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The multilayered parallel-plate structure (MPPS) is a typical structure in microwave engineering. In numerical simulation, MPPS is usually modeled by an infinitely large planarly layered medium (PLM) to simplify the computation. For a realistic microwave circuit, however, the substrate is always finite in spatial extent, i.e., it is a finite-sized MPPS (FS-MPPS). To address this issue, a novel surface integral equation (SIE) formulation is proposed for the efficient simulation of FS-MPPS. In this article, the surface equivalence principle is first revisited. For the exterior and interior problems, different Green’s functions are utilized. In particular, the common homogeneous-medium Green’s function is used for the exterior homogeneous region, whereas the layered-medium Green’s function (LMGF) is applied for the interior multilayered structure. In this manner, FS-MPPS is no longer directly approximated by the infinite PLM model, but instead, it is modeled accurately by SIE whose integration kernel is derived from PLM. Finally, a PMCHWT SIE is established for the electromagnetic analysis of FS-MPPS. Due to the finite nature of the inner problem, the setting of PLM and LMGF is not unique. This nonuniqueness is studied in theory and verified by numerical simulation. It is revealed that such a nonuniqueness of the setting of the inner problem does not change the uniqueness of the final solution. To validate the proposed approach, a multidomain FS-MPPS and an FS-MPPS with embedded dielectric structures are investigated. Compared with the conventional brute-force SIE methods for FS-MPPS, where all the interfaces of the layered structure should be discretized and assigned with degrees of freedom (DOFs), the proposed method is more succinct and efficient.

Axial force has a great influence on the dynamic behavior and the impact resistance of concrete-filled steel tubular (CFST) members. Based on numerical simulation and theoretical analysis, the impact response and deflection calculation method for axially loaded CFST members subjected to lateral impact are investigated in this paper. The nonlinear numerical model of an axially loaded CFST member considering the strain rate effects has been established, and the simulation accuracy has been validated by comparing with existing test results. The contrastive investigation is carried out to illustrate the influence of axial load on the variation pattern of impact force for CFST members under various structural and impact parameters, and its result indicates that the impact force-time histories for CFST members with different axial loads are mainly characterized by rectangular pulse and triangular pulse. Moreover, a simplified calculation method considering the effect of axial force is proposed based on the equivalent single degree of freedom (SDOF) method, devoted to predicting the deflection of axially loaded CFST members subjected to lateral impact. The comparisons with the numerical simulation prove that the deflection calculation method has a reasonable accuracy; thus, the proposed method can be utilized in the damage assessment and anti-impact design for CFST members subjected to lateral impact and axial load.

This paper presents a study of the bending flexibility of fully covered biodegradable polydioxanone biliary stents (FCBPBs) developed for human body. To investigate the relationship between the bending load and structure parameter (monofilament diameter and braid-pin number), biodegradable polydioxanone biliary stents derived from braiding method were covered with membrane prepared via electrospinning method, and nine FCBPBSs were then obtained for bending test to evaluate the bending flexibility. In addition, by the finite element method, nine numerical models based on actual biliary stent were established and the bending load was calculated through the finite element method. Results demonstrate that the simulation and experimental results are in good agreement with each other, indicating that the simulation results can be provided a useful reference to the investigation of biliary stents. Furthermore, the stress distribution on FCBPBSs was studied, and the plastic dissipation analysis and plastic strain of FCBPBSs were obtained via the bending simulation.

OxDNA, as a successful coarse-grain model, has been applied to reproduce the thermodynamic and mechanical properties of both single- and double-stranded DNA. In current simulation, oxDNA is extended to explore the combined effects of temperature and force on the stability of DNA hairpin and its free energy landscape. Simulations were carried out at different forces and temperatures, at each temperature, a 18-base-pair DNA hairpin dynamically transited between folded state and unfolded state, and the separation between two states is consistent with the full contour length of single-stranded DNA in the unfolded state. Two methods were used to identify the critical force of DNA hairpin at each temperature and the critical forces obtained from two methods were consistent with each other and gradually decreased with the increasing temperature from 300 K to 326 K. The critical force at 300 K is reasonably consistent with the single molecule result of DNA hairpin with the same stem length. The two-state free energy landscape can be elucidated from the probability distribution of DNA hairpin extension and its dependence on the force and temperature is totally different. The increasing temperature not only reduces the free energy barrier, but also alters the position of transition point along the extension coordinate, resulting in the reduction of folding distance and the extension of unfolding distance, but their sum is not obviously dependent on the temperature. Generally, an assumption that the location of transition state in two-state energy landscape is independent of the stretching force is used to analyze the data of the single molecule experiment, but current simulation results indicate that effects of stretching forces on the location of transition state in two-state energy landscape are dependent on temperature. At relatively high temperature, stretching force can also change the location of transition state in the free energy landscape.

