Determination of effective medium parameters of an in-line cavity based acoustic metamaterials with visco-thermal losses

The study concerns with defining effective medium parameters-such as refraction index, impedance, mass density and compressibility-of acoustic metamaterials via an inverse method with and without taking of visco-thermal effects into account. This is realized by transfer matrix method using effective medium theory through the homogenization of an acoustic metamaterial by means of a passive material. Visco-thermal effects are approximately defined in the transfer matrix equations by using Zwicker-Kosten model. The model is verified by finite element method in cases with and without visco-thermal effects. In the study, as an example, an in-line spherical cavity based acoustic metamaterial with and without visco-thermal losses is considered. The effect of visco-thermal losses on effective parameters and also on acoustic parameters such as transmission and absorption coefficients is clearly shown for the considered acoustic metamaterial

Stochastic prediction of natural frequencies of laminated composite beams by using a high-order statistical moment based approach

Statistical Moment (SM) based modelling is a quite straightforward approach in stochastic modelling of uncertain structures. However, the method still has deficiencies including determination of SMs of natural frequencies of vibratory structures, and it has not been tested yet for realistic structures. This study aims such verification by employing high-degree statistical moments in stochastic equations. In this respect, SM approach is applied for two different uncertainty cases. In the first case, uncertain parameters are experimentally reproduced from the batch of laminated composite beams. Then, those uncertainties are fed to SM equations used in finite element model to obtain descriptive statistical quantities (mean, variance, skewness, and kurtosis) of stochastic natural frequencies. Next, Pearson model is utilized to obtain probability density function of the natural frequency by using standardized SMs. Beside this, uncertain fundamental natural frequency of fifty samples of composite beams is measured by experimental modal tests. All SM based predictions and modal test results are also compared with numerical Monte Carlo Simulations. The latter case examines composite beams having non normal uncertain thickness. Since the results are in good harmony with each other, it is concluded that high order SM based approach may effectively be used in uncertainty modelling of realistic structures