A New Method for Predicting the Critical Taylor Number in Rotating Cylindrical Flows

1987 ◽  
Vol 54 (3) ◽  
pp. 713-719 ◽  
Author(s):  
J. O. Cruickshank
Keyword(s):  
Dynamic Stability ◽  
Critical Stress ◽  
Stress Condition ◽  
Flow Instability ◽  
Fluid System ◽  
Fluid Systems ◽  
Concentric Cylinders ◽  
Elastic Systems ◽  
Subsequent Motion ◽  
The Stability

A method for determining the boundaries of dynamic stability of a fluid system, as distinct from the prediction of the subsequent motion, is presented. The method is based on well-known approaches to the problem of instability in elastic systems. The extension of these methods to fluid systems, specifically, to the stability of flow between concentric cylinders, confirms that it may be possible in some cases to determine the boundaries of stability of fluid systems without recourse to an Orr-Sommerfeld type treatment. The results also suggest that the concept of apparent (virtual) viscosity may have implications for fluid stability outside the current realm of turbulence modelling. Finally, it is also shown that flow instability may be preceded by the onset of a critical stress condition in analogy with elastic systems.

scholarly journals ABOUT THE STABILITY OF NONCONSERVATIVE UNDAMPED ELASTIC SYSTEMS: SOME NEW ELEMENTS

2009 ◽  
Vol 09 (02) ◽  
pp. 357-367 ◽  
Author(s):  
JEAN LERBET ◽  
ELIE ABSI ◽  
ALAIN RIGOLOT
Keyword(s):  
Dynamic Stability ◽  
Dynamic Instability ◽  
Static Stability ◽  
Stability Domain ◽  
Dynamic Approach ◽  
Linear Approach ◽  
Elastic Systems ◽  
The Common ◽  
The Stability ◽  
New Criterion

It is well-known that the domains of static stability and dynamic stability (even for a linear approach) do not match each other when the system is no more conservative and the dynamic approach is usually privileged, meaning that the dynamic stability domain is included in the static one. Following previous works proposing a new criterion of static stability of nonconservative systems and prolonging a paper of Gallina devoted to linear dynamic instability (flutter), we show in this paper some remarkable relations between the two approaches: contrary to the common thought, the new static stability criterion implies partially the dynamic one.

A Liquid Metal Flow Between Two Coaxial Cylinders System

2019 ◽  
Vol 11 (1) ◽  
pp. 79-86
Author(s):  
Abdelkrim Merah ◽  
Ridha Kelaiaia ◽  
Faiza Mokhtari
Keyword(s):  
Couette Flow ◽  
Metal Flow ◽  
Three Dimensional ◽  
Liquid Sodium ◽  
Taylor Vortex ◽  
Turbulent Regime ◽  
Coaxial Cylinders ◽  
Concentric Cylinders ◽  
Ideal Tool ◽  
The Stability

Abstract The Taylor-Couette flow between two rotating coaxial cylinders remains an ideal tool for understanding the mechanism of the transition from laminar to turbulent regime in rotating flow for the scientific community. We present for different Taylor numbers a set of three-dimensional numerical investigations of the stability and transition from Couette flow to Taylor vortex regime of a viscous incompressible fluid (liquid sodium) between two concentric cylinders with the inner one rotating and the outer one at rest. We seek the onset of the first instability and we compare the obtained results for different velocity rates. We calculate the corresponding Taylor number in order to show its effect on flow patterns and pressure field.

Improved Burn-In Stress Methodology for Stress Uniformity

2018 ◽  
Author(s):  
Jungsuk Ko ◽  
Hoonchang yang ◽  
Hyungchae Jeon ◽  
Gyuyoung Nam ◽  
Youngseok Ryu ◽  
...  
Keyword(s):  
Temperature Stress ◽  
Stress Condition ◽  
Temperature Deviation ◽  
Stress Effects ◽  
Hot Temperature ◽  
Production Environments ◽  
Stress Environment ◽  
The Stability ◽  
Stress Uniformity ◽  
Stress Voltage

