Textile-reinforced mortar (TRM) versus FRP as strengthening material of URM walls: in-plane cyclic loading

In this study the application of a new structural material, namely textile-reinforced mortar (TRM), as a means of increasing the load carrying capacity and deformability of unreinforced masonry walls subjected to cyclic in-plane loading is experimentally investigated. The application of externally bonded TRM is considered in this work as an alternative method to the application of fiber-reinforced polymers (FRP). Hence, the effectiveness of TRM overlays is evaluated in comparison to the one provided by FRPs. Medium-scale tests were carried out on 22 masonry walls subjected to in-plane cyclic loading. Three types of specimens were used: (a) shear walls; (b) beam-columns; and (c) beams. The parameters under investigation included the matrix material (mortar versus resin), the number of textile layers and the compressive stress level applied to shear walls and beam-columns. Compared with their resin-impregnated counterparts, mortar-impregnated textiles may result in generally lower effectiveness in terms of strength, but in much higher in terms of deformability. From the results obtained in this study it is believed that TRMs hold strong promise as a solution for the structural upgrading of masonry structures under in-plane loading.     

Optimized distributed delivery of continuous-media documents over unreliable communication links

Video-on-demand (VoD) applications place very high requirements on the delivery medium. High-quality services should provide for a timely delivery of the data-stream to the clients plus a minimum of playback disturbances. The major contributions of this paper are that it proposes a multiserver, multi-installment (MSMI) solution approach (sending the document in several installments from each server) to the delivery problem and achieves a minimization of the client waiting time, also referred to as the access time (AT) or start-up latency in the literature. By using multiple spatially distributed servers, we are able to exploit slow connections that would otherwise prevent the deployment of video-on-demand-like services, to offer such services in an optimal manner. Additionally, the delivery and playback schedule that is computed by our approach is loss-aware in the sense that it is flexible enough to accommodate packet losses without interrupts. The mathematical framework presented covers both computation and optimization problems associated with the delivery schedule, offering a complete set of guidelines for designing MSMI VoD services. The optimizations presented include the ordering of the servers and determining the number of installments based on the packet-loss probabilities of the communication links. Our analysis guarantees the validity of a delivery schedule recommended by the system by providing a percentage of confidence for an uninterrupted playback at the client site. This, in a way, quantifies the degree of quality of service rendered by the system and the MSMI strategy proposed. The paper is concluded by a rigorous simulation study that showcases the substantial advantages of the proposed approach and explores how optimization of the schedule parameters affects performance.     

Masonry Confinement with Fiber-Reinforced Polymers

The application of fiber-reinforced polymer (FRP) as a means of increasing the axial capacity of masonry through confinement, a subject not addressed before, is investigated in this study. Four series of uniaxial compression tests, with a total of 42 specimens, were conducted on model masonry columns with these variables: number of layers, radius at the corners, cross-section aspect ratio, and type of fibers. It is concluded that, in general, FRP-confined masonry behaves very much like FRP-confined concrete. Confinement increases both the load-carrying capacity and the deformability of masonry almost linearly with the average confining stress. The uniaxial compression test results enabled the development of a simple confinement model for strength and ultimate strain of FRP-confined masonry. This model is consistent with the test results obtained here but should attract further experimental verification in the future to account for types of masonry materials other than those used in this study.     

Dynamic Rate and FEC Adaptation for Video Multicast in Multi-rate Wireless Networks

Video multicast over Wireless Local Area Networks (WLANs) faces many challenges due to varying channel conditions and limited bandwidth. A promising solution to this problem is the use of packet level Forward Error Correction (FEC) mechanisms. However, the adjustment of the FEC rate is not a trivial issue due to the dynamic wireless environment. This decision becomes more complicated if we consider the multi-rate capability of the existing wireless LAN technology. In this paper, we propose a novel method which dynamically adapts the transmission rate and FEC for video multicast over multi-rate wireless networks. In order to evaluate the system experimentally, we implemented a prototype using open source drivers and socket programming. Our experimental results show that the proposed system significantly improves the multicast system performance.     

Capillary flow of low‐density polyethylene

The capillary flow of a commercial low‐density polyethylene (LDPE) melt was studied both experimentally and numerically. The excess pressure drop due to entry (Bagley correction), the compressibility, the effect of pressure on viscosity, and the possible slip effects on the capillary data analysis have been examined. Using a series of capillary dies having different diameters, D, and length‐to‐diameter L/D ratios, a full rheological characterization has been carried out, and the experimental data have been fitted both with a viscous model (Carreau‐Yasuda) and a viscoelastic one (the Kaye—Bernstein, Kearsley, Zapas/Papanastasiou, Scriven, Macosko, or K‐BKZ/PSM model). Particular emphasis has been given on the pressure‐dependence of viscosity, with a pressure‐dependent coefficient β_p. For the viscous model, the viscosity is a function of both temperature and pressure. For the viscoelastic K‐BKZ model, the time‐temperature shifting concept has been used for the non‐isothermal calculations, while the time–pressure shifting concept has been used to shift the relaxation moduli for the pressure‐dependence effect. It was found that only the viscoelastic simulations were capable of reproducing the experimental data well, while any viscous modeling always underestimates the pressures, especially at the higher apparent shear rates and L/D ratios. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

