摘要 :
The classical theory of centralized dynamic characteristics with eight coefficients is adopted for traditional journal bearings. However, under cantilevered load, distributed dynamic characteristics along the axial direction will ...
展开
The classical theory of centralized dynamic characteristics with eight coefficients is adopted for traditional journal bearings. However, under cantilevered load, distributed dynamic characteristics along the axial direction will be generated for the journal bearing with a large aspect ratio (L/D). A double-section loading method simulating shaft bending and excitation was developed, a dynamic characteristic test-rig of a large-aspect-ratio bearing was set-up, the novel four-time vertical excitation method and eight-time cross excitation method were put forward, and the equations of 16 dynamic characteristic coefficients of the bearing were deduced. The dynamic characteristic test was carried out. The results showed that the four-time vertical excitation method had a small response amplitude in the horizontal direction, and was vulnerable to interference by the vibration of the test-rig. The eight-time cross excitation method had a higher SNR with more accurate identification results. When the cantilevered load was not applied, the dynamic characteristic coefficients were evenly distributed along the axial direction, the stiffness coefficients fluctuated slightly as the rotation rate increased, while the main damping coefficients decreased significantly. Shaft bending resulted in a significant increase in local dynamic characteristic coefficients, in which the relative increase in the stiffness coefficients was greater than that of the damping coefficients. Increasing the rotational speed can weaken the effect of shaft bending, and key factors that cause the axially nonuniform distribution of dynamic characteristic coefficients of the bearing are shaft bending and large aspect ratio under low speed and cantilevered load.
收起
摘要 :
The article presents calculations of static and dynamic characteristics and develops a computer model of trolley pole operation. It analyzes the factors affecting the interaction between the trolley pole and the contact wire.
摘要 :
This study presents temporal variations in concentrations of chlorophyll a (Chl a), particulate and dissolved dimethylsulfoniopropionate (DMSPp and DMSPd) in the sea ice and underlying water column in the Canadian Arctic Archipela...
展开
This study presents temporal variations in concentrations of chlorophyll a (Chl a), particulate and dissolved dimethylsulfoniopropionate (DMSPp and DMSPd) in the sea ice and underlying water column in the Canadian Arctic Archipelago during the spring of 2010 and 2011. During both years, bottom ice Chl a, DMSPp and DMSPd concentrations were high (up to 1328 μg L~(-1), 15,082 nmol L~(-1), and 6110 nmol L~(-1), respectively) in May and decreased thereafter. The release of bottom ice algae and DMSPp in the water column was gradual in 2010 and rapid (8 days) in 2011. Bottom brine drainage during the presnowmelt period in 2010 and a rapid loss of the snow cover in 2011 coinciding with rain events explain most of the difference between the 2 years. During both years, less than 13% of the DMSPd lost from the ice was detected in the water column, suggesting a rapid microbial consumption. An under-ice diatom bloom developed in both years. In 2010, the bloom was dominated by centric diatoms while in 2011 pennates dominated, likely reflecting seeding by ice algae following the faster snowmelt progression induced by rainfall events in 2011. Both under-ice blooms were associated with high DMSPp concentrations (up to 185 nmol L~(-1)), but pennate diatoms showed DMSPp/Chl a ratios twice higher than centrics. These results highlight the key role of snowmelt and precipitation on the temporal pattern of ice-DMSP release to the water column and on the timing, taxonomic composition, and DMSP content of phytoplankton under-ice blooms in the Arctic.
收起
摘要 :
The pipeline with a closed side branch is a typical structure in nuclear systems; the shear layer flow and deep cavity in this structure can lead to a complex flow-acoustic coupling, which may result in severe vibration and noise....
展开
The pipeline with a closed side branch is a typical structure in nuclear systems; the shear layer flow and deep cavity in this structure can lead to a complex flow-acoustic coupling, which may result in severe vibration and noise. To clarify the shear layer flow characteristics in this typical structure filled with water, the numerical strategy is established by employing the localized dynamic k-equation large eddy simulation model. Then, the statistical and instantaneous characteristics of the fluid field are analyzed, respectively. The dynamic vortex evolution process is presented by time-frequency domain analysis. Also, the phase relationship of pressure in shear layer is adopted to investigate the formation process of the shear layer mode. The results verify that the pressure fluctuation near the downstream corner is the sound source as the vortex impacts the wall periodically. The convection effect and disturbance intensity effect are separated for the first time through the control of the kinematic viscosity. The results show that the convection velocity influences the vortex shedding frequency (VSF) by changing the vortex moving speed. Also, changes in the disturbance intensity and branch length rectify the VSF by changing the phase relationship of impinging shear layer mode. Finally, a suppressing method of shifting the VSF is put forward by modifying the disturbance only.
收起
摘要 :
An agricultural tractor equipped with both front and rear axle suspensions offers a number of advantages when compared with a traditional tractor fitted only with seat, cab or front axle suspension. A soft suspension with low natu...
展开
An agricultural tractor equipped with both front and rear axle suspensions offers a number of advantages when compared with a traditional tractor fitted only with seat, cab or front axle suspension. A soft suspension with low natural frequencies normally offers better vibration damping capacity than a stiffer one, but needs more travel space to avoid over-travel. The restricted space is therefore the basic constraint when designing effective suspensions.
