Performance of distributed mechanisms for flow admission in wireless adhoc networks

Given a wireless network where some pairs of communication links interfere with each other, we study sufficient conditions for determining whether a given set of minimum bandwidth quality-of-service (QoS) requirements can be satisfied. We are especially interested in algorithms which have low communication overhead and low processing complexity. The interference in the network is modeled using a conflict graph whose vertices correspond to the communication links in the network. Two links are adjacent in this graph if and only if they interfere with each other due to being in the same vicinity and hence cannot be simultaneously active. The problem of scheduling the transmission of the various links is then essentially a fractional, weighted vertex coloring problem, for which upper bounds on the fractional chromatic number are sought using only localized information. We recall some distributed algorithms for this problem, and then assess their worst-case performance. Our results on this fundamental problem imply that for some well known classes of networks and interference models, the performance of these distributed algorithms is within a bounded factor away from that of an optimal, centralized algorithm. The performance bounds are simple expressions in terms of graph invariants. It is seen that the induced star number of a network plays an important role in the design and performance of such networks.Comment: 21 pages, submitted. Journal version of arXiv:0906.378

Adoption of integrated cattle and oil palm farming system in Malaysia

Malaysian government had encouraged the adoption of integrated cattle and oil palm farming system (ICOFS) since the year 1998. This is due to large areas of land suitable for ICOFS in Malaysia that can be well utilised. Rising up the participation in ICOFS is one of the effort in facing the increasing demand of meat nationwide and growing dependency on imported meat. Apart from that, Malaysia needs to increase the self�sufficiency level of national meat production to 32.7% by 2020 as stated in the National Agri-Food Policy 2011-2020. Despite this, only a small number of farmers have adopted ICOFS in Malaysia and less is known so far about the farmer's attitudes towards the ICOFS in Malaysia. Therefore, this study aimed to explore the current practice as well as factors affecting and constraints to the adoption of ICOFS in Malaysia. Interviews of four selected experts in ICOFS were held to examine the current practice of ICOFS in Malaysia. Results from the interviews and literature review have been cross-compared to develop questionnaire instruments. Then, the survey questionnaire was carried out to 153 adopters and non-adopters of ICOFS in Johor, Malaysia to identify the key factors that influence the adoption of ICOFS. The study found that information and know-how and availability of skilled labour are the significant factors that encourage farmers to adopt ICOFS. On the other hand, government support and policy constraint and production and on-farm constraint are the significant constraints hindering the adoption of ICOFS. Based on the findings of this study, one of the measure to increase the rate of adoption of ICOFS are by helping the farmers to hire extra labour and to have skills needed. Furthermore, rate of ICOFS could also be increase by helping the farmers to have better control on weed, pest and diseases on farm as well as not interrupting their oil palm production. In addition, it is suggested to provide them with more opportunity towards incentives, training and technical support from the field officers. It is suggested for future research to explore the effect of factors affecting adoption and constraints to adoption on other attitudinal and psychological outcomes such as satisfaction, successfulness, involvement and degree of use

Prediction of sheet necking with shell finite element models

In sheet forming simulations, the prediction of localised necking is an important goal. A pragmatic\ud approach is to compare calculated principal strains with a forming limit curve (FLC). However, the FLC’s\ud are known to depend on the strain path and most experimental FLC’s are determined for straight deformation\ud paths. Localisation can also be determined numerically with a Marciniak–Kuczynski analysis (M–K). It is\ud recognised that a FEM analysis with shell elements resembles the M–K analysis very much. For uniform\ud deformations a band with slightly reduced thickness is necessary to trigger localisation. In practical forming\ud conditions, however, the non-uniformity of the process automatically triggers localisation and an arbitrary initial\ud imperfection is not needed. FEM models have the additional benefit that boundary conditions, non-proportional\ud deformation and e.g. friction with the tools are completely included

Numerical evaluation of the pipe-pile buckling during vibratory driving in sand

The buckling of steel pipe piles during vibratory driving is numerically studied using the Multi-Material Arbitrary Lagrangian-Eulerian (MMALE) method. This method handles the large soil deformations that occur during pile driving and other geotechnical installation processes. The Mohr-Coulomb and an elastic-perfectly plastic material model are used to model the soil and the pile mechanical behavior, respectively. The result of a small-scale pile driving experiment is used to validate the numerical model. The penetration trend agrees well with the experimental measurements. Thereafter, four case scenarios and their possible effects on pile buckling, namely the presence of heterogeneity in the soil (a rigid boulder inside the soil) and the existence of geometrical imperfection modes in the pile (ovality, out-of-straightness, flatness) are investigated. This study shows that a combination of local and global buckling initiates at the pile tip and the pile shaft, respectively. During the initiation of buckling, a decrease in the penetration rate of the pile is observed compared to the case where no or minimal buckling occurs. It is shown that a less portion of the driving energy is spent on the pile penetration and the rest is spent on other phenomena such as buckling, resulting in less pile penetration. The cross section of the pile tip after buckling takes a form of a “peanut”, yet with a different geometry for each case. In cases where the model was initially symmetric, an asymmetric shape in cross section of the pile tip was obtained at the final stage which can be attributed to complex soil-structure interaction. The results of the numerical approach provide promising results to be used as an evaluation tool to reach reliable predictions in pile installation practice

Initial post-buckling behavior of toroidal shell segments

Initial post-buckling behavior of toroidal shell segment

Overall buckling of lightweight stiffened panels using an adapted orthotropic plate method

The ultimate longitudinal bending strength of thin plated steel structures such as box girder bridges and ship hulls can be determined using an incremental–iterative procedure known as the Smith progressive collapse method. The Smith method first calculates the response of stiffened panel sub-structures in the girder and then integrates over the cross section of interest to calculate a moment–curvature response curve. A suitable technique to determine the strength behaviour of stiffened panels within the Smith method is therefore of critical importance. A fundamental assumption of the established progressive collapse method is that the buckling and collapse behaviour of the compressed panels within the girder occurs between adjacent transverse frames. However, interframe buckling may not always be the dominant collapse mode, especially for lightweight stiffened panels such as are found in naval ships and aluminium high speed craft. In these cases overall failure modes, where the buckling mode extends over several frame spaces, may dominate the buckling and collapse response. To account for this possibility, an adaptation to large deflection orthotropic plate theory is presented. The adapted orthotropic method is able to calculate panel stress–strain response curves accounting for both interframe and overall collapse. The method is validated with equivalent nonlinear finite element analyses for a range of regular stiffened panel geometries. It is shown how the adapted orthotropic method is implemented into an extended progressive collapse method, which enhances the capability for determining the ultimate strength of a lightweight stiffened box girder