A two‐level domain decomposition method with accurate interface conditions for the Helmholtz problem

A new and efficient two‐level, non‐overlapping domain decomposition (DD) method is developed for the Helmholtz equation in the two Lagrange multiplier framework. The transmission conditions are designed by utilizing perfectly matched discrete layers (PMDLs), which are a more accurate representation of the exterior Dirichlet‐to‐Neumann map than the polynomial approximations used in the optimized Schwarz method. Another important ingredient affecting the convergence of a DD method, namely, the coarse space augmentation, is also revisited. Specifically, the widely successful approach based on plane waves is modified to that based on interface waves, defined directly on the subdomain boundaries, hence ensuring linear independence and facilitating the estimation of the optimal size for the coarse problem. The effectiveness of both PMDL‐based transmission conditions and interface‐wave‐based coarse space augmentation is illustrated with an array of numerical experiments that include comprehensive scalability studies with respect to frequency, mesh size and the number of subdomains. Copyright © 2015 John Wiley & Sons, Ltd.     

An Improved Wheeler Residual Stress Model For Remaining Life Assessment of Cracked Plate Panels

In this paper an improved Wheeler residual stress model has been proposed for remaining life assessment of cracked plate panels under variable amplitude loading (VAL). The improvement to the Wheeler residual stress model is in terms of the expressions for the shaping exponent, which is generally obtained through experiments. Simple expressions for the computation of shaping exponent have been proposed for compact tension (CT) specimen and plate panels with a center crack or an edge crack. The remaining life assessment has been carried out by employing linear elastic fracture mechanics (LEFM) principles. In the present study, the degree of influence of overload ratio (OLR) and the shape factor (β) on the shaping exponent have been investigated for remaining life assessment of cracked plate panels under tensile overload. It is observed that the parameters OLR and β have differing influences on the shaping exponent. Crack growth studies have been conducted on CT specimen and plate panels with a center crack or an edge crack subjected to tensile overloads for validating the proposed expressions. It is observed from the studies that the remaining life predicted using the improved Wheeler model for these plate panels are in close agreement with the experimental values reported in literature.     

Slot allocation strategies for delay sensitive traffic support in asynchronous ad hoc wireless networks

Supporting real‐time traffic in ad hoc wireless networks is considered as a challenging problem. Existing bandwidth reservation mechanisms assume a TDMA environment where achieving time synchronisation is expensive in terms of resources. Heuristics that exist for slot allocation schemes assume a CDMA over TDMA model in order to alleviate the presence of hidden terminals. Slot allocation strategies in the presence of hidden terminals assume significance in a single channel system for supporting delay sensitive traffic. In this paper, we propose three heuristics for the slot allocation process in asynchronou single channel multihop wireless networks in the presence of hidden terminals. The heuristics we propose are the early fit reservation (EFR), minimum bandwidth‐based reservation (MBR) and position‐based hybrid reservation (PHR). The EFR heuristic assigns bandwidth link‐by‐link in the forward path. The MBR heuristic allocates bandwidth to the links in the increasing order of free conn‐slots. The PHR heuristic assigns bandwidth for every link proportional to its position in the path. Simulation studies show that EFR performs better in terms of delay characteristics. MBR provides better call blocking performance at the cost of high end‐to‐end delay. PHR provides a better delay performance compared to MBR and better call blocking performance comparedto EFR. Copyright © 2004 John Wiley & Sons, Ltd.     

Using the Fourier series in a circuits or DSP laboratory

Laboratory experiments on circuits that exhibit transient effects are usually performed with a periodic input signal (like a square wave). Three experiments on a resistor capacitor (RC) filter using transient analysis and the solution of a first-order differential equation are described. The results according to a Fourier analysis are then reinterpreted. The similarities and differences in the results from the two theories provide a better understanding of both. Also, using Fourier analysis introduces the basic strengths and weaknesses of discrete signal processing. Further additions to this laboratory are discussed and include (1) the addition of more first-order linear circuits; (2) the addition of second-order (and higher-order) linear circuits; (3) the processing of all data in the frequency domain; and (4) the study of nonlinear circuits     

Component reliability specification

Building reliability into a product is costly and needs to be traded against the consequences of product unreliability. This article is the third in a series of three articles, where the first deals with optimal investment in reliability, which involves executing two tasks—(i) deciding on the reliability requirements and (ii) deciding on component specifications (SP) to achieve the desired reliability. The second article deals with the first task and in this third article, we focus on the second task.     

