Ground effects in time‐domain simulations of outdoor sound propagation.
Finite‐difference, time‐domain (FDTD) methods are precise and powerful tools to solve linearized Euler equations. Interaction between the acoustic waves with local wind or temperature gradients as well as a complex topography can be taken into account. Recently, a time‐domain boundary condition has been proposed [Cotte et al., AIAA J. 47, 2391–2403 (2009)] and has been implemented in a FDTD solver using methods developed in the computational aeroacoustics community [Bogey and Bailly, J. Comput. Phys. 194, 194–214 (2004)]. Propagation of an initial pulse over a distance of 500 m in a two‐dimensional geometry is considered in a frequency band up to 1200 Hz. Surface waves which propagate close to and parallel to impedance grounds are exhibited. To validate the obtained results, a comparison is made in the time‐domain with an analytical solution. Ground effects are discussed depending on the impedance of the ground. [This work was performed using HPC resources from GENCI‐IDRIS (Grant 2009‐022203).]
Padé approximation in time-domain boundary conditions of porous surfaces.
Formulation and implementation of time-domain boundary conditions (TDBCs) at the surface of a reactive porous material are made challenging by the slow decay, complexity, or noncausal nature of many commonly used models of porous materials. In this paper, approaches are described that improve computational efficiency and enforce causality. One approach involves approximating the known TDBC for the modified Zwikker-Kosten impedance model as a summation of decaying exponential functions. A second approach, which can be applied to any impedance model, involves replacing the characteristic admittance with its Padé approximation. Then, approximating fractional derivatives with decaying exponentials, a causal and recursive TDBC is formulated.
Transient nonlinear thermal stability analysis of metro cable under pulse power
The load of metro cable system varies a lot from morning rush hours to the evening closure time. With many trains hopping in the rush hours, the long term and economical short term thermal stability become a big issue as a result of the large difference between thermal time constant and the electrical time constant of cable. Based on a 10kV three-core XLPE metro power cable model, the theoretical thermal capacity was calculated according IEC standard. Then, a Finite Element Method (FEM) was employed to analyze the transient electro-thermal phenomena, taking into account various types of power loss in cable system and the non-linear electrical conductivity of the conductor. Finally, the thermal distribution inside the cable was presented as a function of time, and the maximum thermal capacity was predicted in metro cable.