A Decomposition Scheme for System Optimal Dynamic Traffic Assignment Models

In this paper, a Dantzig-Wolfe decomposition based solution algorithm is developed for the linear programming formulation introduced by Ziliaskopoulos (2000) for System Optimal Dynamic Traffic Assignment problem. The algorithm takes advantage of the network structure in the constraint set of the formulation: the sub-problem is formulated as a minimum-cost-flow problem and the master as a simpler linear programming problem, which allows DTA to be solved more efficiently on meaningful networks. The algorithm is tested on an example network and its performance is analyzed.     

Ultrafast compound Doppler imaging: providing full blood flow characterization

Doppler-based flow analysis methods require acquisition of ultrasound data at high spatio-temporal sampling rates. These rates represent a major technical challenge for ultrasound systems because a compromise between spatial and temporal resolution must be made in conventional approaches. Consequently, ultrasound scanners can either provide full quantitative Doppler information on a limited sample volume (spectral Doppler), or averaged Doppler velocity and/or power estimation on a large region of interest (Doppler flow imaging). In this work, we investigate a different strategy for acquiring Doppler information that can overcome the limitations of the existing Doppler modes by significantly reducing the required acquisition time. This technique is called ultrafast compound Doppler imaging and is based on the following concept: instead of successively insonifying the medium with focused beams, several tilted plane waves are sent into the medium and the backscattered signals are coherently summed to produce high-resolution ultrasound images. We demonstrate that this strategy allows reduction of the acquisition time by a factor of up to of 16 while keeping the same Doppler performance. Depending on the application, different directions to increase performance of Doppler analysis are proposed and the improvement is quantified: the ultrafast compound Doppler method allows faster acquisition frame rates for high-velocity flow imaging, or very high sensitivity for low-flow applications. Full quantitative Doppler flow analysis can be performed on a large region of interest, leading to much more information and improved functionality for the physician. By leveraging the recent emergence of ultrafast parallel beamforming systems, this paper demonstrates that breakthrough performances in flow analysis can be reached using this concept of ultrafast compound Doppler.     

A Cross-Layer Framework for Association Control in Wireless Mesh Networks

The user association mechanism specified by the IEEE 802.11 standard does not consider the channel conditions and the AP load in the association process. Employing the mechanism in its plain form in wireless mesh networks we may only achieve low throughput and low user transmission rates. In this paper we design a new association framework in order to provide optimal association and network performance. In this framework we propose a new channel-quality based user association mechanism inspired by the operation of the infrastructure-based WLANs. Besides, we enforce our framework by proposing an airtime-metric based association mechanism that is aware of the uplink and downlink channel conditions as well as the communication load. We then extend the functionality of this mechanism in a cross-layer manner taking into account information from the routing layer, in order to fit it in the operation of wireless mesh networks. Lastly, we design a hybrid association scheme that can be efficiently applied in real deployments to improve the network performance. We evaluate the performance of our system through simulations and we show that wireless mesh networks that use the proposed association mechanisms are more capable in meeting the needs of QoS-sensitive applications.     

Synthesis of 1D regular arrays of gold nanoparticles and modeling of their optical properties

Self-organized formation of uniform coating of semiconductor substrate by metal nanoparticles offers a convenient and efficient access to large-scale arrays of uniform metal-semiconductor nanostructures. We used a cheap and facile method of photoinduced chemical gold deposition from an aqueous or alcohol gold salt solution onto semiconductor surface (GaAs, InP). By controlling of both the solution composition and the deposition conditions, gold particles of 10-50 nm in diameter were obtained and the gold covering degree of the semiconductor surface was varied in a wide range. Morphology of the nano/micro structures formed was characterized by atomic force microscopy and scanned electron microscopy with local element analysis. The investigations show that the semiconductor surface patterning can be used for the selective deposition of gold nanoparticles, because they are located predominantly at the tops of the microrelief. We have used specially textured by the anisotropic chemical etching microrelief surfaces of semiconductor single crystal as templates and have obtained nanoparticle arrays in the shape of 1D systems of near parallel quasiperiodical wires. For the periodic 1D array of metal nanowires built into the air-semiconductor interface the spectral and angular dependencies of the transmittance/reflectance of the polarized light have been obtained theoretically using differential formalism. These dependencies demonstrate non-monotonic behaviour at surface plasmon polariton excitation conditions and show possibility of designing functional subwavelength devices.