Abstract: This study experimentally investigates the impact of a single piezoelectric (PZT) actuator on a turbulent boundary layer from a statistical viewpoint. The working conditions of the actuator include a range of frequencies and amplitudes. The streamwise velocity signals in the turbulent boundary layer flow are measured downstream of the actuator using a hot-wire anemometer. The mean velocity profiles and other basic parameters are reported. Spectra results obtained by discrete wavelet decomposition indicate that the PZT vibration primarily influences the near-wall region. The turbulent intensities at different scales suggest that the actuator redistributes the near-wall turbulent energy. The skewness and flatness distributions show that the actuator effectively alters the sweep events and reduces intermittency at smaller scales. Moreover, under the impact of the PZT actuator, the symmetry of vibration scales’ velocity signals is promoted and the structural composition appears in an orderly manner. Probability distribution function results indicate that perturbation causes the fluctuations in vibration scales and smaller scales with high intensity and low intermittency. Based on the flatness factor, the bursting process is also detected. The vibrations reduce the relative intensities of the burst events, indicating that the streamwise vortices in the buffer layer experience direct interference due to the PZT control. PubDate: 2019-11-05

Abstract: Time-varying systems are applied extensively in practical applications, and their related parameter identification techniques are of great significance for structural health monitoring of time-varying systems. To improve the identification accuracy for time-varying systems, this study puts forward a novel parameter identification approach in the time–frequency domain using intrinsic chirp component decomposition (ICCD). ICCD is a powerful tool for signal decomposition and parameter extraction, with good signal reconstruction capability in a high-noise environment. To maintain good identification effects for the time-varying system in a noisy environment, the proposed method introduces a redundant Fourier model for the non-stationary signal, including instantaneous frequency (IF) and instantaneous amplitude (IA). The accuracy and effectiveness of the proposed approach are demonstrated by a single-degree-of-freedom system with three types of time-varying parameters, as well as an example of a multi-degree-of-freedom system. The effects of different levels of measured noise on the identified results are also discussed in detail. Numerical results show that the proposed method is very effective in tracking the smooth, periodical, and non-smooth variations of time-varying systems over the entire identification time period even when the response signal is contaminated by intense noise. Graphic abstract PubDate: 2019-11-01

Abstract: The dynamic mechanical properties of concrete and reinforced concrete targets subjected to high-speed projectile impact loading have a significant influence on the impact resistance of protective structures. In this study, high-speed projectile penetration and perforation of concrete and reinforced concrete structures was carried out to determine the high-energy impact loading. The failure behaviors of projectile penetration and perforation of the concrete and reinforced concrete targets were investigated, and the destruction characteristics of the targets were measured. An analytical model was established using the principle of minimum potential energy. The results show that the theoretical predictions are consistent with the experimental data, indicating that the energy method is effective for predicting the dynamic mechanical properties of concrete and reinforced concrete targets under high-speed projectile penetration. PubDate: 2019-10-22

Abstract: Generalized Kelvin–Voigt and Maxwell models using Prony series are some of the most well-known models to characterize the behavior of polymers. The simulation software for viscoelastic materials generally implement only some material models. Therefore, for the practice of the engineer, it is very useful to have formulas that establish the equivalence between different models. Although the existence of these relationships is a well-established fact, moving from one model to another involves a relatively long process. This article presents a development of the relationships between generalized Kelvin–Voigt and Maxwell models using the aforementioned series and their respective relaxation and creep coefficients for one and two summations. The relationship between the singular points (maximums, minimums and inflexion points) is also included. PubDate: 2019-10-15

