates as a cellular structure and then evolves into an oscillating flame when approaching the closed end of the duct. The coupling effect of the flame and acoustic waves induces the flame to oscillate violently as the distance from the ignition source to the closed end of the duct (i.e., the ignition position) increases to a threshold value. The magnitude of pressure oscillation increases as the distance between the ignition source and the closed end of the duct increases and reaches a maximum when the ignition position is 700 mm from the closed end. Additionally, the pressure oscillation is consistent with the flame oscillation. This oscillation results in a large deflagration pressure and a high rate of pressure rise. By contrast, the aluminum foam-laden duct completely suppresses the oscillation of the premixed methane-air deflagrations for all ignition positions. The aluminum foam inhibits the coupling effect and consequently restrains the prominent deformation and oscillation of the flame front. Flame oscillation vanishes as the acoustic damping coefficient increases. In this case, there is no oscillation and the leading position of the left flame exhibits a linear relationship with time. Whether the foam material has an inhibitive effect on the deflagration pressure depends on the ignition position. The aluminum foam enhances the deflagration pressure when the ignition position is 100 mm from the closed end, with a 29% increase in the maximum overpressure. By contrast, the aluminum foam attenuates the deflagration pressure for other ignition positions, with a maximum drop ratio of 61.7% at the ignition position being 700 mm from the closed end.

Dynamic analysis of a helical geared rotor-bearing system with composite rotating shafts

Purpose This study aims to study the dynamic characteristics of a helical geared rotor-bearing system with composite material rotating shafts. Design/methodology/approach A finite element model of a helical geared rotor-bearing system with composite material rotating shafts is developed, in which the rotating shafts of the system are composed of composite material and modeled as Timoshenko beam; a rigid mass is used to represent the gear and their gyroscopic effect is taken into account; bearings are modeled as linear spring-damper; and the equations of motion are obtained by applying Lagrange’s equation. Natural frequencies, mode description, lateral responses, axial responses, lamination angles, lamination numbers, gear mesh stiffness and bearing damping coefficients are investigated. Findings The desired mechanical properties could be constructed using different lamination numbers and fiber included angles by composite rotating shafts. The frequency of the lateral module decreases as the included angle of the fibers and the principal shaft of the composite material rotating shaft increase. Because of the gear mesh stiffness increase, the resonance frequency of the coupling module of the system decreases, the lateral module is not influenced and the steady-state response decreases. The amplitude of the steady-state lateral and axial responses gradually decreases as the bearing damping coefficient increases. Practical implications The model of a helical geared rotor-bearing system with composite material rotating shafts is established in this paper. The dynamic characteristics of a helical geared rotor-bearing system with composite rotating shafts are investigated. The numerical results of this study can be used as a reference for subsequent personnel research. Originality/value The dynamic characteristics of the geared rotor-bearing system had been reported in some literature. However, the dynamic analysis of a helical geared rotor-bearing system with composite material rotating shafts is still rarely investigated. This paper shows some novel results of lateral and axial response results obtained by different lamination angles and different lamination numbers. In the future, it makes valuable contributions for further development of dynamic analysis of a helical geared rotor-bearing system with composite material rotating shafts.

Controlled motion of viscoelastic fiber-reinforced magnetostrictive sandwich plates resting on visco-Pasternak foundation

This article presents a controlled motion investigation of viscoelastic/fiber-reinforced/magnetostrictive/sandwich plates supported via visco-Pasternak foundations. The core of the plate is modeled as a Kelvin-Voigt viscoelastic model. The governing dynamic system is derived using Hamilton’s principle according to simple sinusoidal shear deformation plate theory. A simple feedback control system is utilized to control the structural vibration. Navier’s type solution of the system is obtained to study the influence of significant parameters, especially, the effect of the general thickness ratios, aspect ratio, the thickness ratio of the magnetostrictive layer to viscoelastic layer, sequence staking, foundations, viscoelastic structural damping coefficient, the magnitude of the feedback coefficient, and location of magnetostrictive layers on the vibrational behavior of studied sandwich plate. A comparison with previous studies is performed for validation of the corresponding numerical results and present formulation. Numerical outcomes indicate that increasing the thickness ratio of the viscoelastic layer to the magnetostrictive layer leads to improvement of the system vibration control, as well as the damping coefficient, grows and the damped natural frequency decreases with the increase of viscoelastic structural damping coefficient. The study maybe helps the designers and engineers in development of structural control systems in several applied fields.

