Papers by Keyword: Instability

Abstract: An electrified liquid sheet injected into a dielectric moving through a viscous gas bounded by two horizontal parallel flat plates of a transverse electric field is investigated with the linear analysis method. The liquid sheet velocity profile and the gas boundary layer thickness are taken into account. The relationship between temporal growth rate and the wave number was obtained using linear stability analysis and solved using the Chebyshev spectral collocation method. The effects of the velocity profile on the stability of the electrified liquid sheet were revealed for both sinuous mode and varicose mode. The results show that the growth rate of the electrified Newtonian liquid is greater than that of corresponding Newtonian one under the same condition, and the growth rate of the sinuous mode is greater than that of the varicose mode. Keywords: instability; planar liquid sheet; velocity profile;spectral method;linear analysis

Abstract: Nowadays the disadvantages of large power grids are becoming more and more conspicous. Scholars have come to realize the importance of micro-grids. As the supplement of large power grids, the instability of micro-grids has gathered a huge amount of attention. This paper proposes a solution of the instability problem of micro-grids based on the research on AC-DC power flow. The main idea is to add AC power flow and storage power, which features stable voltage, stable current and stable output, in the micro-grids, thus solving voltage and current fluctuation and other instability problems in micro-grids.

Abstract: Under stretch-bending conditions where the curvature is relatively large, the plane stress assumption leads to a significant inaccuracy in forming limit prediction. In processes where a contact pressure acts on the material combined with a small punch radius it is observed by FE simulations that a significant thickness stress gradient develops. This gradient influences the materials plastic flow behavior through thickness and hence affects the cross-sectional stability of the sheet. Based on this observation a theoretical study has been performed in order to quantify the effects of different process and material parameters on the formability of the sheet. Furthermore, it is observed numerically and experimentally that during stretch-bending the cross-sectional stability is not lost instantaneously but gradually over time. Therefore, the concept of FLC proves to be insufficient as to judge acceptability of the deformed product. Another approach is proposed where an instability criterion is developed that also accounts for the partial loss of stability of the cross-section. The criterion is implemented in FE environment to be used in Shell type elements. The results are compared with stretch-bending experiments on selected HSS and AHSS.

Abstract: It is serious on the degradation of the shallow lake ecosystem at present, while the recovery of the macrophytes vegetations and the increase in water transparency have been the main contents of the ecological restoration. Using a two-variable model, we discuss the instability and sensitivity of the ecosystem to the finite-amplitude perturbations related to the initial condition with CNOP-I method. Results show that the linearly stable clear (turbid) water states can be nonlinearly unstable with the finite-amplitude perturbations. The results also demonstrate that the facilitation interactions between the submerged macrophytes and the water transparency are the main trigger for a shift from the turbid to the clear state.

Abstract: With the help of the commercial FE package ANSYS, this paper presents a finite element (FE) model for analyzing coupled flutter of long-span bridges. This model models the aero-elastic forces acting on the bridge utilizing a specific user-defined element Matrix27 in ANSYS, by which stiffness and damping matrices can be expressed in terms of the reduced wind velocity and flutter derivatives. Taking advantage of this FE model, Transient dynamic analysis is carried out to determine the dynamic response of a structure under the action of pulse load, of which the damping ratio can be obtained by considering response peaks which are several cycles apart. The condition for onset of flutter instability turns into that, at a certain wind velocity, the structural system incorporating fictitious Matrix27 elements does simple harmonic vibration with zero damping ratios or near zero one. The damping ratio is completely calculated in post-analysis of ANSYS and the initial frequency is given by any value and the last frequency can be got by iterating several times. In order to validate the developed procedure, a classical case study on three hundred meter simple supported beam is provided.

