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      Volume 44, Issue 12

      December 2019

    • A review of liquid flow and heat transfer in microchannels with emphasis to electronic cooling


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      Since the realization of microchannel devices more than three and half decades ago with water as the cooling fluid providing heat transfer enhancement, significant progress has been made to improve the cooling performance. Thermal management for electronic devices with their ever-widening user profile remains the major driving force for performance improvement in terms of miniaturisation, long-term reliability, and ease of maintenance. The ever-increasing requirement of meeting higher heat flux density in more compact and powerful electronic systems calls for further innovative solutions. Some recent studies indicate the promise offered by processes with phase change and the use of active devices. But their adoption for electronic cooling still weighs unfavourably against long-term fluid stability and simplicity of device profile with moderate to high heat transfer capability. Applications and reviews of these promising research trends have been briefly visited in this work. The main focus of this review is the flow and heat transfer regime related to electronic cooling in evolving channel forms, whose fabrication are being enabled by the significant advancement in micro-technologies. Use of disruptive wall structures like ribs, cavities, dimples, protrusions, secondary channels and other interrupts along with smooth-walled channels with curved flow passages remain the two chief geometrical innovationsenvisaged for these applications. These innovations target higher thermal enhancement factor since this implies more heat transfer capability for the same pumping power in comparison with the corresponding straight-axis,smooth-wall channel configuration. The sophistication necessary to deal with the experimental uncertainties associated with the micron-level characteristic length scale of any microchannel device delayed the availability of results that exhibited acceptable matching with numerical investigations. It is indeed encouraging that the experimental results pertaining to simple smooth channels to grooved, ribbed and curved microchannels without unreasonable increase in pumping power have shown good agreement with conventional numerical analyses based on laminar-flow conjugate heat-transfer model with no-slip boundary condition. The flow mechanism with the different disruptive structures like dimple, cavity and rib, fin and interruption, vortex generator, converging diverging side walls or curved axis are reviewed to augment the heat transfer. While the disruptions cause heattransfer enhancement by interrupting the boundary layer growth and promoting mixing by the shed vortices or secondary channel flow, the flow curvature brings in enhancement by the formation of secondary rolls culminating into chaotic advection at higher Reynolds number. Besides these revelations, the numerical studies helped in identifying the parameter ranges, promoting a particular enhancement mechanism. Also, the use of modern tools like Poincare section and the analysis of flow bifurcation leading to chaotic advection is discussed. Amongthe different disruptive structures, sidewall cavity with rib on the bottom wall within the cavity plays a significant role in augmenting the thermal performance. Among the different converging-diverging side walls or curved axis, the sinusoidal channel provides the highest mixing by the introduction of secondary vortices or deanvortices to augment the heat transfer with less pressure drop. The optimum geometry in terms of high heat transfer with low pressure plays a major role in the design of heat sink. Directions of some future research are provided at the end.

    • Extended hierarchical key assignment scheme (E-HKAS): how to efficiently enforce explicit policy exceptions in dynamic hierarchies


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      In this paper, we focus on practically motivated flexibility requirements for the hierarchical access control model, namely transitive exception and anti-symmetric exception. Additionally, we motivate a new flexibility requirement called ‘‘class delegation with descendant(s) safety’’ in a practical application scenario.We propose our extended hierarchical key assignment scheme (E-HKAS) that satisfies all three aforementioned flexibility requirements in a dynamic hierarchy of security classes. To propose a generic E-HKAS, we model the hierarchical access control policy as a collection of access groups. E-HKAS enforces transitive and antisymmetric exceptions using an efficient group-based encryption scheme. To enforce class delegation with descendant(s) safety, we propose a novel cryptographic primitive called group proxy re-encryption (GPRE) thatsupports proxy re-encryption between two access groups. We present an IND-CPA-secure construction of our proposed GPRE scheme and formally prove its security. Performance analysis shows that the proposed E-HKASenforces explicit transitive and anti-symmetric exceptions more efficiently than the existing approaches in the literature. Computation cost for key derivation is constant and does not depend on the depth of the hierarchy. Also, to enforce class delegation with descendant(s) safety, the proposed E-HKAS requires constant number of computational steps to be executed.

