Modelling of coupled cross-flow/in-line vortex-induced vibrations using double Duffing and van der Pol oscillators

Many studies have typically applied a linear structural spring–mass–damper oscillator and a van der Pol wake oscillator to model a one-dimensional cross-flow vortex-induced vibration (VIV). In this study, an advanced model for predicting a two-dimensional coupled cross-flow/in-line VIV of a flexibly mounted circular cylinder in a uniform flow is proposed and validated. The ensuing dynamical system is based on double Duffing–van der Pol (structural-wake) oscillators with the two structural equations containing both cubic and quadratic nonlinear terms. The cubic nonlinearities capture the geometrical coupling of cross-flow/in-line displacements excited by hydrodynamic lift/drag forces whereas the quadratic nonlinearities allow the wake–cylinder interactions. Some empirical coefficients are calibrated against published experimental results to establish a new generic analytical function accounting for the dependence of VIV on a physical mass and/or damping parameter. By varying flow velocities in the numerical simulations, the derived low-order model captures several important VIV characteristics including a two-dimensional lock-in, hysteresis phenomenon and figure-of-eight trajectory tracing the periodically coupled in-line/cross-flow oscillations with their tuned two-to-one resonant frequencies. By making use of a newly derived empirical formula, the predicted maximum cross-flow/in-line VIV amplitudes and associated lock-in ranges compare well with several experimental results for cylinders with low/high mass or damping ratios. Moreover, the parametric studies highlight the important effect of geometrical nonlinearities through new displacement coupling terms and the ratio of in-line to cross-flow natural frequencies of the freely vibrating cylinder

Adaptive image steganography based on optimal embedding and robust against chi-square attack

A real-life requirement motivated this case study of secure covert communication. Steganography is a technique used to transfer hidden information in an imperceptible manner. We proposed a novel approach of substitution technique of image steganography. The proposed method is flexible on size of secret message and allows us to embed a large amount of secret messages as well as maintaining good visual quality of stego-image. Using this method, message bits are embedded into uncertain and higher LSB layers, resulting in increased imperceptible and robustness of stego-image. Results show that the proposed algorithm provides large embedding capacity without losing the imperceptibility of the stego-image. The algorithm is also robust against Chi-square attack

T-stress determination using thermoelastic stress analysis

T-stress and mixed-mode stress intensity factors have been determined experimentally using thermoelastic stress analysis and using a finite element method. Pure mode I, strong mixed-mode I and II, and interacting cracks have been used as the case studies. A new technique has been proposed to identify the crack tip from thermoelastic images. It has also been shown that using three terms of Williams's stress field formulation to determine the T-stress, yields a more accurate solution than using only the first two terms of the expansion

Estimation of Equicorrelated Diffusions from Incomplete Data

The paper derives maximum likelihood parameter estimators for symmetrically correlated Weiner processes observed at discrete intervals. Such processes arise when pricing and determining Value-at-Risk for portfolio derivatives. Cases of driftless and mean-reverting state variables are considered. The procedure is applicable to samples with missing data of any pattern and to high dimensional systems. The estimation procedure is illustrated using a sample of stock prices.Maximum likelihood; Equicorrelation; Correlated di usions; Wiener process; Missing data

Dynamical nucleus-nucleus potential and incompressibility of nuclear matter

The dynamical nucleus-nucleus potentials for some fusion reactions are investigated by using the improved quantum molecular dynamics (ImQMD) model with different sets of parameters in which the corresponding incompressibility coefficient of nuclear matter is different. Two new sets of parameters SKP* and IQ3 for the ImQMD model are proposed with the incompressibility coefficient of 195 and 225 MeV, respectively. The measured fusion excitation function for 16O+208Pb and the charge distribution of fragments for Ca+Ca and Au+Au in multi-fragmentation process can be reasonably well reproduced. Simultaneously, the influence of the nuclear matter incompressibility and the range of nucleon-nucleon interaction on the nucleus-nucleus dynamic potential is investigated.Comment: 7 figures, 3 tables, to appear in Phys. Rev.

Proof of a locally limited imperfection of the diaphragm wall based on the numerical method Smoothed-Particle Hydrodynamics (SPH)

