Resource-efficient strategies for mobile ad-hoc networking

The ubiquity and widespread availability of wireless mobile devices with ever increasing inter-connectivity (e. g. by means of Bluetooth, WiFi or UWB) have led to new and emerging next generation mobile communication paradigms, such as the Mobile Ad-hoc NETworks (MANETs). MANETs are differentiated from traditional mobile systems by their unique properties, e. g. unpredictable nodal location, unstable topology and multi-hop packet relay. The success of on-going research in communications involving MANETs has encouraged their applications in areas with stringent performance requirements such as the e-healthcare, e. g. to connect them with existing systems to deliver e-healthcare services anytime anywhere. However, given that the capacity of mobile devices is restricted by their resource constraints (e. g. computing power, energy supply and bandwidth), a fundamental challenge in MANETs is how to realize the crucial performance/Quality of Service (QoS) expectations of communications in a network of high dynamism without overusing the limited resources. A variety of networking technologies (e. g. routing, mobility estimation and connectivity prediction) have been developed to overcome the topological instability and unpredictability and to enable communications in MANETs with satisfactory performance or QoS. However, these technologies often feature a high consumption of power and/or bandwidth, which makes them unsuitable for resource constrained handheld or embedded mobile devices. In particular, existing strategies of routing and mobility characterization are shown to achieve fairly good performance but at the expense of excessive traffic overhead or energy consumption. For instance, existing hybrid routing protocols in dense MANETs are based in two-dimensional organizations that produce heavy proactive traffic. In sparse MANETs, existing packet delivery strategy often replicates too many copies of a packet for a QoS target. In addition, existing tools for measuring nodal mobility are based on either the GPS or GPS-free positioning systems, which incur intensive communications/computations that are costly for battery-powered terminals. There is a need to develop economical networking strategies (in terms of resource utilization) in delivering the desired performance/soft QoS targets. The main goal of this project is to develop new networking strategies (in particular, for routing and mobility characterization) that are efficient in terms of resource consumptions while being effective in realizing performance expectations for communication services (e. g. in the scenario of e-healthcare emergency) with critical QoS requirements in resource-constrained MANETs. The main contributions of the thesis are threefold: (1) In order to tackle the inefficient bandwidth utilization of hybrid service/routing discovery in dense MANETs, a novel "track-based" scheme is developed. The scheme deploys a one-dimensional track-like structure for hybrid routing and service discovery. In comparison with existing hybrid routing/service discovery protocols that are based on two-dimensional structures, the track-based scheme is more efficient in terms of traffic overhead (e. g. about 60% less in low mobility scenarios as shown in Fig. 3.4). Due to the way "provocative tracks" are established, the scheme has also the capability to adapt to the network traffic and mobility for a better performance. (2) To minimize the resource utilization of packet delivery in sparse MANETs where wireless links are intermittently connected, a store-and-forward based scheme, "adaptive multicopy routing", was developed for packet delivery in sparse mobile ad-hoc networks. Instead of relying on the source to control the delivery overhead as in the conventional multi-copy protocols, the scheme allows each intermediate node to independently decide whether to forward a packet according to the soft QoS target and local network conditions. Therefore, the scheme can adapt to varying networking situations that cannot be anticipated in conventional source-defined strategies and deliver packets for a specific QoS targets using minimum traffic overhead. ii (3) The important issue of mobility measurement that imposes heavy communication/computation burdens on a mobile is addressed with a set of resource-efficient "GPS-free" soluti ons, which provide mobility characterization with minimal resource utilization for ranging and signalling by making use of the information of the time-varying ranges between neighbouring mobile nodes (or groups of mobile nodes). The range-based solutions for mobility characterization consist of a new mobility metric for network-wide performance measurement, two velocity estimators for approximating the inter-node relative speeds, and a new scheme for characterizing the nodal mobility. The new metric and its variants are capable of capturing the mobility of a network as well as predicting the performance. The velocity estimators are used to measure the speed and orientation of a mobile relative to its neighbours, given the presence of a departing node. Based on the velocity estimators, the new scheme for mobility characterization is capable of characterizing the mobility of a node that are associated with topological stability, i. e. the node's speeds, orientations relative to its neighbouring nodes and its past epoch time. iiiBIOPATTERN EU Network of Excellence (EU Contract 508803