Abstract: Linear transient growth of optimal perturbations in particle-laden turbulent channel flow is investigated in this work. The problem is formulated in the framework of a Eulerian–Eulerian approach, employing two-way coupling between fine particles and fluid flow. The model is first validated in laminar cases, after which the transient growth of coherent perturbations in turbulent channel flow is investigated, where the mean particle concentration distribution is obtained by direct numerical simulation. It is shown that the optimal small-scale structures for particles are streamwise streaks just below the optimal streamwise velocity streaks, as was previously found in numerical simulations of particle-laden channel flow. This indicates that the optimal growth of perturbations is a dominant mechanism for the distribution of particles in the near-wall region. The current study also considers the transient growth of small- and large-scale perturbations at relatively high Reynolds numbers, which reveals that the optimal large-scale structures for particles are in the near-wall region while the optimal large-scale structures for fluid enter the outer region. PubDate: 2019-10-11

Abstract: Vertical-axis wind turbines (VAWTs) have been widely used in urban environments, which contain dust and experience strong turbulence. However, airfoils for VAWTs in urban environments have received considerably less research attention than those for horizontal-axis wind turbines (HAWTs). In this study, the sensitivity of a new VAWT airfoil developed at the Lanzhou University of Technology (LUT) to roughness was investigated via a wind tunnel experiment. The results show that the LUT airfoil is less sensitive to roughness at a roughness height of < 0.35 mm. Moreover, the drag bucket of the LUT airfoil decreases with increasing roughness height. Furthermore, the loads on the LUT airfoil during dynamic stall were studied at different turbulence intensities using a numerical method at a tip-speed ratio of 2. Before the stall, the turbulence intensity did not considerably affect the normal or tangential force coefficients of the LUT airfoil. However, after the stall, the normal force coefficient varied obviously at low turbulence intensity. Moreover, as the turbulence intensity increased, the normal and tangential force coefficients decreased rapidly, particularly in the downwind region of the VAWT. PubDate: 2019-10-09

Abstract: Soft pneumatic actuators have been widely used for implementing sophisticated and dexterous movements, due to numerous fascinating features compared with their rigid counterparts. Relatively speaking, modeling and analysis of an entire soft pneumatic actuator considering contact interaction between two adjacent air chambers is extremely rare, which is exactly what we are particularly interested in. Therefore, in order to establish an accurate mechanical model and analyze the overall configuration and stress distribution for the soft pneumatic actuator with large deflection, we consider the contact interaction of soft materials rather than hard materials, to produce an effective enhanced model for soft contact of a large deformable pneumatic actuator. In this article, a multiple-point contact approach is developed to circumvent the mutual penetration problem between adjacent air chambers of the soft actuator that occurs with the single-point contact approach employed in linear elastic rigid materials. In contrast to the previous simplified rod-based model that did not focus on contact interaction which was adopted to clarify the entire deformation of the actuator, the present model not only elaborates nonlinear large deformation and overall configuration variations, but also accurately delineates stress distribution law inside the chamber structure and the stress concentration phenomenon. By means of a corresponding static experiment, a comparison of the simulation results with experimental data validates the effectiveness and accuracy of this model employing a multiple-point contact approach. Excellent simulation of the actual bending deformation of the soft actuator is obtained, while mutual penetration is successfully circumvented, whereas the model with single-point contact cannot achieve those goals. Finally, as compared with the rod-based model, the results obtained using the proposed model are more consistent with experimental data, and simulation precision is improved. Graphical abstract PubDate: 2019-10-09

Abstract: In the original publication of this article, Table 2 is incorrectly published due to the negligence of the author’s proofreading. The correct version of Table 2 is provided below. PubDate: 2019-10-01

Abstract: An improved analytical design to investigate the static stiffness of a convoluted air spring is developed and presented in this article. An air spring provides improved ride comfort by achieving variable volume at various strokes of the suspension. An analytical relation is derived to calculate the volume and the rate of change in the volume of the convoluted bellow with respect to various suspension heights. This expression is used in the equation to calculate the variable stiffness of the bellow. The obtained analytical characteristics are validated with a detailed experiment to test the static vertical stiffness of the air spring. The convoluted air bellow is tested in an Avery spring-testing apparatus for various loads. The bellow is modeled in the ABAQUS environment to perform finite element analysis (FEA) to understand and visualize the deflection of the bellow at various elevated internal pressures and external loads. The proposed air spring model is a fiber-reinforced rubber bellow enclosed between two metal plates. The Mooney–Rivlin material model was used to model the hyperelastic rubber material for FEA. From the results, it is observed that the experimental and analytical results match with a minor error of 7.54%. The derived relations and validations would provide design guidance at the developmental stage of air bellows. These expressions would also play a major role in designing an effective active air suspension system by accurately calculating the required stiffness at various loads. PubDate: 2019-10-01

