F. Papa, K T.G., K.J. DeWitt and K. Vaidyanathan
This study is concerned with developing laminar flow of an incompressible, Newtonian fluid, having constant viscosity, rotating in circular and rectangular ducts that contain a…
Abstract
This study is concerned with developing laminar flow of an incompressible, Newtonian fluid, having constant viscosity, rotating in circular and rectangular ducts that contain a 180° bend. The Reynolds number ranges from 100 to 400, the rotation number from 0 to 0.4, and the Dean number from 66 to 264. Positive and negative rotation modes are considered. The artificial compressibility method is used for the numerical calculations and new boundary conditions are developed for these flows. It is shown that rotation causes the secondary flow to occur in ducts of any geometry, and that the strength of the secondary flow in the bend due to both rotation and curvature decreases as compared to the no rotation case.
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F. Papa, K. Vaidyanathan, T.G. Keith and K.J. DeWitt
The artificial compressibility method is used to analyze internal flows in rotating ducts having strong curvature. This study was concerned with the laminar flow of an…
Abstract
The artificial compressibility method is used to analyze internal flows in rotating ducts having strong curvature. This study was concerned with the laminar flow of an incompressible Newtonian fluid having constant viscosity in circular and square ducts with a 908 bend. The emphasis of the present simulation is to determine the effect of rotation and through‐flow rate on the fluid physics and friction characteristics in the straight channel and in the curved geometric regions. The Reynolds numbers ranged from 100 to 790 and the Rossby numbers from 0 to 0.4. Coriolis forces arising from rotation produce a non‐symmetric secondary flow in the bend that increases the loss coefficient as compared with the values for non‐rotation. In addition, the wall friction losses in the straight outlet section are increased, and both effects are directly proportional to the Rossby number.
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Tadashi Dohi, Hiroyuki Okamura and Cun Hua Qian
In this paper, the authors propose two construction methods to estimate confidence intervals of the time-based optimal software rejuvenation policy and its associated maximum…
Abstract
Purpose
In this paper, the authors propose two construction methods to estimate confidence intervals of the time-based optimal software rejuvenation policy and its associated maximum system availability via a parametric bootstrap method. Through simulation experiments the authors investigate their asymptotic behaviors and statistical properties.
Design/methodology/approach
The present paper is the first challenge to derive the confidence intervals of the optimal software rejuvenation schedule, which maximizes the system availability in the sense of long run. In other words, the authors concern the statistical software fault management by employing an idea of process control in quality engineering and a parametric bootstrap.
Findings
As a remarkably different point from the existing work, the authors carefully take account of a special case where the two-sided confidence interval of the optimal software rejuvenation time does not exist due to that fact that the estimator distribution of the optimal software rejuvenation time is defective. Here the authors propose two useful construction methods of the two-sided confidence interval: conditional confidence interval and heuristic confidence interval.
Research limitations/implications
Although the authors applied a simulation-based bootstrap confidence method in this paper, another re-sampling-based approach can be also applied to the same problem. In addition, the authors just focused on a parametric bootstrap, but a non-parametric bootstrap method can be also applied to the confidence interval estimation of the optimal software rejuvenation time interval, when the complete knowledge on the distribution form is not available.
Practical implications
The statistical software fault management techniques proposed in this paper are useful to control the system availability of operational software systems, by means of the control chart.
Social implications
Through the online monitoring in operational software systems, it would be possible to estimate the optimal software rejuvenation time and its associated system availability, without applying any approximation. By implementing this function on application programming interface (API), it is possible to realize the low-cost fault-tolerance for software systems with aging.
Originality/value
In the past literature, almost all authors employed parametric and non-parametric inference techniques to estimate the optimal software rejuvenation time but just focused on the point estimation. This may often lead to the miss-judgment based on over-estimation or under-estimation under uncertainty. The authors overcome the problem by introducing the two-sided confidence interval approach.
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Seyyed Mostafa Hoseinalipour, Hamidreza Shahbazian and Bengt Ake Sunden
The study aims to focus on rotation effects on a ribbed channel of gas turbine blades for internal cooling. The combination and interaction between secondary flows generated by…
Abstract
Purpose
The study aims to focus on rotation effects on a ribbed channel of gas turbine blades for internal cooling. The combination and interaction between secondary flows generated by angled rib geometry and Coriolis forces in the rotating channel are studied numerically.