DYNAMICS MODELING, SIMULATION, AND CONTROL OF ROBOTS WITH FLEXIBLE JOINTS AND FLEXIBLE LINKS

The effects of flexible joints on the dynamic response and control of robot are studied in this paper. Firstly, the spatial robot model consisting of $n$ flexible joints and $n$ flexible links is built, and the dynamic equations of the robot system are derived via the Lagrangian's equations. The tensile deformation, bending deformation, torsional deformation, and nonlinear coupling deformation of the flexible link are considered. Furthermore, the effects of the flexible joint are also considered in order to provide an important theoretical basis for the research of the vibration suppression and control of robots. The flexible joint is simplified as a linear torsion spring with damping, and the mass effect of the flexible joint is also considered in the model. Secondly, the dynamic simulations of the spatial manipulators are done to explore the effects of the joint stiffness and damping on the dynamic response of the robot system. The results show that as the stiffness coefficient increases, the amplitude of dynamic response of the flexible robot decreases, and the vibration frequency of the system becomes larger. As the damping coefficient increases, the dynamic response of the flexible robot decreases, and the dynamic response decays faster. The vibration of the flexible robot can be suppressed by adjusting the values of the stiffness and damping of the flexible joint. Finally, in order to study the effects of the flexibility of the joint on the control system, the rigid-joint manipulator and flexible-joint manipulator are made to move under the same circular motion. Then the joint torques of the two system are obtained respectively by solving the inverse dynamics equations, and the influence of the flexibility of the joint on the dynamics control is studied. The results show that the actuating torques required in the flexible-joint system are reduced compared to that required in the rigid-joint system.

Analysis of temperature oscillations parameters of heat exchanging systems

The oscillations of temperature in the transient states of heat exchanging systems were investigated. The experiments were carried out using a heat exchanger model. The resistance heating cooper rod was an active side and an element (identical in shape) situated above them, but without contact, was the passive side of the model heat exchanger. During the experiments, the oscillatory character of the changing temperature of the passive element versus the active element was observed. The following parameters of these oscillations were investigated: frequency of free oscillations, damping coefficient and relative damping coefficient. The values of heat flux, distances between elements and their shapes (in pair) were changed and their influence on the parameters of the oscillations were investigated. The value of heat flux has the greatest impact on the values of all the examined oscillation parameters; there is an increase in the value of all the parameters along with the increase in the value of the heat flux. There was no influence of the shape of the element on the values of the investigated parameters. The values of the frequency of free oscillations and the damping coefficient decrease at an increasing distance between them, but the relative damping coefficient increases. These dependencies indicate the complex nature of the studied oscillations.

Numerical study of the surfactant-covered falling film flowing down a flexible wall

Despite the extensive literature on liquid falling films, the majority of them have focused on clean falling films on rigid walls. In this study, the falling film contaminated with insoluble surfactant on a flexible substrate is considered in the numerical simulations. The flow dynamics of film is studied by using the Navier–Stokes equations coupled with interface tracking approach. The surfactant concentration is calculated according to the ratio between surfactant mass and the surface area of film. The fully developed solitary-like wave accompanied with front capillary ripples is obtained. The results show that the presences of interfacial surfactant and flexible substrate have a considerable influence upon the dynamics of falling film. The surfactant, which acts to reduce the celerity of interfacial wave, can suppress the dispersion as well. The wall damping plays a role to deplete the deformation energy of short-wavelength deflection. As the wall damping coefficient increases, the proceeding capillary ripples of the interfacial humps can be suppressed, and the secondary circulation flow is intensified. The wall tension, which can inhibit the flexible wall fluctuation, tends to strengthen the proceeding capillary ripples and promote the celerity of interfacial solitary-like wave.