Abstract The necessity of hot temperature stress is widely recognized as the initial stress methodology to maintain the stability of products from infant defects in device [1, 2]. However, hot temperature stress has a disadvantage in terms of stress uniformity because temperature variation according to stress environment such as chamber, board, and tester accelerates different stress effects per chips. In addition, this stress condition can cause serious reliability problem in the mass production environments. Therefore, the stress temperature should be lowered to minimize the temperature deviation due to the production environments. The reduction of stress temperature cause the lack of stress amount, so optimized stress voltage and time to maintain the stress condition is required. In this study, various stress voltage and time with decreasing temperature were evaluated in consideration of lifetime that unit elements such transistors and capacitors did not degrade by any stress conditions. In addition, it was confirmed that stress uniformity can be improved in the stress condition obtained by the evaluation. Furthermore, the enhanced initial failure screen ability was proven with mass evaluations.

To What Extent Does Not Wearing Shoes Affect the Local Dynamic Stability of Walking?: Effect Size and Intrasession Repeatability

2014 ◽  
Vol 30 (2) ◽  
pp. 305-309 ◽  
Author(s):  
Philippe Terrier ◽  
Fabienne Reynard
Keyword(s):  
Dynamic Stability ◽  
Effect Size ◽  
Intraclass Correlation ◽  
Clinical Findings ◽  
Local Dynamic ◽  
Gait Stability ◽  
Local Dynamic Stability ◽  
Gait Parameters ◽  
Absolute Agreement ◽  
The Stability

Local dynamic stability (stability) quantifies how a system responds to small perturbations. Several experimental and clinical findings have highlighted the association between gait stability and fall risk. Walking without shoes is known to slightly modify gait parameters. Barefoot walking may cause unusual sensory feedback to individuals accustomed to shod walking, and this may affect stability. The objective was therefore to compare the stability of shod and barefoot walking in healthy individuals and to analyze the intrasession repeatability. Forty participants traversed a 70 m indoor corridor wearing normal shoes in one trial and walking barefoot in a second trial. Trunk accelerations were recorded with a 3D-accelerometer attached to the lower back. The stability was computed using the finite-time maximal Lyapunov exponent method. Absolute agreement between the forward and backward paths was estimated with the intraclass correlation coefficient (ICC). Barefoot walking did not significantly modify the stability as compared with shod walking (average standardized effect size: +0.11). The intrasession repeatability was high (ICC: 0.73–0.81) and slightly higher in barefoot walking condition (ICC: 0.81–0.87). Therefore, it seems that barefoot walking can be used to evaluate stability without introducing a bias as compared with shod walking, and with a sufficient reliability.

Effect of Load Carrying on Local Dynamic Stability

2007 ◽  
Vol 51 (15) ◽  
pp. 909-913 ◽  
Author(s):  
Jian Liu ◽  
Thurmon E. Lockhart ◽  
Kevin Granata
Keyword(s):  
Dynamic Stability ◽  
Load Condition ◽  
Local Dynamic ◽  
Fall Injuries ◽  
Local Dynamic Stability ◽  
Load Carrying ◽  
Trunk Acceleration ◽  
The Stability ◽  
Major Factors

Occupational load carrying tasks are considered one of the major factors contributing to slip and fall injuries. The objective of the current study was to explore the feasibility to assess the stability changes associated with load carrying by local dynamic stability measures. Twenty-five young participants were involved in a treadmill walking study, with their trunk acceleration profiles measured wirelessly by a tri-axial accelerometer. Finite time local dynamic stability was quantified by maximum Lyapunov exponents (maxLE). The results showed a significant increase in long term maxLE in load condition, indicating the declined local dynamic stability due to the load carrying. Thus, current study confirmed the discriminative validity and sensitivity of local dynamic stability measure and its utility in the load carrying scenario.

scholarly journals Analytical and Experimental Investigation of the Stability of the Rotor-Bearing System of a New Small Turbocharger

1987 ◽  
Author(s):  
H. R. Born
Keyword(s):  
Experimental Investigation ◽  
Dynamic Stability ◽  
Squeeze Film ◽  
Test Results ◽  
Flow Capacity ◽  
Squeeze Film Dampers ◽  
Rotor Bearing ◽  
The Stability ◽  
Turbine Design ◽  
Bearing System