This paper concerns the dissipative control for networked switched linear systems subject to event-triggering strategies and quantization. Multiple event-triggering strategies are adopted to determine whether the sampled data to be transmitted. In order to make full use of network bandwidth, logarithmic quantizers are applied in the networked switched systems. In detail, by using a delay system analysis approach, a time-delay closed-loop switched system is developed. Then, sufficient conditions are obtained to guarantee the dissipative control performance of switched system by utilizing piecewise Lyapunov functional method and average dwell time technique. Furthermore, by designing event-triggered dissipative controllers and event-triggering strategies' parameters, sufficient conditions for the stability of the closed-loop system are provided and the system is dissipative. Finally, numerical simulation results show the effectiveness of the proposed method.

Summary Real-time hybrid simulation (RTHS) combines experimental testing with numerical simulation. It provides an alternative method to evaluate the performance of structures subjected to dynamic loading such as earthquake or wind. During RTHS, a numerical integration algorithm is employed to directly solve the equation of motion and generate command displacements for the experimental test structure online. This paper presents an improved explicit numerical integration method for RTHS based on discrete state space formulation. The improved integration method utilizes an extrapolation-based prediction procedure for command displacement by applying a zero-order hold. The predicted command displacement is then used to compute the final command displacement by applying a first-order hold. Both the stability and the accuracy of the proposed integration method are investigated using control theory and numerical and experimental simulations. The proposed method demonstrates improved performance compared with other integration methods. The robustness and feasibility of the proposed method were verified through experimental RTHS on two different computational platforms including a National Instruments system and a dSPACE embedded controller. The proposed method may also be implemented on other platforms containing an xPC target and MATLAB environment. By placing the algorithm on the dSPACE system, any dynamically rated actuator with a controller that can receive analog signal may be used for RTHS. Copyright © 2015 John Wiley & Sons, Ltd.

Radially encoded MR imaging (MRI) has gained increasing attention in applications such as hyperpolarized gas imaging, contrast-enhanced MR angiography, and dynamic imaging, due to its motion insensitivity and improved artifact properties. However, since the technique collects k-space samples nonuniformly, multidimensional (especially 3D) radially sampled MRI image reconstruction is challenging. The balance between reconstruction accuracy and speed becomes critical when a large data set is processed. Kaiser-Bessel gridding reconstruction has been widely used for non-Cartesian reconstruction. The objective of this work is to provide an alternative reconstruction option in high dimensions with on-the-fly kernels calculation. The work develops general multi-dimensional least square nonuniform fast Fourier transform (LS-NUFFT) algorithms and incorporates them into a k-space simulation and image reconstruction framework. The method is then applied to reconstruct the radially encoded k-space, although the method addresses general nonuniformity and is applicable to any non-Cartesian patterns. Performance assessments are made by comparing the LS-NUFFT based method with the conventional Kaiser-Bessel gridding method for 2D and 3D radially encoded computer simulated phantoms and physically scanned phantoms. The results show that the LS-NUFFT reconstruction method has better accuracy-speed efficiency than the Kaiser-Bessel gridding method when the kernel weights are calculated on the fly. The accuracy of the LS-NUFFT method depends on the choice of scaling factor, and it is found that for a particular conventional kernel function, using its corresponding deapodization function as scaling factor and utilizing it into the LS-NUFFT framework has the potential to improve accuracy. When a cosine scaling factor is used, in particular, the LS-NUFFT method is faster than Kaiser-Bessel gridding method because of a quasi closed-form solution. The method is successfully applied to 2D and 3D in vivo studies on small animals.

A new simulation module is created based on the Simulink S-Function to meet the main characteristic of the active sequential optimal control (SOC) algorithm, in which the gain matrix is time variation, and embed into the Simulink Standard Block Library. Based on this new block library, a numerical simulation is carried out on the seismic responses of a five-storey base-isolated frame structure. Six key indices recommended by the ASCE are calculated to evaluate the control effects among the classical optimal control (COC), linear quadratic Gaussian (LQG) control and SOC in the case of the sensors normal working and sensors contaminated by white noise with different magnitudes, respectively. The results show that the new S-Function module is convenient and efficient in implementing a control algorithm simulation, and comparison with other two, the SOC algorithm has the best noise-resisting ability and the stable advantage in overall control effect.