Longitudinal Vibration of Deep Water Drilling Riser in Disconnected Mode

A drilling riser is a connecting channel in deep water drilling technology. The riser may undergo significant axial vibrations during disconnection, which in turn can lead to failure. In order to investigate the longitudinal vibration behavior of a drilling riser, we constructed a mathematical model based on the classical wave equations, solved using a finite-difference method. We have simulated the longitudinal vibration parameters for a riser under production conditions in the South China Sea as a function of location, top tension, damping coefficient, lower marine riser package (LMRP) weight, riser inner diameter, and riser length. The research results showed that longitudinal vibration increases with distance from the water surface to the LMRP and the vibration amplitude also increases with increasing top tension, riser inner diameter, and riser length, and decreases with increasing LAMP weight. As the damping coefficient increases, over time the vibration amplitude decreases compared with the initial value.

Performance of hydrostatic tilted thrust pad bearings of various recess shapes operating with non-Newtonian lubricant

Hydrostatic thrust bearings are an integral part of hydroelectric power stations. These bearings are usually designed to work under parallel operation, but tilt is inevitable due to manufacturing errors, assembly errors, structural vibrations and structural deformations. Owing to the advent of latest advancements in manufacturing techniques, any geometric shape of recess can be easily manufactured and the designer has a greater flexibility. The geometric shape of recess significantly affects the performance of a bearing. Therefore, the present study is aimed to numerically analyze the influence of the tilt and recess shape on the static and dynamic performance characteristics of the hydrostatic thrust pad bearing system having Rabinowitsch fluid model lubricant. The lubricant obeying Rabinowitsch fluid model with tilt makes the Reynolds equation highly non-linear therefore, finite element method is used to analyze. Three different types of recess shapes of equal area A¯b/A¯oc=4 have been analyzed to model hydrostatic thrust pad compensated by orifice compensator. The numerically simulated results indicate that the tilt angle significantly affects the dynamic and static characteristic parameters. The value of pocket pressure and fluid film reaction of a hydrostatic thrust pad bearing has been found to significantly decrease with tilt whereas the value of lubricant flow, fluid film stiffness coefficient and fluid damping coefficient increase with tilt.

Semi-analytical solution for three-dimensional transient response of functionally graded annular plate on a two parameter viscoelastic foundation

Abstract The three-dimensional transient analysis of functionally graded annular plates with arbitrary boundary conditions is carried out in this paper. The material properties of the FGM plate are assumed to vary smoothly in an exponential law along the thickness direction. The plate is assumed to rest on a two parameter viscoelastic foundation. A semi-analytical method, which integrates the state space method (SSM), Laplace transform and its inversion, as well as the one-dimensional differential quadrature method (DQM), is proposed to obtain the transient response of the plate. The state space method is used to obtain the analytical solution in the thickness direction. The differential quadrature method is employed to approximate the solution in the radial direction. The Laplace transform and the numerical inversion are used to obtain the solution in time domain. Numerical results show a good agreement between the response histories obtained by the present method and finite element method. The effects of the boundary conditions at the edges, the material graded index, the Winkler and shearing layer elastic coefficients, and the damping coefficient are studied. Numerical examples show that the peak response decreases as the material graded index, the Winkler and shearing layer elastic coefficients, and the damping coefficient increase. The results obtained in this paper can serve as benchmark data in further research.

