Parameter Identification of the Nonlinear Piezoelectric Shear d15 Coefficient of a Smart Composite Actuator

The objective of this work is to characterize the nonlinear dependence of the piezoelectric d15 shear coefficient of a composite actuator on the static electric field and include this effect in finite element (FE) simulations. The Levenberg-Marquardt nonlinear least squares optimization algorithm implemented in MATLAB was applied to acquire the piezoelectric shear coefficient parameters. The nonlinear piezoelectric d15 shear constant of the composite actuator integrated with piezoceramic d15 patches was obtained to be 732 pC/N at 198 V. The experimental benchmark was simulated using a three-dimensional piezoelectric FE model by taking piezoelectric nonlinearity into consideration. The results revealed that the piezoelectric shear d15 coefficient increased nonlinearly under static applied electric fields over 0.5 kV/cm. A comparison between the generated transverse deflections of the linear and nonlinear FE models was also performed.

Shear coefficient of spin viscosity estimation through velocity profiles of a ferrofluid under the effect of an external rotating magnetic field of low amplitude and frequency

The present article describes a numerical strategy for the estimation of the shear coefficient of spin viscosity for a ferrofluid sample confined to a cylindrical container and exposed to the effect of an external rotating magnetic field with a low amplitude and frequency. As far as we know, there are no experimental measurements of such coefficient. Furthermore, the few analytical values reported differ in several orders of magnitude. First, we describe briefly the mathematical model of the system and its numerical solution. Then, the definition of the direct and inverse problems is given as a part of the methodology for estimating such coefficient. Finally, we solve the inverse problem using simulated measurements and two global optimization algorithms. We generate this type of measurements by adding white Gaussian noise signals to the numerical solution of the ferrohydrodynamic mathematical model. Several noise levels in the range of 10 to 40 dB were used to increase the number of scenarios for validation purpose. Results showed an excellent agreement between the estimated values and those used in the numerical solution of the mathematical model. A statistical analysis revealed a normal distribution that was dependent on the noise level. This variation did not affect the results, but showed instead the validity of the proposed method. Additionally, this strategy stands as a computational tool for validating experimental results of the future in situ measurements. Tables 7, Figs 11, Refs 17.

KAJIAN NUMERIK FENOMENA UNDULAR TIDAL BORES DALAM MEMPENGARUHI PROSES EROSI PADA DAERAH ALIRAN SUNGAI (Numerical study of undular tidal bores phenomenon in influencing erosion processes of watersheds)

Riverbank erosion is one indication of watershed damage. One of the causes is the phenomenon of tidal bores waves that occur in a river channel.The strength of tidal bores wave's can be measured based on its shear force parameter and dissipation energy. Wave shear force and dissipation energy are the parameters that play a role in a riverbank erosion process. Both of them are characterized by the Froude number (Fr) which is a function of the upstream river flow velocity (V0), the tidal bores flow velocity from the estuary (Vb), the river depth (h1), and the gravity acceleration (g). A numerical study of the phenomenon of undular tidal bores has been carried out in this article. Five undular bores simulations have been built using the open-source Computational Fluid Dynamics (CFD) software, OpenFOAM. This study aims to analyze the effect of the Froude number variations (Fr) on the magnitude of the wave shear coefficient (ϵ) and dissipation energy ( ) on undular bores cases. Five simulations of undular bores have been generated based on five Froude's numbers, Fr = 1.0, 1.1, 1.2, 1.3, and 1.4. The validation has been performed by comparing the experimental and numerical results from the scientific literature. The analysis results show that the increase in Fr has a significant effect on the increase in the ϵ and .These results indicate that the Froude number variations have influenced the wave shear coefficient and dissipation energy on the undular bores cases. Increasing the Fr values have triggered an increase in the value of ϵ linearly and exponentially. Thus, the erosion that occurs on the riverbank in the undular tidal bores phenomenon could be determined based on Froude's number.

