Abstracts:Analytic solutions on twin tunnelling at great depth in viscoelastic geomaterial considering three-dimensional effects of tunnel faces are presented using the complex variable methods. According to the time interval between twin tunnel faces, the proposed solutions can be divided into two cases: A) newly constructing twin tunnels; and B) newly constructing single tunnel influencing an existing one. The analytic solutions can also consider sequential excavations and the mutual influence between twin tunnels. The three-dimensional effects of twin tunnel faces are transformed into equivalent plane-strain problems by introducing dimensionless coefficients. By means of a presented general case, the analytic solutions are verified via comparisons of both stresses and displacements with the numerical solution, in which a good agreement between these two solutions is observed. Subsequently, a parametric investigation is performed to study the influence of several parameters concerning twin tunnel geometry and excavation on stresses and displacements, together with the difference between Cases A and B. On the basis of the results, the analytic solutions provide quantitative insights into the essence of the three-dimensional effects of twin tunnel faces and would have certain values on the preliminary design stage of twin tunnels.

Abstracts:This paper analyses the modal characteristics and dynamic stability of a whirling rotor with flexible blades. A rigid disk with flexible blades, which is called a rotor-blade system in this paper, is modelled as a Jeffcott rotor with Euler-Bernoulli beams. From the nonlinear equations of motion, we derived the equilibrium position and the linearized equations around the equilibrium position by the perturbation method. Based on the linearized equations, a modal analysis was performed for the variations of the supporting stiffness and rotating speed. Furthermore, we investigated the effects of the whirling motion on the natural frequencies and mode shapes and found the threshold speed of instability that cause instability in the system. In addition, we also studied the characteristics of stable and unstable regions in terms of the rotating speed and the supporting stiffness.

Abstracts:The use of an open communication network in a wide-area power system (WAPS) introduces random delays into the transmission of frequency measurements and control signals, which can deteriorate the load frequency control (LFC) performance. Current studies are focusing on developing a suitable delay margin controller to maintain the stability of the WAPS. This paper introduces a new Smith predictor (SP) with tools to accurately predict the input time delays and consequently mitigate the LFC performance loss caused by unreliable communication. The time delays are predicted via the discrete hidden Markov model (DHMM) and the exponentially weighted moving average (EWMA) model. The predicted delays are then input to the SP. The DHMM adopts two different scalar quantization methods, i.e., the uniform technique and the K-means clustering technique. It then maps the time delays on to a discrete observational space. To validate the findings for practical applications, we conduct a case study on a test platform, namely a single-area WAPS with Ethernet, which is implemented via the Truetime simulator. The results indicate that the SP is more effective in eliminating load disturbances and enhancing the robustness against time delays than existing delay-margin-based controllers. The K-means DHMM-based SP achieved better LFC performance than the one with the EWMA. The stability of the LFC system with time-varying delay is discussed using a Lyapunov–Krasovskii-based delay-dependent criterion and the small gain theorem.

Abstracts:Wind-speed forecasting plays an important role in the efficient utilization of wind energy. However, accurate and stable forecasting of wind-speed series is challenging, considering the nonlinearity and chaotic characteristics of wind. Moreover, the limitations of individual forecasting models are ignored, which invariably leads to poor forecasting precision. Therefore, here, a wind-speed forecasting system based on two types of machine learning approaches (decomposition-ensemble and multi-objective optimization) is proposed, which addresses the nonlinearity and chaotic characteristics of wind-speed series well. In this system, the advanced optimization algorithm and no negative constraint theory determine the weights of results decomposed and forecasted by the sub-models. An empirical study using 10 min and 30 min interval datasets shows that the combined forecasting system outperforms comparison models and has advantages for wind-speed forecasting.

Abstracts:A modified energy method is developed to investigate the elastic properties of single-walled carbon nanotubes. The energy of a system is expressed by the force field functions of the molecular mechanics. Under the assumption of a small deformation and the principle of minimum potential energy, the system function is established. In addition to consider the change of bond stretch and bond angle, the inversion energy is also proposed in this study. Based on the established model, the closed-form expressions of temperature-dependent Young's modulus, Poisson's ratio and strain energy of armchair and zigzag carbon nanotubes are obtained. The results show that the effect of tube diameter and inversion energy on the elastic constants of carbon nanotubes (CNTs) is significant when the tube diameter is small. Under different thermal expansion coefficients, the Young's modulus of carbon nanotubes varies nonlinearly with the temperature, whereas the Poisson's ratio is insensitive to the temperature. It is also found that the temperature has a greater effect on the tensile deformation of zigzag CNTs, which provides a more accurate theoretical foundation for the tensile fracture of CNTs in a thermal environment.