Forward Speed Calm Water Roll Decay for Surface Combatant 5415: Global and Local Flow Measurements

Global and local flow measurements for forward speed calm water roll decay are performed in a towing tank for surface combatant model 5415 for both bare hull (BH) and bilge keel (BK) conditions. Roll motion is oscillatory with underdamped exponential decay and linear with respect to initial roll angles less than the average initial roll angle (9°). For larger mean roll angles (> 3°), damped natural frequency is a few percent below the hydrostatic natural roll frequency with larger values for BH than BK condition and increasing Froude number (Fr), whereas for smaller mean roll angles, it sharply increases toward the hydrostatic natural roll frequency. For larger mean roll angles, logarithmic decrement and linear damping linear increase with larger values for BK than BH condition and increasing Fr, whereas for smaller mean roll angles, they sharply increase. For increasing Fr, mean and initial roll angle averaged mean roll angle decreases by 50%; damped natural frequency increases by 6% with larger values for BH than BK condition; and logarithmic decrement/linear damping coefficient increase by a factor of three/four with larger values for BK than the BH condition. Logarithmic decrement and Himeno method linear damping coefficients are qualitatively similar. Nonlinear damping coefficient is two orders of magnitude smaller than linear damping coefficient. Roll reconstruction errors are smallest for Himeno with linear and nonlinear damping. The phase-averaged wave pattern and velocity and axial vorticity fields at x/L = 0.675 initially show larger amplitudes followed by oscillatory exponential decay. Alternating vortex pairs are shed from the bilge keel tip with damped magnitudes for decreasing mean roll angles. The local flow indicates lower frequencies and larger damping than the roll motion.

Forward Speed Calm Water Roll Decay for Surface Combatant 5415: Global and Local Flow Measurements

Global and local flow measurements for forward speed calm water roll decay are performed in a towing tank for surface combatant model 5415 for both bare hull (BH) and bilge keel (BK) conditions. Roll motion is oscillatory with underdamped exponential decay and linear with respect to initial roll angles less than the average initial roll angle (9°). For larger mean roll angles (> 3°), damped natural frequency is a few percent below the hydrostatic natural roll frequency with larger values for BH than BK condition and increasing Froude number (Fr), whereas for smaller mean roll angles, it sharply increases toward the hydrostatic natural roll frequency. For larger mean roll angles, logarithmic decrement and linear damping linear increase with larger values for BK than BH condition and increasing Fr, whereas for smaller mean roll angles, they sharply increase. For increasing Fr, mean and initial roll angle averaged mean roll angle decreases by 50%; damped natural frequency increases by 6% with larger values for BH than BK condition; and logarithmic decrement/linear damping coefficient increase by a factor of three/four with larger values for BK than the BH condition. Logarithmic decrement and Himeno method linear damping coefficients are qualitatively similar. Nonlinear damping coefficient is two orders of magnitude smaller than linear damping coefficient. Roll reconstruction errors are smallest for Himeno with linear and nonlinear damping. The phase-averaged wave pattern and velocity and axial vorticity fields at x/L = 0.675 initially show larger amplitudes followed by oscillatory exponential decay. Alternating vortex pairs are shed from the bilge keel tip with damped magnitudes for decreasing mean roll angles. The local flow indicates lower frequencies and larger damping than the roll motion.

Bifurcation and chaos analysis of multistage planetary gear train

A nonlinear time-varying dynamic model for a multistage planetary gear train, considering time-varying meshing stiffness, nonlinear error excitation, and piece-wise backlash nonlinearities, is formulated. Varying dynamic motions are obtained by solving the dimensionless equations of motion in general coordinates by using the varying-step Gill numerical integration method. The influences of damping coefficient, excitation frequency, and backlash on bifurcation and chaos properties of the system are analyzed through dynamic bifurcation diagram, time history, phase trajectory, Poincare map, and power spectrum. It shows that the multi-stage planetary gear train system has various inner nonlinear dynamic behaviors because of the coupling of gear backlash and time-varying meshing stiffness. As the damping coefficient increases, the dynamic behavior of the system transits to an increasingly stable periodic motion, which demonstrates that a higher damping coefficient can suppress a nonperiodic motion and thereby improve its dynamic response. The motion state of the system changes into chaos in different ways of period doubling bifurcation, and Hopf bifurcation.

