Hydrogen/air supersonic combustion for future hypersonic vehicles

In this work, supersonic hydrogen combustion in the Hyshot II scramjet engine is investigated. In particular, fundamental physics of mixing, combustion and vorticity generation as well as the interaction between shock waves, boundary layer and heat release are analyzed by means of 3D Large Eddy Simulations (LES) using detailed chemistry. Results show very complex structures due to the interaction between the four sonic H₂ crossflow injections and the airstream flowing at M = 2.79. A bow shock forms ahead of each H₂ injector: the interaction between bow shocks and boundary layers leads to separation zones where H₂ recirculates. In these recirculation zones, OH radicals are produced, indicating that a flame already starts upstream of the injectors and downstream of the flow separation. The formation of barrel shocks due to the H₂ expansion and recompressions is also predicted. Comparison of pressure distribution along the wall centreline at 1.3 ms shows agreement with experimental results, mostly in the first part of the combustor, where the grid is very fine. The combustion is very fast and efficient: only 12.35% of hydrogen is found unburned at the combustor exit. This confirms that burning hydrogen is efficient and feasible also in supersonic flows and therefore it is a good candidate for hypersonic airbreathing applications.     

On the blocking performance of EGS networks under multicast traffic

Extended generalized shuffle (EGS) networks are a wide class of interconnection networks introduced by Richards (1993). In this work, we study the blocking performance of EGS networks under point-to-multipoint traffic. Two new routing algorithms for multicast connections in EGS networks are defined, and a theorem proving that these algorithms construct minimum-cost connection trees is enclosed. Simulation results show that the blocking performance of EGS networks under multicast traffic is much better than that of three-stage Clos networks of equal complexity     

A negotiation-based scheme for service level pricing for wireless access

The Next Generation Network (NGN) will enable service providers to offer broadband services with differentiated quality to their subscribers. In this way, the providers will be able to satisfy and retain the critical segment of the top-demanding customers, who generate the largest share of the provider’s revenues. However, service pricing is as important as service quality. In order to offer competitive prices, bilateral per service negotiation between the customer and the supplier is an interesting practical solution that can provide better results than rigid offers based on fixed price-rate lists. In the application scenario of Broadband Wireless Access, the supplier can adapt its offer of wireless access bandwidth to the dynamics of customer-generated traffic, giving more bandwidth per session when traffic is low and less bandwidth per session when traffic is high. We present and study the performance of an algorithm for automatic service level pricing in which two agents negotiate over the service transmission rate and the price on behalf of the user and of the provider. The performance of the proposed negotiation scheme is evaluated with simulations. The paper also compares our algorithm to the fixed-price scheme and to an auction-based strategy based on the Vickrey scheme.     

Closed-form analysis of end-to-end network delay with Markov-modulated Poisson and fluid traffic

This paper develops a method for using traffic sources modelled as a Markov-modulated Poisson process (MMPP) and Markov-modulated fluid process (MMFP) in the framework of the bounded-variance network calculus, a novel stochastic network calculus framework for the approximated analysis of end-to-end network delay. The bounded-variance network calculus is an extension to multi-hop end-to-end paths of the Choe’s and Shroff’s Central-Limit-Theorem-based analysis of isolated network nodes. The input of the analysis is the statistical traffic envelope of sources, which is not available for generic MMPP and MMFP sources. The paper provides two statistical traffic envelopes, named two-moment and linear envelope, for general MMPP and MMFP sources, which can be used as an input of Central-Limit-Theorem-based frameworks for the analysis of network delay and, in turn, make it possible to use the rich collection of MMPP and MMFP models of voice, audio, data and video sources available in the literature. In this way, it is possible to avoid the computational complexity of traditional Markov analysis of end-to-end delay with MMPP and MMFP sources. With the linear envelope we can use simple analytical closed-form solutions for many important schedulers.     