Phase interactions among signals of physical and physiological systems can provide useful information about the underlying control mechanisms of the systems. Physical and biological recordings are often noisy and exhibit nonstationarities that can affect the estimation of phase interactions. We systematically studied effects of nonstationarities on two phase analyses including (i) the widely used transfer function analysis (TFA) that is based on Fourier decomposition and (ii) the recently proposed multimodal pressure flow (MMPF) analysis that is based on Hilbert-Huang transform (HHT) —an advanced nonlinear decomposition algorithm. We considered three types of nonstationarities that are often presented in physical and physiological signals: (i) missing segments of data, (ii) linear and step-function trends embedded in data, and (iii) multiple chaotic oscillatory components at different frequencies in data. By generating two coupled oscillatory signals with an assigned phase shift, we quantify the change in the estimated phase shift after imposing artificial nonstationarities into the oscillatory signals. We found that all three types of nonstationarities affect the performances of the Fourier-based and the HHT-based phase analyses, introducing bias and random errors in the estimation of the phase shift between two oscillatory signals. We also provided examples of nonstationarities in real physiological data (cerebral blood flow and blood pressure) and showed how nonstationarities can complicate result interpretation. Furthermore, we propose certain strategies that can be implemented in the TFA and the MMPF methods to reduce the effects of nonstationarities, thus improving the performances of the two methods.

Circular double stranded DNA has different topological states which are defined by their linking numbers. Equilibrium distribution of linking numbers can be obtained by closing a linear DNA into a circle by ligase. Using Monte Carlo simulation, we predict the temperature dependence of the linking number distribution of small circular DNAs. Our predictions are based on flexible defect excitations resulted from local melting or unstacking of DNA base pairs. We found that the reduced bending rigidity alone can lead to measurable changes of the variance of linking number distribution of short circular DNAs. If the defect is accompanied by local unwinding, the effect becomes much more prominent. The predictions can be easily investigated in experiments, providing a new method to study the micromechanics of sharply bent DNAs and the thermal stability of specific DNA sequences. Furthermore, the predictions are directly applicable to the studies of binding of DNA distorting proteins that can locally reduce DNA rigidity, form DNA kinks, or introduce local unwinding., 15 pages in preprint format, 4 figures

Cerebral autoregulation (CA) is an important vascular control mechanism responsible for relatively stable cerebral blood flow despite changes of systemic blood pressure (BP). Impaired CA may leave brain tissue unprotected against potentially harmful effects of BP fluctuations. It is generally accepted that CA is less effective or even inactive at frequencies >∼0.1 Hz. Without any physiological foundation, this concept is based on studies that quantified the coupling between BP and cerebral blood flow velocity (BFV) using transfer function analysis. This traditional analysis assumes stationary oscillations with constant amplitude and period, and may be unreliable or even invalid for analysis of nonstationary BP and BFV signals. In this study we propose a novel computational tool for CA assessment that is based on nonlinear dynamic theory without the assumption of stationary signals. Using this method, we studied BP and BFV recordings collected from 39 patients with chronic ischemic infarctions and 40 age-matched non-stroke subjects during baseline resting conditions. The active CA function in non-stroke subjects was associated with an advanced phase in BFV oscillations compared to BP oscillations at frequencies from ∼0.02 to 0.38 Hz. The phase shift was reduced in stroke patients even at > = 6 months after stroke, and the reduction was consistent at all tested frequencies and in both stroke and non-stroke hemispheres. These results provide strong evidence that CA may be active in a much wider frequency region than previously believed and that the altered multiscale CA in different vascular territories following stroke may have important clinical implications for post-stroke recovery. Moreover, the stroke effects on multiscale cerebral blood flow regulation could not be detected by transfer function analysis, suggesting that nonlinear approaches without the assumption of stationarity are more sensitive for the assessment of the coupling of nonstationary physiological signals., Author Summary Cerebral autoregulation is an important mechanism that regulates blood supply to brain tissue to match metabolic demands during daily activities. Impaired cerebral autoregulation increases the dependence of cerebral blood flow on systemic blood pressure, and is associated with fatal outcomes in patients after brain injury and acute ischemic stroke. Reliable and noninvasive assessment of cerebral autoregulation is still a major challenge in medical diagnostics and clinic studies, mainly because blood pressure and flow are intrinsically nonstationary (possessing complex oscillations/fluctuations with varying amplitude and frequency) while traditional methods for assessment of the pressure-flow dependence assume stationary signals. We propose a new computational technique that is based on nonlinear theories without the assumption of stationary signals. This technique allows us to detect the degradation of cerebral autoregulation in patients with mild ischemic stroke even at >6 months after the insult. The degradation was present in both stroke and non-stroke sides and was accompanied by an altered pressure-flow interaction over a wide range of frequencies from 0.02–0.38 Hz. Our results challenges the traditionally accepted functional region of autoregulation (