    • Resource ratio based virtual machine placement in heterogeneous cloud data centres


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      Server consolidation through virtualization improves resource utilization significantly in Cloud Data Centres (CDCs). We study the case of a CDC hosting heterogeneous Physical Machines (PMs) as a variable size vector bin-packing problem. The PMs have different configurations of multiple resources like CPU,RAM, Disk Storage and Network Bandwidth. In this paper, we propose PMNeAR-vector heuristic for PM selection in PM-heterogeneity aware Virtual Machine (VM) initial placement. The proposed heuristic is compared with well-known heterogeneity aware FFD-DRR and BFD bin centric heuristics using a dataset withrandom instances of both VMs and PMs of heterogeneous configurations. Fifty rounds of VM initial placement simulation experiments were conducted to validate the average resource wastage. The results show that on average FFD-DRR and BFD bin centric heuristics are wasting 22.62% and 37.27% more resource units compared to the proposed PMNeAR-vector heuristic.

    • Real-time BigData and Predictive Analytical Architecture for healthcare application


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      The amount of data produced within health informatics has grown to be quite vast. The large volume of data generated by various vital sign monitoring devices needs to be analysed in real time to alert the care providers about changes in a patients condition. Data processing in real time has complex challenges for thelarge volume of data. The real-time system should be able to collect millions of events per seconds and handle parallel processing to extract meaningful information efficiently. In our study, we have proposed a real-time BigData and Predictive Analytical Architecture for healthcare application. The proposed architecture comprises three phases: (1) collection of data, (2) offline data management and prediction model building and (3) real-time processing and actual prediction. We have used Apache Kafka, Apache Sqoop, Hadoop, MapReduce, Storm and logistic regression to predict an emergency condition. The proposed architecture can perform early detection of emergency in real time, and can analyse structured and unstructured data like Electronic Health Record (EHR) to perform offline analysis to predict patient’s risk for disease or readmission. We have evaluated prediction performance on different benchmark datasets to detect an emergency condition of any patient in real time and possibility of readmission.

    • Inlet swirl decay of non-Newtonian fluid in laminar flows through tubes


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      Use of swirling velocity at the inlet has been a classical way to enhance the transport properties of flow within pipes. Owing to the laminar nature of flow of fluids in small tubes, enhancing transport within such tubes is very important for applications involving fluid mixing or heat transfer. Non-Newtonian fluids havevaried applications in small pipes ranging from heat pipes to micro heat exchangers. To enhance the transport characteristics, we adopt the method of including swirling of fluid at the inlet of the pipe. Considering this, weinvestigate the effect of the power-law index of the fluid on the decay of the inlet swirl. We find that the length through which swirl decays to 1% of its value at inlet is strongly dependent on the power-law index value. A correlation for the decay length as a function of Reynolds number and power-law index is developed.

    • A Series of ILP models for the optimization of water distribution networks


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      The design of rural drinking water schemes consists of optimization of several network components like pipes, tanks, pumps and valves. The sizing and configuration of these network configurations need to be such that the water requirements are met while at the same time being cost efficient so as to be withingovernment norms. We developed the JalTantra system to design such water distribution networks. The Integer Linear Program (ILP) model used in JalTantra and described in our previous work solved the problem optimally, but took a significant amount of time for larger networks—an hour for a network with 100 nodes. In this current work, we describe a series of three improvements of the model. We prove that these improvements result in tighter models, i.e. the set of points of linear relaxation is strictly smaller than the linear relaxation for the initialmodel. We test the series of three improved models along with the initial model over eight networks of various sizes and show a distinct improvement in performance. The 100-node network now takes only 49 s to solve. These changes have been implemented in JalTantra, resulting in a system that can solve the optimization of real world rural drinking water networks in a matter of seconds. The JalTantra system is free for use, and is available at https://www.cse.iitb.ac.in/jaltantra/.

    • A study on friction and wear characteristics of Fe–Cu–Sn alloy with MoS₂ as solid lubricant under dry conditions


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      Iron-based alloys are materials of choice for engineering applications such as bearings and gears owing to their low cost, ease of manufacture, high strength, availability, and good wear resistance and low coefficient of friction. In this study, Fe–Cu–Sn composite containing varying percentage of molybdenum disulfide (MoS₂) is developed using simple single stage compaction and sintering. The friction and wear behaviors of these composites were studied ball-on-disc tribometer in which EN8 steel ball was used. It was found that with the increase in percentage of MoS₂ from 0 to 3 wt% the coefficient of friction and wear ratesubstantially decreases from around 0.85 to 0.25. The wear mechanism in base composition (0% MoS₂) is observed to be adhesive and abrasive, whereas mild abrasive wear was observed in the 3 wt% MoS₂ composite.The hardness of composite was also found to improve with the increase in MoS₂ weight fraction.