Abstract: Modeling the elastic behavior of solids in energy conversion and storage devices such as fuel cells and lithium-ion batteries is usually difficult because of the nonlinear characteristics and the coupled chemo-mechanical behavior of these solids. In this work, a perturbation finite element (FE) formulation is developed to analyze chemo-elastic boundary value problems (BVPs) under chemical equilibrium. The perturbation method is applied to the FE equations because of the nonlinearity in the chemical potential expression as a function of solute concentration. The compositional expansion coefficient is used as the perturbation parameter. After the perturbation expansion, a system of partial differential equations for the displacement and dimensionless solute concentration functions is obtained and solved in consecutive steps. The presence of a numerical solution enables modeling 3D chemo-elastic solids, such as battery electrodes or ionic gels, of any geometric shape with defects of different shapes. The proposed method is tested in several numerical examples such as plates with circular or elliptical holes, and cracks. The numerical examples showed how the shape of the defect can change the distribution of solute concentration around the defect. Cracks in chemo-elastic solids create sharp peaks in solute concentration around crack tips, and the intensity of these peaks increases as the far field solute concentration decreases. PubDate: 2019-10-01

Abstract: A crystal plasticity model is developed to predict the fatigue crack nucleation of polycrystalline materials, in which the accumulated dislocation dipoles are considered to be the origin of damage. To describe the overall softening behavior under cyclic loading, a slip system-level dislocation density-related damage model is proposed and implemented in the finite element analysis with Voronoi tessellation. The numerical model is applied to calibrate the stress–strain relationship at different cycles before fatigue crack nucleation. The parameters determined from the numerical analysis are substituted into a modified phase transformation model to predict the critical fatigue crack nucleation cycle. Comparing with the experimental results of Sn–3.0Ag–0.5Cu (SAC305) alloy and P91 steel, the proposed method can describe the constitutive behavior and predict the fatigue crack nucleation accurately. PubDate: 2019-10-01

Abstract: A homogenisation model for analysing the effect of micrometre pore sizes on the engineering moduli of elasticity of porous materials was proposed. In the proposed model, the engineering coefficients of localization of total strains (LTS coefficients) are considered instead of the classical strain localization tensors. For a pore, these coefficients represent the ratio of the sum of the strains in the volume of the pore to the sum of the strains in the volume of the porous body. To estimate the elastic moduli of a material with an arbitrary pore size, it is sufficient to have information about the elastic moduli and the LTS coefficient of a material with one basic pore size. Then, in Eshelby’s model of equivalent inclusion, a transition to LTS coefficient for material with arbitrary pore size is achieved, and its elasticity moduli are determined. The results for Young’s modulus of porous titanium, with different sizes of spherical pores, completely conform with the experimental data. We have obtained a model theoretic estimate of the upper bounds of Young’s modulus of porous materials with infinitely small pore size. For the spherical pores, the proposed assessment coincides with the upper limits of the Hashin–Shtrikman bounds. PubDate: 2019-10-01

