A testbed for performance evaluation of load‐balancing strategies for Web server systems

To improve response time of a Web site, one replicates the site on multiple servers. The effectiveness of a replicated server system will depend on how the incoming requests are distributed among replicas. A large number of load‐balancing strategies for Web server systems have been proposed. In this paper we describe a testbed that can be used to evaluate the performance of different load‐balancing strategies. The testbed uses a general architecture which allows different load‐balancing approaches to be supported easily. It emulates a typical World Wide Web scenario and allows variable load generation and performance measurement. We have performed some preliminary experiments to measure the performance of a few policies for load balancing using this testbed. Copyright © 2003 John Wiley & Sons, Ltd.     

Estimation of critical stability parameters by asymptotic analysis in multilayer extrusion

In industrial coextrusion operations, multiple layers are frequently brought in contact with each other at different temperatures to “match” viscosities and to reduce interfacial instability. Here, a linear stability analysis of multilayer plane Poiseuille flow is carried out for real, specific polymer melts in terms of process parameters, such as temperatures and flow rates. However, heat transfer across the layers is neglected as a first approximation. Previously, it was observed that below a certain Reynolds number and above a certain relaxation time, the flow-rate ratio (or the depth ratio), above which the flow is stable to all wavelengths, can be estimated by the long-wavelength asymptotic analysis (1,3). The Reynolds numbers for the industrial flows are smaller, and the relaxation times are larger than the corresponding critical values by several orders of magnitude. Therefore, the critical stability parameters can be estimated in the present study for industrial cases using the asymptotic stability analysis at long wavelengths. Although a large number of earlier parametric studies of model fluids indicate the influence of various theological parameters, only the viscosity stratification plays an important role in the interfacial instability within the operating range of industrial coextrusion processes.     

Alumina Supported Co–K–Mo Based Catalytic Material for Diesel Soot Oxidation

Alumina supported Co–K–Mo based mixed metal oxide type catalytic materials have been prepared by co-impregnation. These catalysts show excellent activity for carbon as well as diesel soot oxidation, which could be due to the redox properties of Mo and Co as well as to a synergistic effect of molybdenum, cobalt, and K contents. The catalyst containing 5 wt% molybdenum shows a lowering of carbon oxidation by about 190 °C under loose contact conditions as compared to the non-catalyzed reaction, as well as to bare alumina. Characterization studies suggest a composite nature of these materials, while thermal stability investigations confirm the stable nature. The selected catalyst has been studied by XPS, however, it is difficult to conclude which are the important factors contributing to the catalytic activity. It appears to be a synergistic effect of Co, K, and Mo components as these catalysts show much improved activity as compared to the individual components in supported and unsupported forms.     

Influence of Exposure Time,Shear Stress,and Surfactants on Detachment of <Emphasis Type="Italic">Escherichia coli</Emphasis> O157:H7 from Fresh Lettuce Leaf Surfaces During Washing Process

Washing and sanitation of fresh produce are critical for detachment and inactivation of pathogenic and spoilage microorganisms. The overall goal of this study was to evaluate the influence of processing conditions such as washing time, shear stress, and a food grade surfactant on the detachment of Escherichia coli O157:H7 from lettuce leaf surfaces during a simulated washing process. A benchtop scale rotating disk washing device was designed to simulate the processing conditions used in commercial washing systems. Numerical simulations were carried out to calculate the flow profile and shear stress near the leaf surface. Removal of E. coli O157:H7 as a function of processing conditions in the rotating disk washing device was evaluated by enumeration of bacterial population using plate counting and changes in bioluminescence intensity and further confirmed by bioluminescence imaging. The results showed that shear stress was necessary to remove attached bacteria from inoculated leaf samples and simple incubation of inoculated leaves with wash water was not effective. Furthermore, detachment of bacteria from inoculated leaf samples increased with increasing shear stress and incubation time. Shear stress values in the range of 0 – 300 mPa were adequate to induce removal of approximately 1.5 log CFU/cm² of bacteria from inoculated samples. The addition of a surfactant increased the removal of bacteria from inoculated leaf samples by 0.5 log CFU/cm². Overall, these results may guide the design of new produce wash systems as well as current industrial practice related to flow conditions and use of surfactants during washing of fresh produce.     

Numerical study of compressible convective heat transfer with variations in all fluid properties

The effects of property variations in single-phase laminar forced micro-convection with constant wall heat flux boundary condition are investigated in this work. The fully-developed flow through micro-sized circular (axisymmetric) geometry is numerically studied using two-dimensional continuum-based conservation equations. The non-dimensional governing equations show significance of momentum transport in radial direction due to μ(T) variation and energy transport by fluid conduction due to k(T) variation. For the case of heated air, variation in C_p(T) and k(T) causes increase in Nu. This is owing to: (i) reduction in T_w, (T_w ? T_m), and (?T/?r)_w and (ii) change in ?T_m/?z results in axial conduction along the flow. The effects of ρ(p,T) and μ(T) variation on convective-flow are indirect and lead to: (i) induce radial velocity which alters u(r) profile significantly and (ii) change in (?u/?r)_w along the flow. It is proposed that the deviation in convection with C_p(T), k(T) variation is significant through temperature field than ρ(p,T), μ(T) variation on velocity field. It is noted that Nu due to variation in properties differ from invariant properties (Nu = 48/11) for low subsonic flow.     

Comparative study of the optimal operation of methane reforming process in cylindrical and spherical reactors using multi-objective optimization

We present a multi-objective optimization (MOO) based study of the optimal operation of methane reformer for spherical reactor and compare the results with the ones for the cylindrical reactor. We considered three objective functions for this comparative study, namely maximization of hydrogen production, minimization of carbon dioxide emission, and minimization of power loss due to pressure drop in the reactor. We solve four MOO problems, which include three 2-objective problems with each pair of the aforementioned three objectives. In addition, we also solve a three objective problem considering all the three objectives. The optimization variables considered for the MOO study correspond to the feed conditions. Specifically, the three variables include the inlet temperature and the molar feed ratios of oxygen to methane & steam to methane.