浮环轴承系统中浮动环涡动运动研究

研究了浮环轴承中内、外层不平衡油膜力作用下浮动环的运动情况。从广义不可压Reynolds方程出发,推导了浮环轴承内外层油膜的瞬态控制方程,结合Hirs整体流动理论和Moody壁面摩擦系数方程,获取浮动环的运动方程,通过算例,较为深入地研究了浮环轴承系统中浮动环的涡动特性：浮动环质心稳定状况下运动轨迹呈椭圆形分布,且幅值随轴颈偏心率增大而增略呈减小趋势。     

微型燃气轮机浮环轴承-悬臂转子系统动力学特性分析

对浮环轴承支承的悬臂转子系统的动力学特性进行分析,建立了浮环轴承双层油膜Reynolds方程和浮环运动方程。采用4节点等参h-精细有限元网格,通过Galerkin方法求解系统Reynolds方程得到双层油膜压力。在小摄动范围内,联合悬臂转子系统的动力学方程、浮环运动方程求出浮环轴承的等效刚度和阻尼系数。应用商业有限元软件ANSYS12.1对实际微型燃气轮机用浮环轴承-悬臂转子系统进行临界转速、谐响应及不平衡响应等转子动力学仿真计算。通过与实验结果对比,验证了此物理计算模型的正确性,并证明了用此方法分析浮环轴承-悬臂转子转子动力学问题具有实际意义。     

微型燃气轮机浮环轴承动力学参数影响因素研究

浮环轴承在高速运转时内层油膜承载力大于外层油膜承载力,在分析浮环轴承稳态内外层油膜压力时,只分析外层油膜压力即可表现出浮环轴承的承载能力.利用转子动力学分析软件DyRoBes与有限差分方法分别对相对重载微型燃气轮机浮环轴承外层油膜压力Reynolds方程进行数值计算,分析并比较其运算结果.由于浮环轴承自身结构的特点,使得浮环轴承内外层油膜刚度和阻尼的相互关系等效于弹簧和阻尼的串联,由此可以算出不同转速下的浮环轴承油膜总刚度和总阻尼及偏心比、轴颈的静平衡位置等影响浮环轴承油膜特性及动力学行为的重要参数,最后利用Routh-Hurwitz判别法分析了对应转速下的油膜稳定性.     

热效应对径向浮环轴承最小油膜厚度及稳定性影响研究

以径向浮环动静压轴承为研究对象,采用有限元法和有限差分法联立求解Reynolds方程、能量方程和温黏关系式,得到内外层油膜的压力分布、温度分布和黏度分布,对油膜压力积分得到轴承的刚度系数和阻尼系数。针对轴颈、浮环建立统一的动力学方程,结合能量方程和Routh-Hurwitz准则推导出单质量刚性对称浮环轴承-转子系统的热失稳判据,分析了油膜热效应对内外膜最小油膜厚度与失稳转速的影响。结果表明:内外膜油腔呈现多个的温度峰值,两端面温度高于油腔中央温度;内外膜最小油膜厚度和系统失稳转速随着进油温度的升高而减小;高速工况下,油膜温升是导致浮环轴承发生油膜破裂和失稳现象的重要因素,计算时需计入油膜热效应对轴承性能的影响。     

径向动压浮环轴承-转子系统多稳定区域研究

以径向动压浮环轴承为研究对象,针对轴颈、浮环建立统一的动力学方程,用Routh-Hurwitz准则给出单质量刚性对称浮环轴承-转子系统稳定性判据。用有限差分计算某高速径向动压浮环轴承刚度、阻尼系数,获得不同工况下系统稳定性曲线。结果表明,小偏心率下系统升速过程中呈现多个稳定区域,且随偏心率、内外膜半径间隙变化而变化。油膜温度变化亦会影响系统稳定性。为高速浮环轴承稳定性整体建模、分析提供借鉴。     