    • Numerical investigation to predict optimum attack angle combination of longitudinal vortex generators in compact heat exchangers for thermo-hydraulic heightened performance


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      The current 3-D numerical analysis explores the effect of combinations of rectangular winglet pairs (RWPs) having different attack angles (i.e. 5 °, 15 ° and 25 °) along a row of the tube array, on the performance of the fin and tube heat exchanger (FTHE). The considered airside Reynolds number Re ranges from 500 to 900. In total, six combinations of three attack angle vortex generators (VGs) have been numerically analysed namely 5 °-15 °-25 °, 5 °-25 °-15 °, 15 °-5 °-25 °, 15 °-25 °-5 °, 25 °-5 °-15 ° and 25 °-15 °-5 °. The performance of the FTHE is represented by area goodness factor. The performance rankings of the FTHEs are also obtained by the MOORA method. Finally, 5 °-25 °-15 ° case provides the best thermal hydraulic performance for which heat transfer coefficient (h) is increased by 68.20% at Re = 500 and 81.78% at Re = 900, with a significant pressure drop penalty.

    • Flow and heat transfer investigation of a circular jet issuing on different types of surfaces


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      In the present study, flow and heat transfer characteristics of an impinging jet issuing from a circular cross-sectional pipe on a rectangular plate, convex and concave hemispheres were investigated numerically for various Reynolds numbers and jet-to-plate distances. Steady and three-dimensional Reynolds-Averaged Navier- Stokes equations were solved iteratively utilizing finite volume method. Turbulence model assessment study reveals that the transitional Shear Stress Transport k-ω turbulence model can be used for such a problem.Regardless of the geometry of the impinging surfaces, two different flow regions were detected around y/D = 3. The first region starts from the stagnation point (y/D = 0) to the edge of the rectangular plate and hemispheres (y/D = 3). In this region, the maximum heat transfer is obtained for the jet impinging on the rectangular plate. However, in the second region that covers the wall jet zone the impinging jet on the convex hemisphere provides the maximum while the concave causes the minimum heat transfer. The most efficient jet-to-plate distance is found as H/D = 2 in the first region while heat transfer does not change with distance in the wall jet zone. It was shown that Nusselt number increases with Reynolds number when the jet impinges on the concave and convex hemispheres as observed for the impinging jet on a flat plate.

    • Testbed evaluation of a seamless handover mechanism for an SDN-based enterprise WLAN


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      Many modern-day applications require seamless connectivity to provide a good quality experience to mobile users. Thus, mobility management mechanism becomes a crucial aspect of all the wireless technologies used to connect to the Internet. In this letter, we present the testbed implementation details of aNetwork Address Translation (NAT)-based, client-unaware, seamless handover mechanism for a Software- Defined Enterprise WLAN framework. The results from the testbed implementation corroborate that the handover mechanism can provide uninterrupted connectivity during a handover process.

    • Natural language processing in mining unstructured data from software repositories: a review


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      With the increasing popularity of open-source platforms, software data is easily available from various open-source tools like GitHub, CVS, SVN, etc. More than 80 percent of the data present in them is unstructured. Mining data from these repositories helps project managers, developers and businesses, in gettinginteresting insights. Most of the software artefacts present in these repositories are in the natural language form, which makes natural language processing (NLP) an important part of mining to get the useful results. The paper reviews the application of NLP techniques in the field of Mining Software Repositories (MSR). The papermainly focuses on sentiment analysis, summarization, traceability, norms mining and mobile analytics. The paper presents the major NLP works performed in this area by surveying the research papers from 2000 to 2018. The paper firstly describes the major artefacts present in the software repositories where the NLP techniques have been applied. Next, the paper presents some popular open-source NLP tools that have been used to perform NLP tasks. Later the paper discusses, in brief, the research state of NLP in MSR field. The paper also lists down the various challenges along with the pointers for future work in this field of research and finally the conclusion.