Influence of Wind Shear Uncertainty in Long-Term Extreme Responses of an Offshore Monopile Wind Turbine

In the field of stochastic dynamics of marine structures, the determination of long-term extreme responses is a crucial aspect to ensure the desired level of structural reliability. The calculation of these responses requires precise knowledge of the environmental conditions and reliable methods to predict the values associated with a reliability target level. While there is a very precise method to determine the value of these extreme values, e. g. the full long-term analysis (FLTA), this approach is computationally expensive. Then, approximated methods are needed. One practical approach for the determination of the most relevant environmental conditions for extreme calculation is the environmental contour method (ECM). However, some limitations have been detected when this method is used for offshore structures that consider survival strategies e. g. offshore wind turbines (OWT). Lastly, a modified ECM procedure (MECM) has been developed with the purpose to bypass the limitations of the traditional ECM. This method is based on short-term simulations and through an iterative process by testing many environmental contours in the operational range allows finding an important wind speed with its corresponding return period and thus, the problem that traditional ECM has, is avoided. The environmental conditions, which are represented by a large number of parameters, are also an important aspect of extreme calculation. Whereas some of them are treated as stochastic values, some are considered deterministic and, therefore the existence of uncertainties in their measured/estimated values is inevitable. These uncertainties are addressed by adopting values recommended by standards and guidelines and, in practice, it is often necessary to be conservative when there is a lack of information about the specific site studied. Therefore, the understanding of the impact that these uncertainties can have on the loads/responses that govern the design of offshore structures, especially wind turbines, is of great relevance. In this work, the influence of uncertainty in the wind shear coefficient (WSC) is studied. This parameter is directly related to one critical environmental condition i. e. wind speed at hub height, and its influence in power production and fatigue loads has been documented in the literature, but, few cases have addressed their influence in bottom fixed OWT responses. This work seeks to highlight the relevance of an accurate selection of shear coefficient and, its influence on the probabilistic analysis of a bottom fixed OWT taking into account that considerable variations from recommended values may occur. Through the use of coupled simulations in FAST, the NREL 5MW wind turbine will be subjected to varying wind shear conditions, and the corresponding 50-yr long-term responses will be calculated considering the MECM to take into account the influence of the wind turbine survival mode. The extreme values are fitted from a Global Maxima Method (GMM). Finally, it is sought to relate the uncertainty in a relevant input parameter (i. e. WSC) with the uncertainties propagated to the output parameters (i. e. extrapolated long-term extreme responses).

Optimum parameters of a five-story building supported by lead-rubber bearings under near-fault ground motions

Seismic response of five-story frame structure supported by lead-rubber bearings isolation system is investigated subjected to near-fault ground motions. The main structure is modeled as a simple linear multi-degrees-of-freedom vibration system with lumped masses, excited by near-fault ground motions in the horizontal direction. The variation curves of peak top floor acceleration and peak bearing displacement of isolated building are plotted under different yield shear coefficient. The objective function selected for optimality is to maximize the seismic energy dissipated by the lead-rubber bearings. The main constraint conditions selected for optimality are the minimization of both peak bearing displacement and peak top floor acceleration. Optimum parameters of lead-rubber bearing isolation system are investigated and found that optimum yield shear coefficient of lead-rubber bearings is found to be in the range of 0.10–0.14 under near-fault ground motions. Optimum yield shear coefficient decreases with the increase of second isolation period. Optimum yield shear coefficient of lead-rubber bearings with higher yield displacement is larger than that of lead-rubber bearings with low yield displacement. Optimum ratio of pre-yield stiffness to post-yield stiffness of lead-rubber bearings is found to be in the range of 16–35. Optimum stiffness ratio increases proportionally with the decrease of yield displacement. Optimum stiffness ratio increases slightly with the increase of yield shear coefficient. Excluding the effect of pre-yield stiffness, the optimum second isolation period is recommended to be in the range of 4–6 s.