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61.
Present acoustic imaging apparatuses give only a virtual image of the insonified object. Several studies have been conducted in order to pick up the information from such an image, such as holography and computer data processing. These techniques are very involved, and so simpler processing systems have been analysed in our laboratory in order to reconstruct a true image. Our first system used optical processing of the acoustic echo. In this paper, a second, acousto-electronic, system is described, which enables like the first the attainment of very high image rates in the B-scan mode. 相似文献
62.
We analyze a discrete-time queueing model where two types of customers, each having their own dedicated server, are accommodated in one single FCFS queue. Service times are deterministically equal to \(s \ge 1\) time slots each. New customers enter the system according to a general independent arrival process, but the types of consecutive customers may be nonindependent. As a result, arriving customers may (or may not) have the tendency to cluster according to their types, which may lead to more (or less) blocking of one type by the opposite type. The paper reveals the impact of this blocking phenomenon on the achievable throughput, the (average) system content, the (average) customer delay and the (average) unfinished work. The paper extends the results of earlier work where either the service times were assumed to be constant and equal to 1 slot each, or the customers all belonged to the same class. Our results show that, in case of Poisson arrivals, for given traffic intensity, the system-content distribution is insensitive to the length (s) of the service times, but the (mean) delay and the (mean) unfinished work in the system are not. In case of bursty arrivals, we find that all the performance measures are affected by the length (s) of the service times, for given traffic intensity. 相似文献
63.
F. Weisbuch V.N. Tokarev S. Lazare C. Belin J.L. Bruneel 《Applied Physics A: Materials Science & Processing》2002,75(6):677-680
A new form of matter removal in laser ablation is reported. Polymethylmethacrylate (PMMA) nanofibers are obtained when a PMMA
target is irradiated with a single pulse of a KrF excimer laser, whose beam is sharply imaged on a square of side the order
of 140 μm, so that a strong intensity gradient is produced. The fluence threshold at which fibers appear, 3 J/cm2, is much larger than the ablation threshold, approximately 0.8–1 J/cm2. Above this fluence, the melt depth is then large enough and the temperature profile is such that explosive boiling is obtained.
The model suggests an expulsion of energetic droplets from the intense pressure of the plume to the exterior of the spot.
For the transient melt of a polymeric viscoelastic liquid resulting from UV-laser excitation, such droplets provide the heads
of the jets pulled from the melt bath, giving rise, after solidification, to nanofibers. The speed of fiber spinning is extremely
high (∼800 m/s) and unusual properties of the laser-produced nanofibers may be expected.
Received: 16 April 2002 / Accepted: 17 April 2002 / Published online: 19 July 2002 相似文献
64.
Analysis of Stop-and-Wait ARQ for a wireless channel 总被引:1,自引:0,他引:1
Stijn De Vuyst Krzysztof Tworus Sabine Wittevrongel Herwig Bruneel 《4OR: A Quarterly Journal of Operations Research》2009,7(1):61-78
In this paper, we study the behavior of the transmitter buffer of a system working under a Stop-and-Wait retransmission protocol.
The buffer at the transmitter side is modeled as a discrete-time infinite-capacity queue. The numbers of information packets
entering the buffer during consecutive slots are assumed to be independent and identically distributed random variables. The
packets are sent over an unreliable channel and transmission errors occur in a correlated manner. Specifically, the probability
of an erroneous transmission is modulated by a two-state Markov chain. An expression is derived for the probability generating
function of the buffer content. This expression is then used to derive several queue-length characteristics and the mean packet
delay. Numerical examples illustrate the strong effect of error correlation on the system performance. The obtained analytical
results are also compared with appropriate simulations.
相似文献
65.