In dieser Dissertation werden die Fehlstellen in den Schlitzwänden untersucht und numerisch simuliert. Die Simulation erfolgt auf Basis des numerischen Verfahrens „Smoothed Particle Hydrodynamics (SPH)“. Die SPH-Methode ist ein gitterfreies Verfahren, das ursprünglich zur numerischen Simulation der astronomischen Probleme erfunden wurde. Diese Methode wurde in den letzten zehn Jahren auch häufig bei geotechnischen Problemen angewendet, insbesondere bei Problemen mit großen Verformungen, die mit herkömmlichen gitterbasierten Methoden wie der Finite-Elemente-Methode sehr schwierig und kompliziert zu simulieren sind. In dieser Forschungsarbeit wurde ein Fortran-Code zur Simulation der Imperfektionen in Schlitzwänden mit der SPH-Methode entwickelt. Um diesen Code zu validieren, werden verschiedene geotechnische Probleme modelliert und die Ergebnisse mit den verschiedenen bekannten Experimenten verglichen. Darüber hinaus wurde früher im Institut für Geo-Engineering der Technischen Universität Clausthal eine Serie von Experimenten durchgeführt, um den Einsturz des historischen Archivgebäudes der Stadt Köln in Deutschland zu erforschen. Auch dieser Versuch wird mit dem entwickelten Fortran-Code simuliert, um die Fähigkeit der SPH-Methode zur Simulation der Imperfektionen in Schlitzwänden auch bei hohem Grundwasserspiegel nachweisen zu können.In this dissertation, the imperfections of diaphragm walls are investigated and numerically simulated. The simulation is carried out based on a numerical method “Smoothed Particle Hydrodynamics (SPH)”. The SPH method is a meshfree method, which is originally invented for numerical simulation of the astronomical problems. This method is also often applied in the last decade in the geotechnical problems, especially the problems with large deformations, which are very difficult and complicated to simulate in the conventional grid-based methods as like as Finite Element method. In this research, a Fortran-Code for simulation of the imperfections in diaphragm walls with SPH method is developed. To validate this code, various geotechnic problems are modelled and the results compared with the different known experiments. Furthermore, an Experiment is carried out earlier in the Institute of Geo Engineering in Technical University of Clausthal in order to observe the collapse of the historical archive building of the city Cologne in Germany. This experiment is also with the developed Fortran-Code simulated, to present the capabilities of the SPH method for simulating the imperfections in diaphragm walls even with high groundwater level

Explore and develop methods for the economic evaluation of school-based interventions to prevent childhood obesity in low and middle income countries

Childhood obesity is a major global public health challenge with associated health, social, and emotional consequences, leading to long term direct and indirect costs. However, there are few published economic evaluations of interventions and only one from a Chinese setting. This thesis aims to explore and develop methods for the economic evaluation of school-based interventions to prevent obesity in children in low and middle income countries, thus making a methodological contribution to the literature. The methods for the economic evaluation were derived from a combination of published literature and guidelines for conducting economic evaluation. The systematic review undertaken within this thesis discovered heterogeneity regarding methods applied. The evaluation, conducted alongside the CHIRPY DRAGON trial, reported the intervention to be highly cost-effective. A number of methodological issues were explored: measuring household cost and outcome data and the construct validity of the CHU-9D in a Chinese sample. Including societal costs and effects increased the incremental cost-effectiveness ratio, however the intervention remained cost-effective using conventional decision making rules and throughout a series of sensitivity analyses. Furthermore, the thesis findings provide support for the construct validity of the CHU-9D within this population

Understanding reservoir engineering aspects of shale oil development on the Alaska North Slope

Thesis (M.S.) University of Alaska Fairbanks, 2014Horizontal drilling and multi-stage hydraulic fracturing have made the commercial development of nano-darcy shale resources a success. The Shublik shale, a major source rock for hydrocarbon accumulations on the North Slope of Alaska, has huge potential for oil and gas production, with an estimated 463 million barrels of technically recoverable oil. This thesis presents a workflow for proper modeling of flow simulation in shale wells by incorporating results from hydraulic fracturing software into hydraulic fracture flow modeling. The proposed approach allows us to simulate fracture propagation and leak-off of fracturing fluid during hydraulic fracturing. This process honors the real proppant distribution, horizontal and vertical variable fracture conductivity, and presence of fracturing fluid in the fractures and surrounding matrix. Data from the Eagle Ford Shale in Texas was used for this modeling which is believed to be analogous to Alaska's Shublik shale. The performance of a single hydraulic fracture using a black oil model was simulated. Simulation results showed that for the hydraulically fractured zone, the oil recovery factor is 5.8% over thirty years of production, to an assumed economic rate of 200 STB/day. It was found that ignoring flowback overestimated oil recovery by about 17%. Assuming a constant permeability in the hydraulic fracture plane resulted in overestimation of oil recovery by almost 25%. The conductivity of the unpropped zone affected the recovery factor predictions by as much as 10%. For the case investigated, about 25% of the fracturing fluid was recovered during the first 2 months of production; in total, 44% of it was recovered over thirty years. Permeability anisotropy was found to have a significant effect on the results. These results suggest that assuming a constant conductivity for the fractures and ignoring the presence of water in the fractures and the surrounding matrix leads to overestimation of initial production rates and final recovery factors. In addition, the modified workflow developed here more accurately and seamlessly integrates the modeled induced fracture characteristics in the reservoir simulation of shale resource plays