Journal of Theoretical and Computational Acoustics

In a recent publication [H. Waubke and C. Kasess, Gaussian closure technique applied to the hysteretic Bouc model with nonzero mean white noise excitation, J. Sound Vibr. 382 (2016) 258–273], the response of a single-degree-of-freedom (SDOF) system under Gaussian white noise and a constant dead load is presented. The system has a hysteresis described by Bouc [R. Bouc, Forced vibration of mechanical systems with hysteresis, in Proc. Fourth Conference on Nonlinear Oscillation (Prague, 1967), p. 315]. New is the usage of a slowly time-varying deterministic load added to the Gaussian white noise process. The transient solution is calculated using the Gaussian closure technique together with an explicit time step procedure. All moments in the Gaussian closure technique are evaluated analytically. The results of the Gaussian closure technique are in good agreement with the results from the Monte-Carlo method.

Boundary element methods (BEM) have obtained a mature state in the last years so that industrial applications are possible. However, to treat real-world problems, the so-called fast methods are necessary to reduce the original quadratic complexity to an almost linear order. Essentially, two methods are popular, the so-called fast multipole method, which uses a kernel expansion, and the algebraic approach based on ${\mathscr{H}}$-matrices with the adaptive cross approximation (ACA) to compress the matrix blocks. The latter is frequently used for scalar-valued problems, but for vector-valued problems, a modification of the pivot strategy is required. It has been suggested to search for the largest singular value out of all minimal singular values of the fundamental solution blocks. This strategy has been proposed by Rjasanow and Weggler and is studied here for elastostatics and elastodynamics. It is shown with numerical experiments that this strategy is mostly robust and results in an almost linear complexity.

In this work, numerical methods to identify the surface areas of a vibrating structure that radiate sound are implemented for cases of structures with inhomogeneous distributions of viscous Rayleigh damping. The intensity-based techniques correspond to acoustic intensity evaluated in terms of the acoustic pressure and particle velocity, non-negative intensity evaluated in terms of the acoustic impedance matrix obtained at the structural surface, and back-calculated non-negative intensity evaluated in terms of the acoustic impedance matrix obtained at a far-field receiver surface. Different configurations of inhomogeneous damping are applied to two elastic structures corresponding to a plate and a cylindrical shell. To examine the influence of inhomogeneous damping on sound radiation, the acoustic intensity on the structural surface, the acoustic intensity on several different far-field receiver surfaces, non-negative intensity and back-calculated non-negative intensity are numerically compared for different inhomogeneous damping cases.

We investigate the influence of uncertain input parameters on the aeroacoustic feedback of cavity flows. The so-called Rossiter feedback requires a direct numerical computation of the acoustic noise, which solves hydrodynamics and acoustics simultaneously, in order to capture the interaction of acoustic waves and the hydrodynamics of the flow. Due to the large bandwidth of spatial and temporal scales, a high-order numerical scheme with low dissipation and dispersion error is necessary to preserve important small scale information. Therefore, the open-source CFD solver FLEXI, which is based on a high-order discontinuous Galerkin spectral element method, is used to perform the aforementioned direct simulations of an open cavity configuration with a laminar upstream boundary layer.

To analyze the precision of the deterministic cavity simulation with respect to random input parameters, we establish a framework for uncertainty quantification (UQ). In particular, a nonintrusive spectral projection method with Legendre and Hermite polynomial basis functions is employed in order to treat uniform and normal probability distributions of the random input. The results indicate a strong, nonlinear dependency of the acoustic feedback mechanism on the investigated uncertain input parameters. An analysis of the stochastic results offers new insights into the noise generation process of open cavity flows and reveals the strength of the implemented UQ framework.

The numerical stability of different formulas for the correction term of the half-space Green’s function is investigated. The formula with complex monopoles is taken as a reference. The expressions of Koh and Yook, tested in a previous publication [R. Piscoya and M. Ochmann, Acoustical Green’s function and boundary element techniques for 3D half-space problems, J. Comput. Acoust. (2017), https://doi.org/10.1142/S0218396X17300018], are rewritten to improve their range of application. The formulas of Sommerfeld and Thomasson are analyzed and its suitability for a BEM implementation is evaluated by comparing their accuracy against our reference. For the sake of completeness, the first and second derivatives of the formulas are explicitly written.

The meshless local Petrov–Galerkin (MLPG) and the local boundary integral equation (LBIE) methods has been introduced approximately three decades ago. These methods are based on writing the local weak form of the governing equation and performing subsequent numerical integration and interpolations in the local subdomains. A key step is the choice of the test function in the local weak form, which has historically led to several different formulations regarding the final form of the local integrals. Considering the application of the methods to acoustics, four different test functions have been employed so far in the literature; all of these approaches resulted in formulations which contain domain integrals. In this paper, we present a new test function to be used in meshless methods, which yields a simple form of the local integral equation without domain integrals and provides significant improvement in terms of the computational time and CPU requirements. The efficiency and the accuracy of the new method are presented and compared with the previous methods.