Design/methodology/approach
A radially outward flow passage as an internal cooling test model with and without ribs is used to perform the investigation. Aspect ratio of the passage is 1:1. Square ribs with e/Dh = 0.1, p/e = 10 and four various rib angles of 90°, 75°, 60° and 45° are configured on both the leading and trailing surfaces along the rotating duct. The study covers a Reynolds number of 10,000 and Rotation number in the range of 0-0.15.
Findings
Nusselt numbers in the ribbed duct are 2.5 to 3.5 times those of a smooth square duct, depending on the Rotation number and rib angle. The maximum value is attained for the 45° ribbed surface. The synergy angle between the velocity and temperature gradients is improved by the angled rib secondary flows and Coriolis vortex. The decrease of the synergy angle is 8.9, 13.4, 12.1 and 10.1 per cent for the 90°, 75°, 60° and 45° ribbed channels with rotation, respectively. Secondary flow intensity is increased by rotation in the 90° and 75° ribbed ducts and is decreased in 45° and 60° ribbed cases for which the rib-induced secondary flow dominates.
Originality/value
The primary motivation behind this work is to investigate the possibility of heat transfer enhancement by vortex flow with developing turbulence in the view point of the field synergy principle and secondary flow intensity.
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This article aims to study numerically three dimensional developing incompressible flow and heat transfer in a fixed curved pipe.
Abstract
Purpose
This article aims to study numerically three dimensional developing incompressible flow and heat transfer in a fixed curved pipe.
Design/methodology/approach
A projection algorithm based on the second order finite difference method is used for discretizing governing equations written in the toroidal coordinate system.
Findings
The effects of curvature and governing non‐dimensional parameters consisting of Reynolds, Prandtl, and Dean numbers on the flow field, entrance length, and heat transfer are studied in detail. The numerical results indicate that the entrance length depends only on the Reynolds number for the curvature ratios greater than 1/7 and therefore, Dean number is not a pertinent parameter in this range.
Research limitations/implications
For heat transfer analysis, two different thermal boundary conditions, i.e. constant wall temperature and constant heat flux at the wall are implemented. The results are calculated for the Dean numbers in the range of 76‐522 and for the two prandtl numbers of 0.5 and 1.
Practical implications
The results can be used in designing heat exchangers, piping systems, and cooling of gas turbine blades.
Originality/value
The numerical results obtained here concentrate on the detailed investigation of flow and temperature field at the entrance region by a quantitative analysis of hydrodynamic and thermal entrance length. The effects of different thermal boundary conditions and different inlet profiles on the flow and temperature fields are studied in the circular curved pipe for the first time.
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Qingyang Wang, Weifeng Wu, Ping Zhang, Chengqiang Guo and Yifan Yang
To guide the stable radius clearance choice of water-lubricated bearings for single screw compressors, this paper aims to analyze the effects of turbulence and cavitation on…
Abstract
Purpose
To guide the stable radius clearance choice of water-lubricated bearings for single screw compressors, this paper aims to analyze the effects of turbulence and cavitation on bearing performance under two conditions of specified external load and radius clearance.
Design/methodology/approach
A modified Reynolds equation considering turbulence and cavitation is adopted, based on the Jakobsson–Floberg–Olsson boundary condition, Ng–Pan model and turbulent factors. The equation is solved using the finite difference method and successive over-relaxation method to investigate the bearing performance.
Findings
The turbulent effect can increase the hydrodynamic pressure and cavitation. In addition, the turbulent effect can lead to an increase in the equilibrium radius clearance. The turbulent region exhibits a higher load capacity and cavitation rate. However, the increased cavitation negatively impacts the frictional coefficient and end flow rate. The impact of turbulence increases as the radius clearance decreases. As the rotating speed increases, the turbulence effect has a greater impact on the bearing characteristics.
Originality/value
The research can provide theoretical support for the design of water-lubricated journal bearings used in high-speed water-lubricated single screw compressors.
Peer review
The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2024-0029/
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Omar Ahmed, Golareh Jalilvand, Scott Pollard, Chukwudi Okoro and Tengfei Jiang
Glass is a promising interposer substrate for 2.5 D integration; yet detailed analysis of the interfacial reliability of through-glass vias (TGVs) has been lacking. The purpose of…
Abstract
Purpose
Glass is a promising interposer substrate for 2.5 D integration; yet detailed analysis of the interfacial reliability of through-glass vias (TGVs) has been lacking. The purpose of this paper is to investigate the design and material factors responsible for the interfacial delamination in TGVs and identify methods to improve reliability.