Abstract: A series of centrifuge model tests has been conduced to examine the behavior of high strength piles composite foundation stability under railway embankment in silt or clay or sand different kind of soils. The high strength composite foundation has a symmetrical plan layout consisting of 3×4, 4×6, and 5×8 piles with a center-to-center spacing of 6 or 4 or 3 times pile width. The piles are under the same height of railway embankment, with the same length, and on the same lying soil layer. The high strength composite foundation stability test results are expressed in terms of soils kinds-pile displacement response of the composite foundation, embankment load experienced by soil between piles and piles in the composite foundation, and bending moment profile along individual pile. It is established that the high strength composite foundation stability efficiency reduces significantly with decreasing of the strength of soil between piles in composite foundation. The tests reveal the shadowing effect phenomenon in which the marginal piles experienced larger laterally and bending moment than that of the central piles. The shadowing effect is most significant for the lead row piles and considerably less significant for subsequent rows of middle piles. The approach adopted by many researchers of taking the average performance of piles in the same row is found to be inappropriate for the middle rows, of piles for composite foundation as the outer piles in the row carrying significantly more laterally load and experience considerably higher bending moment than those of the inner piles.

Abstract: Based on the impact of some horizontal filtering tank’s instability in operation process on production, the present paper discusses the optimal design scheme for horizontal filtering tank structure with the help of finite element. Theoretical guidance will be given to enterprise from the perspective of finite element for the purpose of improving the horizontal filtering tank through constructing the finite element model for horizontal filtering tank with Creo parametric software, conducting simulation with workbench software^[1] and finally arriving at the reasonable design scheme after analysis, thus avoiding the structural instability caused by the over-constraint of structural leg support beam and filter plate under-constraint.

Abstract: In conventional analysis of instability, a rough prediction of uniform deformation was obtained due to taking material parameters as constants. In this study, the constitutive equation with varying parameters for Zn-5%Al alloy at 340 °C is employed to predict the critical values of uniform strain in tension based on Considere criterion and Hart criterion, respectively. It should address the factor of strain rate in the characterization of the capability of uniform deformation on superplastic alloys, or for that matter, on any rate-dependent material. Comparison and analysis indicated that the results on Hart criterion have the better predictability of uniform deformation than Considere criterion. The Considere criterion is dependent on strain path, while Hart crtierion is merely dependent on the values of strain and strain rate in tension, and is independent on the strain path or the deformation condition or the deformation history. Therefore, the uniform strain vs. strain rate relation can be taken as a quantitative reference for designing a reasonable strain path during superplastic forming with increase of formability and reduction of forming time.

Abstract: Compression tests of Mg-13Al-3Ca-3Zn-1Nd-0.2Mn Magnesium alloy as-extruded had been performed in the compression temperature range from 200°C to 400°C and the strain rate range from 0.001 s−1 to 10 s−1 and the flow stress data obtained from the tests were used to develop the power dissipation map, instability map and processing map. The most unsuitable zones in the power dissipation map including 200°C - 315°C and 0.01s^-1 - 0.1s-1 zone, 315°C - 400°C and 0.001s^-1 - 0.01s^-1 zone and 340°C - 360°C and 0.32 s^-1 - 0.56 s^-1 zone. The most unsuitable zones in the instability map are 310°C - 400°C, 0.001s^-1 to 0.56 s^-1 zone and 330°C - 400°C, 1s^-1 to 10 s^-1 zone. The most suitable temperature range is 330°C - 400°C and most optimal strain rate ranges are 1 s^-1 - 10 s^-1 and 0.001s^-1 - 0.56 s^-1.

Abstract: Under stretch-bending conditions, a significant tensile stress gradient through sheet thickness is induced, especially for a small punch radius. The traditional instability theories were developed assuming a uniform tensile stress / strain distribution through thickness; hence, may lead to unreliable prediction of stretch-bending formability. In this study, the instability behavior of sheet metal under stretch-bending is analyzed via FE-simulation of an Angular Stretch-Bend Test (ASBT). In order to reflect the influence of bending, contact normal stress etc., solid elements are used in the simulation. Three deformation stages are identified: (a). stable deformation; (b). strain localization through sheet thickness; (c). localized necking. Based on the instability characteristics, a localized necking criterion is proposed for predicting forming limits of sheet metal under stretch-bending. By combining the proposed criterion and solid element simulation, good agreement between numerical and experimental results is indicated. This work provides a new approach for predicting stretch-bend formability with sufficient accuracy and convenience.