Abstracts:This work deals with pointwise antennas design in hyperthermia treatment. Hyperthermia is a non-invasive therapy usually combined with chemotherapy and/or radiotherapy, which consists in heating the diseased tissue in an attempt to kill the cancerous cells. In particular, we want to find the optimal values of current densities passing through each antenna to selectively heat a specified target. The forward problem is governed by the steady-state heat equation in living tissues which is coupled with the Helmholtz problem modeling the electromagnetism phenomenon. An objective functional measuring the difference between the target temperature and the solution to the model problem is minimized with respect to the current densities by using the topological derivative method. The resulting sensitivities are used to devise first and second order antenna design algorithms as well as a third one that combines both the previous algorithms. Numerical experiments are presented showing different features of the proposed methodology, including its capability in selectively heating the target up to the desired temperature. Finally, a selected result is used in a full transient analysis, where the hot spots are keeping over the diseased tissues during the whole heating process.

Abstracts:A natural food web system involves a diverse community of natural enemies and one of the widespread and common phenomena of such systems is intraguild predation (IGP) (i.e., presence of eating and killing among potential competitors). Here, we investigate the dynamics of a food web system with Allee effect and intraguild predation. In the present study, generalist and specialist natural enemies competing for a shared resource (prey) (intraguild predation) have been introduced. The Allee effect has also been incorporated in the shared prey growth rate. We investigate how parameters defining Allee effect and intraguild predation affect the long-term persistence, extinction and co-existence regions of such species. We outline the conditions under which different types of interior and non-interior equilibria exist and are locally stable. Bi-stable dynamics has also been investigated for the proposed model system for a suitable range of parametric values. A threshold condition on the strength of Allee effect has been obtained assuring the absence of IG predator population along with extinction region of shared prey. To understand the dynamics of the system, a comprehensive study of bifurcation analysis has also been provided taking Allee effect and fertility rate of intraguild predator as bifurcation parameters. These two parameters generate various interesting bifurcations like saddle-node bifurcation and Hopf-bifurcation. We have obtained different parametric regions of Allee parameter for the existence of different boundary and interior equilibria. All the analytical results related with local stability of equilibrium points and all possible successive bifurcations have been supported by different numerical examples, one and two parameter bifurcation diagrams, bi-stability diagram and stability regions of all possible equilibrium points. The impacts of Allee effect on co-existence, stability, extinction of species, their persistence, bistability and bifurcations have been explicitly discussed and the whole dynamics has also been successfully compared with the dynamics of food web without Allee effect. It is observed that the introduction of Allee effect and IG predator induce more rich dynamics and compel the system to be more sensitive to initial population densities.

Abstracts:In this paper, an iterative method is proposed to reduce the order of the coupled eigenvalue problem related to fluid-structure interaction systems. In fact, it is required to solve a smaller eigenvalue problem rather than the larger one (original) to compute the natural frequencies and mode shapes of the system. To this end, all degrees of freedom (DOFs) of the system are divided into master (retained) and slave (eliminated) ones. Then, the problem is re-expressed based on the master DOFs and a transformation matrix is introduced. The results show a remarkable decline in computational costs, whereas the accuracy of the modal outputs does not significantly decrease. A stopping criterion is defined to check whether the iterative process converges. Moreover, three fluid-structure systems are analyzed, including a two-dimensional fully-filled concrete tank, a two-dimensional gravity dam-reservoir, and a three-dimensional arch dam-reservoir, to assess the correctness and performance of the presented method. Findings prove that the proposed method is able to reduce the order of the eigenvalue problem of fluid-structure systems.

Abstracts:This paper presents the modeling, controller design and testing of the integrated permanent magnet toroidal motor drive. As one kind of direct-drive composite motor drive, the integration and configuration for permanent magnet toroidal motor are presented. Based on its special rotor structure, the magnetic flux linkages of the worm stator windings are deduced with its structural and kinematic parameters. The voltage equations and electromagnetic torque in rotary two-phase coordinate are presented. The dynamic machine model for permanent magnet toroidal motor drive is established with nonlinear time-varying electromagnetic parameters, which include inductances and differential information. Then, the dynamic performance with periodic fluctuation is analyzed. Based on the effect analysis for the dynamic response, the electromagnetic torque model of permanent magnet toroidal motor drive is properly simplified. In order to improve the output performance of toroidal motor drive, a terminal sliding surface is introduced in the controller design to accelerate the response speed, and meanwhile, the sliding mode observer is proposed to estimate the angular speed of the rotor in the feedback loop to achieve better tracking performance. The foundation test of inductance was conducted and demonstrates the validity of the machine model. The simulation results show that the proposed control strategy can significantly improve the dynamic behavior and robustness of the permanent magnet toroidal motor drive system.

Abstracts:We analyze the second order, non-linear, one dimensional differential equation describing the steady conduction of heat in convecting–radiating longitudinal fins. We introduce an auxiliary dependent variable, solving a first order differential equation and related to the thickness of the fin, giving the distribution of the heat along the fin. The purely convecting case, corresponding to a linear equation, and the convecting–radiating case, corresponding to a non-linear equation, are treated separately. For the linear case, different solutions, corresponding to different shapes of the fin, are analyzed. For the non-linear case, an explicit solution in terms of the auxiliary variable is obtained. The distribution of the temperature, the efficiency of the fin and the applicability of the results are discussed.