Momentum and Mass Transfer from Atmosphere to Rough Surfaces: Improvement on Drag Partition Theory and Dry Deposition Model

The transfers of momentum and mass from the atmosphere to rough surfaces are fundamental scientific problems for meteorology, environment and industry. The transfer of momentum is crucial for the transfer of mass, heat, etc., in the boundary layer. The mass transfer, e.g., dust dry deposition, is a key process of the dust cycle. Both processes are closely related, but not well understood, particularly on rough surfaces or in unsteady conditions. Momentum and mass flux have been found to be associated with the geometric dimensions of the wake behind roughness elements. The dimensions of these wakes can be determined by the geometry of the obstacles on rough surfaces and wind speed. The objective of this thesis is to improve the theories of momentum transfer and drag partition between roughness elements and the exposed underlying surface, as well as the parameterization of particle deposition by means of numerical simulations of the air flow and dust flux over rough surfaces. To investigate the transfer of momentum and mass to rough surfaces, both two-dimensional Reynold Stress Model (2D RSM) simulations and three-dimensional Large Eddy Simulations (3D LES) are carried out. A rough surface in the simulation refers to a flat surface with regularly distributed identical roughness elements. The wind profile, surface drag, and the geometric dimensions of the wake are determined from the simulation results. Friction velocity (u*) and a friction coefficient (u*/uh) are estimated as functions of roughness density (λ), threshold roughness density (λa), and wind speed (ur). The dimensions of the wake, which influence the drag and are controlled by wind speed, are subject to the roughness density and the dimensions of the elements. Hence, it is important to repeat the numerical experiments for various element heights (h), roughness densities and wind speeds. To study dust dry deposition, particle injections are included in the 3D simulation. It allows for particle tracking in the turbulent flow; thereby, the deposition velocity can be determined under different wind speeds, particle diameters and roughness densities. The 2D simulation consists of 260 runs on 13 distinctive surfaces (1/30 < λ < 2/3, h = 5, 7.5 and 10 mm) at 20 different wind speeds (1 - 20 ms-1). The purpose of the 2D simulation is to analyze the geometric dimensions of the wake in the absence of spanwise disturbance. 2D simulations limit the possible mutual sheltering of elements in the streamwise direction. Without these disturbances, the length of the wake behind isolated element is presented as a function of wind speed. The height-to-length ratio of the wake (λw = hw/Lw) is analyzed and found to be independent of element height. When the wake is a full wake, λw is also independent from wind speed, and λw = λa. The relation among the dimensions of roughness elements, the wake and the drag are estimated. A physical model of drag and drag partition is proposed, based on a resistance method. The drag and drag partitions are expressed as functions of λ, and λa, without empirical parameters. The estimation of the new model are analyzed and compared to classical experimental results and a 3D simulations results. The 3D simulation for air flow over rough surface are conducted for 11 distinctive surfaces (1/30 < λ < 1/2) with identical elements of 10 mm height, at 6 different wind speeds (1-25 ms-1). In the resistance method for the momentum flux, the resistances of the element (Rr), and the underlying surface (Rs) in the canyon layer are respectively determined. The threshold roughness density (λa) is introduced in the expressions of resistances. This threshold is defined as the roughness density of the surface which has equal momentum flux on the element and on the underlying surface. This threshold can be determined by the length of the wake (Lw) on rough surfaces and helps to distinguish elements of different length-to-height ratios (b/h). New expressions of friction velocity and drag partitions (τr + τs) are derived without any empirical parameter. The friction coefficient is determined empirically. Classical wind tunnel data of drag for rough surfaces with various roughness densities, and results from the 3D simulation are successfully reproduced, and in response to different length-to-height ratios of roughness elements. Thus, the new expressions of drag and drag partition on rough surfaces are validated. The discrepancy between the estimation of existing dry deposition model and field measurements reaches 2 orders of magnitude. In the existing models of dust dry deposition, the rough surfaces are treated as a single cylinder. Sensitivity tests show that the possible uncertainty on the deposition velocity generated by this method can reach 337%. To investigate dust dry deposition in more details, 3D simulations of deposition on rough surfaces are conducted and 15 groups of particles with different diameters (0.1 μm < dp < 10 μm) are injected into the simulation domain. Deposition velocity is deduced by counting trapped particle on the surfaces, in fully developed flow. Regression analysis is applied to fit the deposition velocity as functions of wind speed, roughness density or particle size from simulation results. The resulting prediction of the deposition velocity is consistent with field measurements and explains the discrepancies among existing field measurements and previous model estimations. The measurement of deposition process in the natural flow is also studied. The aim of this part is to examine the influence of unsteady dust flux on the measurement of deposition velocity and errors caused by field measuring method. An existing vertical dispersion framework is introduced to simulate the one-dimensional deposition velocity. Intermittent dust flux data from a well-known field measurement during a dust event, as input data. The resulting estimations of deposition velocity are consistent with field measurements. The understanding of momentum and mass transfer on rough surfaces could thereby be improved

