The impact of diffusion on confined oscillated bubbly fluid

We consider the dynamics of monodisperse bubbly fluid confined by two plane solid walls and subjected to small-amplitude high-frequency transversal oscillations. The frequency these oscillations is assumed to be high in comparison with typical relaxation times for a single bubble, but comparable with the eigenfrequency of volume oscillations. A time-averaged description accounting for mutual coupling of the phases and the diffusivity of bubbles is applied. We find nonuniform steady states with the liquid quiescent on average. At relatively low frequencies accumulation of bubbles either at the walls or in planes oriented parallel to the walls is detected. These one-dimensional states are shown to be unstable. At relatively high frequencies the bubbles accumulate at the central plane and the solution is stable.Comment: 12 pages, 9 figures, submitted to Phys. Fluid

A framework for proving the self-organization of dynamic systems

This paper aims at providing a rigorous definition of self- organization, one of the most desired properties for dynamic systems (e.g., peer-to-peer systems, sensor networks, cooperative robotics, or ad-hoc networks). We characterize different classes of self-organization through liveness and safety properties that both capture information re- garding the system entropy. We illustrate these classes through study cases. The first ones are two representative P2P overlays (CAN and Pas- try) and the others are specific implementations of \Omega (the leader oracle) and one-shot query abstractions for dynamic settings. Our study aims at understanding the limits and respective power of existing self-organized protocols and lays the basis of designing robust algorithm for dynamic systems

Self-organization and social science

Abstract Complexity science and its methodological applications have increased in popularity in social science during the last two decades. One key concept within complexity science is that of self-organization. Self-organization is used to refer to the emergence of stable patterns through autonomous and self-reinforcing dynamics at the micro-level. In spite of its potential relevance for the study of social dynamics, the articulation and use of the concept of self-organization has been kept within the boundaries of complexity science and links to and from mainstream social science are scarce. These links can be difficult to establish, even for researchers working in social complexity with a background in social science, because of the theoretical and conceptual diversity and fragmentation in traditional social science. This article is meant to serve as a first step in the process of overcoming this lack of cross-fertilization between complexity and mainstream social science. A systematic review of the concept of self-organization and a critical discussion of similar notions in mainstream social science is presented, in an effort to help practitioners within subareas of complexity science to identify literature from traditional social science that could potentially inform their research

ACHIEVING AUTONOMIC SERVICE ORIENTED ARCHITECTURE USING CASE BASED REASONING

Service-Oriented Architecture (SOA) enables composition of large and complex computational units out of the available atomic services. However, implementation of SOA, for its dynamic nature, could bring about challenges in terms of service discovery, service interaction, service composition, robustness, etc. In the near future, SOA will often need to dynamically re-configuring and re-organizing its topologies of interactions between the web services because of some unpredictable events, such as crashes or network problems, which will cause service unavailability. Complexity and dynamism of the current and future global network system require service architecture that is capable of autonomously changing its structure and functionality to meet dynamic changes in the requirements and environment with little human intervention. This then needs to motivate the research described throughout this thesis. In this thesis, the idea of introducing autonomy and adapting case-based reasoning into SOA in order to extend the intelligence and capability of SOA is contributed and elaborated. It is conducted by proposing architecture of an autonomic SOA framework based on case-based reasoning and the architectural considerations of autonomic computing paradigm. It is then followed by developing and analyzing formal models of the proposed architecture using Petri Net. The framework is also tested and analyzed through case studies, simulation, and prototype development. The case studies show feasibility to employing case-based reasoning and autonomic computing into SOA domain and the simulation results show believability that it would increase the intelligence, capability, usability and robustness of SOA. It was shown that SOA can be improved to cope with dynamic environment and services unavailability by incorporating case-based reasoning and autonomic computing paradigm to monitor and analyze events and service requests, then to plan and execute the appropriate actions using the knowledge stored in knowledge database

Portfolio peak algorithms achieving superior performance for maximizing throughput in WiMAX networks