Parametric instability prediction in a top-tensioned riser in irregular waves

The parametric instability of a top-tensioned riser (TTR) in irregular waves was predicted based on multi-frequency excitation. As the exploration of oil and gas moves into deeper water and the corresponding platforms encounter continuous wave loads, parametric resonance becomes an increasing challenge for TTR design. TTRs may suffer damage due to excessive stress or significant fatigue because of parametric resonance. Studies of the stability of long slender structures have been limited to regular waves, with an emphasis on explaining parametric resonance corresponding to simplified situations in which the related displacement or tension fluctuation is assumed to be an ideal harmonic signal. Waves are irregular in real sea conditions, therefore, the present study proposes a methodology to investigate the instability properties of TTRs in irregular waves. Hill's equation of a TTR system under multi-frequency excitation is derived based on linear-wave theory using a Pierson–Moskowitz wave spectrum. Analytic results are given for the stability limit of the related Hill's equation and they describe the stability of a TTR under irregular wave excitation. These new limits are compared to Mathieu-type stability limits. The stability diagram for Hill's equation with random phase modulation in multi-frequency excitation is obtained. To evaluate the proposed method, an example of the application of the stability diagram to TTR design is provided. Simulation results are compared under two types of wave excitation: single-frequency excitation and multi-frequency excitation, generated according to specific sea spectra. Parametric stability properties of a TTR system predicted by regular wave and irregular wave theory are different from each other. Although the transition curves both move to the upper zone of the parametric plane and the region of unstable zones shrink when the damping coefficient increases, the impact of damping is different. The results show that it is necessary to predict the parametric stability of TTR using irregular waves. Applying the stability diagram to a TTR in irregular waves shows that the introduction of extra damping to an unstable system can suppress the instability. In addition, the single-frequency excitation method predicts that the lower vibration mode is more likely to be excited, whereas the multi-frequency excitation method predicts that the higher vibration mode is more likely to fall into an unstable zone.

Damping of Stay Cable-Passive Damper System with Effects of Cable Bending Stiffness and Damper Stiffness

In the present paper, the asymptotic solution of modal damping ratio of stay cable-passive damper system with the influence of cable bending stiffness and damper stiffness was derived. Maximum modal damping ratio and corresponding optimal damping coefficient, which indicated the relationships of the characteristics of the damper and the cable bending stiffness was theoretically analyzed to obtain their close solutions. On the basis of these close solutions, numerical analysis of modal damping of stay cable-passive damper system with the effects of cable bending stiffness and damper stiffness was conducted. The numerical and analytical results show that the maximum modal damping ratio decrease and the corresponding damping coefficient increase, when considering the influence of the damper stiffness and the cable bending stiffness.

J191061 正方形膜の振動・減衰特性に及ぼすしわ・亀裂の影響([J19106]構造・材料の高度化に向けた宇宙工学と材料力学の展開(6))

Wrinkle and damage on the membrane have an effect on dynamic characteristics. The vibration and damping properties of wrinkled and crack square membranes under the forces at the corner are experimentally investigated. The experiments are performed in vacuum chamber and the responses are measured by laser displacement meter. First, when the tensile forces are the equal, the results indicate that the natural frequency is quite similar to tensile cable and the damping coefficient increase as forces increase. Next, the experiments of membranes under the different tensile forces show that positional relationship between wrinkle and measurement point has effect on natural frequency and damping coefficient. Finally, the results indicate that the effect of crack on natural frequency and damping coefficient is relatively small.

Modal analysis on nanoclay epoxy-based fiber-glass laminates

This paper focuses on the vibration analysis of fiber glass/epoxy/nanoclay nanocomposites. The modal analysis was performed using a grid of 35 response measurement points. For modes 1 and 7, where the bending-twisting shape mode is observed, the damping coefficient increases by about 27%, on average. For modes 2 through 6, where bending is noticed, the average increase is around 68%, with peaks of 169%. A new shape mode comes out at 600 Hz frequency when the nanoclay content reaches 2%. A crossing mode, mode 4 with inferior frequency than mode 3, for the 5% and 10% nanoclay content is also observed. For events where high frequencies are generated, the nanoclay content of 2% leads to a higher energy dissipation.