Performance analysis of the ATM shuffleout switch under arbitrarynonuniform traffic patterns

Shuffleout is a blocking multistage asynchronous transfer mode (ATM) switch using shortest path routing with deflection, in which output queues are connected to all the stages. This paper describes a model for the performance evaluation of the shuffleout switch under arbitrary nonuniform traffic patterns. The analytical model that has been developed computes the load distribution on each interstage link by properly taking into account the switch inlet on which the packet has been received and the switch outlet the packet is addressing. Such a model allows the computation not only of the average load per stage but also its distribution over the different links belonging to the interstage pattern for each switch input/output pair. Different classes of nonuniform traffic patterns have been identified and for each of them the traffic performance of the switch is evaluated by thus emphasizing the evaluation of the network unfairness     

Multistage shuffle networks with shortest path and deflectionrouting for high performance ATM switching: the open-loopshuffleout

A new class of switching architectures for broadband packet networks, called shuffleout, is described and analyzed in the paper. Shuffleout is basically an output-queued architecture with a multistage interconnection network built out of unbuffered b×2b switching elements. Its structure is such that the number of cells that can be concurrently switched from the inlets to each output queue equals the number of stages in the interconnection network. The switching element operates the cell self-routing adopting a shortest path algorithm which, in case of conflict for interstage links, is coupled with deflection routing. The paper presents the basic shuffleout architecture, called open-loop shuffleout, in which the cells that cross the whole interconnection network without entering the addressed output queues are lost. The key target of the proposed architecture is coupling the implementation feasibility of a self-routing switch with the desirable traffic performance typical of output queueing     

Pancreatic trauma: a rare but severe event

INTRODUCTION: High-speed motor vehicle accidents have contributed to an increasing incidence of pancreatic trauma. Early mortality is generally due to associated vascular and other intra-abdominal injury. MATERIALS AND METHODS: The authors report their experience on pancreatic trauma. Six cases were treated during the last five years. Isolated pancreatic injury was observed only in one case. Two patients received simple external drainage. Two patients required distal pancreatectomy. One patient was mistreated with ligature of distal pancreatic duct and, postoperatively, developed pseudocyst. One patient, with associated duodenal injury, died. COMMENTS: Serum amylase is often unreliable, whereas abdominal CT scans have the best sensitivity and specificity in detecting pancreatic injury. Abdominal exploration is always recommended in retroperitoneal trauma evaluation; complete pancreas mobilisation is needed to determine the integrity of the major pancreatic duct. Treatment protocols are based on severity of injury: they range from simple drainage to distal pancreatectomy (in presence of distal transection or parenchymal injury with duct injury) and to pancreaticoduodenectomy (in presence of massive injury, devascularisation, destruction of ampulla). Complications are seen in 20 to 35% of patients and the overall mortality secondary to complications ranges from 10 to 20%. Fistulas and pseudocysts are the most common complications.     

The coupling of turbulence and chemistry in a premixed bluff-body flame as studied by LES

In previous work a subgrid scale fractal model for large eddy simulation of turbulent combustion was developed and validated. In the present article the fractal model applicability is tested by simulating a bluff-body premixed flame anchored in a straight channel. The model assumes that chemical reactions take place only at the dissipative scales of turbulence, i.e., near the so-called “fine structures” (eddy dissipation concept). The model estimates the local spatial dissipative scale η, considering also the growth effect due to heat release, and turns itself automatically off where the local spatial filter Δ equals η. The premixed burner is simulated in 2 and 3 dimensions, both for cold flow and for reacting cases. Results are compared with experimental data and show three-dimensional vortex structures periodically shortening the recirculation zone downstream of the bluff body and entraining fresh mixture into the hot recirculating region. This physical mechanism is involved in flame anchoring. The effect of assuming periodic boundary conditions in the spanwise direction, instead of solid side walls, is also investigated. The analysis shows that periodic boundary conditions cannot capture various effects of side walls, such as the shortening of the recirculation zone and the flow acceleration downstream; furthermore, it also does not allow predictions of wall heat transfer. The 2D reactive case results are also compared with those using RANS κ−? and LES-Smagorinsky models. Finally, comparing kinetic energy spectral densities in the nonreacting and reacting cases it is shown that large-scale fluctuations are damped in the latter and that fast chemical reactions cause a high-frequency energy peak.     

Quality of service for packet telephony over mobile ad hoc networks

IP telephony over mobile ad hoc networks is a topic of emerging interest in the research arena as one of the paths toward the fixed-mobile convergence in telecommunications networks. To investigate the performance characteristics of this service, we propose a complete system architecture, which includes a MAC protocol, a routing protocol, and the treatment of voice packets. The telephone system is analyzed in the case of point-to-point calls inside the ad hoc network, and the end-to-end performance is assessed in terms of the percentage of blocked and dropped calls, packet loss and packet delay. The analysis takes into account network scalability by investigating how; the size of the multihop ad hoc network impacts the quality of service. Moreover, the synthetic mean opinion score of the telephone service is evaluated according to the ITU-T E-model.