径向动静压浮环轴承-转子系统稳定性分析

以径向动静压浮环轴承-转子系统为研究对象,计入浮环质量和转子刚度建立了统一的动力学方程,用Routh-Hurwitz准则推导了单质量弹性对称系统的稳定性判据。用有限差分计算了某高速径向动静压浮环轴承的刚度系数和阻尼系数,在此基础上得到了不同浮环质量和转子刚度下动静压浮环轴承-转子系统的稳定性曲线。计算结果表明,浮环质量对系统稳定性影响不大,而随着转子刚度减小,系统失稳转速迅速降低。该文在高速浮环轴承-弹性转子稳定性整体建模和分析方面有较大的参考意义。     

径向浮环动静压轴承稳定性研究

以径向浮环动静压轴承为研究对象,针对轴颈和浮环建立了统一的动力学方程,用Routh-Hurwitz准则推导了径向浮环轴承的稳定性判据。用有限元法计算了某结构高速径向浮环动静压轴承的刚度系数和阻尼系数,在此基础上得到了不同偏心率下的失稳转速。由计算结果可以看出,浮环轴承具有极佳的稳定性,且随着偏心率的增加,失稳转速迅速提高。文章在高速浮环轴承稳定性整体建模和分析方面有较大的参考意义。     

涡轮增压器动力学建模及振动特性研究

采用结合有限元法(FEM)的多体动力学方法,对某车用浮环轴承涡轮增压器进行多体系统仿真模型的建立:以模态综合法为基础,建立增压器壳体与转子的柔性体子结构模型;以广义不可压缩雷诺方程为基础,建立增压器浮环轴承的EHD弹性液力润滑模型(Elastic Hydro-Dynamic)。计算得到内外油膜压力、转子振动特性以及增压器壳体表面振动速度,并进行实验验证。结果表明,采用柔性体多体动力学与EHD轴承润滑模型相结合的建模方法能够有效用于增压器的振动特性分析,该方法的应用也为研究增压器同步、次同步振动与噪声问题提供了理论依据。     

结构参数对浮环轴承动态特性的影响研究

为了分析结构参数对浮环轴承动态特性的影响,以摩擦学和流体润滑理论为基础,采用浮环转速比为切入点推导出偏心率与结构参数的变化关系,并以此为基础应用有限元法对其进行分析,得出浮环轴承的动态特性系数随结构参数的变化规律.研究结果表明:总交叉刚度的绝对值和总主阻尼随浮环内外半径比或间隙比的增加而减小;总主刚度的绝对值和总交叉阻尼随浮环内外半径比的增大而减小,随间隙比的增加而增加.研究结果为浮环轴承设计提供了参考依据.     

磁流变液润滑浮环轴承及其在转子振动控制中的应用

利用自制磁流变液及其可控简单、成本低等优点,提出了一种使用磁流变液润滑的可控浮环轴承。采用Herschel-Bulkley模型对包含剪切稀化效应的磁流变液剪切特性进行了建模;通过自制的磁流变仪测试系统测试并识别了所制磁流变液的模型参数;利用识别的参数分析了磁流变液润滑浮环轴承的动态特性及可控性;分析了采用此种轴承的转子系统的抑振效果。结果表明,磁流变液润滑浮环轴承在外加磁场后有较好的振动抑制作用。同时,相较于磁流变液润滑普通滑动轴承,磁流变液润滑浮环轴承有更好的抑振效果,且减小了轴颈摩擦力矩,降低了摩擦功耗。     

Effect of formulation variables on the floating properties of gastric floating drug delivery system

