In this paper, an acoustic pressure gradient formula capable of accounting for constant uniform flow effects is suggested. Acoustic pressure gradient calculation is key for acoustic scattering problems, because it may be used to evaluate the hardwall boundary condition. Realistic cases of rotating machines may be evaluated in a moving frame of reference and as such, an acoustic pressure gradient formula capable of accounting for constant uniform flow effects finds significant application. A frequency domain formulation was thus derived for periodic noise source motion located in a moving medium. The suggested formula is mathematically compact and easy to implement. It may offer us significant advantages when tonal noise emissions are dominant, thus finding application potential in acoustic scattering problems in rotating machines in a constant uniform flow. Moreover, the formula contains no Doppler factor, thus facilitating noise prediction for sources in supersonic motion.

The U.S. air transportation network (ATN) is critical to the mobility and the functioning of the United States. It is thus necessary to ensure that it is well-connected, efficient, robust, and secure. Despite extensive research on its topology, the temporal evolution of the network's robustness remains largely unexplored. In the present paper, a methodology is proposed to identify long-term trends in the evolution of the network's topology and robustness over time. The study of the U.S. domestic ATN's robustness was performed based on annual flight data from 1996 to 2016 and network analytics were used to examine the effects of restructuring that followed the 9/11 events. Centrality measures were computed and a node deletion method was applied to assess the network's tolerance to a targeted attack scenario. The outcome of this study indicated that the 9/11 terrorist attacks triggered vast restructuring of the network, in terms of efficiency and security. Air traffic expanded, as new airports and air routes were introduced, allowing the network to recover rapidly and become more efficient. Security concerns resulted in significant improvement of the network's robustness. Since 2001, the global traffic and topological properties of the U.S. ATN have displayed continuous growth, due to the network's expansion. On the other hand, the results suggest that although the system's ability to sustain its operational level under extreme circumstances has lately improved, its tolerance to targeted attacks has deteriorated. The presented methodology has shown its potential to be applied on different network levels or different transportation networks, in order to provide a general perspective of the system's vulnerabilities.

This paper deals with the derivation of an analytical time-domain formulation for the prediction of the acoustic velocity field generated by moving bodies in a medium at rest, according to the Kirchhoff method. The present formulation can be implemented in acoustic pressure codes based on the Farassat's Kirchhoff formula for arbitrary moving bodies, thus allowing direct and fast calculation of the acoustic velocity field in scattering problems. For validation purposes, four test cases are considered, namely a three-dimensional monopole, dipole and quadrupole source, as well as a monopole in uniform flow. Comparison of the results with the analytical solutions proves the remarkable accuracy of the present formulation.