Investigation of the Minor-Element Effects on the Oxidation Behavior of γ-NI + γ’-NI3AL Alloys

The effects of minor-elements, mainly Hf, Y and Si, on the oxidation behavior of Al2O3-scale forming γ-Ni + γ’-Ni3Al-based alloy and coating compositions were investigated. Firstly, the effects of Si addition on Hf+Y co-doped γ+γ’ alloys were assessed on both modified-René N5 superalloy and model alloys. It was found that the addition of Si (≤ 1 at. %) decreased the overall weight gain of modified-René N5 superalloy and Ni-20Al-5Cr-0.05Hf-0.05Y (at. %) model alloy under both isothermal (up to 1000 hours) and cyclic (up to 1000 1h cycles) oxidation conditions at 1150°C. Secondly, the effect of Si (1 at. %) addition on the meta-stable → α-Al2O3 transformation kinetics during the early stage of oxidation was studied on the Ni-20Al-5Cr-0.05Hf-0.05Y model alloy. It was found that the addition of 1 at. % Si delays the θ → α-Al2O3 transformation at temperatures above 900°C. In addition, an alternative mathematical method was developed to determine the θ → α-Al2O3 transformation based on analyzing the time dependence of instantaneous n-value. This method found success in describing the effect of Si addition in delaying the θ → α-Al2O3 transformation on Ni-20Al-5Cr-0.05Hf-0.05Y model alloy. Thirdly, synergistic effect of Si (1 at. %) and Hf/Y on the early stage oxidation behavior of Ni-20Al-5Cr-0.05Hf-0.05Y model alloy was studied by TEM/STEM/EDS. Significantly different microstructures of Al2O3 scale formed on the Si-free and Si-containing alloys were observed after 10 and 30 minutes of oxidation at 1150°C. The Al2O3 scale formed on the Si-free alloy contained a large amount of Ni-rich particles in the region close to the scale/metal interface. By contrast, for the Si-containing alloy, a layer of quickly established large-grained α-Al2O3 was observed in the region close to the scale/metal interface. Segregation behavior of Hf and Y was also influenced by the addition of Si. After 2 hours of oxidation at 1150°C, excessive non-uniform Hf and Y co-segregation was observed at the grain boundaries of Al2O3 scale formed on the Si-free alloy. By contrast, Hf and Y grain boundary segregations were uniform and well-controlled in the Al2O3 scale formed on the Si-containing alloy

A Hierarchical Component-based WebGIS and Its Key Technologies

A practical hierarchical component-based WebGIS model referred to as Geo-Union is presented. Geo-Union consists of four layers: storage layer, service layer, component layer and application layer. Service layer is partitioned into another two layers: Geo-Union client and Geo-Union server. The architectures and object diagram of each layer in Geo-Union are discussed in details. After that, four key technologies adopted in Geo-Union (spatial data model, ORDB, spatial index and spatial cache) are summarized and analyzed, especially the spatial cache framework of Geo-Union. At last, some future works in WebGIS, such as interoperability, security, distributed computing and intelligent computing, are indicated and simply explored