Abstract: The peridynamic motion equation was investigated once again. The origin of incompatibility between boundary conditions and peridynamics was analyzed. In order to eliminate this incompatibility, we proposed a new peridynamic motion equation in which the effects of boundary traction and boundary displacement constraint were introduced. The new peridynamic motion equation is invariant under the transformations of rigid translation and rotation. Meanwhile, it also satisfies the requirements of total linear and angular momentum equilibrium. By this motion equation, three kinds of boundary value problems containing the displacement boundary condition, the traction boundary condition and mixed boundary condition are characterized in peridynamics. As examples, we calculated static tension and longitudinal vibration of a finite rod. The acquired solutions exhibit obvious nonlocal features, and the vibration has the dispersion similar to one dimensional atom chain vibration. PubDate: 2019-10-01

Abstract: The paper investigated the equivalent continuum modeling of beam-like repetitive truss structures considering the flexibility of joints, which models the contact between the truss member and joint by spring-damper with six directional stiffnesses and dampings. Firstly, a two-node hybrid joint-beam element was derived for modeling the truss member with flexible end joints, and a condensed model for the repeating element with flexible joints was obtained. Then, the energy equivalence method was adopted to equivalently model the truss structure with flexible joints and material damping as a spatial viscoelastic anisotropic beam model. Afterwards, the equations of motion for the equivalent beam model were derived and solved analytically in the frequency domain. In the numerical studies, the correctness of the presented method was verified by comparisons of the natural frequencies and frequency responses evaluated by the equivalent beam model with the results of the finite element method model. Graphical abstract PubDate: 2019-10-01

Abstract: To improve the global stiffness and conveniently build a model of a compliant mechanism with spatial multiple degrees of freedom (DOF), the topology optimization method, combined with the isomorphic mapping matrix, is proposed in this paper for structure synthesis of a 6-DOF spatial compliant mechanism. By using the differential approximation method, the Jacobian matrix of the Stewart prototype platform is calculated as the isomorphic mapping matrix, and its eigenvalues and eigenvectors are considered. Combining the isomorphic mapping matrix with the solid isotropic material with the penalization topology optimization method, the topological model of the 6-DOF spatial compliant mechanism is constructed, and a topological structure of the 6-DOF spatial compliant mechanism is derived which has the same differential kinematic characteristics as the Gough–Stewart prototype platform. Piezoelectric actuators are mounted inside the topological structure during the three-dimensional printing manufacturing process, and its driver directions are in accordance with the driver configuration directions of the Gough–Stewart prototype platform. The effectiveness of the proposed method for topological structure synthesis of the 6-DOF spatial compliant mechanism is demonstrated through several numerical examples and experimental studies. PubDate: 2019-10-01

Abstract: Quadrilateral finite elements for linear micropolar continuum theory are developed using linked interpolation. In order to satisfy convergence criteria, the newly presented finite elements are modified using the Petrov–Galerkin method in which different interpolation is used for the test and trial functions. The elements are tested through four numerical examples consisting of a set of patch tests, a cantilever beam in pure bending and a stress concentration problem and compared with the analytical solution and quadrilateral micropolar finite elements with standard Lagrangian interpolation. In the higher-order patch test, the performance of the first-order element is significantly improved. However, since the problems analysed are already describable with quadratic polynomials, the enhancement due to linked interpolation for higher-order elements could not be highlighted. All the presented elements also faithfully reproduce the micropolar effects in the stress concentration analysis, but the enhancement here is negligible with respect to standard Lagrangian elements, since the higher-order polynomials in this example are not needed. PubDate: 2019-10-01

Abstract: As a promising renewable energy, offshore wind energy currently is gaining more attention, by which the economic and efficient operation of floating wind turbine systems is a potential research direction. This study is primarily devoted to the analysis of dynamic response of the NREL-5 MW reference wind turbine supported by an OC3-Hywind SPAR-type platform using a recompiled code which combines FAST with WAMIT. To verify the reliability of the recompiled code, the free decay motions of a floating wind turbine system in still water are examined with satisfactory results. After that, thirteen scenarios with different angles between wind and wave from 0° to 90° are investigated. The dynamic responses of the turbine system in various degrees of freedom (DOFs) for different incident wind/wave directions are presented in both time and frequency domains via the fast Fourier transform. PubDate: 2019-10-01