Design/methodology/approach
The interfacial reliability of TGVs is studied both analytically and numerically. An analytical solution is presented to show the dependence of the energy release rate (ERR) for interfacial delamination on the via design and the thermal mismatch strain. Then, finite element analysis (FEA) is used to investigate the influence of detailed design and material factors, including the pitch distance, via aspect ratio, via geometry and the glass and via materials, on the susceptibility to interfacial delamination.
Findings
ERR for interfacial delamination is directly proportional to the via diameter and the thermal mismatch strain. Thinner wafers with smaller aspect ratios show larger ERRs. Changing the via geometry from a fully filled via to an annular via leads to lower ERR. FEA results also show that certain material combinations have lower thermal mismatch strains, thus less prone to delamination.
Practical implications
The results and approach presented in this paper can guide the design and development of more reliable 2.5 D glass interposers.
Originality/value
This paper represents the first attempt to comprehensively evaluate the impact of design and material selection on the interfacial reliability of TGVs.
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Kristina W. Kintziger and Jennifer A. Horney
Little attention has been given to the mental and physical health impacts of COVID-19 on the academic public health workforce. Academic public health is an important support…
Abstract
Little attention has been given to the mental and physical health impacts of COVID-19 on the academic public health workforce. Academic public health is an important support mechanism for public health practice, providing expertise and workforce training, conducting research, disseminating evidence-based scientific information to both public health and lay audiences, and serving as a supplementary workforce when additional resources are needed. These roles become more important during a public health emergency, particularly during a prolonged public health crisis like the COVID-19 pandemic. As a result of the COVID-19 response, the roles of academic public health have expanded to include developing and implementing contact tracing, surveillance, testing, and vaccination programs for universities and their surrounding communities, all while continuing to prepare students and support the public health practice workforce in their ongoing efforts. As in other responder groups, this has resulted in significant mental health effects and burnout among public health academicians. The authors suggest important steps that can be taken to improve the resilience of the academic public health workforce and to support their contributions during prolonged public health emergencies.
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Richard W. Johnson and Yu-Lin Shen
The purpose of this study is to numerically assess the misalignment-induced deformation and its implications, in the through-silicon via (TSV), silicon chip, solder micro-bump…
Abstract
Purpose
The purpose of this study is to numerically assess the misalignment-induced deformation and its implications, in the through-silicon via (TSV), silicon chip, solder micro-bump, and bonding layer.
Design/methodology/approach
The 3D finite element model features a TSV/micro-bump bonding structure connecting two adjacent silicon (Si) chips, with and without an underfill layer between. A case that the entire solder layer has transformed into an intermetallic layer is also considered.
Findings
The existence of an underfill layer enhances the overall resistance to shear deformation, although with a higher buildup of local stresses. High shear and tensile stresses can develop in the intermetallic and nearby regions of copper and Si if the solder alloy is replaced by an intermetallic layer. The carrier mobility change in Si may be extensively affected by the mechanical action, even in regions far away from the TSV.
Originality/value
This work parametrically explores the trend of stress and deformation fields due to mechanical shear and its influences on the electrical performance of devices. Potential for damage initiation in the TSV/micro-bump is also examined.
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Puneet Pasricha, Dharmaraja Selvamuthu and Viswanathan Arunachalam
Credit ratings serve as an important input in several applications in risk management of the financial firms. The level of credit rating changes from time to time because of…
Abstract
Purpose
Credit ratings serve as an important input in several applications in risk management of the financial firms. The level of credit rating changes from time to time because of random credit risk and, thus, can be modeled by an appropriate stochastic process. Markov chain models have been widely used in the literature to generate credit migration matrices; however, emergent empirical evidences suggest that the Markov property is not appropriate for credit rating dynamics. The purpose of this article is to address the non-Markov behavior of the rating dynamics.
Design/methodology/approach
This paper proposes a model based on Markov regenerative process (MRGP) with subordinated semi-Markov process (SMP) to obtain the estimates of rating migration probability matrices and default probabilities. Numerical example is given to illustrate the applicability of the proposed model with the help of historical Standard & Poor’s (S&P) credit rating data.
Findings
The proposed model implies that rating of a firm in the future not only depends on its present rating, but also on its previous ratings. If a firm gets a rating lower than its previous ratings, there are higher chances of further downgrades, and the issue is called the rating momentum. The model also addresses the ageing problem of credit rating evolution.
Originality/value
The contribution of this paper is a more general approach to study the rating dynamics and overcome the issues of inappropriateness of Markov process applied in rating dynamics.