Time Domain Room Acoustic Solver with Fourth-Order Explicit FEM Using Modified Time Integration

This paper presents a proposal of a time domain room acoustic solver using novel fourth-order accurate explicit time domain finite element method (TD-FEM), with demonstration of its applicability for practical room acoustic problems. Although time domain wave acoustic methods have been extremely attractive in recent years as room acoustic design tools, a computationally efficient solver is demanded to reduce their overly large computational costs for practical applications. Earlier, the authors proposed an efficient room acoustic solver using explicit TD-FEM having fourth-order accuracy in both space and time using low-order discretization techniques. Nevertheless, this conventional method only achieves fourth-order accuracy in time when using only square or cubic elements. That achievement markedly impairs the benefits of FEM with geometrical flexibility. As described herein, that difficulty is solved by construction of a specially designed time-integration method for time discretization. The proposed method can use irregularly shaped elements while maintaining fourth-order accuracy in time without additional computational complexity compared to the conventional method. The dispersion and dissipation characteristics of the proposed method are examined respectively both theoretically and numerically. Moreover, the practicality of the method for solving room acoustic problems at kilohertz frequencies is presented via two numerical examples of acoustic simulations in a rectangular sound field including complex sound diffusers and in a complexly shaped concert hall

An explicit time-domain finite element method for room acoustics simulations: Comparison of the performance with implicit methods

This paper presents the applicability of an explicit time-domain finite element method (TD-FEM) using a dispersion reduction technique called modified integration rules (MIR) on room acoustics simulations with a frequency-independent finite impedance boundary. First, a dispersion error analysis and a stability analysis are performed to derive the dispersion relation and the stability condition of the present explicit TD-FEM for three-dimensional room acoustics simulations with an infinite impedance boundary. Secondly, the accuracy and efficiency of the explicit TD-FEM are presented by comparing with implicit TD-FEM using MIR through room acoustics simulations in a rectangular room with infinite impedance boundaries. Thirdly, the stability condition of the explicit TD-FEM is investigated numerically in the case with finite impedance boundaries. Finally, the performance of the explicit TD-FEM in room acoustics simulations with finite impedance boundaries is demonstrated in a comparison with the implicit TD-FEM. Although the stability of the present explicit TD-FEM is dependent on the impedance values given at boundaries, the explicit TD-FEM is computationally more efficient than the implicit method from the perspective of computational time for acoustics simulations of a room with larger impedance values at boundaries

Implosion Simulation by Hydro Code Coupled with Laser Absorption using New Raytrace Algorithm

The calculation of the laser absorption is very important for implosion simulations to capture precisely its dynamics. In many implosion simulations, the laser absorption is computed by the use of ray tracing. However, the conventional ray tracing method has the problem that it generates a non-physical absorption distribution because it represents a laser beam by a finite number of rays. Such a non-physical distribution on the target surface could be numerical perturbations that grow drastically due to Rayleigh-Taylor instability. An enormous number of rays are required to avoid such a non-physical distribution. This results in high computational costs. Thus, we have developed a new method of ray tracing that essentially generates no non-physical absorption distribution. In the new algorithm, rays are inversely traced from grid points unlike the conventional method. This paper presents the new algorithm and a preliminary implosion simulation where the pressure perturbation due to the non-uniformity of the irradiation is computed by the use of this new ray tracing

Single domain growth and charge ordering of epitaxial YbFe2O4 films

YbFe2O4 is a charge-ordered ferroelectric that exhibits coupling between magnetization and electric polarization near room temperature and crystallizes in a rhombohedral structure (R3¯m). This study presents an attempt to fabricate stoichiometric and epitaxial YbFe2O4-δ films with a nearly single-domain structure using an RF magnetron sputtering method. The (0001)-oriented epitaxial films of YbFe2O4-δ on YSZ (111) substrates via reactive sputtering method exhibited clear three-fold symmetry normal to the substrate without the formation of twin domains rotated by 60°. The oxygen stoichiometry of the epitaxial YbFe2O4-δ was improved by controlling an oxygen partial pressure (PO2) during the deposition. The films showed a sharp ferrimagnetic transition, and the transition temperature (TN) increased linearly to approximately 245 K with decreasing PO2. The magnitude of magnetization of the obtained films was comparable to that of bulk single crystals. Further, the electron diffraction pattern of the stoichiometric films confirmed the presence of three-dimensional charge order, which is consistent with the behavior of the bulk crystals as well

Time-domain extended-reaction microperforated panel sound absorber modeling for acoustics simulation by finite element method

For precise wave-based room acoustics modeling, an accurate extended-reaction (ER) sound absorber model must be formulated to assess frequency and incident-angle dependences of a sound absorber. Two novel efficient time-marching schemes with implicit time-domain FEM (TD-FEM) are presented to model the extended reacting boundary of microperforated panel (MPP) sound absorbers. Generally, MPP absorbers (MPPAs) have an air cavity behind them, which causes ER behavior. Formulating the ER behavior of MPPAs is necessary for simulating room acoustics. A hindrance to the time-domain modeling of the ER of MPPAs is the need to treat its complex impedance on the microperforations. The proposed schemes model MPPs as interior boundary conditions and deal with the complex transfer impedance with auxiliary differential equations (ADEs), producing stable schemes after the Crank–Nicolson solver is applied. For scheme verification, the impedance tube model with a single-leaf MPPA is analyzed. Additionally, the effectiveness of the proposed schemes is assessed by practical room acoustics modeling involving MPPAs and comparison with a frequency-domain FEM solver, which can address complex transfer impedance exactly. The results show excellent performance of the proposed methods. The TD-FEMs can model room acoustics, including the MPPA, O(100) times faster while maintaining accuracy comparable to that of FD-FEM