This paper presents an overview of the development of a reliable bearing system for a new line of small turbochargers where the bearing system has to be compatible with a new compressor and turbine design. The first part demonstrates how the increased weight of the turbine, due to a 40 % increase in flow capacity, influences the dynamic stability of the rotor-bearing system. The second part shows how stability can be improved by optimizing important floating ring parameters and by applying different bearing designs, such as profiled bore bearings supported on squeeze film dampers. Test results and stability analyses are included as well as the criteria which led to the decision to choose a squeeze film backed symmetrical 3-lobe bearing for this new turbocharger design.

The Stability-Limiting Flow Mechanisms in a Subsonic Axial-Flow Compressor and Its Passive Control With Casing Treatment

2008 ◽  
Author(s):  
Xingen Lu ◽  
Junqiang Zhu ◽  
Chaoqun Nie ◽  
Weiguang Huang
Keyword(s):  
Flow Instability ◽  
State Of The Art ◽  
Axial Flow ◽  
Flow Mechanism ◽  
Blade Passage ◽  
Casing Treatment ◽  
Compressor Rotor ◽  
Flow Compressor ◽  
The Stability ◽  
End Wall

The phenomenon of flow instability in the compression system such as fan and compressor has been a long-standing “bottle-neck” problem for gas turbines/aircraft engines. With a vision of providing a state-of-the-art understanding of the flow field in axial-flow compressor in the perspective of enhancing their stability using passive means. Two topics are covered in this paper. The first topic is the stability-limiting flow mechanism close to stall, which is the basic knowledge needed to manipulate end-wall flow behavior for the stability improvement. The physical process occurring when approaching stall and the role of complex tip flow mechanism on flow instability in current high subsonic axial compressor rotor has been assessed using single blade passage computations. The second topic is flow instability manipulation with casing treatment. In order to advance the understanding of the fundamental mechanisms of casing treatment and determine the change in the flow field by which casing treatment improve compressor stability, systematic studies of the coupled flow through a subsonic compressor rotor and various end-wall treatments were carried out using a state-of-the-art multi-block flow solver. The numerically obtained flow fields were interrogated to identify complicated flow phenomenon around and within the end-wall treatments and describe the interaction between the rotor tip flow and end-wall treatments. Detailed analyses of the flow visualization at the rotor tip have exposed the different tip flow topologies between the cases with treatment casing and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by end-wall treatments is a result of the tip flow manipulation. Compared to the smooth wall case, the treated casing significantly dampen or absorb the blockage near the upstream part of the blade passage caused by the upstream movement of tip clearance flow and weakens the roll-up of the core vortex. These mechanisms prevent an early spillage of low momentum fluid into the adjacent blade passage and delay the onset of flow instability.

Convection in a rotating spherical fluid shell with an inhomogeneous temperature boundary condition at infinite Prandtl number

1993 ◽  
Vol 250 ◽  
pp. 209-232 ◽  
Author(s):  
Keke Zhang ◽  
David Gubbins
Keyword(s):  
Numerical Solutions ◽  
Unstable Mode ◽  
Mode Analysis ◽  
Mathematical Framework ◽  
Fluid Systems ◽  
Critical Wavelength ◽  
Resonance Solution ◽  
Inhomogeneous Temperature ◽  
Spherical Fluid ◽  
The Stability

We examine thermal convection in a rotating spherical shell with a spatially non-uniformly heated outer surface, concentrating on three distinct heating modes: first, with wavelength and symmetry corresponding to the most unstable mode of the uniformly heated problem; secondly, with the critical wavelength but opposite equatorial symmetry; and thirdly, with wavelength much larger than that of the most unstable mode. Analysis is focused on boundary-locked convection, the associated spatial resonance phenomena, the stability properties of the resonance solution, and time-dependent secondary convection. A number of new forms of instability and convection are found: the most interesting is perhaps the saddle-node bifurcation, which is the first to be found for realistic fluid systems governed by partial differential equations. An analogous Landau amplitude equation is also analysed, providing an important mathematical framework for understanding the complicated numerical solutions.

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