Radiative Calibration of Heat-Flux Sensors at NIST: Facilities and Techniques

We present an overview of the National Institute of Standards and Technology high temperature blackbodies, both in operation and in development, suitable for heat-flux sensor calibration. Typical results of calibrations using the transfer technique in the 25 mm Variable-Temperature Blackbody are presented to demonstrate the long-term repeatability of the calibration technique. A comparative study of the absolute and transfer calibrations of a Gardon gage in a spherical blackbody with a cooled enclosure surrounding the gage housing was conducted. Results of this study demonstrated the influence of convection associated with absolute calibration of sensors in a cooled enclosure. Plans for further development of the transfer technique to higher heat-flux levels and the associated technical issues are discussed.     

The exponentiated Weibull family: a graphical approach

The exponentiated Weibull family extends the two-parameter Weibull distribution. The shape of the Weibull plotting-paper plots are discussed, and a parametric characterization of the pdf and the failure rate for the exponentiated Weibull family are carried out. Such a study is very relevant to deciding if a given data set can be adequately modeled by such a distribution     

Comparative performance of HFC134a- and HCFC22-based vapour absorption refrigeration systems

A comparative thermodynamic study of the vapour absorption refrigeration systems (VARS) working with HFC134a and HCFC22 is presented. Due to its superior performance in HCFC22-based VARS, dimethyl acetamide is chosen as the solvent for both the refrigerants. It is observed that the HCFC22-based system yields significantly better COP than the HFC134a system. However, since the latter operates at lower pressures, the possibility exists to improve its COP by resorting to two-stage operation. © 1998 John Wiley & Sons, Ltd.     

Optimal performance of double-effect series flow vapour absorption refrigeration systems with new working fluids

In a double-effect series flow vapour absorption refrigeration system (VARS), an optimum value of the low-pressure generator temperature exists at which all the vapour generated at the high-pressure generator is condensed. At these conditions, a comparative study of the performance of VARS using environment friendly refrigerants such as, R32, R134a, and R124 with N,N′-dimethyl acetamide (DMAC) as the absorbent is made. It is found that the system with R32-DMAC gives the best performance at high evaporator temperatures. R124-DMAC may be preferred at extreme operating conditions like low evaporator and high heat rejection temperatures. Influence of operating temperatures (high-pressure generator, evaporator, condenser and absorber) and the effectiveness of heat exchangers on the optimum low-pressure generator temperature, cut-off temperature, circulation ratio and coefficient of performance are studied. © 1998 John Wiley & Sons, Ltd.     

HCFC22-based vapour absorption refrigeration systems. Part II: Influence of component effectiveness

A thermodynamic analysis was performed on a single-stage vapour absorption refrigeration system (VARS) operating with monochlorodifluouromethane (HCFC22) as a refrigerant and dimethylether of tetraethyleneglycol (DMETEG) or dimethyl acitamide (DMA) as a working fluid. Influence of solution heat exchanger effectiveness (E_h) and mass transfer effectivenesses of absorber (E_a) and generator (E_g) on the performance of VARS were studied. The variations in heat quantities at solution heat exchanger, generator and absorber as well as performance characteristics, namely CR, COP_th and second law efficiency (ϵ) at various operating temperatures are reported. As expected, low CR and high COP_th and ϵ can be obtained at high values of E_h, E_a and E_g. The effects of E_h on the performance is more pronounced when compared to that of E_a and E_g. Also, the change in the circulation ratio (CR) due to a given change in E_a is higher than that due to the same change in E_g. CR is not a function of E_h. A comparison of the influence of E_h, E_a, and E_g on the VARS performance for HCFC22-DMA and HCFC22-DMETEG pairs was made.