PURPOSE: To evaluate the contribution of formulation variables on the floating properties of a gastric floating drug delivery system (GFDDS) using a continuous floating monitoring system and statistical experimental design. METHODS: A modified continuous floating monitoring system, which consisted of an electric balance interfacing with a PC, was designed to perform the continuous monitoring of floating kinetics of GFDDS. Several formulation variables, such as different types of hydroxypropyl methylcellulose (HPMC), varying HPMC/Carbopol ratio, and addition of magnesium stearate, were evaluated using Taguchi design, and the effects of these variables were subjected to statistical analysis. RESULTS: The continuous floating monitoring system developed was validated, using capsules with different density, and a good correlation between theoretical and experimental values was obtained (R2 = 0.9998), indicating the validity of the setup. The statistical analysis indicated that magnesium stearate had a significant effect on the floating property of GFDDS (p < 0.05), and addition of magnesium stearate could significantly improve the floating capacity of the GFDDS. It was found that the HPMC of higher viscosity grade generally exhibited a greater floating capacity, but the effect was not statistically significant. For polymers with the same viscosity, i.e., K4M and E4M, the degree of substitution of the function group did not show any significant contribution. A better floating behavior was achieved at higher HPMC/Carbopol ratio. Carbopol appeared to have a negative effect on the floating behavior of GFDDS. CONCLUSIONS: It was concluded that by using a validated continuous floating monitoring system, the effect of formulation variables on the floating property of the delivery system and their ranges could be identified. Incorporation of hydrophobic agents, such as magnesium stearate, could significantly improve the floating capacity of the GFDDS.     

Stability of a floating cylinder

A cylinder having a length less than 0.707 of its diameter will float with its axis vertical while one of length exceeding one diameter floats with its axis horizontal. This transition range was investigated and under certain circumstances, two stable positions of the cylinder axis were found in each quadrant of rotation. Details of behaviour in the transition range were attributed to the edges of the square-ended cylinder.     

Capillary attraction of floating rods

The capillary attraction of two parallel cylinders with circular cross-section representing slender particles floating at the interface between two immiscible fluids is considered. Given the particle separation, the elevation of the particle centers in hydrostatics is computed to satisfy the vertical force balance involving the buoyancy force, the capillary force, and the particle weight. A numerical procedure is developed for calculating the horizontal force exerted on a pair of cylinders in solitary or periodic arrangement. The results confirm that the particles attract each other under the conditions considered. The particle motion and transient flow due to the particle attraction are computed using a boundary-integral method for Stokes flow. In the algorithm, the particle center velocity of translation and angular velocity of rotation are calculated to satisfy force and torque balances. Numerical simulations using a boundary-element method subject to an initial state provided by hydrostatics illustrate the nature of the motion and furnish estimates for the particle velocity induced by capillarity.     

Ponding on floating membranes

Membranes subjected to ponding loads and floating on a liquid are analyzed. The initially flat membrane, which may be prestressed by edge in-plane tractions or displacements, is subjected to the weight of a liquid (e.g. rain water) filling the space created by the deflection of the membrane. Large deflections of membranes are considered, which result from nonlinear kinematic relations. The three coupled nonlinear equations in terms of the displacements governing the response of the membrane are solved using the analog equation method, which reduces the problem to the solution of three uncoupled Poisson's equations with fictitious sources. The problem is strongly nonlinear. In addition to the geometrical nonlinearity, the ponding problem is itself nonlinear, because the ponding load and the liquid reaction are not a priori known as they depend on the produced deflection surface. Iterative schemes are developed which converge to the equilibrium state of the membrane. Example problems are presented, which illustrate the method and demonstrate its efficiency. The method has all the advantages of the pure BEM.     

Evaluation and floating enhancement of levodopa sustained release floating minitablets coated with insoluble acrylic polymer

This article describes the in vitro evaluation and the enhancement of the floating properties of coated sustained release (SR) minitablets (MTs). The evaluated system consisted of a 3-mm drug-containing gas-generating core prepared by melt granulation and subsequent compression, which was then coated with a flexible polymeric membrane. Eudragit RL30D and acetyl triethylcitrate were used as a film former and a plasticizer, respectively. The coating level was fixed at 20% (wt/wt). The optimally coated floating MTs floated within 10 min and remained buoyant for more than 13 h, regardless of the pH of the test medium. By evaluating the dissolution profiles of levodopa at different pH, it was found that the release of levodopa was sustained for more than 12 h regardless of the pH, even if the coating did not cancel the effect of the pH-dependent solubility of the active drug. Finally, the robustness of the coated floating MTs was assessed by testing the drug release variability in function of the stirring conditions during dissolution tests.