收起
摘要 :
Dynamical characteristics of tip vortices shed from a 1 m diameter, four-bladed rotor in hover are investigated using various aperiodicity correction techniques. Data are acquired by way of stereo-particle image velocimetry and co...
展开
Dynamical characteristics of tip vortices shed from a 1 m diameter, four-bladed rotor in hover are investigated using various aperiodicity correction techniques. Data are acquired by way of stereo-particle image velocimetry and comprises measurements up to 260 vortex age with 10 offsets. The nominal operating condition of the rotor corresponds to Re _c = 248,000 and M = 0.23 at the blade tip. With the collective pitch set to 7.2 and a rotor solidity of 0.147, blade loading (C _T/σ) is estimated from blade element momentum theory to be 0.042. The findings reveal a noticeable, anisotropic, aperiodic vortex wandering pattern over all vortex ages measured. These findings are in agreement with recent observations of a full-scale, four-bladed rotor in hover operating under realistic blade loading. The principal axis of wander is found to align itself perpendicular to the slipstream boundary. Likewise, tip vortices trailing from different blades show a wandering motion that is in phase instantaneously with respect to one another, in every direction and at every wake age in the measurement envelope.
收起
摘要 :
The harmonic vibration characteristics of a deeply buried spherical methane tank in viscoelastic soil subjected to cyclic loading in the frequency domain are investigated.Thedynamic behavior of the soil is described based on the t...
展开
The harmonic vibration characteristics of a deeply buried spherical methane tank in viscoelastic soil subjected to cyclic loading in the frequency domain are investigated.Thedynamic behavior of the soil is described based on the theory of fractional derivatives. By introducing potential functions, the closed-form expressions for the displacement and the stress of the viscoelastic soil surrounding the deeply buried spherical methane tank are obtained. Two die structures are considered: a homogeneous elastic medium and a shell structure. Based on the theory of elastic motion and the Flügge theory, analytic solutions for the dynamic responses of the spherical methane tank in a fractional-derivative viscoelastic soil are derived explicitly. Analytic solution expressions of the undetermined coefficients are determined by using the continuum boundary conditions. The system dynamic responses to the homogeneous elastic medium and the shell structure and the influences of the parameters of the fractional derivative, soil, and die on the dynamic characteristic of the system are compared and analyzed. The results indicate a significant difference between the dynamic responses of the die structures for the two models.
收起
摘要 :
Impinging Janus drops can be stably produced by adding a high-viscosity drop to a low-viscosity drop. Here, we investigate the dynamic features of bouncing Janus drops on a solid substrate for an exploration of the effects of the ...
展开
Impinging Janus drops can be stably produced by adding a high-viscosity drop to a low-viscosity drop. Here, we investigate the dynamic features of bouncing Janus drops on a solid substrate for an exploration of the effects of the viscosity ratio, initial drop shape, and impact velocity on altering the hydrodynamics. Numerical results show that the low-viscosity component evolves into liquid alignment along the principal direction with the help of a preferential flow, thereby resulting in the partial detachment from the mother Janus drops. We establish a regime map of the separation ratio of the drop and discuss how the parameters affect the asymmetry in the bounce and separation behavior. The low-viscosity components can be more likely to be detached from Janus drops as the viscosity ratio, drop's ellipticity, and/or impact velocity increase. This phenomenon is explained by the residence time and breakup of symmetry in the horizontal momentum between the low-and high-viscosity components. The peculiar dynamic characteristics of the Janus drop can provide potential for various applications, such as liquid purification and separation.
收起
摘要 :
The results of investigations of the structure and dynamics of the atmospheric planetary boundary layer in the transitional "ocean-continent" zone in the winter period, restored from the remote lidar soundings, are given. The char...
展开
The results of investigations of the structure and dynamics of the atmospheric planetary boundary layer in the transitional "ocean-continent" zone in the winter period, restored from the remote lidar soundings, are given. The characteristic parameters of PBL are ascertained, as well as ways of formation of its structure and dynamics. The distinctive features of PBL for a given region are shown by the examples of December 13 and 14, 2011. The mean PBL height, upper boundary of the convective layer, and the mean height of the stable layer in January and February are presented. The fact is grounded that the well-developed convection observed at nighttime is caused by existence of a low-level jet stream and high cooling altitudes of the planetary boundary layer.
收起
摘要 :
The structures and the dynamic characteristics of the lattices of two compositions of solid solutions of multiferroic BiFeO_3 with ferroelectric KNbO_3 and antiferroelctric NaNbO_3, namely, (1 –x)BiFeO_3–xKNbO_3 and (1 – x)BiFe...
展开
The structures and the dynamic characteristics of the lattices of two compositions of solid solutions of multiferroic BiFeO_3 with ferroelectric KNbO_3 and antiferroelctric NaNbO_3, namely, (1 –x)BiFeO_3–xKNbO_3 and (1 – x)BiFeO_3–xNaNbO_3, have been studied using X-ray powder diffraction and Raman spectroscopy. For these systems with x = 0.3, 0.5, and 0.7, the symmetry and unit cell parameters at room temperature have been determined. An analysis of the vibrational spectra has revealed sequences of rotational distortions with variations in the concentrations of components.
收起
原文获取