Cristobal G Arbouet L Sarrazin F Talaga D Bruneel JL Joanicot M Servant L 《Lab on a chip》2006,6(9):1140-1146
Sub-nanolitre droplets engineered in microfluidic devices constitute ideal microreactors to investigate the kinetics of chemical reactions on the millisecond time scale. Up to date, fluorescence detection has been extensively used in chemistry and biology to probe reactants and resultant products within such nanodroplets. However, although fluorescence is a very sensitive technique, it lacks intrinsic specificity as frequently fluorescent labels need to be attached to the species of interest. This weakness can be overcome by using vibrational spectroscopy analysis. As an illustrative example, we use confocal Raman microspectroscopy in order to probe the concentration profiles of two interdiffusing solutes within nanolitre droplets transported through a straight microchannel. We establish the feasibility of the experimental method and discuss various aspects related to the space-time resolution and the quantitativeness of the Raman measurements. Finally, we demonstrate that the droplet internal molecular mixing is strongly affected by the droplet internal flow. 相似文献
66.
Herwig Bruneel 《European Journal of Operational Research》1985,21(3):379-386
The paper considers an infinite discrete-time buffer system with one single output channel. Unlike most analyses of such a buffer system, the present study uses a ‘dynamic’ model for the arrival process of data units into the system. More specifically, the distribution of the number of arrivals per discrete time-unit is allowed to fluctuate in time, in a periodical fashion, whereas in classical models this distribution remains the same, as time goes by. The probability generating function of the number of data units in the buffer, at various time instants, is derived under such dynamic arrival conditions. An extended illustrating example, comparing ‘static’ and ‘dynamic’ arrival models, concludes the paper. 相似文献
67.
Herwig Bruneel 《Operations Research Letters》1984,3(3):149-152
The letter considers a discrete buffered system with one randomly interrupted output channel and stochastic interruptions of the arrival stream which are correlated to the output interruptions. The behaviour of this buffer system is studied, and in particular the influence of the correlation is derived. 相似文献
68.
In novel switching approaches such as Optical Burst Switching, the involved buffers can only provide a degenerate waiting
room, with delays restricted to multiples of a basic value, the granularity. Although the resulting performance loss was already
studied analytically, previous work is either limited by the assumption of independent arrivals, or it involves a matrix with
size growing fast with buffer size or arrival process complexity.
Overcoming this, we developed a generic and accurate loss performance model for a degenerate GI/G/1 buffer in discrete time,
that yields results instantly for any constellation of burst sizes, inter-arrival times, granularity, load and buffer size.
This paper presents our model and compares its results to simulations, illustrating the impact of different types of correlation
in the arrival process on loss performance. Our basic model is general and accurate, it can thus serve as a basic tool for
optical switch design.
相似文献
69.
B. Steyaert K. Laevens D. De Vleeschauwer H. Bruneel 《Annals of Operations Research》2008,162(1):159-169
A critical component in the delivery of VBR streaming video over packet-based networks, is the playout buffer at the receiver
side. As packets will incur a variable delay within the network, a mechanism is needed to dejitter the stream and assure that
packets are played out at exactly the same rate at which they were generated by the sender.
Packet loss at the playout buffer can be induced by either underflow or overflow, and we first identify the general conditions
under which these events occur. These general rules require some knowledge on the network delay characteristics of an arbitrary
packet, and are then further developed and refined, by taking a closer look at the case where a single delay bottleneck exists
within the network.
This analysis allows us to derive useful design rules, where we consider two cases, depending on whether or not the receiver
has knowledge about the delay the first packet of the stream has incurred. We conclude by demonstrating that the traditional
playout paradigm can be enhanced to guard against large initial delays. 相似文献
70.
Peixia Gao Sabine Wittevrongel Herwig Bruneel 《4OR: A Quarterly Journal of Operations Research》2008,6(3):279-290
We consider a discrete-time multiserver queueing system with infinite buffer size, constant service times of multiple slots
and a first-come-first-served queueing discipline. A relationship between the probability distributions of the partial system
contents and the packet delay is established. The relationship is general in the sense that it doesn’t require knowledge of the exact nature of the arrival process. By means of the relationship,
results for the distribution of the partial system contents for a wide variety of discrete-time queueing models can be transformed
into corresponding results for the delay distribution. As a result, a separate full analysis of the packet delay becomes unnecessary.
相似文献