Abstract: Blast-induced traumatic brain injury (b-TBI) is a kind of significant injury to soldiers in the current military conflicts. However, the mechanism of b-TBI has not been well understood, and even there are some contradictory conclusions. It is crucial to reveal the dynamic mechanism of brain volume and shear deformations under blast loading for better understanding of b-TBI. In this paper, the numerical simulation method is adopted to carry out comprehensive and in-depth researches on this issue for the first time. Based on the coupled Eulerian–Lagrangian method, the fluid–structure coupling model of the blast wave and human head is developed to simulate two situations, namely the head subjected to the frontal and lateral impacts. The simulation results are analyzed to obtain the underlying dynamic mechanisms of brain deformation. The brain volume deformation is dominated by the local bending vibration of the skull, and the corresponding frequency for the forehead skull under the frontal impact and the lateral skull faced to the lateral impact is as high as 8 kHz and 5 kHz, respectively. This leads to the high-frequency fluctuation of brain pressure and the large pressure gradient along the skull, totally different from the dynamic response of brain under head collisions. While the brain shear deformation mainly depends on the relative tangential displacement between the skull and brain and the anatomical structure of inner skull, being not related to the brain pressure and its gradient. By further comparing the medical statistics, it is inferred that diffuse axonal injury and brain contusion, the two most common types of b-TBI, are mainly attributed to brain shear deformations. And the von Mises stress can be adopted as the indicator for these two brain injuries. This study can provide theoretical guidance for the diagnosis of b-TBI and the development of protective equipment. PubDate: 2019-10-01

Abstract: Characteristics of convective heat transfer of a supersonic model combustor with variable inlet flow conditions were studied by numerical simulation in this paper. The three-dimensional flow and wall heat flux at different air inlet Mach numbers of 2.2, 2.8 and 3.2 were studied numerically with Reynolds-averaged Navier–Stokes equations with a shear-stress transport (SST) k − ω turbulence model and a three-step reaction model. Meanwhile, ethylene was chosen as the fuel, and the fixed fuel-to-air equivalence ratio is 0.8 in all cases in this paper. The results of the simulations indicate that wall heat flux distribution of the combustor is very non-uniform with several peaks of wall heat flux at varied locations. For the low inlet Mach number of 2.2, a shock train structure is formed in the isolator, and three peaks of wall heat flux are located respectively on the backward face of the cavity, on the side wall near the fuel injection and on the bottom wall near the injection holes, and a maximum wall heat flux reaches 5.4 MW/m2. For the medium inlet Mach number of 2.8, there exists a much shorter shock structure with three peaks of wall heat flux similar to that of Mach number 2.2. However, as the inlet Mach number increased to 3.2, there is no shock structure upstream of fuel injections, and the combustor flow is in a supersonic mode with different locations and values of wall heat flux peaks. The statistical results of wall heat loading show that the change of total wall heat is not monotonic with the increase of inlet Mach number, and the maximum appears in the case of Mach number being 2.8. Meanwhile, for all the cases, the bottom wall takes up more than 50% of the total heat loading. PubDate: 2019-10-01

Abstract: This study experimentally and numerically investigated the effect of pulsatile flow of different frequencies and outflow resistance on wall deformation in a lateral aneurysm. A method for constructing a flexible aneurysm model was developed, and a self-designed piston pump was used to provide the pulsatile flow conditions. A fluid–structure interaction simulation was applied for comparison with and analysis of experimental findings. The maximum wall displacement oscillation increased as the pulsation frequency and outflow resistance increased, especially at the aneurysm dome. There is an obvious circular motion of the vortex center accompanying the periodic inflow fluctuation, and the pressure at the aneurysm dome at peak flow increased as the pulsatile flow frequency and terminal flow resistance increased. These results could explain why abnormal blood flow with high frequency and high outflow resistance is one of the risk factors for aneurysm rupture. PubDate: 2019-09-06