Nonlinear modeling of the dynamic effects of arterial pressure and CO2 variations on cerebral blood flow in healthy humans

The effect of spontaneous beat-to-beat mean arterial blood pressure fluctuations and breath-to-breath end-tidal CO2 fluctuations on beat-to-beat cerebral blood flow velocity variations is studied using the Laguerre-Volterra network methodology for multiple-input nonlinear systems. The observations made from experimental measurements from ten healthy human subjects reveal that, whereas pressure fluctuations explain most of the high-frequency blood flow velocity variations (above 0.04 Hz), end-tidal CO2 fluctuations as well as nonlinear interactions between pressure and CO2 have a considerable effect in the lower frequencies (below 0.04 Hz). They also indicate that cerebral autoregulation is strongly nonlinear and dynamic (frequency-dependent). Nonlinearities are mainly active in the low-frequency range (below 0.04 Hz) and are more prominent in the dynamics of the end-tidal CO2-blood flow velocity relationship. Significant nonstationarities are also revealed by the obtained models, with greater variability evident for the effects of CO2 on blood flow velocity dynamics.     

An ontological metamodel for cyber-physical system safety,security, and resilience coengineering

Software and Systems Modeling - Cyber-physical systems are complex systems that require the integration of diverse software, firmware, and hardware to be practical and useful. This increased...     

Water quality of Vistonis Lagoon, Northern Greece: seasonal variation and impact of bottom sediments

The aim of this study is to present the seasonal variation of nutrients in the water column and the bottom sediments of Vistonis Lagoon, a hypereutrophic Mediterranean coastal lagoon located in Northern Greece, and to estimate the impact of bottom sediments on the water quality of this lagoon. Nutrient concentrations in the water column and in bottom sediments were determined throughout seven seasonal sampling cruises from May 2003 to October 2004. Physicochemical parameters, such as transparency, temperature, salinity, pH and dissolved oxygen, were measured in situ in the water column using suitable equipment. Nutrient concentrations in bottom water were found generally higher than those in surface water. Nitrogen, mainly as nitrates, and phosphorus are released into the water column from the bottom sediments, especially during the summer period. Anoxia in the bottom water, as well as resuspension of the sediments are the main factors affecting nutrient internal loading in this lagoon. An approximate calculation showed that total phosphorus release was about 80 mg m⁻² d⁻¹ for the period March–August 2004. Vistonis Lagoon restoration will be possible only through the minimization or elimination of both external and internal nutrient loadings.     

Exposing mobile malware from the inside (or what is your mobile app really doing?)

It is without a doubt that malware especially designed for modern mobile platforms is rapidly becoming a serious threat. The problem is further multiplexed by the growing convergence of wired, wireless and cellular networks, since virus writers can now develop sophisticated malicious software that is able to migrate across network domains. This is done in an effort to exploit vulnerabilities and services specific to each network. So far, research in dealing with this risk has concentrated on the Android platform and mainly considered static solutions rather than dynamic ones. Compelled by this fact, in this paper, we contribute a fully-fledged tool able to dynamically analyze any iOS software in terms of method invocation (i.e., which API methods the application invokes and under what order), and produce exploitable results that can be used to manually or automatically trace software’s behavior to decide if it contains malicious code or not. By employing real life malware we assessed our tool both manually, as well as, via heuristic techniques and the results we obtained seem highly accurate in detecting malicious code.     

Hydrology and Nutrient Enrichment at Two Coastal Lagoon Systems in Northern Greece

The lagoons of N.E. Greece, located on the western side of NestosRiver, and of N.W. Greece, located at the lower reaches of Kalamas River, are among the most important shallow, semi-enclosed ecosystems in Northern Greece. The temporal variability of nutrients at both lagoonal systems shows the strong influence of fresh water discharge on water quality. Nutrient enrichment factors showed that nitrites and ammonium were six times higher at the lagoons of N.W. Greece than those observed at N.E. Greece, while phosphates were forty times higherat Nestos River lagoons. The flushing half-life was calculatedbased on a combination of hydrological and tidal processes, foreach lagoon of these two systems, allowing for the assessment ofwater quality changes. Proper management measures for both systems should focus on the control of fresh water quality entering the lagoons, the reduction of phosphoric fertilizers used by agriculture and the better oxygenation of the water column. One way to eliminate massive fish deaths during the winter in N.E. Greece is also the transfer of fresh, warm groundwater, while bathymetric modifications and channel wideningare needed at the lagoons of N.W. Greece.     

PEM fuel cell stack testing in the framework of an EU-harmonized fuel cell testing protocol: Results for an 11 kW stack

Fuel cell testing and standardization thematic network (FCTESTNET) was a Thematic Network funded by the European Commission under the Fifth Framework Program (FP5), which was comprised of 55 European partners. The project concluded in 2006 and the main output was the collection and compilation of agreed testing procedures for different fuel cell technologies (PEM, SOFC, MCFC), applications (stationary, portable, transport), as well as balance of plant.     

The Lost Value of Groundwater: The Case of Asopos River Basin in Central Greece

Due to the unique nature of the environmental damage, mainly by Cr(VI), the pollution of the Asopos river basin aquifer in Greece has been recently characterized as an ‘environmental crisis’. This paper presents the results of the first attempt in Greece to value the environmental damage to groundwater and to estimate its total economic value using environmental economics. Towards this direction a Contingent Valuation study was conducted, covering the settlements mostly affected in the area. The results indicate that local households are willing to pay an extra monthly charge in voluntary basis for ten years, in order to restore the aquifer under investigation. Different household profiles show different willingness to pay, depending on attitudes against the environmental damage, population age and place of residence. The elicited estimates seem to reflect mostly non-use values and, although smaller than actual private and public expenses for the replacement of the lost public good, the findings are promising and may serve as a spark for future research.     

An Unbiased Method for Probabilistic Fire Safety Engineering,Requiring a Limited Number of Model Evaluations

The rise of Performance Based Design methodologies for fire safety engineering has increased the interest of the fire safety community in the concepts of risk and reliability. Practical applications have however been severely hampered by the lack of an efficient unbiased calculation methodology. This is because on the one hand, the distribution types of model output variables in fire safety engineering are not known and traditional distribution types as for example the normal and lognormal distribution may result in unsafe approximations. Therefore unbiased methods must be applied which make no (implicit) assumptions on the PDF type. Traditionally these unbiased methods are based on Monte Carlo simulations. On the other hand, Monte Carlo simulations require a large number of model evaluations and are therefore too computationally expensive when large and nonlinear calculation models are applied, as is common in fire safety engineering. The methodology presented in this paper avoids this deadlock by making an unbiased estimate of the PDF based on only a very limited number of model evaluations. The methodology is known as the Maximum Entropy Multiplicative Dimensional Reduction Method (ME-MDRM) and results in a mathematical formula for the probability density function (PDF) describing the uncertain output variable. The method can be applied with existing models and calculation tools and allows for a parallelization of model evaluations. The example applications given in the paper stem from the field of structural fire safety and illustrate the excellent performance of the method for probabilistic structural fire safety engineering. The ME-MDRM can however be considered applicable to other types of engineering models as well.     

Performance analysis of IEEE 802.11 ad hoc networks in the presence of exposed terminals

The paper evaluates the performance effects of exposed terminals in IEEE 802.11 ad hoc networks in finite load conditions. It derives analytical models for the estimation of channel utilization and media access delay for IEEE 802.11 ad hoc networks in finite load conditions with and without exposed terminals. The simulation results show that the analytical estimated channel utilization and media access delay metrics are fairly accurate.