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This paper is concerned with the relationship between the onset and the development of the Taylor instabilities and their treatment as turbulent flows in the most accepted turbulence models (Constantinescu (1); Ng-Pan (2); Hirs (3)) used with the Reynolds equation, in the range of 41.3√R/C < Re < 2000. The authors show that in between these limits there is a transition regime where the velocity and pressure profiles are fundamentally different from either a Couette flow or a fully developed turbulent flow. Thus the issue under consideration is whether the flow formations observed during Taylor instability regimes should be simulated using the widely accepted turbulence models as they presently are modeled in microscale clearance flows. We are considering the flow of light silicone oil in gaps varying from 3.302 mm (0.13 in.) to 0.127 mm (0.005 in.) between two concentric cylinders, with the inner cylinder rotating. The computational engine used in this study is a well-established and a tried software package: CFD-ACE+. It was found that the Taylor vortices (cells) begin to form at certain, but different, “critical” speeds, function of clearance size, and as the speed grows, the vortices become fully developed and evolve further into wavy vortices. Calculations show that both the 1st and 2nd critical Taylor numbers and Reynolds numbers are functions of the clearance size. The Taylor numbers decrease, while the Reynolds numbers increase with the decrease in clearance size. The onset of both instabilities is clearly characterized by the discontinuities in the Torque-√Ta (or Torque – Re) curve slope. The calculations presented here show that the slope changes in the above-mentioned graphs are due to the changes in the average velocity gradient on the outer cylinder and not to a change in the actual viscosity as it is implemented by the turbulence models mentioned above. Finally a comparison is made between present calculations and the data of Roberts (4), Cole (5), Walowit et al. (6), Weinstein (7), Koschmieder (8), and DiPrima (9). 相似文献
64.
圆杯无压边锥模拉深皱曲强度与拉深极限预测 总被引:1,自引:0,他引:1
采用能量法导出圆杯无压边锥模拉深的皱曲强度,考虑了材料性 能,径向和周向弯曲作用,厚度变化及加工硬化效应。应用该强度预测无皱极限拉深比,与 Sowerby等人使用3种材料3种板厚作的无压边锥模拉深试验结果完全一致,最大相对误差不 超过8%。提出的研究方法同样适用于轴对称非直壁零件拉深凸凹模间悬空区的切向压缩失稳 分析。 相似文献
65.
Effect of response surface parameter variation on structural reliability estimates 总被引:28,自引:0,他引:28
In the reliability estimation of complex structures, response surface methodology has been suggested as a way to estimate the actual but implicit limit state function. Typically the response surface is constructed from a polynomial function and fitted to the implicit function at a number of points. The location of these points has been noted as being an issue but the effect of varying their location has had little attention in the literature. In the present paper some simple examples are used to indicate possible effects. It is noted that the probability can be both under- and over-estimated, depending on the choice of points, but that no clear guidance for point selection can be given in any one case. A particularly disturbing feature is that for some types of problems there can be instability in the probability estimate as the location of the points is changed. This is demonstrated through a previously well-discussed example. 相似文献
66.
利用线性不稳定性理论研究了旋转气体介质对黏性环膜液体射流破碎的影响。研究结果表明,无论是轴对称模式还是非轴对称模式,由液体环膜内部气体介质旋转所产生的离心力是液体射流的失稳因素,有助于液体射流的破碎。另外,由液体环膜外部气体介质旋转所产生的离心力是液体射流的促稳因素,不利于液体射流的破碎。当相同强度的旋转同时存在于内部和外部气体介质中时,对于轴对称模式,内部气体介质的影响显著,而对于非轴对称模式,则外部气体介质的影响更为明显。通常情况下,非轴对称模式的扰动增长率强于轴对称模式的扰动增长率,因此会在环膜液体射流的破碎中占据主导地位。 相似文献
67.
68.
综合航空涡轮发动机失稳先兆波的幅值特性与频率特性,设计出能够提取失稳先兆特征的处理方法,并据此对发动机失稳数据进行处理与分析。通过使用不同的参数,此方法可实现对模态波和尖脉冲型失稳的预警。处理结果表明,此提取方法能够在失稳前15~20个转子周期内,有效地对尖脉冲型失稳进行预报,对模态波型失稳能够在60~80个转子周期前预警。并且此提取方法的算法复杂度较低,有利于失稳的在线预警,对航空燃气涡轮发动机主动控制技术的发展具有十分重要的意义。 相似文献
69.
Gongtian Gu Jin Huang Wenhu Han Cheng Wang 《International Journal of Hydrogen Energy》2021,46(21):12009-12015
Flame propagation in Hele-Shaw cells with a micro-sized gap was experimentally investigated. The evolution of flame front morphology was recorded via Schlieren photographs as the hydrogen-oxygen (H2–O2) mixture was ignited at ambient temperature and pressure. By varying gap size, two different regimes of flame propagation are identified: 1) the non-accelerating flame in narrow gaps; 2) the self-accelerating flame in relatively wide gaps. For the former, the initial flame front is globally circular, and subsequently evolves into branches separated from the surface, exhibiting dendritic-growth and fingering shapes. In the latter regimes, the flame front exhibits a cellular structure and accelerates nearly sonic speed due to hydrodynamic instabilities. It is found that the flame acceleration depends non-monotonically on the gap size due to the competing mechanisms of viscosity friction and heat loss through the walls. The effect of equivalence ratio on the non-accelerating flame is studied to identify the mechanism controlling the local extinction flame. 相似文献
70.
G. Stournaras G. Tsimpidis P. Tsoumanis G. Yannatos R. Guillande 《Bulletin of Engineering Geology and the Environment》1998,57(1):65-68
The geomorphic and instability conditions of the Neogene formations in Northern Peloponnesus are examined and related to
the lithologies and hydrological data. Instability phenomena caused by erosion are briefly described and a qualitative analysis
of the run-off conditions within the Finikas and Meganeitos catchments is presented.
Received: 31 July 1997 · Accepted: 12 January 1998 相似文献