    • Using game theory to model DoS attack and defence


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      Denial of service (DoS) or distributed denial of service (DDoS) attacks based on bandwidth depletion remain a persistent network security threat and have always been an important issue for system administrators and researchers. Defence mechanisms proposed so far to defend against such attacks could not address the problem adequately and efficiently due to lack of quantitative approaches in modelling defence strategies against DoS/DDoS attacks. Game theory is a microeconomic and mathematical tool that provides a quantitative framework to model such attacks. A model based on game theory can act as a decision supportsystem to the defender and augments its capabilities to take best decisions for maintaining an optimum level of network security round the clock against such attacks. Inspired by this, different DoS/DDoS scenarios, where game theory has been used to represent the strategic interaction between the attacker and a defender, are investigated. Based on the strategic interactions, a game theoretical defence mechanism is proposed to mitigate DoS/DDoS attacks. The proposed mechanism is based on two-player zero-sum game. It considers DoS/DDoSattack based on bandwidth depletion where an attacker wants to occupy maximum bandwidth of a link having a limited capacity. The attacker does so by flooding the network with unsolicited or malicious flows. The attacker has to decide an effective attack rate per flow. It has to choose an optimal size of botnet also for a cost-effective attack. It does trade-off analysis prior to attack. If its payoff or benefit obtained is less than the attack cost, it chooses to refrain from launching such a costlier DoS/DDoS attack. On the other hand, to set an upper bound on network traffic, the defender needs to set an optimum threshold per flow so that maximum attack flows are either dropped or redirected to a honeypot deployed in the network. Arbitrary setting of a threshold for flow rates can also cause a loss of legitimate flows. The defender chooses the optimum threshold value with precise estimation to minimize loss of legitimate flows. The defender also does trade-off analysis and sets the threshold in a way that can minimize the attacker’s payoff. This optimization problem is presented as a game between the attackerand defender. Action sets and objective functions of both players are defined. The network constrains are modelled and payoffs are calculated. The game converges to Nash equilibrium. The best course of actions is deduced from the Nash strategies. Results obtained by simulation and numerical calculations are in favour of the proposed game theoretical defence mechanism and strongly advocate the worthiness of using game theory to defend against DoS and DDoS attacks to strengthen network security.

    • On the numerical solution of fractional differential equations with cubic nonlinearity via matching polynomial of complete graph


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      This study deals with a generalized form of fractional differential equations with cubic nonlinearity, employing a matrix-collocation method dependent on the matching polynomial of complete graph. The method presents a simple and efficient algorithmic infrastructure, which contains a unified matrix expansion of fractional-order derivatives and a general matrix relation for cubic nonlinearity. The method also performs a sustainable approximation for high value of computation limit, thanks to the inclusion of the matching polynomial in matrix system. Using the residual function, the convergence and error estimation are investigated viathe second mean value theorem having a weight function. In comparison with the existing results, highly accurate results are obtained. Moreover, the oscillatory solutions of some model problems arising in several applied sciences are simulated. It is verified that the proposed method is reliable, efficient and productive.

    • Classifier combination approach for question classification for Bengali question answering system


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      Question classification (QC) is a prime constituent of an automated question answering system. The work presented here demonstrates that a combination of multiple models achieves better classification performance than those obtained with existing individual models for the QC task in Bengali. We have exploited stateof-the-art multiple model combination techniques, i.e., ensemble, stacking and voting, to increase QC accuracy. Lexical, syntactic and semantic features of Bengali questions are used for four well-known classifiers, namelyNaĩve Bayes, kernel Naı¨ve Bayes, Rule Induction and Decision Tree, which serve as our base learners. Singlelayer question-class taxonomy with 8 coarse-grained classes is extended to two-layer taxonomy by adding 69 fine-grained classes. We carried out the experiments both on single-layer and two-layer taxonomies. Experimental results confirmed that classifier combination approaches outperform single-classifier classification approaches by 4.02% for coarse-grained question classes. Overall, the stacking approach produces the best results for fine-grained classification and achieves 87.79% of accuracy. The approach presented here could be used in other Indo-Aryan or Indic languages to develop a question answering system.

    • Effects of the physical parameter on gate all around FET


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      As the devices are getting compact, the size of transistors reduces day by day; however, with certain limitations. Due to miniaturization, the characteristics of the transistor change due to quantum mechanical effects and the present scenario, analytically modeled surface potential-based gate all around (GAA) FET modelby solving 1-D Poisson’s equation, approximation method and using necessary boundary condition. Here, the change in channel material (Si, InP, GaAs, InAs and Ge), channel radius (varied from 6 nm to 10 nm), oxide thickness (changed from 2 nm to 5 nm), drain to source voltage (varied from -0.5 V to 0.5 V), Source/Draindoping (varied from 10¹⁷ to 10²²/cm³) and temperature (from 0 to 300 K) of the transistor, surface potential changes from -1.6 V to 1.3 V approx. respectively, considered as the GAA FET parameters. The proposed novel model exhibits better control over hot carrier effect, Drain Induced Barrier Lowering (DIBL), reduced threshold voltage and other such short channel effects in the GAA FET. Moreover, the I–V characteristics of the GAA FET were analyzed. The MATLAB code is used for modeling of the GAA FET nanowire transistor.

    • Volume Contents

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    • Acknowledgements

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