The Mobile WiMAX IEEE 802.16 standards ensure provision of last mile wireless access, variable and high data rate, point to multi-point communication, large frequency range and QoS (Quality of Service) for various types of applications. The WiMAX standards are published by the Institute of Electric and Electronic Engineers (IEEE) and specify the standards of services and transmissions. However, the way how to run these services and when the transmission should be started are not specified in the IEEE standards and it is up to computer scientists to design scheduling algorithms that can best meet the standards. Finding the best way to implement the WiMAX standards through designing efficient scheduler algorithms is a very important component in wireless systems and the scheduling period presents the most common challenging issue in terms of throughput and time delay. The aim of the research presented in this thesis was to design and develop an efficient scheduling algorithm to provide the QoS support for real-time and non-real-time services with the WiMAX Network. This was achieved by combining a portfolio of algorithms, which will control and update transmission with the required algorithm by the various portfolios for supporting QoS such as; the guarantee of a maximum throughput for real-time and non-real-time traffic. Two algorithms were designed in this process and will be discussed in this thesis: Fixed Portfolio Algorithms and Portfolio Peak Algorithm. In order to evaluate the proposed algorithms and test their efficiency for IEEE 802.16 networks, the authors simulated the algorithms in the NS2 simulator. Evaluation of the proposed Portfolio algorithms was carried out through comparing its performance with those of the conventional algorithms. On the other hand, the proposed Portfolio scheduling algorithm was evaluated by comparing its performance in terms of throughput, delay, and jitter. The simulation results suggest that the Fixed Portfolio Algorithms and the Portfolio Peak Algorithm achieve higher performance in terms of throughput than all other algorithms. Keywords: WiMAX, IEEE802.16, QoS, Scheduling Algorithms, Fixed Portfolio Algorithms, and Portfolio Peak Algorithms.The Mobile WiMAX IEEE 802.16 standards ensure provision of last mile wireless access, variable and high data rate, point to multi-point communication, large frequency range and QoS (Quality of Service) for various types of applications. The WiMAX standards are published by the Institute of Electric and Electronic Engineers (IEEE) and specify the standards of services and transmissions. However, the way how to run these services and when the transmission should be started are not specified in the IEEE standards and it is up to computer scientists to design scheduling algorithms that can best meet the standards. Finding the best way to implement the WiMAX standards through designing efficient scheduler algorithms is a very important component in wireless systems and the scheduling period presents the most common challenging issue in terms of throughput and time delay. The aim of the research presented in this thesis was to design and develop an efficient scheduling algorithm to provide the QoS support for real-time and non-real-time services with the WiMAX Network. This was achieved by combining a portfolio of algorithms, which will control and update transmission with the required algorithm by the various portfolios for supporting QoS such as; the guarantee of a maximum throughput for real-time and non-real-time traffic. Two algorithms were designed in this process and will be discussed in this thesis: Fixed Portfolio Algorithms and Portfolio Peak Algorithm. In order to evaluate the proposed algorithms and test their efficiency for IEEE 802.16 networks, the authors simulated the algorithms in the NS2 simulator. Evaluation of the proposed Portfolio algorithms was carried out through comparing its performance with those of the conventional algorithms. On the other hand, the proposed Portfolio scheduling algorithm was evaluated by comparing its performance in terms of throughput, delay, and jitter. The simulation results suggest that the Fixed Portfolio Algorithms and the Portfolio Peak Algorithm achieve higher performance in terms of throughput than all other algorithms. Keywords: WiMAX, IEEE802.16, QoS, Scheduling Algorithms, Fixed Portfolio Algorithms, and Portfolio Peak Algorithms

Modelling bubble clusters in compressible liquids

We present a new model for bubbly cavitating flows. Based on volume-averaged equations, a subgrid model is added to account for a bubble, or multiple bubbles, within each computational cell. The model converges to the solution of ensemble-averaged bubbly flow equations for weak oscillations and monodisperse systems. In the other extreme, it also converges to the theoretical solution for a single oscillating bubble, and captures the bubble radius evolution and the pressure disturbance induced in the liquid. A substantial saving of computational time is achieved compared to ensemble-averaged models for polydisperse mixtures

Detailed simulations of bubble-cluster dynamics

The violent collapse of bubble clusters can be an important mechanism of damage to adjacent material surfaces in both engineering and biomedical applications. Because of their complexity, past theoretical studies have generally been restricted to significantly simplified models, such as homogenized continuum models based upon volume averages or arrays of strictly spherical bubbles, which neglect detailed bubble dynamics. However, the details of the bubble-scale dynamics are potentially important locally. For example, wall or tissue damage is expected to depend upon peak pressures rather than the average pressure that might be computed with a homogeneous model. Here, we simulate the expansion and subsequent collapse of hemispherical clusters of 50 bubbles adjacent to a planar rigid wall and viscous fluids as models for soft tissues in therapeutic ultrasound using a computationally efficient diffuse-interface numerical scheme for compressible multiphase flows. It represents in detail the coupled asymmetric dynamics of each bubble within the cluster. The development of this scheme and its application to simulate detailed bubble-cloud collapse are the principal contributions of this dissertation. The numerical scheme represents multi-fluid interfaces using field variables (interface functions) with associated transport equations. In our formulation, these are augmented, with respect to an established formulation, to enforce a selected interface thickness. The resulting interface region can be set just thick enough to be resolved by the underlying mesh and numerical method, yet thin enough to provide an efficient model for dynamics of well-resolved scales. A key advance in our method is that the interface regularization is asymptotically compatible with the thermodynamic laws of the mixture model upon which it is constructed. It incorporates first-order pressure and velocity non-equilibrium effects while preserving interface conditions for equilibrium flows, even within the thin diffused mixture region. The finite-volume numerical solver is also integrated in a multi-resolution Adaptive Mesh Refinement (AMR) framework that allows efficient resolution of individual bubbles of the cluster in a sufficiently large domain. We first quantify the improved convergence of this formulation in an air-helium shock-tube problem and an air-water bubble-collapse problem, then show that it enables fundamentally better simulations of single-bubble dynamics. Demonstrations include both a spherical-bubble collapse, which facilitates comparison with a semi-analytic solution, and a jetting-bubble collapse adjacent a wall. For the spherical collapse, we show agreement with the semi-analytic solution, and the preservation of symmetry despite the Cartesian mesh. Comparisons for the near-wall case show that without the new formulation the re-entrant jet is suppressed by numerical diffusion leading to qualitatively incorrect results. Next, the method is applied to simulate cluster dynamics adjacent to material surfaces. Simulations near the rigid wall show that collapse propagates inward, and a geometrical pressure focusing occurs, which generates impulsive pressures near the focus. The peak pressures depend strongly on the arrangement of the bubbles, particularly those near the focus. The initial acceleration of the bubbles that drives their expansion is identified as an important parameter governing the bubble interactions, and hence the pressure focusing. The simplified models we compare with provide good agreement for the gross cluster behavior, for example gas volume history, but fail to predict the same peak pressures seen in the detailed simulations during the collapse. Replacing the rigid wall with a viscous fluid, as a crude model for tissue, shows significantly different dynamics compared to the rigid wall. Simulations show weaker pressure focusing with substantially lower peak pressures