325 research outputs found
Polynomial-Chaos-based Kriging
Computer simulation has become the standard tool in many engineering fields
for designing and optimizing systems, as well as for assessing their
reliability. To cope with demanding analysis such as optimization and
reliability, surrogate models (a.k.a meta-models) have been increasingly
investigated in the last decade. Polynomial Chaos Expansions (PCE) and Kriging
are two popular non-intrusive meta-modelling techniques. PCE surrogates the
computational model with a series of orthonormal polynomials in the input
variables where polynomials are chosen in coherency with the probability
distributions of those input variables. On the other hand, Kriging assumes that
the computer model behaves as a realization of a Gaussian random process whose
parameters are estimated from the available computer runs, i.e. input vectors
and response values. These two techniques have been developed more or less in
parallel so far with little interaction between the researchers in the two
fields. In this paper, PC-Kriging is derived as a new non-intrusive
meta-modeling approach combining PCE and Kriging. A sparse set of orthonormal
polynomials (PCE) approximates the global behavior of the computational model
whereas Kriging manages the local variability of the model output. An adaptive
algorithm similar to the least angle regression algorithm determines the
optimal sparse set of polynomials. PC-Kriging is validated on various benchmark
analytical functions which are easy to sample for reference results. From the
numerical investigations it is concluded that PC-Kriging performs better than
or at least as good as the two distinct meta-modeling techniques. A larger gain
in accuracy is obtained when the experimental design has a limited size, which
is an asset when dealing with demanding computational models
Dynamic behaviour of an electrolyser with a two phase solid-liquid electrolyte : Part I : spectral analysis of potential fluctuations
The dynamic behaviour of a simplified electrolyser with a two phase solid-liquid electrolyte was investigated. The current-voltage curve and the impedance of the working electrode (current collector) was measured. In addition the fluctuations of the working electrode potential and of the electrolyte resistance close to the electrode were analysed, especially by spectral analysis. Conductive zinc-coated polystyrene beads and insulating glass beads were compared. For glass beads the kinetics were controlled by ohmic drop fluctuations due to the collisions of the beads onto the collector. With zinc beads the charge exchanges between the beads and the current collector were also observed and contributed to the potential fluctuations. A complementary analysis of the elementary potential and electrolyte resistance transients will be reported in Part II; the electrochemical behaviour of the collector will then be modelled
The impact of inlet boundary layer thickness on the unsteady aerodynamics of S-duct intakes
The need to reduce aero-engine emissions and direct operating costsis driving the civil aerospace sectortowards considering more integrated propulsion systems. Many of the proposed novel aircraft architectures employ convoluted intakes for either the aero-engine or propulsion system. These intakes are characterized by unsteady distortion that can hinder the performance and operability of the propulsion system. This work assessesthe impact of the inlet boundary layer on the unsteady aerodynamics of an S-duct intake using time-resolved particle image velocimetry at the aerodynamic interface plane.An increase in the boundary layer thickness at the intake inlet increasesthe flow unsteadiness on the swirl angle by up to 40% relativeto the baseline case. The azimuthal orientation of the inlet boundary layer modifies the intensity and topology of the most frequent swirl distortion pattern. For a relatively thick inlet boundary layer, the reduction of the dominant frequencies associated withthe unsteady swirl angle is postulated to be beneficial for the engine stability. Overall, this works gives guidelines for the integration between the intake and the engine across the range of potential inlet operating conditions
The association between real-life markers of phone use and cognitive performance, health-related quality of life and sleep
INTRODUCTION: The real-life short-term implications of electromagnetic fields (RF-EMF) on cognitive performance and health-related quality of life have not been well studied. The SPUTNIC study (Study Panel on Upcoming Technologies to study Non-Ionizing radiation and Cognition) aimed to investigate possible correlations between mobile phone radiation and human health, including cognition, health-related quality of life and sleep. METHODS: Adult participants tracked various daily markers of RF-EMF exposures (cordless calls, mobile calls, and mobile screen time 4 h prior to each assessment) as well as three health outcomes over ten study days: 1) cognitive performance, 2) health-related quality of life (HRQoL), and 3) sleep duration and quality. Cognitive performance was measured through six "game-like" tests, assessing verbal and visuo-spatial performance repeatedly. HRQoL was assessed as fatigue, mood and stress on a Likert-scale (1-10). Sleep duration and efficiency was measured using activity trackers. We fitted mixed models with random intercepts per participant on cognitive, HRQoL and sleep scores. Possible time-varying confounders were assessed at daily intervals by questionnaire and used for model adjustment. RESULTS: A total of 121 participants ultimately took part in the SPUTNIC study, including 63 from Besancon and 58 from Basel. Self-reported wireless phone use and screen time were sporadically associated with visuo-spatial and verbal cognitive performance, compatible with chance findings. We found a small but robust significant increase in stress 0.03 (0.00-0.06; on a 1-10 Likert-scale) in relation to a 10-min increase in mobile phone screen time. Sleep duration and quality were not associated with either cordless or mobile phone calls, or with screen time. DISCUSSION: The study did not find associations between short-term RF-EMF markers and cognitive performance, HRQoL, or sleep duration and quality. The most consistent finding was increased stress in relation to more screen time, but no association with cordless or mobile phone call time
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